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A large-volume mesocosm-based nutrient perturbation experiment was conducted off the island of Hawai‘i, USA, to investigate the response of surface ocean phytoplankton communities to the addition of macronutrients, trace metals, and vitamins and to assess the feasibility of using mesocosms in the open ocean. Three free-drifting mesocosms (~60 m3) were deployed: one mesocosm served as a control (no nutrient amendments); a second (termed +P) was amended with nitrate (N), silicate (Si), phosphate (P), and a trace metal + vitamin mixture; and a third (termed -P) was amended with N, Si, and a trace metal + vitamin mixture but no P. These mesocosms were unreplicated due to logistical constraints and hence differences between treatments are qualitative. After 6 d, the largest response of the phytoplankton community was observed in the +P mesocosm, where chlorophyll a and 14C-based primary production were 2-3× greater than in the -P mesocosm and 4-6× greater than in the control. Comparison between mesocosm and ‘microcosm’ incubations (20 l) revealed differences in the magnitude and timing of production and marked differences in community structure with a reduced response of diatoms in microcosm treatments. Notably, we also observed pronounced declines in Prochlorococcus populations in all treatments, although these were greater in microcosms (up to 99%). Overall, this study confirmed the feasibility of deploying free-drifting mesocosms in the open ocean as a potentially powerful tool to investigate ecological impacts of nutrient perturbations and constitutes a valuable first step towards scaling plankton manipulation experiments.

Global warming is increasing the frequency, duration and intensity of extreme events such as marine heat waves (MHWs). The effects of MHWs include a variety of negative environmental impacts. Extreme weather events can interact with other environmental stressors such as invasion by marine non-indigenous species (NIS). The aim of this study was to (1) compare the responses of fouling assemblages recruited within a harbour (highly invaded) with the responses of those recruited in natural habitats (not invaded) to simulated MHWs of different temperatures and durations, and (2) evaluate the legacy effects of those MHWs on the invasibility of both types of assemblage by deploying them in a marina environment (high NIS propagule pressure). Experimental assemblages were sampled after 5 or 10 d of exposure to 1 of 3 different temperature conditions to examine the effects of varying MHW conditions. Later those assemblages were deployed inside a marina facility to test the invasibility of heat-stressed assemblages. The results revealed that higher temperatures and longer MHWs had an overall negative impact on both native and non-indigenous assemblages. Shorter MHWs had greater effects on assemblages dominated by NIS, while longer MHWs affected native species more. Increasing MHW duration promoted increased variability in the resulting invasive assemblages. Winner and loser species and homogenisation could potentially alter the legacy effects of MHWs on the pattern of NIS recruitment. This study highlights the importance of interactions between environmental stressors to the conservation of coastal communities, crucial ecosystems on oceanic islands.

Monitoring the occurrence and density of parasites and pathogens can identify high infection-risk areas and facilitates disease control and eradication measures. Environmental DNA (eDNA) techniques are increasingly used for pathogen detection due to their relative ease of application. Since many factors affect the reliability and efficacy of eDNA-based detection, rigorous validation and assessment of method limitations is a crucial first step. We evaluated an eDNA detection method usingin situfiltration of large-volume water samples, developed to detect and quantify aquatic wildlife parasites by quantitative PCR (qPCR). We assessed method reliability usingBatrachochytrium dendrobatidis,a pathogenic fungus of amphibians and the myxozoanTetracapsuloides bryosalmonae, causative agent of salmonid proliferative kidney disease, in a controlled experimental setup. Different amounts of parasite spores were added to tanks containing either clean tap water or water from a semi-natural mesocosm community. Overall detection rates were higher than 80%, but detection was not consistent among replicate samples. Within-tank variation in detection emphasises the need for increased site-level replication when dealing with parasites and pathogens. Estimated parasite DNA concentrations in water samples were highly variable, and a significant increase with higher spore concentrations was observed only forB. dendrobatidis. Despite evidence for PCR inhibition in DNA extractions from mesocosm water samples, the type of water did not affect detection rates significantly. Direct spiking controls revealed that the filtration step reduced detection sensitivity. Our study identifies sensitive quantification and sufficient replication as major remaining challenges for the eDNA-based methods for detection of parasites in water.

International audience; Aquaculture is becoming a relevant and productive source of seafood, and production is expected to double in the near future. However, bivalve activities can significantly impact coastal ecosystem functioning. To study the direct and indirect impacts of oysters on the microbial food web, a 0D biogeochemical modelling approach was adopted. The model was adjusted by parameter optimisation, assimilating data from several mesocosm observations of concentrations of nitrate, phosphate, silicate, dissolved organic carbon, chlorophyll, and bacterial biomass. The optimisation method provided a set of optimal parameters to fit the experimental observations of ‘control’ (i.e. natural water without oysters) and ‘oyster’ (i.e. natural water with oysters) mesocosms. The modelling results showed good accordance with the experimental observations, suggesting that the oysters directly reduced phytoplankton community biomass, thus constraining the ecosystem to a more heterotrophic state. Oysters also reduced competition between bacteria and phytoplankton for nutrient uptake, favouring higher bacterial biomass than in the control experiment. Additionally, the presence of oysters strongly increased large micro-zooplankton biomass (50-200 µm; mainly ciliates and large flagellates). This was a consequence of bacterivory by small zooplankton (5-50 µm; mostly flagellates and small ciliates), providing a trophic link between bacteria and larger zooplankton. In conclusion, parameter optimisation showed good capacity to manage experimental data in order to build a more realistic model. Such models, in connection with future developments in aquaculture and global change scenarios, could be a promising tool for exploited ecosystem management and testing different environmental scenarios.

Anthropogenic disturbances in the Chesapeake Bay (USA) have depleted eastern oysterCrassostrea virginicaabundance and altered the estuary’s environment and water quality. Efforts to rehabilitate oyster populations are underway; however, the effect of oyster biodeposits on water quality and plankton community structure are not clear. In July 2017, we used 6 shear turbulence resuspension mesocosms (STURMs) to determine differences in plankton composition with and without the daily addition of oyster biodeposits to a muddy sediment bottom. STURM systems had a volume-weighted root mean square turbulent velocity of 1.08 cm s-1, energy dissipation rate of ~0.08 cm2s-3, and bottom shear stress of ~0.36-0.51 Pa during mixing-on periods during 4 wk of tidal resuspension. Phytoplankton increased their chlorophyllacontent in their cells in response to low light in tanks with biodeposits. The diatomSkeletonema costatumbloomed and had significantly longer chains in tanks without biodeposits. These tanks also had significantly lower concentrations of total suspended solids, zooplankton carbon, and nitrite +nitrate, and higher phytoplankton carbon concentrations. Results suggest that the absence of biodeposit resuspension initiates nitrogen uptake for diatom reproduction, increasing the cell densities ofS. costatum. The low abundance of the zooplankton population in non-biodeposit tanks suggests an inability of zooplankton to graze onS. costatumand negative effects ofS. costatumon zooplankton. A high abundance of the copepodAcartia tonsain biodeposit tanks may have reducedS. costatumchain length. Oyster biodeposit addition and resuspension efficiently transferred phytoplankton carbon to zooplankton carbon, thus supporting the food web in the estuary.

