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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]

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]

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]

Awarding CO2 offset credits may incentivize seagrass restoration projects and help reverse greenhouse gas (GHG) emissions from global seagrass loss. However, no study has quantified net GHG removal from the atmosphere from a seagrass restoration project, which would require coupled Corg stock and GHG flux enhancement measurements, or determined whether the creditable offset benefit can finance the restoration. We measured all of the necessary GHG accounting parameters in the 7-km2Zostera marina (eelgrass) meadow in Virginia, U.S.A., part of the largest, most cost-effective meadow restoration to date, to provide the first seagrass offset finance test-of-concept. Restoring seagrass removed 9,600 tCO2 from the atmosphere over 15 years but also enhanced both CH4 and N2O production, releasing 950 tCO2e. Despite tripling the N2O flux to 0.06 g m−2 yr−1 and increasing CH4 8-fold to 0.8 g m−2 yr−1, the meadow now offsets 0.42 tCO2e ha−1 yr−1, which is roughly equivalent to the seagrass sequestration rate for GHG inventory accounting but lower than the rates for temperate and tropical forests. The financial benefit for this highly successful project, $87 K at $10 MtCO2e−1, defrays ~10% of the restoration cost. Managers should also consider seagrass co-benefits, which provide additional incentives for seagrass restoration. [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]

Most information on seagrass carbon burial derives from point measurements, which are sometimes scaled by meadow area to estimate carbon stocks; however, sediment organic carbon (Corg) concentrations may vary with distance from the meadow edge, resulting in spatial gradients that affect the accuracy of stock estimates. We mapped sediment Corg concentrations throughout a large (6 km2) restored seagrass meadow to determine whether Corg distribution patterns exist at different spatial scales. The meadow originated from ≤1-acre plots seeded between 2001 and 2004, so we expected Corg to vary spatially according to the known meadow age at sample sites and with proximity to the meadow edge. Applying spatial autoregressive models allowed us to control for spatial autocorrelation and quantify the relative effects of edge proximity and age on Corg concentrations. We found that edge proximity, not age, significantly predicted the meadow-scale Corg distribution. We also evaluated relationships between Corg and a variety of specific explanatory variables, including site relative exposure, shoot density, sediment grain size, and bathymetry. Factors known to affect carbon burial at the plot-scale, such as meadow age and shoot density, were not significant controls on the meadow-scale Corg distribution. Strong correlations between Corg, grain size, and edge proximity suggest that current attenuation increases fine-sediment deposition and, therefore, carbon burial with distance into the meadow. By mapping the sediment Corg pool, we provide the first accurate quantification of an enhanced carbon stock attributable to seagrass restoration. The top 12 cm of the bed contain 3660 t Corg, approximately 1200 t more Corg than an equal area of bare sediment. Most of that net increase is concentrated in a meadow area with low tidal current velocities. Managers should account for the effects of meadow configuration and current velocity when estimating seagrass blue carbon stocks. Our results suggest that a large, contiguous meadow should store more blue carbon than an equal area of small meadow patches. [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]

The dynamics of shallow-water coastal environments are controlled by external drivers--sea level rise, storms, and sediment and nutrient supplies--and by internal feedbacks. Interactions of biotic processes (vegetation growth, trophic dynamics) and abiotic drivers can lead to nonlinear responses to changing conditions and to the emergence of thresholds, hysteresis, and alternative stable states. We develop a conceptual framework for studying interactions between the dynamics of marshes and habitats in shallow coastal bays with unconsolidated sediments (seagrass, oyster reefs). Using examples primarily from the Virginia Coast Reserve Long Term Ecological Research site, we show that in the subtidal part of the landscape, two alternative stable states can exist--one dominated by seagrass up to a certain depth that represents a tipping point to the second, unvegetated stable state. The depth limit of the seagrass stable state is influenced by (1) the positive feedback of vegetation on reducing sediment suspension and improving the light environment for growth, (2) climate (e.g., temperature), and (3) water quality. Two stable states are also present in intertidal areas, with salt marshes lying above mean sea level and tidal flats below mean sea level. State transitions are driven by sediment availability, sea level rise, the relative strength of wind waves with respect to tidal currents, and the biotic feedback of vegetation on sediment stabilization and accretion. State-change dynamics in one system may propagate to adjacent systems, and this coupling may influence the landscape-scale response to environmental change. Seagrass meadows and oyster reefs affect adjacent marshes both positively (wave attenuation) and negatively (reduced sediment supply), and marsh-edge erosion could negatively influence the light environment for seagrass growth. Forecasting the resilience of coastal ecosystems and the landscape-scale response to environmental change in the next century requires an understanding of nonlinear dynamics, including the possibility of multiple stable states, the coupled evolution of adjacent systems, and potential early warning signs of thresholds of change. [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]

