Search

Storms strongly affect coral reefs; one unstudied but potentially important outcome may be a decrease in herbivory, presumably through changes to freshwater, sediment and nutrient influx. Algal turfs are sensitive early indicators of reef condition, and experimental evidence demonstrates low sediment loads and strong herbivory maintain short, healthy turf. While unknown, storms likely disrupt these controlling forces. We have observed storms that generate frequent, visible sediment plumes in Moorea, French Polynesia. To evaluate the effects of storms on herbivory, we conducted a set of field experiments manipulating herbivore access to naturally occurring turf under three rainfall regimes: no rain, light rain, and heavy rain that generated a plume event. We found strong effects of herbivores except following the storm, indicating disruption of typically strong top-down control by herbivores on algal turfs. Further research into the underlying mechanisms is critical as storm intensities and watershed development increase in many tropical regions.

Worldwide, many coral reefs are at risk of shifting to degraded algal-dominated states, due to compromised ecological conditions. Functional diversity of herbivorous fishes maintains coral reef health and promotes reef resilience to disturbances. Given previous evidence, it appears the functional roles of herbivorous fishes differ across geographical locations, indicating a need for further assessment of macroalgal consumption by herbivorous fishes. We assessed functional diversity by examining foraging behavior of herbivorous fish species on macroalgae on a fringing reef in Moorea, French Polynesia. We video-recorded choice experiments containing seven common macroalgae and used Strauss' linear resource selection index to determine macroalgal selectivities. We used cluster analysis to identify any distinct groups within herbivorous fish species, given the macroalgal species they targeted, and fitted generalized linear mixed-effects models to identify factors that best predicted the number of bites taken on macroalgae. Seven species from 3 fish families/tribes took a total of 956 bites. Fish species differed in their selectivity with some species (Naso lituratus, N. unicornis, Calatomus carolinus) strongly preferring one or two macroalgal species, while other fish species (Acanthurus nigrofuscus, Ctenochaetus striatus, Chlorurus sordidus, Balistapus undulatus) were less selective. This resulted in fish species forming two clusters. Only 3 of 7 macroalgae were preferred by any fish species, with two fish species both preferring the same two macroalgae. The limited differences in fish species' preferences for different macroalgae suggests limited functional complementarity. Two models (macroalgal species identity+fish functional group, macroalgal species identity+fish species) best predicted the number of bites taken on macroalgae compared to models incorporating only a single explanatory factor or fish family. In the context of this Moorean fringing reef, there is greater functional redundancy than complementarity of herbivorous fishes consuming macroalgae, and the fishes grouped together according to their relative selectivity. We observed fish species that are not classified as browsers consuming macroalgae, suggesting diets of herbivorous fishes may be broader than previously thought. Finally, we observed macroalgal selectivities and consumption that differed from previous studies for the same fish species. Our results contribute to the understanding of functional diversity of herbivorous fishes across coral reefs, and also highlight the need for additional research to further elucidate the role of context and functional diversity of herbivorous fishes consuming macroalgae on coral reefs.

Humans have increased nitrogen inputs to estuaries, necessitating research on the patterns of these resource subsidies. When originating outside an estuary, nitrogen is a resource subsidy. This subsidy can be delivered with a variety of temporal patterns, ranging from presses to pulses. In other communities, press versus pulse subsidies has substantial and divergent impacts; however, this framework is not typically used in estuaries. This study documents nitrogen concentration-frequencies in 5 Southern California estuaries that demonstrate these nutrients subsidies can often be characterized as pulse-interpulse. There were extremely high pulses of nitrogen (> 500 uM NH4, > 2500 uM NO3), even compared with other eutrophic estuaries, that were likely driven by strong seasonal and rare rainfall typical of Mediterranean climates. High nutrient concentrations were occasionally decoupled from rainfall, likely due to land use. Extreme eutrophy may have the potential to dampened pulses, producing uniform, elevated concentration-frequency inputs. These data suggest managers should target rainfall events in nutrient sampling protocols as well as monitor changes in biological indicators following these events to characterize the impact of these inputs. Climate change, population size, and coastal development will likely increase nutrient subsidies to estuaries, necessitating further research on the timing, frequency, and duration of these inputs as well as their ecological consequences. In particular, our work highlights the need to integrate pulse events into work on biological indicators in estuaries.

Coral reefs are highly productive ecosystems that typically exist in nutrient-poor waters, stimulating considerable research on the sources of nutrients that may support this productivity, including nutrient recycling. We examined the potential for resident herbivorous damselfish ( Stegastes nigricans ) to recycle nutrients within their territories by conducting an exclusion experiment on S. nigricans territories and measuring algal turf growth on shallow fringing reefs in Mo'orea, French Polynesia. A nutrient bioassay was used to evaluate whether the enhanced growth was due to increased nutrients. We chose 15 isolated damselfish territories for a caging experiment, with a large cage obstructing all fish access on one half of the territory (−Damselfish, −Herbivory) and a small low-profile cage allowing access but not herbivory to a portion on the other half (+Damselfish −Herbivory). Additionally, we measured turf in a randomly-chosen uncaged area within each territory where both damselfish and herbivory were present (+Damselfish +Herbivory). Behavioral observations confirmed damselfish rarely swam over the large-caged half of the territory, effectively eliminating their physical presence, but continued to interact with the other half. Over the 16-day experiment, time and treatment had a significant effect on turf growth. By day 16, the turf receiving the +Damselfish −Herbivory treatment experienced average net turf growth of 5.34 mm, which was significantly greater than turf growth within the territories receiving the −Damselfish −Herbivory (2.13 mm) and the +Damselfish +Herbivory (1.50 mm) treatments. The macroalgae in the nutrient bioassay grew 21% more on the side of the territory where damselfish were present compared to the side where they were excluded, supporting our conclusion that the mechanism that supported faster turf growth was increased nutrients. Thus, our study was the first to establish experimentally that the presence of damselfish increased algal growth by enhancing nutrient supplies via recycled waste products, a mechanism that has been advanced but never tested. Surveys established that damselfish territories covered 28–64% of the fringing reef flat along the north shore of Mo'orea, suggesting this increased productivity within territories could be a significant contribution to total reef productivity. These findings highlight the importance of continuing research into the long-term and spatial-scale effects of damselfish as nutrient recyclers on coral reefs.

Closely cropped algal turfs are characteristic of healthy coral reefs, but unchecked growth can cause transitions into long sediment-laden turfs, which may be an alternative degraded state. While control by herbivores is well established, potential interactions between nutrients and macroalgae have not been evaluated. We varied macroalgal presence and nutrient addition and measured turf height and sediment accumulation over 18 d on an algal-dominated fringing reef. We found a novel facilitative effect of macroalgal presence on turf height as macroalgal presence increased growth by 34% over its absence. Nutrient addition also significantly, but independently, increased turf height by 127% compared to no nutrient addition. Macroalgal presence reduced sediment accumulation, possibly by trapping the sediment and/or by a whiplash effect. Thus, overfished coral reefs experiencing macroalgal blooms and/or nutrient additions may be at risk of developing long algal turfs, which could maintain a persistent shift to a degraded state.

Worldwide, many coral reef ecosystems have shifted from coral to algal dominance, yet the ecological function of these emergent communities remains relatively unknown. Turbinaria ornata, a macroalga with a rapidly expanding range in the South Pacific, forms dense stands on hard substrate, likely providing ecological services unique from corals. While generally unpalatable, T. ornata can function as a secondary foundation species and hosts an epibiont community that may provide overlooked trophic resources in phase shifted reef ecosystems. Results from video recorded field experiments designed to quantify consumer pressure on T. ornata epibionts showed that both consumer pressure and epibiont cover increased with thallus size. Additionally, most fish species, including herbivores, omnivores, and detritivores, exhibited higher bite rates on thalli with epibionts compared to thali with epibionts experimentally removed. Juvenile parrotfishes were responsible for 50% of total bites recorded and also had the highest bite rates. Results indicate that epibionts, particularly on large T. ornata, are a food resource for a diversity of fishes, representing a previously undescribed function of this macroalga in coral reef ecosystems. Exploring the functions of macroalgal dominated reef communities will be increasingly important as reefs continue to phase shift toward macroalgal dominance in the Anthropocene.