Few studies have investigated the effects of ocean warming and acidification on marine benthic organisms over ecologically relevant time scales. We used an environmentally controlled coral reef mesocosm system to assess growth and physiological responses of the sea urchin species Echinometra sp. A over 2 generations. Each mesocosm was controlled for temperature and pCO2 over 29 mo under 3 climate change scenarios (present day and predicted states in 2050 and 2100 under RCP 8.5). The system maintained treatment conditions including annual temperature cycles and a daily variation in pCO2. Over 20 mo, adult Echinometra exhibited no significant difference in size and weight among the treatments. Growth rates and respiration rates did not differ significantly among treatments. Urchins from the 2100 treatment had elevated ammonium excretion rates and reduced O2:N ratios, suggesting a change in catabolism. We detected no difference in spawning index scores or oocyte size after 20 mo in the treatments, suggesting that gonad development was not impaired by variations in pCO2 and temperature reflecting anticipated climate change scenarios. Larvae produced from experimentally exposed adults were successfully settled from all treatments and raised for 5 mo inside the mesocosm. The final size of these juveniles exhibited no significant difference among treatments. Overall, we demonstrated that the mesocosm system provided a near natural environment for this urchin species. Climate change and ocean acidification did not affect the benthic life stages investigated here. Importantly, in previous short-term (weeks to months) experiments, this species exhibited reductions in growth and gonad development, highlighting the potential for short-term experiments with non-acclimated animals to yield contrasting, possibly erroneous results.

Rhodolith beds are highly diverse benthic communities organized around the physical structure and primary productivity of red coralline algae. Despite a worldwide distribution and growing recognition that rhodolith beds are important calcium carbonate (CaCO₃) bio-factories, little is known of the factors and processes that regulate their structure, function, and stability. One prevalent and largely untested paradigm is that beds develop in environments where water motion is strong enough to prevent burial by sediments. Observations over seven months and three weeks in the centre and near the upper and lower margins of a Newfoundland rhodolith (Lithothamnion glaciale) bed, as well as a laboratory mesocosm experiment with rhodoliths and dominant macrofauna from the bed, were used to characterize, parse, and model spatial and temporal variation in rhodolith sediment load (RSL) and movement among presumably important abiotic and biotic factors. RSL and movement were largely mediated by a few dominant benthic invertebrates. Hydrodynamic forces were insufficient to displace rhodoliths. Daisy brittle stars (Ophiopholis aculeata) and small common sea stars (Asterias rubens) contributed to dislodgement of sediment from rhodoliths. Large green sea urchins (Strongylocentrotus droebachiensis) easily displaced rhodoliths in mesocosms. Results provide the first quantitative demonstration that rhodolith beds need not be exposed to threshold hydrodynamic conditions to avoid burial. Beds can simply occur in areas where burial is unlikely because of low sedimentation rates. In such cases, select resident bioturbators operating simultaneously at different spatial scales (within and outside rhodoliths) appear to suffice to maintain RSL below lethal quantities, contributing to stability of beds.

Pelagic mussel collectors provide an alternative to fishing for mussel seed from natural beds. These collectors, which have been recently introduced in the Dutch part of the Wadden Sea, facilitate the settlement and survival of blue mussel Mytilus edulis larvae. We assessed the removal of plankton by juvenile mussels and the recovery of plankton after filtration. A mesocosm experiment, using natural sea water, was executed on 12 occasions from June to October in 2010 and 2011. Mussel filtration resulted in large reductions in nanophytoplankton, ciliate and total chlorophyll biomasses (65-62%), while picophytoplankton and bacterial biomasses were reduced to a lower extent (38 and 18%). After filtration, mussels were removed and the plankton community was allowed to recover for 8 d, which is the average residence time of water in the area. During this recovery period, net growth rates of bacteria, pico- and nanophytoplankton increased initially in the mussel-filtered mesocosms, but at the end of the recovery period, growth rates were similar in mussel-filtered and control mesocosms. At the end of the recovery period, plankton concentrations between control and mussel mesocosms were not statistically different despite the initial large reduction due to mussel filtration. Our results suggest that nutrients released by mussels during filtration might have stimulated the filtered plankton community, enabling recovery to filtration within 8 d.

Studies on the responses of tropical microbial communities to changing hydrographic conditions are presently poorly represented. We present here the results from a mesocosm experiment conducted in ...

The seawater samples were stained with primuline (Direct Yellow 59, Sigma-Aldrich Co), fixed with a solution containing 3.6% (v/v) glutaraldehyde and 10% (v/v) glycerol (final concentrations), and gently filtered onto black-stained 25 mm diameter 0.4-µm pore-size Nucleopore filters. Filters were mounted on slides and analyzed by epifluorescence microscopy. The method is a modification from Grebecki (1962), Hobbie et al. (1977) and Caron (1983) with the glycerol added to reduce the damage of particularly small delicate protists during filtration as described in Sazhin et al. (2007). FlowCAM analysis of mesocosm seawater The FlowCAM was run in autoimage-mode, using 4x magnification to analyze particles ranging between 15 and 1000 µm. The water samples were kept in dim light at 4°C until analyzed, within 4 h after sampling. Each sample was run for ca. 30 min, corresponding to 5.7 ml of analyzed volume. The relevant context capture property chosen to do the analysis was distance to nearest neighbor = 50 µm. All the image collages were manually post-analyzed in order to identify the particles in question. Particle sizes were estimated from area-based diameter determined by the FlowCAM VISP 2.2 software following Jakobsen & Carstensen (2011). Colonies of P. pouchetii are composite clusters of cells spaced in colonies of various sizes. When the colonies are forming dense blooms, the FlowCAM cannot distinguish whether a cluster is a part of colony or if a cluster is a part of a neighbouring colony. The FlowCAM therefore captures clusters of colonies. S. marinoi and P. pouchetii are colonial structures of multiple cells, and we developed scaling relationships between colony area and cell number by manually counting the number of cells in either a diatom chain or a P. pouchetii colony (n > 100) and related the cell number to particle area. We further developed scaling relationships between cluster areas by manually counting the number of cells in P. pouchetii clusters (n > 100). The same procedure was used for estimating S. marinoi cells, where we manually counted the number of cells per chain. The cell numbers were related to either cluster area (P. pouchetii clusters) or chain length (S. marinoi), and the relationships were then used to estimate the total number of P. pouchetii or S. marinoi cells in colonies or chains, respectively, by sample. Cell concentrations were estimated by correcting for the analyzed volume. The resulting regression equation used for estimation the number of P. pouchetii cells in colonies and colony fragments was Cells = ABD 1.834 * 0.028 (R 2 = 0.83), where ABD (area base diameter) is an output parameter from