Disturbance and habitat destruction due to human activities is a pervasive problem in near-shore marine ecosystems, and restoration is often used to mitigate losses. A common metric used to evaluate the success of restoration is the return of ecosystem services. Previous research has shown that biodiversity, including genetic diversity, is positively associated with the provision of ecosystem services. We conducted a restoration experiment using sources, techniques, and sites similar to actual large-scale seagrass restoration projects and demonstrated that a small increase in genetic diversity enhanced ecosystem services (invertebrate habitat, increased primary productivity, and nutrient retention). In our experiment, plots with elevated genetic diversity had plants that survived longer, increased in density more quickly, and provided more ecosystem services (invertebrate habitat, increased primary productivity, and nutrient retention). We used the number of alleles per locus as a measure of genetic diversity, which, unlike clonal diversity used in earlier research, can be applied to any organism. Additionally, unlike previous studies where positive impacts of diversity occurred only after a large disturbance, this study assessed the importance of diversity in response to potential environmental stresses (high temperature, low light) along a water-depth gradient. We found a positive impact of diversity along the entire depth gradient. Taken together, these results suggest that ecosystem restoration will significantly benefit from obtaining sources (transplants or seeds) with high genetic diversity and from restoration techniques that can maintain that genetic diversity. [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.

Examines the porewater profiles of inorganic and organic nutrients in seagrass-vegetated sediments in a shallow bay in Bermuda Islands. Concentration of dissolved and inorganic nutrient; Use of stoichiometric nutrient regeneration model; Depletion of sediment porewaters.

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]

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]

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]

As part of a long-term study on the effects of nitrogen (N) loading in a shallow temperate lagoon, we measured rates of N2 fixation associated with seagrass (Zostera marina) epiphytes during the summer from 2005 to 2019, at two sites along a gradient from where high N groundwater enters the system (denoted SH) to a more well-flushed outer harbor (OH). The data presented here are the first such long-term N2 fixation estimates for any seagrass system and one of the very few reported for the phyllosphere in a temperate system. Mean daily N2 fixation was estimated from light and dark measurements using the acetylene reduction assay intercalibrated using both incorporation of 15N2 into biomass and a novel application of the N2:Ar method. Surprisingly, despite a large inorganic N input from a N-contaminated groundwater plume, epiphytic N2 fixation rates were moderately to very high for a seagrass system (OH site 14-year mean of 0.94 mmol N m−2 d−1), with the highest rates (2.6 mmol N m−2 d−1) measured at the more N-loaded eutrophic site (SH) where dissolved inorganic N was higher and soluble reactive phosphorus was lower than in the better-flushed OH. Over 95% of the total N2 fixation measured was in the light, suggesting the importance of cyanobacteria in the epiphyte assemblages. We observed large inter-annual variation both within and across the two study sites (range from 0.1 to 2.6 mmol N fixed m−2 d−1), which we suggest is in part related to climatic variation. We estimate that input from phyllosphere N2 fixation over the study period contributes on average an additional 20% to the total daily N load per area within the seagrass meadow. [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]

Seagrass meadows are highly productive habitats that provide important ecosystem services in the coastal zone, including carbon and nutrient sequestration. Organic carbon in seagrass sediment, known as “blue carbon,” accumulates from both in situ production and sedimentation of particulate carbon from the water column. Using a large-scale restoration (>1700 ha) in the Virginia coastal bays as a model system, we evaluated the role of seagrass, Zostera marina, restoration in carbon storage in sediments of shallow coastal ecosystems. Sediments of replicate seagrass meadows representing different age treatments (as time since seeding: 0, 4, and 10 years), were analyzed for % carbon, % nitrogen, bulk density, organic matter content, and 210Pb for dating at 1-cm increments to a depth of 10 cm. Sediment nutrient and organic content, and carbon accumulation rates were higher in 10-year seagrass meadows relative to 4-year and bare sediment. These differences were consistent with higher shoot density in the older meadow. Carbon accumulation rates determined for the 10-year restored seagrass meadows were 36.68 g C m-2 yr-1. Within 12 years of seeding, the restored seagrass meadows are expected to accumulate carbon at a rate that is comparable to measured ranges in natural seagrass meadows. This the first study to provide evidence of the potential of seagrass habitat restoration to enhance carbon sequestration in the coastal zone. [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]