Foundation species facilitate associated communities and provide key ecosystem functions, making anthropogenically driven phase-shifts involving these species critically important. One well-documented such phase-shift has been from coral to algal domination on tropical reefs. On South Pacific coral reefs, the macroalga Turbinaria ornata has expanded its range and habitat but, unlike algae that often dominate after phase-shifts, T. ornata is structurally complex and generally unpalatable to herbivores. Therefore, it may serve a foundational role on coral reefs, such as providing habitat structure to more palatable primary producers and corresponding trophic support to fishes. We predicted increasing T. ornata density would facilitate growth of associated algae, resulting in a positive trophic cascade to herbivorous fish. An experiment manipulating T. ornata densities showed a unimodal relationship between T. ornata and growth of understory algae, with optimal growth occurring at the most frequent natural density. Epiphyte cover also increased with density until the same optimum, but remained high with greater T. ornata densities. Foraging by herbivorous fishes increased linearly with T. ornata density. An herbivore exclusion experiment confirmed T. ornata facilitated epiphytes, but resource use of epiphytes by herbivores, though significant, was not affected by T. ornata density. Therefore, T. ornata performs foundational roles because it provides novel habitat to understory and epiphytic macroalgae and trophic support to consumers, though likely this function is at the expense of the original foundational corals.

Herbivores balance resource requirements with predation risk, which can differ among landscapes; hence, landscape can shape these trade-offs, influencing herbivore distribution and behavior. While this paradigm has been well established on coral-dominated reefs, tropical reefs worldwide are shifting to algal dominance. If herbivores avoid algae due to higher risk and forage in coral, these algal states may be stabilized. However, if herbivores forage more in resource-rich algal states, this may promote coral recovery. We assessed the distribution and behavior of herbivorous fishes in Moorea, French Polynesia in coral and algal turf-dominated fringing reef sites. Acanthuridae were more abundant in coral states and Labridae, tribe Scarinae, in algal turf states, though total fish abundances were equivalent in the two states. Fish in both families spent more time feeding in algal states and hiding/swimming in coral states. Thus, behavior reflects the trade-off between resource acquisition and refuge in these two landscapes and may promote recovery to coral.

Ecological communities are subjected to multiple anthropogenic stressors at both global and local scales that are increasing in number and magnitude. Stressors can interact in complex ways and are classified as additive, synergistic or antagonistic; the nature of the interaction is key to predicting changes and understanding community resilience. Coral reefs are among the most impacted communities and have shifted from coral- to algal-dominated states, and overfishing, nutrient enrichment and sedimentation are local stressors that often co-occur and may support degraded algal states. Short algal turfs are abundant benthic space holders on healthy reefs that may be pushed by local stressors to long algal turfs, a more degraded state that may prevent recovery to coral dominance. We conducted a fully crossed three-factor field experiment on short algal turf communities manipulating herbivory pressure (+/−cages), nutrients (+/−fertilizer) and sediments (natural accumulation/removal). We applied stressors for 16 days, removed them and monitored turf height during and after manipulations. We found that significant pair-wise interactions between all stressors pushed the community towards a degraded state with longer algal turfs. All three types of interactions (additive, synergistic and antagonistic) were common and occurred in equal frequency, suggesting more investigations into all types are needed to accurately predict community responses to multiple stressors. For example, when herbivores were present, nutrients and sediments interacted additively, while in the absence of herbivores, nutrients and sediments interacted synergistically. All interactions broke down following termination of experimental manipulations and all effects were undetectable after 49 days, indicating that this reef may be resilient, at least when stressors are applied on a short time-scale. Synthesis. Because management of local stressors is often more tractable than global stressors, local management has been proposed as a means to offset global stressors. However, ecological communities often experience multiple local stressors simultaneously, and interactions between stressors, including synergisms and antagonisms, may be the source of nonlinear shifts in communities or “ecological surprises.” The majority of interactions in our study were both strong and nonlinear, and we suggest that, if pervasive across systems, nonlinear interactions may drive the recent global increase in “ecological surprises.”

Resource subsidy regimes, which range from presses to pulses, are common structuring forces in communities, yet research contrasting their effects is lacking. Many coastal marine ecosystems, including estuaries and coral reefs, have experienced increased nutrient subsidies while concurrently shifting to macroalgal dominance; however, the role of subsidy regime in transitions remains unknown. We created concentration–frequency distributions of nutrients in Cook’s Bay, Moorea, French Polynesia, and Carpinteria Salt Marsh Reserve, CA, USA. Both showed relatively high pulses interspersed by press concentrations. We grew dominant macroalgae alone and together in microcosms approximating these subsidy regimes to quantify individual performance and competitive outcomes. Subsidy regime changed growth and competitive abilities of macroalgae from both ecosystems but with divergent effects. In nutrient-limited reefs, different species were favoured under each enrichment regime, suggesting a fluctuating nutrient environment enhances diversity. In contrast, in eutrophic estuaries, enrichment of both regimes facilitated a single competitive dominant, suppressing diversity. Functional form groups did not predict responses to subsidy regime, likely because classifications ignore temporal variability in resource supplies. Because climate change will alter rainfall patterns globally, further accelerating nutrient subsidies from land to sea, understanding species’ responses to nutrient subsidy regimes is key to predicting the fate of coastal communities.

Coral-dominated reefs typically have abundant refuges and limited resources for herbivores; conversely, algal-dominated reefs have abundant resources but often have limited refuges. This difference results in a trade-off between risk and reward for herbivorous fishes. However, disturbances that kill coral but retain topographic complexity can decouple this tradeoff, challenging this paradigm. Many fringing reefs in Moorea, French Polynesia are algal-dominated with heterogeneous structural complexity likely driven by disturbances associated with crown-of-thorns predation and typhoons, which occurred between 2006 and 2010. Short turfs are a good recruitment surface for coral larvae, and thus has a greater potential to recover to coral dominance than long turfs. To explore how variation in complexity impacts the behavior of herbivorous fishes, we compared abundance, residence time, and time budgets of Acanthurus nigrofuscus, a common herbivore species, in paired high versus low complexity areas. We chose areas in close proximity to maximize the likelihood fish could choose between complexity types. We also measured turf height in high versus low complexity plots. Fish were ~3× more abundant, spent roughly double the time in, and budgeted more than 4× as much time to hiding in high complexity reef areas. Although, on average, individual herbivores spent the same proportion of time foraging in both areas, their greater abundance and longer residence time suggests their local herbivory rates could be 7.5 times higher in high complexity areas. We also found turfs in high complexity plots were roughly 75% the height of turfs in low complexity plots. This provides some evidence for stronger overall herbivory pressure in high complexity areas, albeit differences in absolute turf height were modest (~0.5 mm). Combined, these results suggest reefs that retain structural complexity following disturbance may be more likely to recover due to the higher localized herbivory rates resulting from uncoupling of the tradeoff between structural complexity and resource limitation.

Shifts from coral-dominated to macroalgal-dominated states on coral reefs are typically attributed to overfishing, often in combination with natural or other anthropogenic disturbances. However, a recent study attributed increased macroalgal cover to enhanced nutrient supplies alone without associated reductions in herbivorous fishes. This highlights the need to understand macroalgal responses to terrestrial runoff and how this can contribute to their proliferation on coral reefs. We explored the responses of two common brown macroalgae, Sargassum mangarevense (Grunow) Setchell and Padina boryana Thivy to nutrient addition, sediment addition, and mechanical damage in 3 field experiments. In the first experiment, we exposed all experimental units to herbivores and found that nutrient addition ameliorated biomass loss of both S. mangarevense and P. boryana by 79% and 47%, respectively, while sediment addition had no effect on either species. Exploring the underlying causes of these findings, we determined nutrient addition caused a 5% increase in toughness of S. mangarevense and a 51% increase in calcification of P. boryana. In a final experiment, both mechanical damage and nutrient addition independently increased calcification of P. boryana, while an antagonistic interaction between these factors for S. mangarevense suggests an upper limit to thallus toughness. Overall, we demonstrate that nutrients from anthropogenic terrestrial runoff may support macroalgal proliferation on coral reefs by ameliorating short-term biomass losses in the presence of herbivores. Further, we suggest the mechanism behind this amelioration may be changes in algal physical characteristics, specifically thallus toughness and calcification, in response to anthropogenic or natural disturbances.