We exposed a natural phytoplankton community to combined present (390 µatm, low carbon [LC]) and future CO2 levels predicted for the year 2100 (900 µatm, high carbon [HC]), and ambient (4.5 nM, -DFB [desferoxamine B]) and high (12 nM, +DFB) dissolved iron (dFe) levels, for 25 d in mesocosms. We report on the physiological response of the community dominated by the coccolithophore Emiliania huxleyi. The community structure shifted on Day 10, leading to 2 different phases (1 and 2), i.e. before and after Day 10, respectively. We focussed on the massive bloom of E. huxleyi that developed in Phase 2, in the LC+DFB treatment. In high dFe conditions, pigments and photosynthetic parameters increased compared to the control (LC-DFB). Cell death was only detected during the community shift (Days 10-12) and mostly increased in the presence of high CO2. The accumulation of reactive oxygen species (ROS) decreased under high dFe, pointing to an efficient, rather than a stressed, metabolism. DNA lesions, caused by excess irradiance, were minimised under high Fe. E. huxleyi is known for its low Fe requirements for growth. However, we demonstrate that Fe is essential to E. huxleyi for DNA repair and ROS management, and to maintain optimal functioning of the photosynthetic machinery, with implications for carbon cycling and future ecosystem functioning.

Aerobic anoxygenic phototrophic (AAP) bacteria are photoheterotrophic prokary- otes that use light as a secondary energy source to complement the consumption of organic mat- ter. Despite this metabolic flexibility and their widespread distribution, their low relative abun- dances suggest that they may be subjected to strong regulatory processes. However, there is still little information on the regulation of AAP abundance, particularly in freshwaters. Here, we used a lake mesocosm experiment to address the top-down regulation of freshwater AAP by protists and zooplankton under 2 contrasting nutrient regimes. Our results support the hypothesis that freshwater AAP are subject to intense top-down regulation, and are selectively removed by grazers. The average gross growth rate of AAP was ca. 1.5 times higher, and grazing loss rates 1.6 times higher than those of the bulk bacterial community. Our results further indicate that whereas protists are the main predators of AAP, zooplankton may account for over a third of AAP losses, and both exhibit a greater selectivity for AAP relative to total bacteria. The mechanistic under - pinning of this selectivity is still unclear, but it may be related to the average larger cell size of AAP, and to their higher potential growth rates relative to the bulk bacterial community. Our results further suggest that AAP may play a disproportionate role in the nutrition of lake zooplank- ton, and in the trophic transfer of organic carbon in lake food webs.

International audience; The impact of fish and oysters on components of the pelagic microbial food web (MFW) was studied in a 10 d mesocosm experiment using Mediterranean coastal waters. Two mesocosms contained natural water only (‘Controls’), 2 contained natural water with Crassostrea gigas (‘Oyster’), and 2 contained natural water with Atherina spp. (‘Fish’). Abundances and biomasses of microorganisms (viruses, bacteria, phytoplankton, heterotrophic flagellates, and ciliates) were measured to estimate their contribution to the total microbial carbon biomass. Two MFW indices, the microbial autotroph:heterotroph C biomass ratio (A:H) structural index and the gross primary production:respiration ratio (GPP:R) functional index, were defined. In the Fish mesocosms, selective predation on zooplankton led to a trophic cascade with 51% higher phyto- plankton C biomass and consequently higher A:H and GPP:R than in the Controls. By the end of the experiment, the Oyster mesocosms had a bacterial C biomass 87% higher and phytoplankton C biomass 93% lower than the Controls, giving significantly lower A:H and GPP:R (1), whereas oyster activities made the MFW more hetero- trophic (both indices

Eastern oysters Crassostrea virginica are dominant ecosystem engineers that construct complex reefs in estuarine systems. Reef persistence relies on reef growth, which must outpace reef degradation and sediment deposition. The quantitative impact of burial on oyster survival and sublethal effects of sediment deposition are undefined. In this mesocosm study, we quantified effects of partial and complete burial (0, 50, 70, 90, and 110% of oyster shell height) on survival, biodeposition, condition index and growth of oysters (shell height = 25�75 mm). Survival only declined significantly when 90% or more of an oyster was buried; the critical burial depth inducing 50% mortality was 108%. This finding will allow scaling of mortality rates of oysters as a function of sediment load and oyster size. Biodeposition and condition index were negatively related to burial depth, whereas shell growth increased with burial depth. Decreases in biodepo- sition and condition index were likely related to disruption of feeding by burial and reallocation of energy resources to shell growth. The increase in shell growth was analogous to vertical migration of infaunal species—buried oysters increased vertical shell growth to breach the sediment surface to feed and respire, at the expense of tissue growth. While oysters are tolerant of partial burial in terms of survival, burial adversely affects metabolic processes that influence demographic rates and reef persistence. Future management efforts should consider the effects of partial burial, which often occur more frequently than catastrophic burial events causing mass mortality.

Complex interactions between bacteria and protists are essential to the ecosystem ecology of peatbogs. However, little is known about how short-term changes in environmental conditions may influence microbial and metazoan communities. Microbial processes and para - meters may be used as sensitive indicators of eutrophication. We address the hypothesis that an increase in the concentration of nutrients (in an experiment simulating eutrophication) will affect species richness and abundance of microorganisms and small metazoans and will cause changes in food web structure in different types of peatbogs. The experiments were performed in a Sphag- num peatland and a carbonate fen. Four experimental treatments were established (control and fertilised: +P, +N, P+N). An increase in habitat fertility was found to modify the taxonomic compo- sition and functioning of microbial communities. This was reflected in a decrease in the species richness of testate amoebae and a substantial increase in the abundance of bacteria, flagellates and ciliates in both types of peatbogs. A better understanding of which parameters regulate microbial populations in peatbogs is critical for more accurate prediction of how peatbogs will respond to global change or anthropogenic disturbances.

