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Net global losses of seagrasses have accelerated efforts to understand recovery from disturbances. Stressors causing disturbances (e.g., storms, heatwaves, boating) vary temporally and spatially within meadows potentially affecting recovery. To test differential recovery, we conducted a removal experiment at sites that differed in thermal stress for a temperate seagrass (Zostera marina). We also synthesized prior studies of seagrass recovery to assess general patterns. Seagrass shoots were removed from 28.3 m2 plots at edge and central sites of a meadow in South Bay, Virginia, USA. We hypothesized faster recovery for edge plots where greater oceanic exchange reduces thermal stress. Contrary to our hypothesis recovery was most rapid in the central meadow matching control site shoot density in 24 months. Recovery was incomplete at the meadow edge and estimated to require 158 months. Differences in recovery were likely due to storm‐driven sediment erosion at the edge sites. Based on data from prior recovery studies, which were primarily on monospecific meadows of Zostera, seagrasses recover across a broad range of conditions with a positive, nonlinear relationship between disturbance area and recovery time. Our experiment indicates position within a seagrass meadow affects disturbance susceptibility and length of recovery. Linking this finding to our literature synthesis suggests increased attention to spatial context will contribute to better understanding variation in recovery rates. [ABSTRACT FROM AUTHOR]

Restoration is accelerating to reverse global declines of key habitats and recover lost ecosystem functions, particularly in coastal ecosystems. However, there is high uncertainty about the long‐term capacity of restored ecosystems to provide habitat and increase biodiversity and the degree to which these ecosystem services are mediated by spatial and temporal environmental variability. We addressed these gaps by sampling fishes biannually for 5–7 years (2012–2018) at 16 sites inside and outside a rapidly expanding restored seagrass meadow in coastal Virginia (USA). Despite substantial among‐year variation in abundance and species composition, seine catches in restored seagrass beds were consistently larger (6.4 times more fish, p < 0.001) and more speciose (2.6 times greater species richness, p < 0.001; 3.1 times greater Hill–Shannon diversity, p = 0.03) than seine catches in adjacent unvegetated areas. Catches were particularly larger during summer than autumn (p < 0.01). Structural equation modeling revealed that depth and water residence time interacted to control seagrass presence, leading to higher fish abundance and richness in shallow, well‐flushed areas that supported seagrass. Together, our results indicate that seagrass restoration yields large and consistent benefits for many coastal fishes, but that restoration and its benefits are sensitive to the dynamic seascapes in which restoration is conducted. Consideration of how seascape‐scale environmental variability affects the success of habitat restoration and subsequent ecosystem function will improve restoration outcomes and the provisioning of ecosystem services. [ABSTRACT FROM AUTHOR]

Seagrass meadows are important carbon sinks in the global coastal carbon cycle yet are also among the most rapidly declining marine habitats. Their ability to sequester carbon depends on flow–sediment–vegetation interactions that facilitate net deposition, as well as high rates of primary production. However, the effects of seasonal and episodic variations in seagrass density on net sediment and carbon accumulation have not been well quantified. Understanding these dynamics provides insight into how carbon accumulation in seagrass meadows responds to disturbance events and climate change. Here, we apply a spatially resolved sediment transport model that includes coupling of seagrass effects on flow, waves, and sediment resuspension in a seagrass meadow to quantify seasonal rates of sediment and carbon accumulation in the meadow. Our results show that organic carbon accumulation rates were largely determined by sediment accumulation and that they both changed non‐linearly as a function of seagrass shoot density. While seagrass meadows effectively trapped sediment at meadow edges during spring–summer growth seasons, during winter senescence low‐density meadows (< 160 shoots m−2) were erosional with rates sensitive to density. Small variations in winter densities resulted in large changes in annual sediment and carbon accumulation in the meadow; meadow‐scale (hundreds of square meters) summer seagrass dieback due to marine heatwaves can result in annual erosion and carbon loss. Our findings highlight the strong temporal and spatial variability in sediment accumulation within seagrass meadows and the implications for annual sediment carbon burial rates and the resilience of seagrass carbon stocks under future climate change. [ABSTRACT FROM AUTHOR]