Seagrasses are important foundational species that frequently display distinct depth distributions, although the drivers of these patterns can be spatially and temporally variable. While the pantropical seagrass Halophila decipiens is known from waters as shallow as 1 m deep, in Moorea, French Polynesia we only found it > 6.4 m deep. To explore factors affecting depth distribution, we transplanted H. decipiens into 3 habitats: the existing seagrass bed (control), just outside the seagrass bed, and shallower habitat adjacent to a fringing coral reef. Results showed that growth was not significantly different between the control and just outside of the seagrass bed; however, number of shoots and rhizome length were significantly reduced adjacent to the reef. Transplanting seagrass into the shallow reef site with and without herbivore exclusion cages showed that H. decipiens grew in herbivore exclusion treatments, but lost both shoots and rhizomes in the control. These results indicate that H. decipiens can grow in shallow habitats adjacent to reefs on Moorea, but that herbivory pressure, presumably from the reef, limits its depth distribution.

While the importance of top-down control by grazers in maintaining tropical reef community structure is well known, the effect of ‘fishing down the food web’, which simultaneously changes the abundance and size of herbivorous fishes, has received less attention. As many fishing practices target the biggest fish and regulations often set minimum size limits, understanding size-dependency of this controlling force is important. We evaluated the hypothesis that reduction in the abundance and size of fish, regardless of species identity, reduces the role of herbivory in controlling algal abundance on reefs and assessed variation in efficacy of this top-down control on different types of common macroalgae. We quantified herbivory rates within cages of differing opening sizes on assemblages of four common algal species (Padina boryana, Dictyota bartayresiana, Halimeda opuntia and Galaxaura fasciculata) on two fringing reefs in Moorea, French Polynesia. Small acanthurids ( 50% in the largest opening cages (6 × 6 cm) where the maximum size fish entering was 12 cm in length (mean = 7.6, SE = 0.4). Fish entering medium (4.5 × 4.5 cm openings, maximum fish length = 8 cm, mean = 6.3, SE = 0.4) and small (3 × 3 cm openings, no fish observed entering) cages had herbivory rates approximately equal to the control treatment (1 × 1 cm openings). Consumption varied among algal species, with minimal consumption of physically and chemically defended algae and no pattern across treatments. Our results demonstrate a need for management plans to not only maintain the overall abundance of herbivorous fish but to protect the largest sizes for effective top-down control of algal communities.

Nitrogen (N) or phosphorus (P) limits primary productivity in nearly every ecosystem worldwide, yet how limitation changes over time, particularly in connection to variation in environmental drivers, remains understudied. We evaluated temporal and species-specific variability in the relative importance of N and P limitation among tropical macroalgae in two-factor experiments conducted twice after rains and twice after dry conditions to explore potential linkages to environmental drivers. We studied three common macroalgal species with varying ecological strategies: a fast-growing opportunist, Dictyota bartayresiana; and two calcifying species likely to be slower growing, Galaxaura fasciculata and Padina boryana. On the scale of days to weeks, nutrient responses ranged among and within species from no limitation to increases in growth by 20 and 40 % over controls in 3 d with N and P addition, respectively. After light rain or dry conditions, Dictyota grew rapidly (up to ~60 % in 3 d) with little indication of nutrient limitation, while Padina and Galaxaura shifted between N, P, or no limitation. All species grew slowly or lost mass after a large storm, presumably due to unfavorable conditions on the reef prior to the experiment that limited nutrient uptake. Padina and Galaxaura both became nutrient limited 3 d post-storm, while Dictyota did not. These results suggest that differing capabilities for nutrient uptake and storage dictate the influence of nutrient history and thus drive nutrient responses and, in doing so, may allow species with differing ecological strategies to coexist in a fluctuating environment. Moreover, the great variability in species’ responses indicates that patterns of nutrient limitation are more complex than previously recognized, and generalizations about N versus P limitation of a given system may not convey the inherent complexity in governing conditions and processes.

Coral populations and structural coral reefs have undergone severe reductions and losses respectively over large parts of the Galapagos Islands during and following the 1982-83 El Nino event. Coral tissue loss amounted to 95% across the Archipelago. Also at that time, all coral reefs in the central and southern islands disappeared following severe degradation and eventual collapse due primarily to intense bioerosion and low recruitment. Six sites in the southern islands have demonstrated low to moderate coral community (scattered colonies, but no carbonate framework) recovery. The iconic pocilloporid reef at Devil's Crown (Floreana Island) experienced recovery to 2007, then severe mortality during a La Nina cooling event, and is again (as of 2017) undergoing rapid recovery. Notable recovery has occurred at the central (Marchena) and northern islands (Darwin and Wolf). Of the 17 structural reefs first observed in the mid-1970s, the single surviving reef (Wellington Reef) at Darwin Island remains in a positive growth mode. The remainder either degraded to a coral community or was lost. Retrospective analyses of the age structure of corals killed in 1983, and isotopic signatures of the skeletal growth record of massive corals suggest the occurrence of robust coral populations during at least a 500-year period before 1983. The greatest potential threats to the recovery and persistence of coral reefs include: ocean warming and acidification, bioerosion, coral diseases, human population growth (increasing numbers of residents and tourists), overfishing, invasive species, pollution, and habitat destruction. Such a diverse spectrum of disturbances, acting alone or in combination, are expected to continue to cause local and archipelago-wide mortality and degradation of the coral reef ecosystem.

Theoretical predictions of a direct positive association between productivity and diversity (P/D) have received considerable empirical support in marine systems, although this relationship tends to break down at smaller scales (< 25 km). Another theory commonly advanced in terrestrial systems, the species-energy hypothesis (SEH), relates productivity (= energy) to diversity indirectly through an increase in population abundances, which then reduces the likelihood of localized extinction and therefore enhances richness. The SEH has been supported on both global and local scales but has not been experimentally tested in estuarine systems or at the scale of a patch. We tested whether the predictions of the SEH (increased abundance and diversity) or the P/D hypothesis (increased diversity, no predictions for abundance) described the relationship between macroalgal productivity and the abundance and diversity of infauna and site-attached epifauna (hereafter macrofauna) and the microphytobenthos (MPB) in an estuarine benthic community. We conducted a single-factor experiment comparing the effects over time of three treatments comprised of two levels of macroalgae as an added energy source and a plastic mimic as a control that provided structure and habitat but no energy. Contrary to predictions of both the SEH and the P/D hypothesis, there were no measurable differences in taxonomic richness among treatments. However, in partial support of the SEH, we found that total macrofaunal abundance was 50% higher in both macroalgal treatments compared to the mimic and these were largely comprised of detritivores such as spionid polychaetes and oligochaetes. Phaeopigments increased in macroalgal plots but not the mimic, suggesting that added productivity decomposed, supplying more food for detritivores. As this taxonomic group is present in all plots, although in differing abundances, increasing energy flow through this pathway did not enhance taxonomic richness. Macroalgae stimulated the MPB (measured as chl a) more than did the mimic, and elevated chl b in macroalgal treatments indicated the increase was due to sporeling green macroalgae. However, benthic diatoms and dinoflagellates (estimated from chl c) were not affected by treatment. Thus, we found that at the patch level, neither the SEH nor the P/D hypothesis predicted patterns of diversity for estuarine macrofauna; rather, in this system, increased productivity simply supported higher abundances within the same taxonomic groups that occurred at all productivity levels.