Fine-scale behaviour such as foraging is difficult to quantify in free-swimming wild fish yet has important basic and applied implications. Here, we used tri-axial accelerometer biologgers to determine accelerometric predictors of bonefish Albula vulpes behaviours (resting, swimming, bursting, coasting, and foraging) in a wetland mesocosm in Eleuthera, The Bahamas. We also used a swim flume to estimate the relationship between acceleration and swimming speed for bonefish (n = 9). In the wetland study, 5 bonefish were tagged externally with accelero - meter loggers and monitored for a 5 d period during which visual behavioural observations were conducted for 4 h. Classification tree models were used to identify accelerometric criteria for bonefish behaviours, and a classification algorithm was applied to estimate behavioural frequen- cies for bonefish in the wetland for the 5 d period. Bonefish spent the majority of time resting (57%), followed by swimming (26%) and coasting (17%), and foraged an average of 11 times h �1 . Bonefish exhibited primarily slow swimming speeds (average 0.18 m s �1 ) while in the wetland, with occasional burst swimming events (14 h �1 ) to maximum swimming speeds ranging from 4.3 to 6.4 m s �1 across individuals. Swimming and foraging behaviours varied among individuals and over the diel cycle. Bonefish generally swam most at dawn and foraged most at night. Tempera- ture and tide were also significant predictors of swimming behaviour, and fish were generally most active at lower temperatures (~24°C) and during ebbing tidal periods despite the fact that the wetland was disconnected from the ocean. The ability to estimate fine-scale behaviours such as foraging and activity levels has important implications for understanding energy dynamics, which is fundamental to the fitness of wild fish.

The intermediate disturbance hypothesis (IDH) predicts that disturbances of moder- ate frequency or intensity result in maximum diversity, while the dynamic equilibrium hypothesis (DEH) proposes that disturbance and productivity interact in their effects on diversity. These hypotheses were tested with a marine ciliate community in a mesocosm study conducted in a Nor- wegian fjord. Two nutrient regimes, 6 disturbance frequencies (undisturbed to daily disturbed) and 2 replicates for each treatment were used. To disturb the system, we repeatedly lowered and raised a disk to mix the water column. The experiment lasted 3 wk, with samples (7 including the initial sample) taken for nutrients and physical parameters, chl a, ciliates and mesozooplankton. The system reacted more strongly to nutrient addition than to our chosen range of disturbance frequencies. Nutrient addition resulted in higher chl a levels and ciliate abundance but reduced ciliate diversity without changing the mean ciliate species richness. The reduction in diversity was due to the very strong, positive response to nutrient addition by the mixotrophic Mesodinium rubrum. In contrast, metazoan predators on ciliates (primarily copepods) did not respond to any of the experimental manipulations, at least partially due to the duration of the experiment rela- tive to their generation times. While disturbance had varied and at times marked effects on the mesocosm communities, the expected unimodal response to disturbance frequency was not seen, nor was the expected interaction between disturbance and productivity. While the IDH and DEH have intuitive appeal, they appear to have limited power to predict natural ciliate community patterns.

Greater attention to habitat-specific species interactions should improve our ability to predict the context-dependency of species interactions. Predator avoidance depends upon resource traits that can vary predictably between habitats, but few studies have linked predator avoidance to habitat-specific resource traits. For example, predator avoidance in herbivores may depend upon wave exposure, because wave-exposed and wave-protected seaweeds display strong intraspecific differences in morphology and physiology that likely influence their value to herbivores. In 2 sets of laboratory mesocosm experiments we compared predator avoidance in the herbivorous snail Littorina obtusata offered the bladder wrack Fucus vesiculosus from either wave-exposed or protected shores. The effect of predation risk by the green crab Carcinus mae- nas on avoidance by L. obtusata depended on wave exposure, with a stronger effect of predation risk for snails on wave-exposed F. vesiculosus. To explore the traits responsible for this pattern, we conducted multi-choice feeding experiments with F. vesiculosus from 3 wave-exposed and 3 pro- tected sites. F. vesiculosus from protected sites (where avoidance behaviors were weak) was more palatable than F. vesiculosus from wave-exposed sites. Although several morphological (e.g. blade width and vesicle number) and physiological traits (e.g. C:N ratio) of F. vesiculosus depended upon wave exposure, these were unrelated to snail feeding and predator avoidance. Therefore, L. obtusata displayed stronger predator avoidance on less palatable, wave-exposed seaweed. We suggest that the different effects of predators on rocky shores may partly reflect the influence of exposure-specific seaweed traits that determine predator avoidance.

To test the interacting effects of hard clams Mercenaria mercenaria and bottom shear stress on nutrient- and ecosystem dynamics, we performed a 4 wk experiment in six 1000 l shear turbulence resuspension mesocosms (STURM). Three tanks each contained 50 hard clams (RC set-up), and 3 tanks had no clams (R set-up). All tanks contained defaunated muddy sediment and estuarine water and had similar water column turbulence intensities (~1 cm s �1 ), energy dissipa- tion rates (~0.08 cm 2 s �3 ), and tidal cycles (4 h mixing on and 2 h off). The same high instantaneous bottom stress (0.35 to 0.4 Pa) was applied to all tanks during the mixing-on cycles. Hard clams in interaction with high bottom shear stress initially destabilized the sediments and increased seston levels to ~200 mg l �1 in the RC tanks during the mixing-on cycles. Over time, seston concentrations declined in the RC tanks until they reached levels similar to the R tanks of ~60 mg l �1 . Bivalve feeding in the RC tanks significantly reduced phytoplankton biomass and shifted the phytoplank- ton community structure to Chlorophyceae/Prasinophytes. Nutrient (particulate phosphorus, nitro- gen, and carbon, dissolved inorganic nitrogen, nitrate + nitrite, phosphate) concentrations were significantly en hanced in the RC tanks, mediated by high sediment resuspension and bivalve excretion. A brown tide organism, Aureococcus anophagefferens, bloomed in 2 of 3 RC tanks. Bivalve feeding and light limitation reduced microphytobenthos biomass in the RC tanks. Micro- phytobenthos biomass was low overall but significantly higher in the R tanks. Phytoplankton abundance, microphytobenthos biomass, seston concentrations, and nitrogen dynamics were sig- nificantly affected by interactions between hard clams and bottom shear stress.

The effects of grazing and nutrients on epiphyte biomass in seagrass beds have received much attention, yet less is known about effects on other metrics of epiphyte growth dynamics, such as epiphyte productivity and biomass turnover rates. To help address this gap, we present here a number of mesocosm experiments in which we manipulate grazer presence and nutrient concentrations under initially moderate eutrophic conditions. We also examine the potential bias contained in estimates of epiphyte grazing that disregard the effects of grazing on epiphyte productivity. The effects of grazing and nutrient enrichment on the epiphyte growth metrics examined were disparate. Namely, the effects of grazing on epiphyte biomass predominated over those of nutrient enrichment, but grazing and nutrients had similar importance as controls of epiphyte productivity and biomass turnover rates. The results illustrate that the effects of environmental and biological factors on one given metric of epiphyte growth dynamics can be quite different from those on other metrics. Thus, we suggest that the biomass-centered view mostly used to date in studies of epiphyte dynamics should perhaps shift toward a broader approach including other metrics of epiphyte growth, such as productivity and turnover rates, to better understand epiphyte dynamics in coastal systems and cascading functional consequences in the system. Another risk of biomass-centered studies lies in the calculation of grazing rates since, as also shown here, estimates of grazing rates derived as the difference in biomass accrual rates between non-grazed and grazed conditions can be highly biased.

