Estuarine Gradient Research Articles

Community disassembly and reassembly following experimental storm surge and wrack application

AbstractQuestionsThe structure of coastal vegetation could be altered by a regime of more intense hurricanes stemming from global climate change: (1) How do plant communities and environmental conditions across an estuarine gradient change in relation to simulated storm surge and wrack deposition? and; (2) Are there differences in the magnitude of compositional shifts along an underlying soil water conductivity gradient?LocationCoastal northwest Florida, USA.MethodsFour plant communities spanning an estuarine gradient (brackish marsh, freshwater marsh, wetland forest and pine savanna) were surveyed prior to and following flooding with artificial seawater and manual application of wrack. Environmental data (soil water conductivity and light transmission to soil surface) were also collected before and after treatment applications. Soil samples for texture analysis were collected prior to experimental treatments.ResultsThe storm surge treatment reduced cover of most species, causing browning and eventual loss of leaves, particularly in pine savanna species. Soil water conductivity remained elevated for 6 mo following the experimental storm surge in the wetland forest and pine savanna. Wrack deposition resulted in substantial mortality of underlying vegetation and reduced species richness in all communities. The composition of the pine savanna plant community shifted in response to experimental storm surge and wrack application, with ‘hardy’ species capable of resprouting becoming more abundant and sensitive species dropping out of the assemblage. Vegetation zones with high background soil salinity were more resilient to the effects of storm surge and wrack deposition.ConclusionsThe results suggest that the combined effects of storm surge and wrack deposition can drive compositional shifts, particularly in more upland assemblages. We conclude that vegetation consisting of species ‘naïve’ to increased salinity and/or burial by wrack are highly vulnerable to tropical storm intensification.

Decoupling of recruitment from adult clam assemblages along an estuarine shoreline

The maintenance of populations of benthic species requires both effective (if not regular) recruitment plus sufficient post-recruitment survival through vulnerable juvenile phases. Decoupling of abundances of settlers and adults often occurs when conditions for juvenile survival are poor. For infaunal bivalves, predation can be a key process that alters the abundance or distribution of adults relative to early recruits. We examined spatial patterns of recruitment of intertidal clams along the marine-estuarine axis in Puget Sound, Washington, USA. Abundances and species richness of clam recruits were variable in space and time, but during the period of high recruitment (summer) were equal across all sites. Juvenile and adult clams, however, were consistently rare at the inner-estuarine sites relative to the outer. We hypothesized that predation was higher at our inner sites, and manipulated predator access to newly recruited clam assemblages along the estuarine gradient. Predators >1cm were excluded with mesh net from intertidal PVC tubes where recruitment of clams was allowed to occur naturally, and abundances and species richness of clams were measured after one summer season. Tubes with mesh nets over the top had significantly higher overall abundances of clams than unprotected tubes after 4months, but showed no difference in clam species richness. More strikingly, the clam assemblage at the inner-estuarine sites comprised very few individuals relative to the outer-estuarine sites, regardless of predator exclusion. Multivariate analyses showed that clam assemblages in the tubes were better explained by position along the estuarine gradient than by predator treatment. Parallel studies at these sites suggested that growth of suspension feeders is actually higher in the inner estuary. Physical gradients are much less extreme in this well-mixed fjord than in many other estuaries, but we hypothesize that physical stresses such as higher temperatures at the inner beaches in the summer may be a major contributor to juvenile mortality, decoupling recruitment from adult populations.