The role of herbivorous fishes in coral reef resilience has increased interest in the process of herbivory and has focused attention on herbivore feeding behavior, making it important to evaluate experimental methods used to assess herbivore decisions. We tested whether small-scale differences in collection site play a role in within-species palatability of macroalgae. Baseline grazing assays using algae collected on a fringing reef in Moorea, French Polynesia, revealed that herbivore preferences among three common species ranked Padina boryana > Sargassum mangarevense ≫ Amansia rhodantha. Comparing grazing preferences between individual thalli of the same species collected

Local anthropogenic stressors such as overfishing, nutrient enrichment and increased sediment loading have been shown to push coral reefs toward greater dominance by algae. In a few cases this shift has been temporary, with the ability to recover to a healthy coral-dominated community after disturbance, suggesting some systems have considerable resilience. However, an understanding of the circumstances under which reefs may recover is only beginning to emerge. We monitored recovery of a coral-dominated reef in the Eastern Tropical Pacific (ETP) after cessation of a ∼6 month multiple stressor experiment (with herbivore exclosure, nutrient addition, and sediment addition). We observed substantial recovery from small-scale disturbances, though there were differences in both the extent and temporal dynamics of recovery between treatments. Plots that had been caged showed the largest recovery in absolute terms and recovery was quite rapid, while nutrient and sediment addition plots were slower to recover. We also observed different recovery patterns depending on the type of algae that replaced coral during or after disturbances. Macroalgae that established during manipulation were almost completely removed within 2 weeks, revealing that a significant proportion had covered still-living coral. Turf algae persisted longer, but were almost completely replaced by regenerating coral within 18 months. Very little crustose coralline algae were apparent during manipulations, but coverage did increase during recovery. This rapid recovery of corals after simulated anthropogenic disturbance to ETP reefs underscores the value of management of local stressors for short-term recovery and perhaps as a buffer for longer-term global stressors.

Closely cropped algal turfs support key ecosystem functions on healthy coral-dominated reefs, yet how this important reef component is affected by sedimentation, a key stres-sor on reefs worldwide, is relatively unknown. We used a 2-factor caging experiment to evaluatethe effects of varying sediment depth and presence of herbivorous fish on algal turf height on afringing reef in Mo’orea, French Polynesia. Without herbivory, 2 mm of sediment reduced turfgrowth by ~50% compared to sediment removal treatments; in contrast, growth with 4 mm of sed-iment was low or negligible regardless of herbivory treatment. Negative effects of sediment werelinked to the development of black basal layers of sediment, indicating accumulation of hydrogensulfide. Black sediment occurred in 60 to 70% of all 4 mm plots and in 43% of caged 2 mm plotsbut was not found in open 2 mm plots, implying that grazing ameliorated development of blacksediment under 2 mm loads. Sediment levels of 2 mm did not deter herbivory, evidenced by thesignificant decrease in turf height in open compared to caged plots. Under 4 mm of sediment,black sediment inhibited both growth and herbivory where it occurred. Without black sediment,however, fish grazing balanced algal growth, resulting in negligible algal height changes across4 mm plots but with differing underlying mechanisms. Field surveys on other sedimented reefswith healthy herbivore communities confirmed an increase in the presence of black sediment atdepths over 3 mm. Thus, deeper sediment depths inhibit turf growth, yet under moderate levels ofsedimentation, intact herbivorous fish communities may maintain closely cropped, healthy turfcommunities by preventing the negative effects of black sediment.KEY WORDS: Sediments · Algal turf · Herbivory · Fringing reef · Tropical algae

Multiple natural and anthropogenic stressors impact coral reefs across the globe leading to declines of coral populations, but the relative importance of different stressors and the ways they interact remain poorly understood. Because coral reefs exist in environments commonly impacted by multiple stressors simultaneously, understanding their interactions is of particular importance. To evaluate the role of multiple stressors we experimentally manipulated three stressors (herbivore abundance, nutrient supply, and sediment loading) in plots on a natural reef in the Gulf of Panama in the Eastern Tropical Pacific. Monitoring of the benthic community (coral, macroalgae, algal turf, and crustose coralline algae) showed complex responses with all three stressors impacting the community, but at different times, in different combinations, and with varying effects on different community members. Reduction of top–down control in combination with sediment addition had the strongest effect on the community, and led to approximately three times greater algal biomass. Coral cover was reduced in all experimental units with a negative effect of nutrients over time and a synergistic interaction between herbivore exclosures and sediment addition. In contrast, nutrient and sediment additions interacted antagonistically in their impacts on crustose coralline algae and turf algae so that in combination the treatments limited each other’s effects. Interactions between stressors and temporal variability indicated that, while each stressor had the potential to impact community structure, their combinations and the broader environmental conditions under which they acted strongly influenced their specific effects. Thus, it is critical to evaluate the effects of stressors on community dynamics not only independently but also under different combinations or environmental conditions to understand how those effects will be played out in more realistic scenarios.

Confidence in the use of macroalgae as an indicator of estuarine eutrophication is limited by the lack of quantitative data on the thresholds of its adverse effects on benthic habitat quality. In the present study, we utilized sediment profile imagery (SPI) to identify thresholds of adverse effects of macroalgal biomass, sediment organic carbon (% OC) and sediment nitrogen (% N) concentrations on the apparent Redox Potential Discontinuity (aRPD), the depth that marks the boundary between oxic near-surface sediment and the underlying suboxic or anoxic sediment. At 16 sites in eight California estuaries, SPI, macroalgal biomass, sediment percent fines, % OC, and % N were analyzed at 20 locations along an intertidal transect. Classification and Regression Tree (CART) analysis was used to identify step thresholds associated with a transition from "reference" or natural background levels of macroalgae, defined as that range in which no effect on aRPD was detected. Ranges of 3–15 g dw macroalgae m−2, 0.4–0.7 % OC and 0.05–0.07 % N were identified as transition zones from reference conditions across these estuaries. Piecewise regression analysis was used to identify exhaustion thresholds, defined as a region along the stress–response curve where severe adverse effects occur; levels of 175 g dw macroalgae m−2, 1.1 % OC and 0.1 % N were identified as thresholds associated with a shallowing of aRPD to near zero depths. As an indicator of ecosystem condition, shallow aRPD has been related to reduced volume and quality for benthic infauna and alteration in community structure. These effects have been linked to reduced availability of forage for fish, birds and other invertebrates, as well as to undesirable changes in biogeochemical cycling.

Eutrophication, defined as the accumulation of organic matter typically in response to anthropogenically enhanced nutrient inputs, often takes the form of macroalgal blooms in shallow estuaries and causes a cascade of adverse ecosystem effects. Confidence in the use of macroalgae as an indicator of eutrophication in estuaries is limited by the lack of quantitative data on thresholds of adverse effects. Field experiments can provide "benchmarks" of no effect or adverse effects that can be used to validate thresholds derived statistically from field data. To determine a benchmark of adverse effects of macroalgal abundance on macrobenthic faunal communities in intertidal flats, experiments were conducted in two sites in Bodega Harbor (BOD) and two sites in Upper Newport Bay (UNB), California, USA. At each site, 24 cages maintained six treatments of macroalgae for eight weeks, with mat depths of 0, 1.0, 1.5, 2.5, 3.5, and 5.0 cm composed mostly of bloom-forming green macroalgae in the genus Ulva. Every two weeks, cores of sediment (10 cm deep) were collected, and macrofauna were quantified. Mats 1 cm deep, equivalent to a biomass of 110-120 g dry mass (dm)/m2 or 840-930 g wet mass/m2, resulted in the reduction of macrofaunal abundance by at least 67% and species richness by at least 19% within two weeks at three of four sites. Loss was attributed to the decline of key functional groups. Surface-deposit feeders were eliminated from one site at BOD within four weeks and at one site in UNB within six weeks, while 1-cm mats negatively affected suspension feeders and herbivores in the second site at BOD. In contrast, the other site at UNB was not affected by macroalgal treatment, likely due to an initial community composed of a high proportion of subsurface-deposit feeders tolerant of stressful environments. Macroalgal abundances as low as 110-120 g dm/m2 had significant and rapid negative effects on macrobenthic invertebrates, providing a clear benchmark of adverse effects of macroalgal blooms on macrofaunal abundance and community structure, two indicators of ecosystem health. This information can inform the establishment of appropriate metrics for macroalgal abundance in eutrophic estuaries.