Since pre-industrial times, uptake of anthropogenic CO2 by surface ocean waters has caused a documented change of 0.1 pH units. Calcifying organisms are sensitive to elevated CO2 concentrations due to their calcium carbonate skeletons. In temperate rocky intertidal environments, calcifying and noncalcifying macroalgae make up diverse benthic photoautotrophic communities. These communities may change as calcifiers and noncalcifiers respond differently to rising CO2 concentrations. In order to test this hypothesis, we conducted an 86 d mesocosm experiment to investigate the physiological and competitive responses of calcifying and noncalcifying temperate marine macroalgae to 385, 665, and 1486 mu atm CO2. We focused on comparing 2 abundant red algae in the Northeast Atlantic: Corallina officinalis (calcifying) and Chondrus crispus (noncalcifying). We found an interactive effect of CO2 concentration and exposure time on growth rates of C. officinalis, and total protein and carbohydrate concentrations in both species. Photosynthetic rates did not show a strong response. Calcification in C. officinalis showed a parabolic response, while skeletal inorganic carbon decreased with increasing CO2. Community structure changed, as Chondrus crispus cover increased in all treatments, while C. officinalis cover decreased in both elevated-CO2 treatments. Photochemical parameters of other species are also presented. Our results suggest that CO2 will alter the competitive strengths of calcifying and noncalcifying temperate benthic macroalgae, resulting in different community structures, unless these species are able to adapt at a rate similar to or faster than the current rate of increasing sea-surface CO2 concentrations.

Two distinct climatic seasons in the tropics generate extreme environmental fluctua- tions in land-based aquaculture and corresponding challenges for integrated aquaculture. Here we assessed the influence of environmental and culture variables on productivity of Cladophora coelothrix at multiple scales, firstly in an operational settlement pond and subsequently in labora- tory and flow-through mesocosm experiments. Classification and regression tree (CART) analyses were used to partition the influence of key variables on growth. In situ productivity correlated pri- marily with nitrogen concentration, position in the pond and stocking density, with a lesser influ- ence of salinity, temperature and the ratio of nitrate-nitrogen (NO3 � -N) and total ammonium nitro- gen (TAN). We subsequently evaluated the resilience of C. coelothrix in laboratory and mesocosm environments by manipulating salinity, temperature and nutrients across a broader range of val- ues derived from a year-long survey of the settlement pond. Here temperature had the greatest influence on growth, followed by nitrogen concentration and salinity. Growth rates of C. coelothrix were high, irrespective of season, except at the extremes of each season, for example, at the lowest annual temperature and with the combination of high temperature and low salinity. Growth was also limited by ~20% when NO3 � -N and TAN were available simultaneously. The use of CART analyses for both in situ and controlled environment data provided a formal dissection of the relative contribution of each environmental variable to growth, demonstrating the year-round potential for bioremediation and identifying the critical combinations of environmental variables for management of this system.

Bacterioplankton growth is often directly or indirectly controlled by external energy subsidies via organic matter inputs or solar radiation. We carried out a mesocosm experiment to assess how bact ...

Data from 7 mesocosms with a gradient in daily nutrient loading rate (LN) and inverse modelling were used to estimate all C, N and P flows in an idealised food web. The gradient in LN created autotrophic production (AP) of 7 to 314 μg C l−1 d−1 (mean values over 18 d). The production of dissolved organic C (DOC) decreased from 240 to 40% of AP, and was dominated by heterotrophs (80 to 58%), with heterotrophic bacteria (BAC) producing 48 ± 8% (means ± SD) by excretion or lysis. The consumption of DOC by BAC was 133 to 86% of AP, and DOC accumulated when experimental LN ≥ natural LN. C of BAC origin constituted 5.5 to 0.2% of the C consumed by copepods. The production of dissolved N and P was mainly by heterotrophs (92 ± 2% of DN, 92 ± 4% DP), but autotrophs produced 26 ± 7% of dissolved organic N (DON) and 21 ± 8% of dissolved organic P (DOP). For the production of inorganic N and P (DIN and DIP, respectively), BAC predators produced >50% of DIN and >70% of DIP at low or moderate LN (averages 37 ± 16% of DIN and 66 ± 14% of DIP). The contribution by BAC was low for DIN (12 ± 6%) and zero for DIP. For the re-mineralisation of N and P, the significance of BAC was as N and P rich food particles. DOP accumulated more than DOC, despite the high demand for P. Turnover time of dissolved organic matter (DOM) decreased from around 80 to 12 d with increasing LN. These data provide new insights into the role of various functional groups of organisms for the cycling of DOM, and to the differences in C, N and P cycling.

Through the use of mesocosm experiments, we show that an unusually early spring phytoplankton bloom can be induced by intermittent high-light periods. We performed mesocosm experiments where plankton assemblages from Kiel Bight (Western Baltic Sea) received a light regime based on the natural seasonal irradiance dimmed to 43% of surface irradiance of cloudless days, starting with irradiance levels of mid-January (6 mesocosms) and mid-February (6 mesocosms). After 6 d, half of the mesocosms received a ca. 2-fold increase in irradiance. In the January mesocosms, a phytoplankton bloom developed only in the treatments with the high-light episode, whereas in the February mesocosms a phytoplankton bloom also developed in the controls. Phytoplankton net growth rates, production:biomass ratios and biomass at the end of the high irradiance episodes were positively correlated to the daily light dose. The relative biomass of diatoms increased with increasing light, whereas the relative biomass of cryptophytes decreased. A bottom-up transmission to mesozooplankton (mainly copepods of the genera Acartia and Oithona) was evident by increased densities of copepod nauplii and egg production under higher light conditions, whereas copepodids and adults showed no responses during the experimental period. The taxonomic composition of the nauplii was shifted to the advantage of Acartia/Centropages (not distinguished at the naupliar stage) under higher light conditions.

The combined effects of increased UV-B and temperature on natural marine phyto- plankton from the St. Lawrence Estuary (Canada) were examined in an 8 d mesocosm experiment carried out in Rimouski (Quebec, Canada) in August 2008. We tested the hypothesis that in - creased temperature (+ 3°C) will offset algal growth suppression by UV-B (78% UV-B increase) using duplicate mesocosm experiments containing natural phytoplankton assemblages. The re - sponse of the entire phytoplankton community, in terms of HPLC pigment-based phytoplankton bio mass, community composition (CHEMTAX), xanthophyll cycles photoprotection and quantum yield of photosystem II (the ratio of variable to maximum fluorescence: Fv/Fm), showed a signifi- cant influence of temperature (negative on small phytoplanktonic cells, < 5 µm, and positive on larger diatoms) but only after the peak of the diatom bloom, when nutrients became limited. Inter- actions between temperature and UV-B treatments were significant only for small cells during post-bloom; UV-B induced an increase in phytoplankton biomass at the normal temperature but had no effect at warmer temperatures. Enhancing UV-B delayed the bloom slightly under the nor- mal temperature and spread it over a longer period of time, with no sign of major cellular damage. Our re sults do not support the tested hypothesis, and they suggest that temperature plays a greater role than UV-B radiation in structuring phytoplankton communities, possibly favouring diatoms rather than small cells in a warmer climate scenario. Other effects such as grazing or coastal eutrophication should be considered in future studies.