Accommodation of the sex-ratio in eastern oysters Crassostrea virginica to variation in growth and mortality across the estuarine salinity gradient

Protandric oysters generate a relatively uniform reproductive potential over a wide range of environmental conditions that impose variations in growth rate and life span. Sex-at-length keys applied to survey data show that the female fraction routinely fell between 0.4 and 0.5, regardless of location in the salinity gradient. About 70% of population biomass is female over the same salinity range. Due to the necessary local modulation of the rate of male-to-female conversion to limit the influence of varying growth and life span over the salinity gradient, the number of males always exceeds by a small amount the number of females; thus improving the likelihood of a female having neighbouring males, a necessity for an immobile broadcast spawner. However, oysters at the extremes of the estuarine gradient all yielded populations with divergent sex-ratios. One consequence of reduced generation time brought about by increased mortality from disease should be selection favouring the switch from male to female at smaller size, if disease mortality is strongly female-biased. The site with the longest record of high mortality manifests such an increase. Sites up coastal rivers, putative refuges from disease, harbour animals with the slowest male-to-female conversion rates. Arguably these animals are most similar to the ancestral oyster pre-disease. Marketed animals range from 62% to 69% female. The principal influence of the fishery, and of oyster disease, would seem to be a reduction in lifetime egg production. Dermo disease may have reduced lifetime fecundity of females by nearly a factor of four.

Mechanisms mediating plant distributions across estuarine landscapes in a low‐latitude tidal estuary

Understanding of how plant communities are organized and will respond to global changes requires an understanding of how plant species respond to multiple environmental gradients. We examined the mechanisms mediating the distribution patterns of tidal marsh plants along an estuarine gradient in Georgia (USA) using a combination of field transplant experiments and monitoring. Our results could not be fully explained by the "competition-to-stress hypothesis" (the current paradigm explaining plant distributions across estuarine landscapes). This hypothesis states that the upstream limits of plant distributions are determined by competition, and the downstream limits by abiotic stress. We found that competition was generally strong in freshwater and brackish marshes, and that conditions in brackish and salt marshes were stressful to freshwater marsh plants, results consistent with the competition-to-stress hypothesis. Four other aspects of our results, however, were not explained by the competition-to-stress hypothesis. First, several halophytes found the freshwater habitat stressful and performed best (in the absence of competition) in brackish or salt marshes. Second, the upstream distribution of one species was determined by the combination of both abiotic and biotic (competition) factors. Third, marsh productivity (estimated by standing biomass) was a better predictor of relative biotic interaction intensity (RII) than was salinity or flooding, suggesting that productivity is a better indicator of plant stress than salinity or flooding gradients. Fourth, facilitation played a role in mediating the distribution patterns of some plants. Our results illustrate that even apparently simple abiotic gradients can encompass surprisingly complex processes mediating plant distributions.

Habitat Related Variation in Reproductive Traits among Intertidal Crabs from the Southwestern Atlantic

The reproductive traits of three coastal grapsoid crabs from the southwestern Atlantic, Neohelice granulata (Dana, 1851), Cyrtograpsus angulatus Dana, 1851 and C. altimanus Rathbun, 1914, were compared under the hypothesis that more energy is diverted to maintenance (and less to reproduction) in the upper intertidal and/or in estuaries than in the lower marine intertidal or subtidal, in order to cope with the harsh, and frequently variable, environmental conditions of semiterrestrial and brackish water habitats. Each species occupies a different habitat along intertidal, as well as estuarine gradients: N. granulata lives in the upper and middle intertidal of salt marshes and mud flats, especially in estuarine waters; C. angulatus lives in the low intertidal and subtidal of estuarine and marine habitats, and C. altimanus is predominantly an intertidal and subtidal marine crab. Results suggested that energy demands imposed by the harsh environmental conditions of the estuarine and semiterrestrial habitats resulted in less energy available for reproduction in N. granulata, evidenced by a shorter breeding season, a reduced reproductive output per clutch and an increased clutch interval. The opposite occurred with C. altimanus: less energy was necessary to cope with the stable marine water, allowing the presence of a long breeding season, a higher reproductive output per clutch and a very short clutch interval. Differences observed between marine and estuarine populations of C. angulatus are comparable with differences between C. altimanus, and N. granulata.

Genotypic Distribution of a Specialist Model Microorganism, Methanosaeta, along an Estuarine Gradient: Does Metabolic Restriction Limit Niche Differentiation Potential?