Cyanobacteria, an increasingly important epiphyte on macroalgae and seagrass, have been shown to have strong effects on its hosts; this association has been identified as a driving mechanism that maintains algal blooms on coral reefs. We examined both the costs and benefits of epiphytism on 2 algal congeners of Halimeda (H. tuna and H. opuntia), both of which are abundant members of tropical reef communities in the Caribbean. To evaluate potential benefits of an associational defense as well as costs to growth, we manipulated herbivore access to (uncaged/caged) and cyanobacteria presence on (epiphytized/cleaned) 2 species of Halimeda on shallow patch reefs in Belize and measured change in branch length and segment number after 10 (H. tuna) and 5 (H. opuntia) days. Cyanobacterial epiphytes did not serve as an associational defense from herbivory as there were no differences between caged and uncaged treatments for either response variable. The presence of cyanobacterial epiphytes did not affect the growth of branches or net generation of new segments, demonstrating there was also no cost to growth. The robustness of both species of Halimeda to epiphytism contrasts strongly with recent research that found strong effects of epiphytes on several other species of tropical algae. Our results may be attributed to the unique characteristics of Halimeda, a heavily physically and chemically defended algal genus, and the shallow nature of the patch reefs reducing the potential for significant light limitation. These findings suggest that close interactions such as epiphytism may not be as generalizable as originally assumed; studies must consider differences among host species, as this may lead to a better understanding of community-wide effects.

Rapidly growing human populations have caused heavy modifications to the watersheds of many Mediterranean climate estuaries, subjecting them to excessive nutrient enrichment and harmful macroalgal blooms. Despite these impacts, comprehensive studies in these systems are rare and comparisons between systems are lacking. We surveyed five southern California estuaries that ranged in size from 93 to 1,000 ha and incorporated differing land usages and watershed sizes. We sampled environmental variables (sediment redox potential, organic content, total nitrogen and total phosphorus, water column nitrate, ammonium, and salinity) and macroalgal cover and biomass quarterly at three locations within each estuary over 15 months to compare spatial and wet vs. dry season patterns. Maximum mean water column nitrate concentration across all estuaries ranged from 47 to 1,700 μM, showing that all estuaries were highly enriched with nitrogen, at least at some times. Mean macroalgal biomass ranged from 0 to 1,500 g wet wt m−2. However, neither nutrient concentrations nor algal biomass showed consistent seasonal patterns as maximum values occurred in different seasons in different estuaries. Three-dimensional principal components analysis followed by regression analyses confirmed that macroalgal abundance was not directly related to water or sediment N concentrations. Rather each of these southern California estuaries showed individual patterns in all measured variables, which were most likely induced by a suite of physical modifications unique to each system and its watershed.

Algae from two kingdoms and four major phyla form productive benthic communities that play key ecological roles on coral reefs of the eastern tropical Pacific (ETP). The diversity of algae that comprise these communities is being actively explored throughout the region, with new species still being discovered. Due to its physical setting, there are several distinct algal habitats in the ETP, including reef crests dominated by crustose coralline algae and algal turfs, reef flats that support a higher abundance of foliose macroalgae, unconsolidated accumulations of dead coral rubble that may support rhodoliths and cryptic macroalgae, and deeper-water refuges from herbivory dominated by more of the palatable forms of macroalgae rarely found on reefs. Past work has identified a number of ecological processes that control algal populations and communities at play in the ETP, but research elucidating their relative roles is still limited in this region. Recent work suggests that these reefs are supported by allochthonous nutrient supplies such as upwelling and thermocline shoaling, though reduced light during these events may limit productivity to shallow reef zones. Though at least partially sheltered from major disturbances that structure other reef systems, such as hurricanes and outbreaks of crown-of-thorns sea stars, reefs of the ETP are strongly affected by sea surface temperature extremes associated with El-Nino-Southern Oscillation (ENSO) leading to major coral mortality with the opening of space for colonization by algae. Biotic processes that control algal communities include herbivory and competition with coral, both of which limit algal proliferation and may enhance recovery after disturbance. However, facilitative interactions within algal communities, such as reduced herbivory and amelioration of nutrient limitation by internal recycling that act as positive feedbacks within areas dominated by algae, may stabilize algal communities after disturbance. There is some empirical and modeling evidence that suggests these processes may drive reefs of the ETP to display alternative stable states. This evidence includes a bifurcated pattern of recovery after ENSO disturbance, the patchy nature of this recovery within certain reefs, and the existence of stabilizing feedbacks that support resilience of both coral and algal states. Further, a simulation model (cellular automaton) of ETP reefs parameterized with experimentally-derived feedback relationships suggests these reefs exist under environmental conditions that produce bistability. However, much research is still needed to fully understand the physical and ecological processes that structure algal communities, and how these controls may shift with anthropogenic impacts.

While nutrient enrichment increases herbivory rates on highly palatable species of tropical algae, it is unclear how nutrient enrichment affects species with differing initial palatabil- ities. We determined that there was a wide range in relative palatability among 5 perennial and common species of Caribbean coral reef algae in a cafeteria-style choice experiment, with overall preference for Acanthophora spicifera the highest, Dictyota dichotoma and Caulerpa sertulari- oides intermediate, and Turbinaria turbinata and Sargassum polyceratium the lowest, although there were significant differences in grazing rates among algal species between the 2 fringing reef sites tested. We then tested the hypothesis that increased nutrient content in algae increases palatability using choice experiments between enriched and ambient algal samples for each of 3 species along this palatability gradient: A. spicifera, D. dichotoma, and S. polyceratium. For all species, herbivores selected enriched over ambient samples across the 2 sites despite contrasting herbivore guilds (i.e. dominated by sea urchin Diadema antillarum versus herbivorous fishes). However, despite increased palatability of enriched S. polyceratium, overall herbivory on this spe- cies was still relatively low compared to other studies, suggesting that S. polyceratium in this sys- tem may have stronger chemical defenses, or that a specialist herbivore guild may be missing on this Caribbean reef. Regardless of the mechanism, our study implies that anthropogenic or natural events that result in high supplies of nutrients in coral reef seawater will increase overall seaweed palatability, even of unpalatable species.

Localized declines in coral condition are commonly linked to land-based sources of stressors that influence gradients of water quality, and the distance to sources of stressors is commonly used as a proxy for predicting the vulnerability and future status of reef resources. In this study, we evaluated explicitly whether proximity to shore and connections to coastal bays, two measures of potential land-based sources of disturbance, influence coral community and population structure, and the abundance, distribution, and condition of corals within patch reefs of the Florida Reef Tract. In the Florida Keys, long-term monitoring has documented significant differences in water quality along a cross-shelf gradient. Inshore habitats exhibit higher levels of nutrients (DIN and TP), TOC, turbidity, and light attenuation, and these levels decrease with increasing distance from shore and connections to tidal bays. In clear contrast to these patterns of water quality, corals on inshore patch reefs exhibited significantly higher coral cover, higher growth rates, and lower partial mortality rates than those documented in similar offshore habitats. Coral recruitment rates did not differ between inshore and offshore habitats. Corals on patch reefs closest to shore had well-spread population structures numerically dominated by intermediate to large colonies, while offshore populations showed narrower size-distributions that become increasingly positively skewed. Differences in size-structure of coral populations were attributed to faster growth and lower rates of partial mortality at inshore habitats. While the underlying causes for the favorable condition of inshore coral communities are not yet known, we hypothesize that the ability of corals to shift their trophic mode under adverse environmental conditions may be partly responsible for the observed patterns, as shown in other reef systems. This study, based on data collected from a uniform reef habitat type and coral species with diverse life-history and stress-response patterns from a heavily exploited reef system, showed that proximity to potential sources of stressors may not always prove an adequate proxy for assigning potential risks to reef health, and that hypothesized patterns of coral cover, population size-structure, growth, and mortality are not always directly related to water quality gradients.