The present study investigates the relation between community characteristics, input of organic matter and drill cuttings. Drill cuttings are discharged from offshore oil and gas exploration, and settle on soft bottom sediments where the benthic fauna may be affected. In a 3- factorial mesocosm experiment, 2 benthic communities were treated with either water-based drill cuttings or natural sediment in combination with addition or no addition of organic matter, and biochemical and biological responses were studied. The biogeochemical response of organic mat- ter and drill cuttings additions resembled each other, in both cases resulting in enhanced sedi- ment-water fluxes of oxygen, nitrate and ammonium, and reduced concentration of oxygen in sed- iment pore water. This finding indicated degradation of an organic compound in the water-based drill cuttings. Regarding the biological response, benthic community composition was signifi- cantly different for all treatment factors, evidenced by PERMANOVA. Abundance and biomass were reduced in boxes without addition of organic matter, probably as a response to starvation, while abundance and taxa richness were reduced in boxes with drill cuttings. The particular effect of water-based drill cuttings on the environment seems to be complex, and should be investigated further.

A culture-independent analysis of the bacterial assemblages in the sediments of a lugworm Arenicola marina L. bioturbated mesocosm was carried out using a molecular RNA-gene based approach. Clone libraries were constructed for 4 spatial locations in the mesocosm: deep yeast layer (DYL), mid sand (MS), new casts (NC) and old casts (OC) after 29 d of bioturbation. In total, 495 clones were analysed and grouped into 159 unique phylotypes. All libraries were dom- inated by Epsilonproteobacteria, with Alpha-, Delta- and Gammaproteobacteria, Verrucomicro- bia and members of the Cytophaga-Flavobacteria-Bacteroides group also prevalent. Phyologe- netic comparison of the sites with respect to evolutionary distance showed that the libraries fell into 2 groups: (1) DYL and MS and (2) NC and OC. There were significant differences in species composition between but not within the 2 groupings. Bacteria involved with sulphur cycling are most likely to be affected by lugworm bioturbation. An Epsilonproteobacterium phylotype with >95% 16S similarity to Sulfurimonas denitrificans was abundant in the DYL sand layer but was not present in the egested sand (NC, OC). Promotion of the growth of sulphide-oxidising bacteria and their subsequent digestion would be consistent with the 'gardening' hypothesis and suggests that chemoautotrophic processes in shallow-water anaerobic sediments may be enhanced by the activities of the lugworm.

The objective of the present study was to examine how N and P availability interact with C metabolism in marine heterotrophic plankton and whether or not heterotrophic groups are likely to be subjected to essential nutrient limitations in natural coastal waters. The nutrients stud- ied were mineral P and N; the latter is a proxy for essential amino acids. We present a general the- oretical framework and criteria for evaluating essential nutrient limitations in heterotrophs. Data were derived from a comprehensive mesocosm experiment, where food-web flows were esti- mated using inverse modelling. Bacteria of the mesocosm communities were severely P-deficient throughout. The heterotrophic nanoplankton was most likely limited by another essential nutrient or by food C availability. Ciliates were most likely P-limited, whereas copepods might experience P limitation, but were more likely limited by another essential nutrient or food C in the present experiments. The N contents of the food were close to the theoretical requirements for copepods and ciliates. All planktonic groups released dissolved inorganic nitrogen (DIN), and all except bacteria released dissolved inorganic phosphorus (DIP). Our results suggested that P limitation might be readily experienced by bacteria, ciliates and perhaps also by copepods, but not by het- erotrophic nanoplankton in natural North East Atlantic coastal waters. A predator must balance its energy metabolism and growth under the variable availabilities of essential nutrients, and we pro- pose that C growth efficiency is a dynamic variable mainly dependent on the availability of the most limiting nutrient. We support the view that C availability alone cannot be used as a proxy for the food limitation of bacterial and zooplankton growth. Specific essential nutrients should be regarded as potential limiting factors, as for phytoplankton. The variable nutritional requirements of heterotrophic predators will then represent a major driver of heterotrophic species diversity, allowing a broad diversity of heterotrophic species in plankton communities.

Climate models predict increases in frequency of summer heat waves. In Europe, such events have already caused declines in seagrass meadows, highlighting the importance of short-term responses of local communities to climate stress. Understanding the variability among populations along the European thermal gradient in response to heat waves is crucial for seagrass conservation and management. Using a mesocosm we compared effects of a simulated heat wave on the photophysiology of Zostera marina populations coming from low (43° N, Adriatic Sea) and high latitudes (56° N, North and Baltic Seas). Measurements before, during and up to 4 wk after the heat wave included photophysiological parameters derived from light response curves generated by PAM fluorometry and gene expression using qRT-PCR. In all 3 populations, initial exposures to thermal stress were characterized by increases in dark adapted effective quantum yield (Y0), maximum electron transfer rate of PSII (ETRmax) and slope of the light response curve (α), coinciding with upregulations of the gene superoxidase dismutase [Mn]. With continuation of the heat wave these initial effects disappeared, demonstrated by declines in Y0, ETRmax and α relative to controls. Z. marina from the Adriatic suffered from the simulated heat wave as much as its high-latitude counterparts. However, we also demonstrate slight photophysiological differences between the populations during the recovery phase, where performance of high-latitude populations continued declining even after water temperatures returned to control levels, while photochemical activity fully recovered in the Adriatic population. These results might draw the attention of future studies and seagrass conservation efforts.

The metabolism and biogeochemical cycles of aquatic ecosystems are largely mediated by microbial communities, with biofilm assemblages dominating in stream ecosystems. To determine the effects of the availability of resources on the structure and function of heterotrophic stream biofilms, we created an enrichment gradient by amending darkened stream channel mesocosms with a stoichiometrically balanced solution of sucrose, NO3 - and PO4 3- . A total of 1902 bacterial partial 16S rRNA gene sequences yielded 293 unique operational taxonomic units (OTUs, 97% sequence similarity). Significant differences (p < 0.005) were detected between communities from all treat- ments with the exception of the 2 highest enrichment levels, with increasing enrichment resulting in progressive community divergence and declining diversity. Both the productivity and function of the biofilm community increased exponentially with enrichment, with exponents of 1.5 for areal mass, 2.3 for live-cell density, and 2.5 to 3.5 for the activities of 5 extracellular enzymes. The nonlinear increase in functional capacity suggests that heterotrophic biofilms are highly responsive to the avail- ability of resources, probably as a result of the physical structures and synergistic social interactions found in biofilm assemblages.