A reductionist ecological approach of using a model genus was adopted in order to understand how microbial community structure is driven by metabolic properties. The distribution along an estuarine gradient of the highly specialised genus Methanosaeta was investigated and compared to the previously determined distribution of the more metabolically flexible Desulfobulbus. Methanosaeta genotypic distribution along the Colne estuary (Essex, UK) was determined by DNA- and RNA-based denaturing gradient gel electrophoresis and 16S rRNA gene sequence analyses. Methanosaeta distribution was monotonic, with a consistently diverse community and no apparent niche partitioning either in DNA or RNA analyses. This distribution pattern contrasts markedly with the previously described niche partitioning and sympatric differentiation of the model generalist, Desulfobulbus. To explain this difference, it is hypothesised that Methanosaeta's strict metabolic needs limit its adaptation potential, thus populations do not partition into spatially distinct groups and so do not appear to be constrained by gross environmental factors such as salinity. Thus, at least for these two model genera, it appears that metabolic flexibility may be an important factor in spatial distribution and this may be applicable to other microbes.

Predictive distribution modeling of the sandy-beach supralittoral amphipod Atlantorchestoidea brasiliensis along a macroscale estuarine gradient

Exposed marine beaches are physically rigorous habitats in which macrofaunal patterns have been well correlated with physical factors. In this context, the habitat safety hypothesis (HSH) predicts an increase in abundance of supralittoral species from dissipative to reflective conditions in microtidal oceanic beaches. However, the HSH has not been adequately tested in estuarine sandy beaches. Here, we build a predictive model based on the supralittoral talitrid amphipod Atlantorchestoidea brasiliensis that allowed us to test the HSH along a macroscale estuarine gradient. Fifteen Uruguayan sandy beaches along a salinity gradient (from 0.1 to 34.3) generated by the Río de la Plata estuary (SW Atlantic Ocean) were sampled over a two-year period. A conditional two-step procedure using a General Additive Model (GAM) was performed in order to model A. brasiliensis occurrence (1st-step) and abundance (2nd-step), based on a comprehensive set of environmental variables [salinity, water temperature, beach face slope (BFS), mean grain size, wave height and sand compaction, moisture and organic matter]. Each GAM was parameterized using generalized linear models (GLMs). An external validation procedure was used. Data were divided randomly into training (75%) and test (25%) sets. The 1st-step GAM/GLM retained 5 physical descriptors in the model (decreasing order of importance): wave height, salinity, BFS, organic matter and sand compaction. The 2nd-step GAM/GLM retained 6 physical descriptors: sand compaction, water content, salinity, wave height, water temperature and organic matter. The predictive ability of these models and the resulted combined model, as well as their external validity, was highly significant and supported the predictions of the HSH. However, the highest and lowest abundances were observed, respectively, on reflective and dissipative beaches in the outer estuary; i.e., the predictions based on the HSH did not account for the high levels of abundance observed on these beaches compared with oceanic ones. Therefore, the HSH was expanded to include microtidal estuarine beaches.

Grazing and viral lysis vary for different components of the microbial community across an estuarine gradient

The balance between grazing and viral lysis of both photosynthetic and heterotro- phic microbes determines the food resources that are available, and how much of each resource is present, for higher trophic levels. Mobile Bay is the fourth largest estuary in the USA, based on river discharge, and is home to a variety of nursery habitats for larvae and juveniles of economi- cally important species which depend for their food on production of organisms of microbial size. To characterize the patterns of growth and removal rates of this fraction of organisms in Mobile Bay, we carried out modified dilution experiments in summer and winter at 3 stations along a salinity and nutrient gradient. Rates of growth, grazing and viral lysis for prokaryotes and domi- nant phytoplankton (cyanobacteria, diatoms and dinoflagellates) were determined for each exper- iment. Grazing and viral lysis of prokaryotes did not appear to be related to season or location, and both processes were detected within the bay as well as on the shelf. For phytoplankton, grazing was the dominant removal process inshore in the summer, while viral lysis of phytoplankton was detected in the fall and in winter. In winter, an apparent increase in dinoflagellates, and in the overall biomass of phytoplankton, was concurrent with increased viral lysis, suggesting that recy- cling by viruses may be an important source of nutrients in coastal waters.