Macroalgal dominance of some tropical reef communities in the Eastern Pacific after coral mortality during the 1997-1998 El Niño Southern Oscillation (ENSO) was facilitated by protection from herbivory by epiphytic cyanobacteria. Our results do not support that reduction in number of herbivores was a necessary precursor to coral reef decline and shifts to algal reefs in this system. Rather, macroalgae dominated the community for several years after this pulse disturbance with no concurrent change in herbivore populations. While results of microcosm experiments identified the importance of nutrients, especially phosphorus, in stimulating macroalgal growth, nutrient supply alone could not sustain macroalgal dominance as nutrient-stimulated growth rates in our in situ experiments never exceeded consumption rates of unprotected thalli. In addition, thalli with nutrient-enriched tissue were preferentially consumed, possibly negating the positive effects of nutrients on growth. These tropical reefs may be ideal systems to conduct experimental tests distinguishing phase shifts from alternative stable states. Shifts were initiated by a large-scale disturbance with no evidence of a changing environment except, perhaps, dilution in herbivory pressure due to increased algal cover. Community establishment was most likely stochastic, and the community was likely maintained by strongly positive interaction between macroalgal hosts and cyanobacterial epiphytes that uncoupled consumer control of community structure.

In many temperate estuaries, mats of opportunistic macroalgae accumulate on intertidal flats and in lower elevations of salt marshes, perhaps playing a role in linking water column nitrogen (N) supply to these benthic habitats. Using a flow-through seawater system and tidal simulator, we varied densities (equivalent to 0, 1, 2, or 3 kg m 2 wet mass) of 15 N-labelled macroalgae (Enteromorpha intestinalis) on estuarine sediments in microcosms with/without pickleweed (Salicornia virginica) to assess N transfers from algae. In the 6-week experiment, macroalgal biomass increased from initial levels in the lower density treatments but all algae lost N mass, probably through both leakage and decomposition. With all densities of algae added, sediments and pickleweed became enriched in 15 N. With increasing mat density, losses of algal N mass increased, resulting in stepwise increases in 15 N labeling of the deeper sediments and pickleweed. While we did not detect a growth response in pickleweed with macroalgal addition during the experiment, N losses from algal mats that persist over many months and/or recur each year could be important to the mineral nutrition of N-limited marsh plants. We conclude that N dynamics of intertidal sediments and lower salt marsh vegetation are linked to the N pools of co-occurring macroalgae and that further study is needed to assess the magnitude and importance of N transfers. D 2005 Elsevier B.V. All rights reserved.

The roles of co-occurring herbivores that modify habitat structure and ecosystem processes have seldom been examined in manipulative experiments or explored in early successional communities. In a created marsh in southern California (USA), we tested the individual and combined effects of two epibenthic invertebrates on nutrient and biomass pools, community structure, and physical habitat features. We manipulated snail (Cerithidea californica) and crab (Pachygrapsus crassipes) presence in field enclosures planted with pickleweed (Salicornia virginica) at elevations matching the plant's lower extent in an adjacent natural marsh. In the 4-month experiment, C. californica altered habitat structure by reducing sediment surface heterogeneity and shear strength (a measure of sediment stability) markedly throughout the enclosures. Both invertebrates had strong negative effects on a group of correlated sediment physicochemical characteristics, including nitrogen and organic matter concentrations and soil moisture. In addition, both invertebrates greatly reduced benthic chlorophyll a, a proxy for biomass of microphytobenthos. Compared to controls, macroalgal cover was up to sixfold lower with crabs present, while snails increased cover at low elevations of enclosures. Unexpectedly, macroalgal cover was eliminated with both species present, perhaps through P. crassipes consumption of larger thalli and C. californica reduction in cover of recruits. Neither species influenced the S. virginica canopy (quantified with an index of branch length and number); however, at the lower elevation of enclosures, the two species together negatively impacted the plant canopy. The two invertebrates' modifications to our experimental marshes led to distinct suites of biotic and physicochemical features depending on their presence or co-occurrence, with the latter producing several unexpected results. We propose that the roles and interactions of habitat-modifying fauna deserve further attention, particularly in the context of efforts to conserve and restore the processes found in natural systems.

Quarterly field sampling was conducted to characterize variations in water column and sediment nutrients in a eutrophic southern California estuary with a history of frequent macroalgal blooms. Water column and sediment nutrient measures demonstrated that Upper Newport Bay (UNB) is a highly enriched estuary. High nitrate (NO3 −) loads from the river entered the estuary at all sampling times with a rainy season (winter) maximum estimated at 2,419 mol h−1. This resulted in water NO3 − concentration in the estuary near the river mouth at least one order of magnitude above all other sampling locations during every seasons; maximum mean water NO3 − concentration was 800 μM during springer 1997. Phosphorus (P)-loading was high year round (5.7–90.4 mol h−1) with no seasonal pattern. Sediment nitrogen (N)-content showed a seasonal pattern with a spring maximum declining through fall. sediment and water nutrients, as well as percent cover of three dominant macroalgae, varied between the main channel and tidal creeks. During all seasons, water column NO3 − concentrations were higher in the main channel than in tidal creeks while tidal creeks had higher levels of sediment total Kjeldhal nitrogen (TKN) and P. During each of the four sampling periods, percent cover ofEntermorpha intestinalis andCeramium spp. was higher in tidal creeks than in the main channel, while percent cover ofUlva expansa was always higher in the main channel. Decreases in sediment N in both creek and channel habitats were concurrent with increases in macroalgal cover, possibly reflecting use of stored sediment TKN by macroalgae. Our data suggest a shift in primary nutrient sources for macroalgae in UNB from riverine input during winter and spring to recycling from sediments duirng summer and fall.

Enteromorpha intestinalis is a bloom-forming species of macroalgae associated with eutrophication. The objective of this study was to investigate how this alga performs osmoregulation and nutrient uptake in order to proliferate under environmental conditions that covary with eutrophication. We quantified the response ofE. intestinalis to salinity, light, and nutrients. We performed two short-term (48 h) laboratory experiments (salinity alone and salinity × nutrients × light) to examine the algal responses of tissue water, potassium (K+), and nutrient (NO 3 − and total N) content. Tissue water content decreased with increasing salinity, and although K+ concentration decreased from the initial concentration, it decreased less with increased salinity treatment demonstrating two mechanisms to withstand short-term salinity fluctuation. The salinity × nutrient × light experiment showed that, in the short term, light had an interaction with tissue K+. Total tissue N content was positively related to N treatment level, and light did not affect total nutrient concentration. The effect of light was present whether the nutrients were present in the tissue as inorganic or organic forms. With reduced light, we hypothe size that the assimilation of inorganic to organic N was energy limited. The ability of this alga to take up available nutrients rapidly for growth and short-term osmoregulation, even under low light and salinity levels, helps to explain the bloom potential ofE. intestinalis.

We conducted a laboratory experiment to quantify nutrient (nitrogen and phosphorus) limitation of macroalgae collected along a gradient in water column nutrient availability in Upper Newport Bay estuary, a relatively nutrient-rich system in southern California, United States. We collectedEnteromorpha intestinalis and water for use in the experiment from five sites ranging from the lower end of the estuary to the head. Initial algal tissue N and P concentrations and molar N∶P ratios—as well as water column NO3 and total Kjeldahl nitrogen (TKN)—increased along a spatial gradient from the lower end toward the head. Water column soluble reactive phosphorus (SRP) varied among sites as well but did not follow a pattem of increasing from the seaward end toward the head. Algae from each site were assigned to one of four experimental treatments: control (C), nitrogen enrichment (+N), phosphorus enrichment (+P), and nitrogen and phosphorus enrichment (+N+P). Each week for 3 wk we replaced the water in each unit with the appropriate treatment water to mimic a poorly flushed estuary. After 3 wk, the degree of nutrient limitation ofE. intestinalis varied spatially with distance from the head of the estuary. Growth ofE. intestinalis collected from several sites increased with N enrichment alone and increased further when P was added in combination with N This indicated that N was limiting and that when N was sufficient, P became limiting. Sites from whichE. intestinalis exhibited nutrient limitation spanned the range of background water column NO3 (12.9±0.4 to 55.2±2.1 μM) and SRP (0.8±0.0 to 2.9±0.2 μM) concentrations. Algae that were N limited had initial tissue N levels ranging from 1.18±0.03 to 2.81±0.08% dry weight and molar N∶P ratios ranging from 16.75±0.39 to 26.40±1.98.