Macroalgae structure coastal ecosystems affecting metabolism, nutrient dynamics, and food webs. Spatially explicit prediction of macroalgal abundance is critical for understanding coastal ecosystems and trajectories. However, models of macroalgal distribution tend to be mechanistic and generalize poorly, or biogeographic and too coarse to use over spatial scales most appropriate to ecosystem research and management (1–100 km2). Our objective was to develop spatial distribution models for benthic macroalgae in soft‐sediment environments. We compared macroalgal abundance quantified as percent cover, with environmental drivers on 1 ha intertidal flats in a > 900 km2 lagoon system along the Atlantic Coast of Virginia, U.S.A. Physical drivers of macroalgae (e.g., depth‐mediated light availability, exposure to waves) are related to bed morphology. We developed a novel topographic index (τ) to determine whether bed morphology predicts macroalgal abundance. This topographic index described variation in elevation occurring over spatial scales relevant to macroalgae, ranging from smooth to hummocky (τ = 0.01–1.07). Models tested τ along with mean elevation, fetch, and water residence time as predictors of macroalgal abundance. τ, and the interaction with water residence time, were most strongly related to macroalgal abundance. Hummocky flats accumulated less macroalgae than smoother flats, but exceptions occurred with short residence times. Model error (root mean square error) was low, varying between 8% and 18% across models. These models, based on readily measured physical features, are a useful approach for assessing macroalgal abundance in relation to shoreline hardening, species invasions, sea‐level rise, and changing sedimentation affecting coastal ecosystems. [ABSTRACT FROM AUTHOR]

Seagrass meadows are valued for their ecosystem services, including their role in mitigating anthropogenic CO2 emissions through 'blue carbon' sequestration and storage. This study quantifies the dynamics of whole ecosystem metabolism on daily to interannual timescales for an eelgrass (Zostera marina) meadow using in situ benthic O2 flux measurements by aquatic eddy covariance over a period of 11 yr. The measurements were part of the Virginia Coast Reserve Long‐Term Ecological Research study, and covered a relatively stable period of seagrass ecosystem metabolism 6–13 yr after restoration by seeding (2007–2014), a die‐off event likely related to persistently high temperatures during peak growing season in 2015, and a partial recovery from 2016 to 2018. This unique sequence provides an unprecedented opportunity to study seagrass resilience to temperature stress. With this extensive data set covering 115 full diel cycles, we constructed an average annual oxygen budget that indicated the meadow was in metabolic balance when averaged over the entire period, with gross primary production and respiration equal to 95 and −94 mmol O2 m−2 d−1, respectively. On an interannual scale, there was a shift in trophic status from balanced to net heterotrophy during the die‐off event in 2015, then to net autotrophy as the meadow recovered. The highly dynamic and variable nature of seagrass metabolism captured by our aquatic eddy covariance data emphasizes the importance of using frequent measurements throughout the year to correctly estimate trophic status of seagrass meadows. [ABSTRACT FROM AUTHOR]

Seagrass meadows perform an important ecological function as filters for incoming nutrients from surrounding watersheds, especially nitrogen (N). By enhancing N removal processes, including N burial in sediments and denitrification, seagrass meadows improve water quality. With accelerating losses of seagrass meadows worldwide, seagrass restoration plays a key role in reestablishing these coastal ecosystem functions. However, few measurements exist of N burial rates in temperate seagrass meadows and none have been published for restored meadows. In this study, we measured N burial rates in a large (6.9 km2) restored eelgrass (Zostera marina) meadow and compared N removal through burial to previous measurements of removal via denitrification. We also compared N removal to inputs from external loading and fixation and to N assimilation in seagrass biomass. We found that, in this meadow, burial was the dominant process of N removal; the burial rate of 3.52 g N m−2 yr−1 was comparable to rates in natural meadows within 10 yr after seeding and was more than 20× the rate in adjacent bare sediments (0.17 g N m−2 yr−1). We also found that the high rates of N assimilation (2.62 g N m−2 yr−1) created a substantial though temporary sink for nitrogen during the growing season. Our results highlight how seagrass meadows mediate N cycling through high rates of burial, which to date has been understudied in the literature. The successful return of the N filter function after restoration, shown here for the first time, can motivate continued efforts for seagrass restoration and conservation. [ABSTRACT FROM AUTHOR]