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]

In shallow coastal bays where nutrient loading and riverine inputs are low, turbidity, and the consequent light environment are controlled by resuspension of bed sediments due to wind-waves and tidal currents. High sediment resuspension and low light environments can limit benthic primary productivity; however, both currents and waves are affected by the presence of benthic plants such as seagrass. This feedback between the presence of benthic primary producers such as seagrass and the consequent light environment has been predicted to induce bistable dynamics locally. However, these vegetated areas influence a larger area than they footprint, including a barren adjacent downstream area which exhibits reduced shear stresses. Here we explore through modeling how the patchy structure of seagrass meadows on a landscape may affect sediment resuspension and the consequent light environment due to the presence of this sheltered region. Heterogeneous vegetation covers comprising a mosaic of randomly distributed patches were generated to investigate the effect of patch modified hydrodynamics. Actual cover of vegetation on the landscape was used to facilitate comparisons across landscape realizations. Hourly wave and current shear stresses on the landscape along with suspended sediment concentration and light attenuation characteristics were then calculated and spatially averaged to examine how actual cover and mean water depth affect the bulk sediment and light environment. The results indicate that an effective cover, which incorporates the sheltering area, has important controls on the distributions of shear stress, suspended sediment, light environment, and consequent seagrass habitat suitability. Interestingly, an optimal habitat occurs within a depth range where, if actual cover is reduced past some threshold, the bulk light environment would no longer favor seagrass growth. [ABSTRACT FROM AUTHOR]

Coastal ecosystems play a disproportionately large role in society, and climate change is altering their ecological structure and function, as well as their highly valued goods and services. In the present article, we review the results from decade-scale research on coastal ecosystems shaped by foundation species (e.g. coral reefs, kelp forests, coastal marshes, seagrass meadows, mangrove forests, barrier islands) to show how climate change is altering their ecological attributes and services. We demonstrate the value of site-based, long-term studies for quantifying the resilience of coastal systems to climate forcing, identifying thresholds that cause shifts in ecological state, and investigating the capacity of coastal ecosystems to adapt to climate change and the biological mechanisms that underlie it. We draw extensively from research conducted at coastal ecosystems studied by the US Long Term Ecological Research Network, where long-term, spatially extensive observational data are coupled with shorter-term mechanistic studies to understand the ecological consequences of climate change. [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]

Abstract: Herbivory has an important influence on seagrass abundance and distribution, yet there is limited understanding of grazing behavior in communities at geographic margins of species ranges. We used 24-h seagrass-tethering experiments to measure the effect of foliar nitrogen content and time of year on bucktooth parrotfish Sparisoma radians herbivory in high-latitude Thalassia testudinum meadows of Bermuda. Parrotfish preferentially grazed nitrogen-enriched tethered shoots especially at offshore sites that had relatively low background nutrient levels. Selective feeding was pronounced during periods of high grazing intensity. We found strong seasonal patterns, likely because Bermuda is at the northerly limit of these two species. Herbivory rates peaked in late summer when the mean leaf removal rate by S. radians was 3.38±1.11cmshoot−1 day−1, which exceeded the daily seagrass growth rate by 10-fold. Seagrass meadows are in decline in this region and we suggest that seasonal episodes of clustered, intensive grazing could make these habitats more vulnerable to other environmental or human-induced stresses, such as storms and runoff of sediment and nutrients. [Copyright &y& Elsevier]