Large blooms of opportunistic green macroalgae such as Enteromorpha intestinalis are of ecological concern in estuaries worldwide. Macroalgae derive their nutrients from the water column but estuarine sediments may also be an important nutrient source. We hypothesized that the importance of these nutrient sources to E. intestinalis varies along a nutrient-resource gradient within an estuary. We tested this in experimental units constructed with water and sediments collected from 3 sites in Upper Newport Bay estuary, California, US, that varied greatly in water column nutrient concentrations. For each site there were three treatments: sediments + water; sediments + water + Enteromorpha intestinalis (algae); inert sand + water + algae. Water in units was exchanged weekly simulating low turnover characteristic of poorly flushed estuaries. The importance of the water column versus sediments as a source of nutrients to E. intestinalis varied with the magnitude of the different sources. When initial water column levels of dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) were low, estuarine sediments increased E. intestinalis growth and tissue nutrient content. In experimental units from sites where initial water column DIN was high, there was no effect of estuarine sediments on E. intestinalis growth or tissue N content. Salinity, however, was low in these units and may have inhibited growth. E. intestinalis growth and tissue P content were highest in units from the site with highest initial sediment nutrient content. Water column DIN was depleted each week of the experiment. Thus, the water column was a primary source of nutrients to the algae when water column nutrient supply was high, and the sediments supplemented nutrient supply to the algae when water column nutrient sources were low. Depletion of water column DIN in sediment + water units indicated that the sediments may have acted as a nutrient sink in the absence of macroalgae. Our data provide direct experimental evidence that macroalgae utilize and ecologically benefit from nutrients stored in estuarine sediments.

Salicornia virginica (common pickleweed) is the dominant vascular plant of many saline marshes of the US west coast, yet little is known about seasonal patterns or abiotic factors controlling it. In a southern California salt marsh, quarterly sampling revealed strong seasonal trends, with 2x greater S. virginica biomass in summer than in winter. Tissue nitrogen (N) and phosphorus (P) concentrations were highest in winter and lower in spring and summer, suggesting a dilution of nutrients as plants accumulated biomass during the growing season. Despite high sediment nutrient levels in this marsh, an experiment examining N and P effects still found strong S. virginica responses to N applied biweekly for > 1 year. Increases in succulent tissue biomass after N addition were first seen in April 1998 (after fertilization for 11 months); two-fold increases in biomass and the number of branches resulted by the end of the experiment in August 1998. Addition of N increased N concentration in the woody tissues when sampled in August. The N:P ratio increased with N addition beginning in winter (7 months after fertilization began) and continuing through the remainder of the experiment. Effects of P addition were less marked, as adding P did not result in biomass responses; however, it did influence tissue nutrient levels. These amendments increased P concentrations in the woody tissue in August 1998. In contrast to N amendments, which did not affect root nutrient concentrations, P addition led to increases in P content of root tissues in the latter portion of the growing season. These data suggest that increases in nutrients (especially N, but also P) can lead to large changes in S. virginica characteristics even in estuaries with high sediment nutrient levels.

To investigate whether tissue N and P content of morphologically distinct macroalgae reflect different processes controlling nutrient availability, we measured water column nutrients and collected 5 species of algae for tissue N and P analysis from 18 stations along the southwestern coast of Puerto Rico. Nutrient content of sediments was also determined for a subset of stations. South- western Puerto Rico was chosen because the literature suggests that gradients in sediment type and organic matter content, advection, and terrestrial influence occur in this region. Stations were either inshore or offshore areas with 3 stations per area chosen a priori as High Nutrient Stations (HNS). Water column and sediment nutrient concentrations were elevated inshore and in HNS. Species were of 3 morphological forms: upright thalli with open branches, densely packed mats, and rhizophytic thalli. In the first category, Acanthophora spicifera had higher N content inshore compared to off- shore while both A. spicifera and Hypnea musciformis had higher tissue N and P contents in HNS. In contrast, mat-forming algae (Dictyota dichotoma and D. cervicornis) had higher tissue N and P con- tents offshore compared to inshore. Although these species had high nutrients in many of the HNS, samples from some offshore reefs were equally high. Halimeda incrassata, a rhizophytic form, had greater tissue N content inshore than offshore. H. incrassata tissue nutrients were also elevated in some HNS, but not others. There were significant correlations between water column and sediment nutrients and the tissue N and P content of A. spicifera and H. incrassata, but not for either Dictyota. These results suggest algae with upright thalli and open-branching patterns may have a more direct relationship between tissue nutrient content and water column nutrient concentration than other forms. In contrast, mat-forming species may deplete nutrients within the mat, relying on strong cur- rents found offshore to penetrate dense mats and replenish nutrients. Rhizophytic algae have access to both water column and sediment nutrients, and higher inshore tissue contents and at some HNS may reflect enhanced nutrient supplies from these sources. Our findings suggest that if other envi- ronmental factors are carefully taken into consideration, the tissue N and P content of macroalgae may prove to be an effective indicator of different nutrient sources in tropical systems.

The objective of this paper was to modify an existing size-structured pop- ulation model for the coral Gardineroseris planulata developed from field measurements at the Uva Island reef, Panama, for use at the regional scale (Tropical Eastern Pacific). The modified model can be used to aid in prediction of changes in coral population dynamics in response to ENSO events of different frequency and magnitude. The earlier model incorporates coral growth, predation by the sea star Acanthaster planci, and mortality associated with El Ninio-Southern Oscillation (ENSO) events. The regional model was modified from the earlier local version by extending the coral growth-temperature rela- tionship to include the lower temperatures observed in areas of strong seasonal upwelling. In addition, ENSO-associated mortality was modified by incorporating a dynamic function that relates partial mortality of corals in different size classes to the rate of temperature change during an ENSO event. This function is based on data from three reef areas (Uva Island and the Pearl Islands, Panama, and the Galapagos Islands, Ecuador), which represent a regional gradient from an area characterized by relatively high annual temperature with low variability to areas with lower annual temperature subject to seasonal upwelling. Model predictions of coral population size structure in different areas agreed with field obser- vations. Predictions for coral recovery after an ENSO event were most accurate for the Uva Island population and least accurate for the Galapaigos Islands population. We also tested the effect of various ENSO frequencies on the size structure of the coral population. Increasing ENSO frequency in the upwelling area resulted in an increase in the proportion of small colonies, while the same increase in ENSO frequency in the thermally stable reef area had little effect on the population. Sensitivity analysis suggested that more accurate estimates of growth of the smallest and largest colonies in upwelling areas as well as predation on intermediate-size colonies are important to improve predictive capability.

Our objective was to begin to investigate sources, sinks, and flux rates of nitrogen (N) and phosphorus (P) in Famosa Slough, a small (12 ha) highly modified urban estuary in San Diego, California, U.S.A. The hydrology of Famosa Slough has been modified by culverts that dampen tidal influence and seasonal runoff from two urban watersheds, each of which has been implicated as a nutrient source that generates nuisance algal blooms. In 1995 and 1996, the ranges of nutrients measured in the water column were extremely wide; upper values exceeded those in other estuaries identified as eutrophic. Average dissolved inorganic nitrogen ranged from 2 to 250 μM, while dissolved inorganic phosphorus ranged from

We explored the use of an opportunistic green alga, Enteromorpha intestinalis (L. Link), as an indicator of N enrichment in a southern California salt marsh. In conjunction with N additions to cordgrass ( Spartina foliosa , Trin) in April, June and August 1995, mesh bags containing N-starved algal tissue were placed within cordgrass patches, at their edges along islands, and in adjacent channels. After 1 week in the field, recovered algal tissue was used to test detection of two levels of total N supply (one twice as high as the other), as well as no added N (control). Tissue N concentration, calculated as the percentage change in N, was the best of several algal measures at discerning differences in N availability in any month. In both April and June, tissue N declined from the marsh plain to the channels, reflecting declining N supply. Tissue N concentration also reflected differences in the total quantity of N added. Within the channels adjacent to fertilized areas, algal tissue N was similar to control areas, suggesting that N additions to cordgrass are not resulting in eutrophication of open waters. In August, the algae detected N additions on the marsh plain, but survivorship was poor; other algal species may be better indicators of enrichment in late-summer. With further investigation, the technique presented in this paper has the potential to be developed into a useful bioassay for detecting eutrophication of coastal salt marshes and lagoons.