Flows of carbon and nitrogen in a natural phytoplankton assemblage from the plume of the Scheldt estuary were followed during and after a light-induced bloom by measuring changes in concentrations and stable isotope ratios in mesocosms after addition of 13C (as bicarbonate or glucose) and 15N (as ammonium or nitrate). Addition of 13C-bicarbonate showed a strong coupling between algal primary production and bacterial secondary production, while addition of 13C-glucose revealed bacteria as a carbon source for eukaryotes. Tracing the 13C into mesozooplankton, we identified selective grazing on algae during the phytoplankton bloom, transfer of carbon from bacteria to mesozooplankton and opportunistic feeding strategies after the bloom. Nitrogen labeling confirmed carbon labeling findings, identified bacteria rather than mesozooplankton as ammonium regenerators and allowed quantification of nitrification. (Resume d'auteur)

Diatoms typically constitute the majority of large phytoplankton in coastal systems, where they dominate upwelling conditions. The exact mechanism for diatom success has never been explicitly demonstrated, and we hypothesize that it is attributable to their early, rapid response to newly upwelled nitrate. This strategy allows diatoms to grow quickly and consume a disproportion- ate fraction of the available nutrients. To test this hypothesis and investigate differential growth of phytoplankton during the early stages of upwelling, an upwelling event — and the subsequent phyto- plankton bloom — was simulated in a mesocosm experiment in Monterey Bay. Nitrate, ammonium and inorganic carbon uptake by 3 size fractions (0.7-5 µm, 5-20 µm, >20 µm) was measured daily through stable isotope tracer incubations. The particulate nitrogen and carbon biomass was initially dominated by the smallest phytoplankton, but the largest fraction increased most rapidly to dominate the biomass and nitrogen and carbon assimilation after a few days. All size fractions achieved similar maximum specific nitrate uptake rates (VNO3), but this occurred most rapidly and was maintained longest by the largest fraction. This initial acceleration of VNO3 appears to be the mechanism by which diatoms exploit upwelling conditions. All size fractions assimilated carbon and nitrogen in a nearly constant ratio of ~6.3 as the assemblage returned to balanced growth. Pigment measurements and light microscopy independently documented changing phytoplankton abundance, with diatoms demonstrating a characteristic pattern of succession. Changes in diatom diversity and assemblage composition occurred early, providing evidence of a faster response than observable changes in nitrate or biomass implied.

International audience; The response of the microbial community to Saharan dust deposition was investigated in 6 large mesocosms (52 m(3)) deployed at an oligotrophic coastal site in the NW Mediterranean Sea in June 2008 (DUNE project). The mesocosms represented well the environmental conditions observed at the study site during the 8 d experimental period, and the triplicate mesocosms exhibited high reproducibility for each treatment. Dust deposition resulted in an increase in chlorophyll a concentration (0.22 +/- 0.03 mu g l(-1)), as compared to that in the control treatments (0.12 +/- 0.01 mu g l(-1)), but no treatment effect was observed for bacterial heterotrophic abundance at 5 m depth. Results from the fingerprinting technique CE-SSCP indicate a temporal evolution of the structure of the total (16S rRNA gene) and active (16S rRNA transcripts) bacterial community, and Saharan dust deposition had a noticeable structuring effect on the active bacterial community. Combining results from 16S rRNA gene clone libraries and CE-SSCP indicates that the relative contribution of Alteromonas macleodii to the active bacterial community was enhanced 2-fold following dust addition. The 2 operational taxonomic units (OTUs) Thiothrix and Alteromonas, belonging to Gammaproteobacteria, and the Bacteroidetes OTU NS5-1 were specific to the clone libraries from the dust-amended mesocosms or more abundant in these than in the control ones. CARD-FISH analyses, however, indicate that these OTUs had overall low abundances (1 to 5% of total DAPI-counts). Despite the pronounced temporal trend observed during the experimental period, dust deposition had a small, but noticeable structuring effect on the heterotrophic bacterial community that was detectable only at the OTU level at 99% similarity of the 16S rRNA gene.

Uptake of inorganic and organic nitrogen (N) by phytoplankton and bacteria was inves- tigated during a mesocosm study conducted in Raunefjord, Norway in April 2005. One mesocosm was fertilized with nitrate and phosphate at a ratio of 16:1 and maintained in the light, while one un- amended light mesocosm served as a control. Dissolved nutrients, phytoplankton and bacterial bio- mass, and phytoplankton community composition were monitored throughout the 26 d experiment. Uptake of 15 N-labeled ammonium and nitrate, and dual-labeled ( 15 N and 13 C) urea and dissolved free amino acids (DFAA) was measured for phytoplankton and bacteria using 2 methods: size fractiona- tion into 0.2-0.8 and >0.8 μm size classes, and flow cytometric sorting based on chlorophyll autoflu- orescence. Prior to fertilization, dissolved inorganic N concentrations were low and comprised ~5% of total dissolved N. Added nitrate was completely utilized in the amended mesocosm within 10 d, stimulating a large bloom of colonial Phaeocystis pouchetii. Ammonium contributed over half of total measured N uptake by phytoplankton and bacteria in both enclosures, while nitrate and urea each supplied roughly 10 to 25%. Overall, DFAA were a negligible N source to phytoplankton but con- tributed 11% to total bacterial N uptake. Bacterial uptake represented a significant portion of total uptake of all N forms, especially urea and DFAA. Comparison of the 2 methods for measuring phyto- plankton versus bacterial uptake demonstrates how the use of 0.8 μm filters can lead to significant overestimation of phytoplankton N uptake due to the retention of bacterial biomass.

Dissolved organic matter (DOM) concentrations in many northern European freshwater systems have been increasing during the past decades. DOM affects the marine plankton community where rivers discharge into the sea. Large DOM molecules have been suggested to be more available to aquatic plankton than smaller ones due to their more recent origin in the degradation process. In this study, we investigated the effect of riverine DOM molecular size on coastal plankton with the hypothesis that nitrogen associated with large molecules stimulates the plankton more than nitrogen in smaller molecules. Three size fractions of riverine DOM were isolated with tangential ultrafiltration and introduced at similar nitrogen concentrations to mesocosms with a natural coastal marine plankton community under nitrogen limiting conditions. The results show that growth of bacteria and dinoflagellates, but not diatoms, was stimulated by addition of large DOM molecules. Even though organic nitrogen concentrations tended to decrease more in large DOM treatments compared to smaller DOM treatments, no significant differences were detected. However, proteolytic enzyme activities were elevated in treatments with the largest DOM molecules, suggesting that more organic nitrogen was utilized in this treatment. We suggest that input of larger river DOM molecules to nitrogen limited coastal systems may influence the composition of the coastal phytoplankton community in favour of dinoflagellates.