Nitrogen (N) limitation of primary production is common in temperate salt marshes, even though conservative N recycling can fulfill a large proportion of plant N demand. In nutrient poor young marshes, N limitation may be more severe and new N sources, such as N fixation, more important for plant growth. We measured N fixation and the response of salt marsh primary producers (Spartina alterniflora and benthic microalgae) to N fertilization in one mature (>150 year) and two young (7 and 15 year) naturally developing marshes at the Virginia Coast Reserve LTER site. S. alterniflora aboveground biomass in the mature marsh (1,700±273 g m-2) was 1.8 and 2.8 times higher than in the 15 year and 7 year old marshes, respectively. Fertilization significantly increased S. alterniflora biomass in the two young marshes (160–175%) and areal aboveground tissue N in the youngest marsh (260%). Microalgal chlorophyll a (Chl a) in the mature marsh was nearly 2-fold lower than in the 7-year-old marsh, and there was no evidence that this was due to light limitation. However, Chl a in fertilized plots was 30% higher than control plots at the youngest site. Daily N fixation decreased with increasing marsh age in summer, when rates were highest at all sites. Autotrophic N fixation (difference between rates in the light and dark) was most important in the summer, but we saw no indication of a shift in dominance between autotrophic and heterotrophic N fixers during marsh development. Estimated annual N fixation was 2- to 3-fold higher in the young marsh (18.3±1.5 g N m-2 year-1), than in the intermediate-aged (9.0±0.7) or mature marsh (6.1±0.5). In the young marshes, N fixation was sufficient to provide a substantial proportion of aboveground S. alterniflora N demand. Our results suggest that both benthic microalgae and S. alterniflora in young salt marshes are N limited, and that this limitation decreases as the marsh matures. The high rates of N fixation by autotrophic and heterotrophic bacteria in the sediment could provide an important source of N for primary producers during marsh development. [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]

Dinitrogen (N2) fixation rates were determined in the phyllosphere and in sediments with and without the presence of eelgrass (Zostera marina) in San Quintín Bay, an upwelling-influenced coastal lagoon in the NE Pacific. Samples were collected during winter 2015 at 4 sites with a gradient of oceanic influence and contrasting impact from oyster aquaculture. N2 fixation rates were determined with the acetylene reduction assay. Treatments under light and in the dark, and with and without sodium molybdate resulted in similar fixation rates, suggesting that heterotrophic nonsulfate-reducing bacteria made the largest contribution to N2 fixation, while sulfate-reducing bacteria had low fixation activity at most stations. N2-fixation rates in sediments ranged from 7 to 12 µmol m-2 h-1 and were similar to those in other temperate seagrass-dominated estuaries. Winter conditions were likely responsible for small spatial differences in N2 fixation rates throughout the lagoon, even between vegetated and unvegetated sites and among depth sections of sediment cores. During winter, Z. marina growth rates and biomass are low, resulting in low and less variable release of labile organic carbon, which acts as substrate for diazotrophs. The lowest N2 fixation rates were measured at a site where high denitrification rates have been observed, probably reflecting a competition between diazotrophs and denitrifiers. The highest fixation rates were measured at the innermost station where oceanic nitrate is scarce. Epiphytic bacteria contributed ~7% of the total N2 fixation, with rates of <0.5 µmol m-2 h-1. N2 fixation potentially supplied 5-10% of Z. marina N requirements and could supply ~30% of the N loss via denitrification in winter. [ABSTRACT FROM AUTHOR]

Salt marshes are among the most productive ecosystems on Earth, providing nurseries for fish species and shelter and food for endangered birds. Salt marshes also mitigate the impacts of hurricanes and tsunamis, and sequester large volumes of carbon in their peat soil. Understanding the mechanisms responsible for marsh stability or deterioration is therefore a key issue for society. Sea level rise is often viewed as the main driver of salt marsh deterioration. However, while salt marshes can reach equilibrium in the vertical direction, they are inherently unstable in the horizontal direction. Marsh expansion driven by sediment supply rarely matches lateral erosion by waves, creating a dynamic landscape. Recent results show that marsh collapse can occur in the absence of sea level rise if the rate at which sediment is eroded at marsh boundaries is higher than the input of sediment from nearby rivers or from the continental shelf. We propose that the horizontal dynamics and related sediment fluxes are key factors determining the survival of salt marshes. Only a complete sediment budget between salt marshes and nearby tidal flats can determine the fate of marshes at any given location, with sea level rise being only one among many external drivers. Ancient Venetians understood this dynamic very well. They manipulated the supply of sediment to the Venice lagoon, Italy, in order to control the long-term evolution of the intertidal landscape. [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]