Fish assemblages of interior tidal marsh channels in relation to environmental variables in the upper San Francisco Estuary

Tidal marsh wetlands represent critical habitat for many estuarine fishes and are particularly important to conserving and restoring native and at-risk species. We describe the seasonal and regional variation in the composition and abundance of fishes in interior tidal marsh channels in the upper San Francisco Estuary (SFE), and relate these to variation in environmental conditions. Fish were sampled quarterly using modified fyke nets from October 2003 to June 2005 in 18 interior tidal marsh channels spanning 3 distinct river systems: Petaluma River, Napa River and West Delta. We collected 116 samples and 9452 individuals of 30 fish species. Four non-native species dominated—Mississippi silverside, western mosquitofish, yellowfin goby, and rainwater killifish—with an additional 13 species occurring commonly (represented equally by natives and non-natives, residents and transients). Large seasonal differences in composition and abundance of fishes occurred, with the lowest abundances in winter and spring and highest abundances in summer and fall. Correlation of ordination scores and environmental variables further supported the importance of season, as well as fish species’ status (native vs. non-native), feeding preference (pelagic vs. demersal), and marsh utilization (resident vs. transient), as factors influencing fish assemblage composition. The proximity of the marsh systems to freshwater and marine influences, which largely control salinity and temperature variation, explained 26% of the variation in fish composition, while channel geomorphology explained 22%. We recommend that both edge habitat (which may be beneficial to fish foraging success) and the extent of tidal connectivity (which allows access for fishes), in addition to location along the estuarine gradient, be considered in designing and managing tidal marsh restoration.

Contrasting patterns of niche partitioning between two anaerobic terminal oxidizers of organic matter

Understanding the ecological principles underlying the structure and function of microbial communities remains an important goal for microbial ecology. We examined two biogeochemically important taxa, the sulfate-reducing bacterial genus, Desulfobulbus, and the methanogenic archaeal genus, Methanosaeta, to compare and contrast niche partitioning by these two taxa that are ecologically linked as anaerobic terminal oxidizers of organic material. An observational approach utilizing functional gene pyrosequencing was combined with a community-based reciprocal incubation experiment and characterization of a novel Desulfobulbus isolate. To analyze the pyrosequencing data, we constructed a data analysis pipeline, which we validated with several control data sets. For both taxa, particular genotypes were clearly associated with certain portions of an estuarine gradient, consistent with habitat or niche partitioning. Methanosaeta genotypes were generally divided between those found almost exclusively in the marine habitat (∼30% of operational taxonomic units (OTUs)), and those which were ubiquitously distributed across all or most of the estuary (∼70% of OTUs). In contrast to this relatively monotonic distribution, for Desulfobulbus, there were many more genotypes, and their distributions represented a wide range of inferred niche widths from specialist genotypes found only at a single site, to ubiquitous or generalist genotypes found in all 10 sites examined along the full estuarine gradient. Incubation experiments clearly showed that, for both taxa, communities from opposite ends of the estuary did not come to resemble one another, regardless of the chemical environment. Growth of a Desulfobulbus isolated into pure culture indicated that the potential niche of this organism is significantly larger than the realized niche. We concluded that niche partitioning can be an important force structuring microbial populations, with biotic and abiotic components having very different effects depending on the physiology and ecology of each taxon.