Seagrass meadows play an important role in "blue carbon" sequestration and storage, but their dynamic metabolism is not fully understood. In a dense Zostera marina meadow, we measured benthic O2 fluxes by aquatic eddy covariance, water column concentrations of O2, and partial pressures of CO2 (pCO2) over 21 full days during peak growing season in April and June. Seagrass metabolism, derived from the O2 flux, varied markedly between the 2 months as biomass accumulated and water temperature increased from 16°C to 28°C, triggering a twofold increase in respiration and a trophic shift of the seagrass meadow from being a carbon sink to a carbon source. Seagrass metabolism was the major driver of diurnal fluctuations in water column O2 concentration and pCO2, ranging from 173 to 377 μmol L−1 and 193 to 859 ppmv, respectively. This 4.5‐fold variation in pCO2 was observed despite buffering by the carbonate system. Hysteresis in diurnal water column pCO2 vs. O2 concentration was attributed to storage of O2 and CO2 in seagrass tissue, air–water exchange of O2 and CO2, and CO2 storage in surface sediment. There was a ~ 1:1 mol‐to‐mol stoichiometric relationship between diurnal fluctuations in concentrations of O2 and dissolved inorganic carbon. Our measurements showed no stimulation of photosynthesis at high CO2 and low O2 concentrations, even though CO2 reached levels used in IPCC ocean acidification scenarios. This field study does not support the notion that seagrass meadows may be "winners" in future oceans with elevated CO2 concentrations and more frequent temperature extremes. [ABSTRACT FROM AUTHOR]

Abstract: Non‐seagrass sources account for ∼ 50% of the sediment organic carbon (SOC) in many seagrass beds, a fraction that may derive from external organic matter (OM) advected into the meadow and trapped by the seagrass canopy or produced in situ. If allochthonous carbon fluxes are responsible for the non‐seagrass SOC in a given seagrass bed, this fraction should decrease with distance from the meadow perimeter. Identifying the spatial origin of SOC is important for closing seagrass carbon budgets and “blue carbon” offset‐credit accounting, but studies have yet to quantify and map seagrass SOC stocks by carbon source. We measured sediment δ13C, δ15N, and δ34S throughout a large (6 km2), restored Zostera marina (eelgrass) meadow and applied Bayesian mixing models to quantify total SOC contributions from possible autotroph sources, Z. marina, Spartina alterniflora, and benthic microalgae (BMA). Z. marina accounted for < 40% of total meadow SOC, but we did not find evidence for outwelling from the fringing S. alterniflora salt‐marsh or OM advection from bare subtidal areas. S. alterniflora SOC contributions averaged 10% at sites both inside and outside of the meadow. The BMA fraction accounted for 51% of total meadow SOC and was highest at sites furthest from the bare subtidal‐meadow edge, indicative of in situ production. 210Pb profiles confirmed that meadow‐enhanced sedimentation facilitates the burial of in situ BMA. Deducting this contribution from total SOC would underestimate total organic carbon fixation within the meadow. Seagrass meadows can enhance BMA burial, which likely accounts for most of the non‐seagrass SOC stored in many seagrass beds. [ABSTRACT FROM AUTHOR]

In this study, we developed a push-pull incubation method to measure denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in subsurface sediments of subtidal seagrass meadows. This subtidal push-pull technique, adapted from a push-pull method developed for intertidal salt marshes, used mini-piezometers to directly sample pore water in the root zone of the seagrass during an in situ incubation. The pore water was amended with 15 [ABSTRACT FROM AUTHOR]

Ecosystem restoration is often costly, but can be effective at increasing biodiversity and ecosystem services. We used a case study-reseeding seagrass to a coastal lagoon-to demonstrate the value of enhanced ecosystem services as a result of restoration. We modeled the recovery of areal plant coverage in a system where seagrasses were lost due to disease and disturbance, and estimated the value of the returned functions of nitrogen removal and carbon sequestration. We estimated, as of 2010, that this restoration removes 170 ton of nitrogen per year via denitrificiation and sequesters carbon at a rate of 630 tons carbon per year in the sediment. Further, we estimated that natural recovery would take more than 100 years to reach the areal coverage achieved by restoration using seeds in just 10 years. Restoration enhanced this recovery, and the earlier establishment of plants results in a net gain of at least 4,100 ton of nitrogen removed from the system via denitrification and 15,000 ton of carbon sequestered in the sediment. These services have significant ecological and societal value. [ABSTRACT FROM AUTHOR]