Seagrasses are important habitat-formers and ecosystem engineers that are under threat from bloom-forming seaweeds. These seaweeds have been suggested to outcompete the seagrasses, particularly when facilitated by eutrophication, causing regime shifts where green meadows and clear waters are replaced with unstable sediments, turbid waters, hypoxia, and poor habitat conditions for fishes and invertebrates. Understanding the situations under which seaweeds impact seagrasses on local patch scales can help proactive management and prevent losses at greater scales. Here, we provide a quantitative review of available published manipulative experiments (all conducted at the patch-scale), to test which attributes of seaweeds and seagrasses (e.g., their abundances, sizes, morphology, taxonomy, attachment type, or origin) influence impacts. Weighted and unweighted meta-analyses (Hedges d metric) of 59 experiments showed generally high variability in attribute-impact relationships. Our main significant findings were that (a) abundant seaweeds had stronger negative impacts on seagrasses than sparse seaweeds, (b) unattached and epiphytic seaweeds had stronger impacts than 'rooted' seaweeds, and (c) small seagrass species were more susceptible than larger species. Findings (a) and (c) were rather intuitive. It was more surprising that 'rooted' seaweeds had comparatively small impacts, particularly given that this category included the infamous invasive Caulerpa species. This result may reflect that seaweed biomass and/or shading and metabolic by-products like anoxia and sulphides could be lower for rooted seaweeds. In conclusion, our results represent simple and robust first-order generalities about seaweed impacts on seagrasses. This review also documented a limited number of primary studies. We therefore identified major knowledge gaps that need to be addressed before general predictive models on seaweed-seagrass interactions can be build, in order to effectively protect seagrass habitats from detrimental competition from seaweeds. [ABSTRACT FROM AUTHOR]

Physiological measurements were used to investigate the dependence of photosynthesis on light, temperature, and intercellular carbon dioxide (CO) levels in the C marsh grass Spartina alterniflora. Functional relationships between these environmental variables and S. alterniflora physiological responses were then used to improve C-leaf photosynthesis models. Field studies were conducted in monocultures of S. alterniflora in Virginia, USA. On average, S. alterniflora exhibited lower light saturation values (~1000 μmol m s) than observed in other C plants. Maximum carbon assimilation rates and stomatal conductance to water vapor diffusion were 36 μmol (CO) m s and 200 mmol (HO) m s, respectively. Analysis of assimilation-intercellular CO and light response relationships were used to determine Arrhenius-type temperature functions for maximum rate of carboxylation ( V), phosphoenolpyruvate carboxylase activity ( V), and maximum electron transport rate ( J). Maximum V values of 105 μmol m s were observed at the leaf temperature of 311 K. Optimum V values (80.6 μmol m s) were observed at the foliage temperature of 308 K. The observed V values were lower than those in other C plants, whereas V values were higher, and more representative of C plants. Optimum J values reached 138 μmol (electrons) m s at the foliage temperature of 305 K. In addition, the estimated CO compensation points were in the range of C or C-C intermediate plants, not those typical of C plants. The present results indicate the possibility of a C-C intermediate or C-like photosynthetic mechanism rather than the expected C-biochemical pathway in S. alterniflora under field conditions. In a scenario of atmospheric warming and increased atmospheric CO concentrations, S. alterniflora will likely respond positively to both changes. Such responses will result in increased S. alterniflora productivity, which is uncharacteristic of C plants. [ABSTRACT FROM AUTHOR]

The importance of positive interactions is increasingly acknowledged in contemporary ecology. Most research has focused on direct positive effects of one species on another. However, there is recent evidence that indirect positive effects in the form of facilitation cascades can also structure species abundances and biodiversity. Here we conceptualize a specific type of facilitation cascade—the habitat cascade. The habitat cascade is defined as indirect positive effects on focal organisms mediated by successive facilitation in the form of biogenic formation or modification of habitat. Based on a literature review, we demonstrate that habitat cascades are a general phenomenon that enhances species abundance and diversity in forests, salt marshes, seagrass meadows, and seaweed beds. Habitat cascades are characterized by a hierarchy of facilitative interactions in which a basal habitat former (typically a large primary producer, e.g., a tree) creates living space for an intermediate habitat former (e.g., an epiphyte) that in turn creates living space for the focal organisms (e.g., spiders, beetles, and mites). We then present new data on a habitat cascade common to soft-bottom estuaries in which a relatively small invertebrate provides basal habitat for larger intermediate seaweeds that, in turn, generate habitat for focal invertebrates and epiphytes. We propose that indirect positive effects on focal organisms will be strongest when the intermediate habitat former is larger and different in form and function from the basal habitat former. We also discuss how humans create, modify, and destroy habitat cascades via global habitat destruction, climatic change, over-harvesting, pollution, or transfer of invasive species. Finally, we outline future directions for research that will lead to a better understanding of habitat cascades. [ABSTRACT FROM AUTHOR]