Intertidal meiofauna communities along an estuarine gradient in a protected area (Ria de Foz, north-west Spain)

The northern Galician rias provide important economic and social services from fisheries, shellfish harvesting and recreational activities. Consequently, these rias have been included in the Nature 2000 Network as a Special Conservation Zone. The aims of this study were to describe the meiofauna spatial structure in terms of diversity and abundance of major taxa, as well as its relationship with the environmental characteristics in the Ria de Foz nature reserve. For this aim, environmental gradients and distribution patterns of intertidal meiobenthic communities at the Ria de Foz nature reserve were studied using multivariate methods. Ria de Foz showed to be a complex transitional habitat with significant changes in its environment along the estuarine gradient. An abundant and rich meiofauna community was reported at Ria de Foz. A total of 21 higher taxa of meiofauna were found. The most common taxa were nematodes, copepods, ostracods and turbellarians. Three clear different meiobenthic communities were defined. These three communities showed a clear spatial distribution pattern along the estuarine gradient. Moreover, a high degree of similarity between distribution patterns of meiobenthic communities and previously studied macrobenthic communities was found. Regarding the role of environmental factors shaping meiobenthic community distribution patterns distance to the river mouth was the environmental variable that best explained meiobenthic community structure and distribution. Therefore, changes on hydrodynamics that affect environmental factors related to the estuarine gradient will change the distribution pattern and diversity of benthic communities. This work provides the first information about meiobenthos diversity and structure in the Ria de Foz nature reserve and should be useful for future management and conservation plans on this and other similar protected areas.

Abundance and diversity patterns of the sessile macrobenthic community associated with environmental gradients in Vitória Harbor, southeastern Brazil

Harbor terminals and urban sewage effluents affect the composition and distribution of epibenthic organisms. In this study, we hypothesized that the benthic community structure at the Vitoria Harbor changes spatially in a ~3 km scale, and that these changes are associated with environmental gradients resulting from point-source sewage and differences in the physical and chemical parameters of the water along the harbor access channel. Four sites, internal (PI), intermediate-internal (PMI), intermediate-external (PME) and external (PE), varying from 0.5 to 4.0 km off the harbor, were sampled on five quadrats at six sampling dates (N = 30 per site). The epibenthic community on the shallow sublitoral rocky shore was sampled fortnightly from December 2005 to February 2006 by point-intersection method. A total of 27 taxa were registered with higher richness and diversity values at the external sites. The similarity analysis indicated two distinct systems, with the internal sites PI and PMI apart from the external PME and PE, which showed 97% of dissimilarity. While the internal sites presented some estuarine characteristics and a high coverage (> 60%) of hydrozoans and bryozoans with silt/clay, the external ones showed coastal water influence and higher amounts of sedimentary material substrate (> 50%). This pattern reflects the estuarine gradient and the suspended sedimentary material at the internal sites, which is carried out to the external parts of the channel. The data showed two distinct benthic communities and support the hypothesis that the community structure varies along the harbor access channel in a gradient from the inner to the outer portion of the estuary.

The Arctic Ocean Estuary

Large freshwater contributions to the Arctic Ocean from a variety of sources combine in what is, by global standards, a remarkably small ocean basin. Indeed, the Arctic Ocean receives ∼11% of global river discharge while accounting for only ∼1% of global ocean volume. As a consequence, estuarine gradients are a defining feature not only near-shore, but throughout the Arctic Ocean. Sea-ice dynamics also play a pivotal role in the salinity regime, adding salt to the underlying water during ice formation and releasing fresh water during ice thaw. Our understanding of physical–chemical–biological interactions within this complex system is rapidly advancing. However, much of the estuarine research to date has focused on summer, open water conditions. Furthermore, our current conceptual model for Arctic estuaries is primarily based on studies of a few major river inflows. Future advancement of estuarine research in the Arctic requires concerted seasonal coverage as well as a commitment to working within a broader range of systems. With clear signals of climate change occurring in the Arctic and greater changes anticipated in the future, there is good reason to accelerate estuarine research efforts in the region. In particular, elucidating estuarine dynamics across the near-shore to ocean-wide domains is vital for understanding potential climate impacts on local ecosystems as well as broader climate feedbacks associated with storage and release of fresh water and carbon.