North American Atlantic salt marshes are generally considered to be nitrogen (N) limited systems, and plants within these marshes were historically thought to use only inorganic forms of N, such as NH4+. Recent research has suggested that Spartina alterniflora may take up some organic nitrogen compounds directly. To determine the availability of dissolved organic nitrogen (DON) to S. alterniflora in marshes along the North American Atlantic coast, porewater was sampled at eight field sites along the North American Atlantic Coast, ranging from Maine (44° N) to Florida (30° N). To determine if a latitudinal gradient existed in the assimilation of either DON or NH4+, replicated mesocosm experiments were conducted with S. alterniflora plants at three field sites (Massachusetts, Virginia, South Carolina), which represented three distinct ecotypes in the latitudinal range. To determine how microbial activity may influence DON availability, we also assessed rates of microbial activity. We found that porewater DON availability increased linearly with latitude. While S. alterniflora assimilated DON at all sites, plants from the southernmost site had 50% greater NH4+ assimilation rates than the other two sites, suggesting that low-latitude S. alterniflora may rely more strongly on NH4+ as an N source. In contrast, high-latitude salt marshes contain greater pools of available DON, and high-latitude S. alterniflora plants exhibit similar uptake rates of DON and NH4+, suggesting a greater reliance upon DON as an N source. We also found that microbial activity decreased by an order of magnitude with increasing latitude along our latitudinal gradient. This suggests that DON uptake is more important at high-latitude locations, where temperature constrains microbial mineralization of organic N. Our analysis also suggests that N limitation of S. alterniflora increases with decreasing latitude. We suggest that other detritus-based ecosystems may exhibit similar latitudinal patterns regarding DON availability and use. [ABSTRACT FROM AUTHOR]

Ecological restoration assists the recovery of degraded ecosystems; however, restoration can have deleterious effects such as outbreeding depression when source material is not chosen carefully and has non-local adaptations., We surveyed 23 eelgrass ( Zostera marina L.) populations along the North American Atlantic coast to evaluate genetic structure and connectivity among restored and naturally recruited populations., While populations along the North America Atlantic coast were genetically distinctive, significant migration was detected among populations. All estimates of connectivity ( FST, migration rate base on rare alleles, and Bayesian modelling) showed a general north to south pattern of migration, corresponding to the typical long-shore currents in this region., Individual naturally recruited meadows in the Virginia coastal bays appear to be the result of dispersal from different meadows north of the region. This supports the hypothesis that recruitment into this region is typically a slow, episodic process rather than a permanent, continuous connection between the populations., While natural recovery of populations that were catastrophically lost in the 1930s has been slow, large-scale seed-based restoration has been very successful at quickly restoring landscape-scale areal coverage (over 1600 ha in just 10 years). Our results show that restoration was also successful at restoring meadows with high genetic diversity. Naturally recruited meadows were less diverse and exhibited signs of genetic drift., Synthesis. Our analyses demonstrate that metapopulation dynamics are important to the natural recovery of seagrass ecosystems that have experienced catastrophic loss over large spatial scales; however, natural recovery processes are slow and inefficient at recovering genetic diversity and population structure when recruitment barriers exist, such as a limited seed source. Seed-based restoration provides a greater abundance of propagules, rapidly facilitates the recovery of populations with higher genetic diversity, and when seed sources are chosen carefully protects regional genetic structure. First-order estimates indicated that the genetic diversity achieved by active restoration in 10 years would have otherwise taken between 125 and 185 years to achieve through natural recruitment events. [ABSTRACT FROM AUTHOR]