Routine determination of dissolved organic nitrogen (DON) is performed in numerous laboratories around the world using one of three families of methods: UV oxidation (UV), persulfate oxidation (PO), or high temperature combustion (HTC). Essentially all routine methods measure total dissolved nitrogen (TDN) and calculate DON by subtracting the dissolved inorganic nitrogen (DIN). While there is currently no strong suggestion that any of these methods is inadequate, there are continuing suspicions of slight inaccuracy by UV methods.This is a report of a broad community methods comparison where 29 sets (7 UV, 13 PO, and 9 HTC) of TDN analyses were performed on five samples with varying TDN and DIN concentrations. Analyses were done in a “blind” procedure with results sent to the first author. With editing out one set of extreme outliers (representing 5 out of 145 ampoules analyzed), the community comparability for analyzing the TDN samples was in the 8–28% range (coefficient of variation representing one standard deviation for the five individual samples by 28 analyses). When DIN concentrations were subtracted uniformly (single DIN value for each sample), the comparability was obviously worse (19–46% cv). This comparison represents a larger and more diverse set of analyses, but the overall comparability is only marginally better than that of the Seattle workshop of a decade ago. Grouping methods, little difference was seen other than inconclusive evidence that the UV methods gave TDN values for several of the samples higher than HTC methods. Since there was much scatter for each of the groups of methods and for all analyses when grouped, it is thought that more uniformity in procedures is probably needed. An important unplanned observation is that variability in DIN analyses (used in determining the final analyte in most UV and PO methods) is essentially as large as the variability in the TDN analyses.This exercise should not be viewed as a qualification exercise for the analysts, but should instead be considered a broad preliminary test of the comparison of the families of methods being used in various laboratories around the world. Based on many independent analyses here, none of the routinely used methods appears to be grossly inaccurate, thus, most routine TDN analyses being reported in the literature are apparently accurate. However, it is not reassuring that the ability of the international community to determine DON in deep oceanic waters continues to be poor. It is suggested that as an outgrowth of this paper, analysts using UV and PO methods experiment and look more carefully at the completeness of DIN conversion to the final analyte and also at the accuracy of their analysis of the final analyte. HTC methods appear to be relatively easy and convenient and have potential for routine adoption. Several of the authors of this paper are currently working together on an interlaboratory comparison on HTC methodology. [Copyright &y& Elsevier]

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]

Patterns of dissolved organic carbon (DOC) and nitrogen (DON) delivery were compared between times of stormflow and baseflow in Paine Run, an Appalachian stream draining a 12.4 km^2 forested catchment in the Shenandoah National Park (SNP), Virginia. The potential in-stream ecological impact of altered concentrations and/or chemical composition of DOM during storms also was examined, using standardized bacterial bioassays. DOC and DON concentrations in Paine Run were consistently low during baseflow and did not show a seasonal pattern. During storms however, mean DOC and DON concentrations approximately doubled, with maximum concentrations occurring on the rising limb of storm hydrographs. The rapid response of DOM concentration to changes in flow suggests a near-stream or in-stream source of DOM during storms. Stormflow (4% of the time, 36% of the annual discharge) contributed >50% of DOC, DON and NO3^- flux in Paine Run during 1997. In laboratory bacterial bioassays, growth rate constants were higher on Paine Run stormflow water than on baseflow water, but the fraction of total DOM which was bioavailable was not significantly different. The fraction of the total stream DOC pool taken up by water column bacteria was estimated to increase from 0.03 1 0.02% h^-1 during baseflow, to 0.15 1 0.04% h^-1 during storms. This uptake rate would have a minimal effect on bulk DOM concentrations in Paine Run, but storms may still have considerable impact on the bacterial stream communities by mobilizing them into the water column and by supplying a pulse of DOM. [ABSTRACT FROM AUTHOR]