On the west coast of British Columbia (BC), Canada, the intertidal is under pressure bycommercial farming of the non-native Manila clam Venerupis philippinarum . We applied a 3-tieredapproach to determine how farming practices were affecting the geochemical attributes of the inter-tidal. We compared farmed and reference sites with respect to % silt, organic matter, ammonium,phosphorus, iron and manganese in surface (0 to 3 cm) and bulk (0 to 10 cm) sediments. Ammoniumporewater profiles and sediment sorption coefficients were determined for 1 of the 3 regions wherethe greatest intensity of shellfish farming occurred (Tier I). Mesocosm studies were conducted inwhich intertidal species were manipulated to match the community composition on the farmed ascompared to reference sites (Tier II). Microcosm studies were applied to determine how shifts in com-munity structure as observed on farmed versus reference sites and as manipulated in the mesocosmstudies, altered the rate of ammonium diffusion to overlying surface waters (Tier III). All geochemicalattributes and ammonium sorption coefficients were greater in either surficial and/or bulk sedimentsof farmed as compared to reference beaches. Tier III studies indicated that the rate of ammonium dif-fusion from sediments to surface waters was less in microcosms with only bivalves compared to thosewith mussels, barnacles and bivalves. Tier I and II studies suggest that greater recovery of ammo-nium and phosphorus is related to increased bivalve density and % plant coverage on farmed versusreference sites. In contrast, greater amounts of organic matter and silt are due to the presence ofantipredator nets. Farming practices that ensure well aerated sediments and support a diverse inter-tidal community would be advised so that important nutrient geochemical cycles are not altered suchto be a detriment to the quality of habitat or to interfere with primary productivity.KEY WORDS: Intertidal · Community composition · Shellfish aquaculture · Nutrients · Organicmatter · Nitrogen · Phosphorous · Iron

Plant biodiversity affects primary productivity and resource utilization in terrestrial and aquatic ecosystems, and may also influence other key ecological processes such as predator-prey interactions. We tested the hypotheses that predation intensity is negatively related to plant species richness, and that prey density is positively related to plant species richness. We performed one field and one mesocosm experiment based on a benthic estuarine food web to measure the effects of macroalgal species richness on the predation of a herbivorous amphipod by a carnivorous fish. In the mesocosm experiment, prey mortality was significantly lower in the 5 species algal polyculture than in the average monoculture, suggesting that increasing algal richness decreased predation intensity. In the field experiment, prey colonization and density were positively related to algal richness in the presence of a diverse guild of predators. Combined, these results suggest that primary producer rich- ness can influence animal community structure and food web dynamics, emphasizing the need for biodiversity-ecosystem functioning research in realistic, multitrophic systems.

The complex structure of seagrass beds provides increased protection for prey com- pared to unvegetated substrates. Additionally, most studies support the notion that predation inten- sity is negatively correlated with seagrass density. However, virtually every study to previously test this idea has used the same number of predators and prey across a gradient of seagrass density treat- ments. This design ignores another well-established fact: invertebrate and fish abundance increase with increasing seagrass density. Therefore, it makes sense to carry out predation experiments that increase the number of both predators and prey as seagrass density increases. Outdoor tank experi- ments (7 density treatments: 0 to 3000 leaves m -2 ) were carried out using artificial seagrass units modeled after turtlegrass Thalassia testudinum and field mesocosm experiments (4 density treat- ments: 0 to 3000 leaves m -2 ) in living T. testudinum in Big Lagoon (Florida, USA). In these experi- ments the absolute number of predators (the pinfish Lagodon rhomboides) and prey (grass shrimp Palaemonetes pugio) were increased with increasing seagrass density. Both experiments yielded similar results: the only significant difference in predation rate was between the unvegetated treat- ment and the seagrass treatments as a group. These results suggest that dense seagrass does not nec- essarily provide increased protection for prey organisms.

12 pages, 4 figures, 2 tables, Growing evidence of inorganic nutrient limitation on oceanic bacteria suggests a global dependence of bacterial activity and production on rates of nutrient supply. The present study examined whether surface bacterial abundance is significantly related to water column stability, and whether bacterial activity and growth rate are related to the rate of diffusive supply of inorganic nutrients to the mixed layer in the Central Atlantic during 2 meridional cruises. The 2 cruises were run under very different oceanic conditions, with relatively low values of bacterial activity in spring 1995 and relatively higher values in fall 1995. We obtained depth-resolved data in the second cruise and found that the integrated value of bacterial production was also related to the rate of nutrient supply, while integrated particulate primary production and chlorophyll concentration were not. There was also no relationship between particulate primary production and bacterial production. The relationship between nutrient supply and integrated bacterial production was tested with data from a mesocosm experiment showing a good fit to the pattern obtained in the Atlantic. Average bacterial production was ~21% of primary production in the Central Atlantic, with values ranging between 5 and 100%, and higher values in the tropical areas. The demonstration of a direct relationship between nutrient supply and bacterial activity helps to explain a relatively large bacterial biomass as compared to phytoplankton biomass, a low bacterial growth efficiency, and a high bacterial carbon demand relative to contemporaneous primary production often measured in the open ocean, as well as the accumulation of dissolved organic carbon (DOC) observed in nutrient-limited oligotrophic seas, This research was part of the LATITUD project (AMB94-0739), funded by the Spanish Interministerial Commission for Science and Technology, while writing was supported by projects COCA (REN2000-1471-CO2-01/MAR) and MODIVUS (CTM2005-04795/MAR). We thank the crew of RV ‘Hespérides’ for assistance

Cyst germination of strains of the harmful algal species Heterosigma akashiwo and Chattonella subsalsa (Raphidophyceae) from Delaware's (USA) Inland Bays was studied both in the field and laboratory during the spring and early summer seasons. Quantitative real-time PCR was employed for detection and quantification of cells in natural sediments and of germinated vegetative cells in the water column. Temperature, salinity, and dissolved nutrient concentrations were exam- ined in field mesocosm experiments to identify physicochemical factors associated with germination, while the effects of temperature and light on germination were examined in laboratory experiments. We detected and monitored a wide range of cyst abundances of H. akashiwo (from 164 to 2820 cysts cm -3 wet sediment) and C. subsalsa cysts (from 2 to 135 cysts cm -3 wet sediment) in environmental sediments. Germinated H. akashiwo cells were detected in situ after temperatures reached 15°C. However, in laboratory studies, H. akashiwo germination occurred at even lower temperatures (10°C), which was considerably lower than typical germination temperatures from similar Japanese strains. In contrast, a temperature of 20°C stimulated C. subsalsa germination in both field and labo- ratory studies, although germination still occurred at low temperatures (10°C). The presence or absence of light did not affect the germination of C. subsalsa. The low quantities of detected vegeta- tive cells from cyst germination for both H. akashiwo and C. subsalsa suggest the inoculation of a small number of vegetative cells into the water column during the spring and early summer months.