Based on noninvasive eddy correlation measurements at a marine and a freshwater site, this study documents the control that current flow and light have on sediment--water oxygen fluxes in permeable sediments. The marine sediment was exposed to tidal-driven current and light, and the oxygen flux varied from night to day between -29 and 78 mmol m-2 d-1. A fitting model, assuming a linear increase in oxygen respiration with current flow, and a photosynthesis--irradiance curve for light-controlled production reproduced measured fluxes well (R² = 0.992) and revealed a 4-fold increase in oxygen uptake when current velocity increased from - 0 to 20 cm s-1. Application of the model to a week-long measured record of current velocity and light showed that net ecosystem metabolism varied substantially among days, between -27 and 31 mmol m-2 d-1, due to variations in light and current flow. This variation is likely typical of many shallow-water systems and highlights the need for long-term flux integrations to determine system metabolism accurately. At the freshwater river site, the sediment--water oxygen flux ranged from -360 to 137 mmol m-2 d-1. A direct comparison during nighttime with concurrent benthic chamber incubations revealed a 4.1 times larger eddy flux than that obtained with chambers. The current velocity during this comparison was 31 cm s-1, and the large discrepancy was likely caused by poor imitation by the chambers of the natural pore-water flushing at this high current velocity. These results emphasize the need for more noninvasive oxygen flux measurements in permeable sediments to accurately assess their role in local and global carbon budgets [ABSTRACT FROM AUTHOR]

Gracilaria vermiculophylla (Ohmi) Papenfuss is an invasive alga that is native to Southeast Asia and has invaded many estuaries in North America and Europe. It is difficult to differentiate G. vermiculophylla from native forms using morphology and therefore molecular techniques are needed. In this study, we used three molecular markers ( rbcL, cox2- cox3 spacer, cox1) to identify G. vermiculophylla at several locations in the western Atlantic. RbcL and cox2- cox3 spacer markers confirmed the presence of G. vermiculophylla on the east coast of the USA from Massachusetts to South Carolina. We used a 507 base pair region of cox1 mt DNA to (i) verify the widespread distribution of G. vermiculophylla in the Virginia ( VA) coastal bays and (ii) determine the intraspecific diversity of these algae. Cox1 haplotype richness in the VA coastal bays was much higher than that previously found in other invaded locations, as well as some native locations. This difference is likely attributed to the more intensive sampling design used in this study, which was able to detect richness created by multiple, diverse introductions. On the basis of our results, we recommend that future studies take differences in sampling design into account when comparing haplotype richness and diversity between native and non-native studies in the literature. [ABSTRACT FROM AUTHOR]

Macroalgae, often the dominant primary producers in shallow estuaries, can be important regulators of nitrogen (N) cycling. Like phytoplankton, actively growing macroalgae release N to the water column; yet little is known about the quantity or nature of this release. Using 15N labeling in laboratory and field experiments, we estimated the quantity of N released relative to assimilation and gross uptake by Gracilaria vermiculophylla (Ohmi) Papenfuss (Rhodophyta, Gracilariales), a non-native macroalgae. Field experiments were carried out in Hog Island Bay, a shallow back-barrier lagoon on the Virginia coast where G. vermiculophylla makes up 85%–90% of the biomass. There was good agreement between laboratory and field measurements of N uptake and release. Daily N assimilation in field experiments (32.3±7.2 μmol N·g dw−1·d−1) was correlated with seasonal and local N availability. The average rate of N release across all sites and dates (65.8±11.6 μmol N·g dw−1·d−1) was 67% of gross daily uptake, and also varied among sites and seasons (range=33%–99%). Release was highest when growth rates and nutrient availability were low, possibly due to senescence during these periods. During summer biomass peaks, estimated N release from macroalgal mats was as high as 17 mmol N·m−2·d−1. Our results suggest that most estimates of macroalgal N uptake severely underestimate gross N uptake and that N is taken up, transformed, and released to the water column on short time scales (minutes–hours). [ABSTRACT FROM AUTHOR]