Using a long-term data set of Gardineroseris planulata (Dana) on Uva Island reef, Panama, we developed a simulation model that relates size-specific schedules of growth and partial mortality to predation by Acanthaster planci (Linnaeus) and El Nino–Southern Oscillation (ENSO)-related elevation of water temperature. We compared the model predictions to field observations for both the subpopulation of colonies that was used for model development and for the entire population. No statistically significant differences in the size-frequency distributions of the real and modeled coral populations were found for the subpopulation during any of 9 yr or for the entire population during 4 yr. These results suggested that the model relationships were reflecting field conditions. Longer-term (100 yr) simulations were conducted to assess the relative importance of predation and ENSO-related colony losses in determining the size structure of the coral population. Predation by A. planci was of overwhelming importance due to both stronger effects of predation (larger transitions) and the frequency of predation (yearly) compared to ENSO (episodically). Even the least-frequent predation scenario skewed the distribution toward smaller colonies, while simulations where populations were subjected to frequent ENSO events (≥3 yr) still maintained colonies in the largest size class. The model results suggested that sea star predators may not have been in the present abundance on this reef prior to the last 30 yr; with predators present, the model predicts that the distribution would be skewed toward smaller colonies.

Upper Newport Bay estuary, Orange County, California, US, was added to the Clean Water Act303(d) list for nutrients, due in part to the excessive growth of macroalgae during certainseasons. In 1998, a Total Maximum Daily Load for nitrogen and phosphorus was adopted forSan Diego Creek, the largest tributary to the estuary. As part of the implementation of theTMDL, the N and P water quality objectives were evaluated. Insufficient data were available todetermine whether they were appropriate; therefore, mechanistic studies investigating theprocesses in UNB that control macroalgal biomass were needed. We 1) investigated thecontribution of nutrients from estuarine sediments to macroalgal growth and tissue nutrientcontent; 2) determined if N or P is the nutrient most limiting to macroalgae; 3) measured rates ofN and P uptake by Enteromorpha intestinalis and Ulva expansa, the dominant, green, bloomformingmacroalgal species; and 4) investigated the effect! s of variation in the frequency and concentration of nutrient pulses on macroalgal growth andtissue nutrient content. Sediments were an important source of nutrients to macroalgae,particularly where water column nutrient availability was low. N was the nutrient most limitinggrowth of macroalgae from UNB and P was the next most limiting nutrient after N. Rates ofnutrient uptake varied with external substrate concentration, initial tissuenutrient status, and phase of nutrient uptake. E. intestinalis grew most with daily additions ofnutrients but also took up and stored nutrients from the episodic pulses to maintain growth in alow nutrient environment for up to 28 d. This work furthers our understanding of nutrientmacroalgaldynamics in a southern California estuary.

Differential tolerance of low salinity, competition for nitrogen (N), and facilitation by altering N supply all may act to determine the pattern of seasonal succession of Enteromorpha intestinalis (L.) Link and Ulva expansa (Setch) S. and G. in estuaries and lagoons of southern California. Low salinity negatively affected both of these algae. However, when N was in sufficient supply, salinities of 15 ppt favored E. intestinalis while oceanic salinity (35 ppt) favored U. expansa; neither alga had a clear advantage at 25 ppt. When starved of N, E. intestinalis and U. expansa competed directly for nutrients. When grown alone, they had similar N uptake and growth rates; when grown together, E. intestinalis was the superior competitor, negatively affecting growth of U. expansa. In addition, U. expansa facilitated the growth of E. intestinalis when N was in short supply; when grown together, there was a positive effect of U. expansa on E. intestinalis. The mechanism of this effect may have been the release or ‘leaking’ of DON when U. expansa no longer had sufficient tissue N to grow. Thus, E. intestinalis would be favored immediately after a rain, but would be replaced by U. expansa when N is available and tidal action reestablishes oceanic salinity. However, at the end of the rainy season when N becomes scarce, E. intestinalis would outcompete U. expansa. We hypothesize that U. expansa may facilitate the dominance of E. intestinalis by leaking N that can be assimilated by E. intestinalis.

Three mechanisms aiding recovery and expansion of a population of Acropora palmata on a patch reef that was directly in the path of Hurricane Andrew were documented: rapid wound healing, high rates of asexual recruitment, and rapid growth rate of the new recruits. In addition, the growth pattern of new recruits was assessed in order to quantify the initial sequence of Structural changes during the transition from recruit to adult morphology. Wound healing was initially rapid (1.59 cm of linear growth per month). but slowed with time. Nine months after the storm, 72% of 218 hurricane-generated fragments of A. palmata had cemented to the bottom, becoming new asexual recruits. Within 18 months of the storm, the number of fragments in the same area had increased to 271, average fragment size was larger, and 94% had become recruits. Recruits showed complex patterns of branch formation, including rapid growth of solitary proto-branches, differential growth of proto-branches dependent on recruit orientation, and dominance within aggregates of proto-branches. We present a conceptual model that suggests A. palmata is adapted to disturbances of both low intensity and high frequency (conditions typical of reef flat zones) and episodic high intensity and low frequency events (hurricanes and tropical storms).

Benthic algae play a key role in regulating carbon and nutrient turnover and in supporting food webs in shallow-water coastal environments. They are especially important in the wide diversity of estuarine habitats foundworldwide. Benthic producers are generally divided into macroalgae and microalgae (also known as microphytobenthos). Macroalgae contribute between4.8%and5.9%of the totalmarinenet primary productivity (calculated from Duarte and Cebrian, 1996). Although lower than total oceanic (81.1%) and coastal (8.5%) phytoplankton, the local macroalgal productivity (per squaremeter) is comparable to some of the most productive terrestrial ecosystems such as tropical forests. Benthic microalgae often form visible brown or green mats on the sediment surface. These mats are thin, as the microalgae are confined to the photic zone (1–3mm) of the sediment, yet the density of algae and other microorganisms is usually high, often 100–1000 times higher than in thewater column. In shallow estuaries, benthic microalgae normally account for 20–50% of the total primary production (Underwood and Kromkamp, 1999). In this chapter, we discuss the benthic algal communities that inhabit the soft-bottom (mud/sandflat, seagrass bed, marsh), hard-bottom (rocky intertidal, shallow subtidal), and coral reef habitats that lie within estuarine ecosystems.

Competition for nutrients among estuarine phytoplankton and algal mats (a combination of floating and attached green macroalgae and attached cyanobacterial mats) was studied using replicate expenmental microcosms. At high nutnent loading (nitrate-N = 77 FM dl ) , the growth of phytoplankton was reduced by a factor of 10 in the presence of the algal mats. Without the algal mats the phytoplankton was very abundant (> 5 X 106 cells rill-l) and dominated by small flagellates, while in the presence of the algal mats the phytoplankton assemblage was sparse and diatoms, flagellates, and unicellular blue-greens were common. The competition hierarchy was cyanobacterial mats >> attached green macroalgae > floating green macroalgae > phytoplankton. When nutrient supply rate was low (nitrate-N = 1.2 pM d-l), the presence of the algal mats shifted the phytoplankton from flagellates to blue-green algae, but did not affect total biomass of the phytoplankton. We conclude that the attached forms of macroalgae as well as the cyanobacterial mats were better competitors for high levels of nutrients than the phytoplankton. This resource competition may explain the negative correlation found in field s tudes between phytoplankton and macroalgae growing in shallow nutrient-enriched estuaries.