Coastal lagoons are a common land-margin feature worldwide and function as an important filter for nutrients entering from the watershed. The shallow nature of lagoons leads to dominance by benthic autotrophs, which can regulate benthic-pelagic coupling. Here we demonstrate that both microalgae and macroalgae are important in controlling dissolved inorganic as well as organic nitrogen (DIN and DON) fluxes between the sediments and the water column. Fluxes of nitrogen (NH[sup+, sub4], NO[sup-, sub3] DON, urea, and dissolved free and combined amino acids [DFAA, DCAA]) and O[sub2] were measured from October 1998 through August 1999 in sediment cores collected from Hog Island Bay, Virginia. Cores were collected from four sites representing the range of environmental conditions across this shallow lagoon: muddy, high-nutrient and sandy, low-nutrient sites that were both dominated by benthic microalgae, and a mid-lagoon site with fine sands covered by dense macroalgal mats. Sediment-water column DON fluxes were highly variable and comparable in magnitude to DIN fluxes; fluxes of individual compounds (urea, DFAA, DCAA) often proceeded simultaneously in different directions. Where sediment metabolism was net autotrophic because of microalgal activity, TDN (total dissolved nitrogen) fluxes, mostly comprised of DIN, urea, and DFAA, were directed into the sediments. Heterotrophic sediments, including those underlying macroalgal mats, were a net source of TDN, mostly as DIN. Macroalgae intercepted sediment-water column fluxes of DIN, urea, and DFAA, which accounted for 27-75% of calculated N demand. DON uptake was important in satisfying macroalgal N demand seasonally and where DIN concentrations were low. Up to 22% of total N uptake was released to the water column as DCAA. Overall, macroalgae assimilated, transformed, and rereleased to the water column both organic and inorganic N on short (minutes-hours) and long (months) time scales. Microalgae and macroalgae... [ABSTRACT FROM AUTHOR]

Presents a study that investigated the effect of ultraviolet radiation on the production of inorganic nitrogen, urea and amino acids from aquatic dissolved organic matter (DOM) for waters from Hog Island Bay, a coastal lagoon in Virginia. Methodology; Initial characteristics of DOM; Light-induced changes.

Investigates the role of macroalgal blooms in the decline of seagrass in nutrient-enriched coastal waters. Increase of phytoplankton biomass; Eutrophication of shallow estuaries and lagoons; Use of model calculations for estimating light attenuation by different autotrophs.

Marine invertebrate grazing on temperate macroalgae may exert a significant “top-down” control on macroalgal biomass. We conducted two laboratory experiments to test (1) if consumption by the omnivorous mud snail Ilyanassa obsoleta (Say) on the macroalga Ulva lactuca Linnaeus was a function of food quality (nitrogen content) and (2) if grazing on benthic macroalgae occurred at significant rates in the presence of alternative food sources in the sediment (detritus, larvae, benthic microalgae). Grazing rates were higher for N-enriched macroalgae; however, all snails lost weight when grazing on macroalgae alone, indicating that U. lactuca was a poor food source. The presence of sediment from two sites, a sandy lagoon and an adjacent organic-rich muddy tidal creek, did not affect consumption of macroalgae in microcosm experiments, and the grazing snails were capable of significantly reducing macroalgal biomass associated with both sediment types. Grazing rates by this omnivore were as high as 10.83 mg wet weight·individuals-1·d-1 and were similar to those recorded for herbivorous species. In situ loss rates calculated from average grazing rates per individual and snail abundances (up to 3.5 g dry weight·m-2·d-1) also were comparable with those calculated for herbivorous species. This level of grazing could remove up to 88% of new macroalgal growth at the lagoon site where the N supply was relatively low but had a much smaller effect (18% of new growth) at the high-nutrient creek site. Snails facilitated macroalgal growth at both sites by increasing tissue N content by 40%–80%. Consumption and digestion of macroalgae aided in the recycling of nutrients temporarily bound in the algae and resulted in enrichment of surficial sediments. Increased N sequestration in the sediments also was associated with an interruption of snail burrowing behavior due to persistent anoxia in sediments rich in decaying algal material. Our data suggest that in shallow lagoons where mud snails and benthic macroalgae coexist, grazing may influence N retention in macroalgal biomass. [ABSTRACT FROM AUTHOR]

Examines the influence of growth irradiance on the non-steady-state relationship between photosynthesis and tissue carbon and nitrogen (N) pools in Chaetomorpha linum (Chlorophyta) Kutzing in response to abrupt changes in external N availability. Acclimation of algae to low irradiance for a given thallus N content; Finding that N-saturated algae had the same photosynthesis-irradiance relationship regardless of light level.

Changes in the size of intracellular nitrogen pools and the potential feedback by these pools on etaximum N uptake (NH[SUB4] and NO[SUB3]) rates were determined for Chaetomorpha linum (Müller) Kützing grown sequentially under nutrient-saturating and nutrient-limiting conditions. The size of individual pools in N-sufficient could be ranked as residual organic N (RON) comprised mainly of amino acids and compounds > protein N > NO[SUB3] > NH[SUB4] chlorophyll N. When the external N supply was removed, growth rates remained high and individual N pools were depleted at exponential rates that reflected both dilution of existing pools by the addition of new biomass from growth and movement between the pools. Calculated fluxes between the tissue N pools showed that the protein pool increased throughout the N depletin period and thus did not serve a storage function. RON was the largest storage reserve; nitrate was the second largest, but more temporary, storage pool that was depleted within to days. Upon N resupply, the RON pool increased 3 × faster that either the inogrance or protein pools, suggesting that prtein synthesis was the rate-limiting step in N assimilation and caused a buildup of intermediate storage compuinds. Maximum uptake rates for both NH[SUB4] And NO[SUB3,SUP-] varied inversely with macroalagal N status and appeared to be controlled by changes in small intracellular N pools. Uptake of NO[SUB3,SUP-] showed an initial lag phase, but the initial uptake of NH[SUB4,SUP+] Pool was depleted in the absence of an external N supply. A strong negative correlation between the RON pool the and maximum assimilation uptake rates for both NH[SUB4,SUP+] And NO[SUB3,SUP-] suggested a feedback control or assimilatin uptake to the buildup and depletin of organic compounds. Enhanced uptake and the accumulatin of N as simple organic compounds or nitrate both provide a temporary mechanism to buffer against the asynchrony of N supply and demans in C. linum. [ABSTRACT FROM AUTHOR]

Gracilaria in Virginia, USA, is abundant and composed of thalli either having relatively flat or cylindrical branches. These two morphologies were referred to previously as G. foliifera (Forsskål) Bøgesen and G. verrucosa (Hudson) Papenfuss. However, G. verrucosa is regarded an invalid name, and the flat specimens are now referred to as G. tikvahiae McLachlan. This has created confusion about the nomenclature of Gracilaria from this region. Here we document that the cylindrical form that dominates Hog Island Bay, Virginia, is G. vermiculophylla (Ohmi) Papenfuss, an alien macroalga from the West Pacific. Most of the ecological studies performed at the Long Term Ecological Research (LTER) site in Hog Island Bay used this cylindrical species. The present study clarifies the taxonomical status of this species, and we identify attributes that make this alien successful in turbid coastal lagoons. [ABSTRACT FROM AUTHOR]

Sediment-bound phosphorus (P) is a potential nutrient source for P-limited seagrasses inhabiting carbonate sediments. We explored the role of organic acid (OA) exudation by seagrasses in liberating mineral P from carbonate sediments. Organic acids can act to increase available P by dissolving carbonate sediment, competing with P for binding sites and complexing dissolution end products, and also by fueling microbial processes that change pore-water pH. We used dialysis tubing placed around individual roots in situ to quantify dissolved species immediately adjacent to roots (root zone) and compared these to bulk pore-water concentrations in vegetated and nonvegetated sediments. Total OA concentrations were highest in the root zone (29.8 ± 1.8 µmol L-1) compared to bulk measures of 15.5 ± 1.9 and 7.5 ± 0.6 µmol L-1 in vegetated and nonvegetated sediments, respectively. Phosphate concentrations were also highest in the root zone and were linearly related to OA concentrations (R² = 0.63). Organic acid concentrations increased along a seagrass productivity gradient, and ratios of OA concentrations to productivity showed a significant response to a gradient in P-limitation of seagrasses. Organic acid concentrations found in and around roots, compared to those found in bulk sediment measures, indicate that seagrasses are a significant source of OA. Sampling at small spatial scales (mm) immediately adjacent to the roots is critical, because bulk sediment pore-water measures did not capture the observed fluctuations caused by the rapid reaction and consumption of OA in the sediment. Root-zone processes can liberate considerable quantities of P, and OA exudates likely contribute significantly to the success of T. testudinum in P-limited environments. [ABSTRACT FROM AUTHOR]