Terrigenous nutrient and organic matter in a subtropical river estuary, Okinawa, Japan: origin, distribution and pattern across the estuarine salinity gradient

Estuaries and their associated rivers are organically rich sites that play an important role in the removal and transformation of organic matter and nutrients derived from terrigenous and anthropogenic sources. This study conducted comprehensive field surveys from March 2010 to January 2011 to study the downstream pattern and distribution of nutrients (NO, NO, NH and PO) and dissolved organic carbon (DOC) and particulate organic carbon (POC) and nitrogen (PN) in the surface waters of the Manko estuary, a subtropical estuary in Okinawa, Japan. Fatty acid (FA) analysis and POC–C/N ratios were used to further identify variations in the sources of particulate organic matter along the estuary during summer (July) and winter (January). The results suggest that the estuary contains high concentrations of dissolved inorganic (N and P) and organic (DOC) nutrients, which are largely influenced by terrestrial sources from the Kokuba and Noha rivers, and moderate levels of particulate nitrogen and carbon. In general, suspended particles and dissolved nutrients followed sedimentation and biotic uptake patterns common in other subtropical estuaries. Thus, an important fraction of terrestrial materials was rapidly sinking along the estuary and was replaced with estuarine and marine-derived materials at mid- to high-salinity along the estuary. The FA signatures suggested that bacteria, domestic and agricultural waste-derived organic matter were the dominant sources of suspended organic matter in the Manko estuary. Overall, despite the relatively high terrestrial and anthropogenic influences in the Manko estuary, effective processing of different sources and forms of terrigenous organic matter in the estuarine salinity gradient significantly reduces their signatures prior to export to the coastal ocean.

Ability of benthic indicators to assess ecological quality in estuaries following management

Robust approaches to evaluate ecological quality in aquatic ecosystems become demanding concomitantly with the degradation resulting from increasing human-induced pressures. Aiming at a successful management, numerous evaluation tools are currently available and have been widely used for quality status assessment in estuaries mainly through the analysis of macrobenthic communities. The Mondego estuary (Portugal) has been under severe environmental pressure since the early 1990s due to different anthropogenic stresses. To improve the ecological condition and mainly to ameliorate the risk of eutrophication symptoms in an estuarine section (south arm), in 2006, the upstream connection between the north and south estuarine arms was re-established. Data on subtidal macrobenthic communities and environmental variables over a 5-year interval, covering the pre-management (from 2004 to 2006) and post-management (2007 and 2008) periods were used to assess restoration effectiveness. Firstly, temporal differences in the structure and composition of the macrobenthic assemblages between periods were assessed. Secondly, the relative performance of different ecological indicators (Shannon–Wiener, Margalef, Pielou, Simpson, AMBI, taxonomic diversity measures and Eco-exergy based indicators) to capture potential ecological changes between periods was analyzed. Lastly, indicators’ performance was compared with the information given by the analysis of macrofauna composition and structure to distinguish their usefulness for addressing specific management objectives.In the post-management, water quality did not reflect considerable changes while macrobenthic assemblages responded to restoration variably along the estuarine gradient. Overall, ecological indicators effectively detected the prevailing ecological conditions and behaved consistently with the subtidal community ecological context. Most indicators were able to capture temporal changes in macrobenthic assemblages structure and performed differently for the distinct estuarine zones assessed, not always indicating a quality improvement. Further monitoring and assessment on subtidal macrobenthic communities’ structure and functioning is needed to understand better the restoration effects. Recognize the success of such actions in aquatic systems is crucial for their adequate management.

Context-Dependent Eelgrass–Macroalgae Interactions Along an Estuarine Gradient in the Pacific Northwest, USA

Land-based eutrophication is often associated with blooms of green macroalgae, resulting in negative impacts on seagrasses. The generality of this interaction has not been studied in upwelling-influenced estuaries where oceanic nutrients dominate seasonally. We conducted an observational and experimental study with Zostera marina L. and ulvoid macroalgae across an estuarine gradient in Coos Bay, Oregon. We found a gradient in mean summer macroalgal biomass from 56.1 g dw 0.25 m−2 at the marine site to 0.3 g dw 0.25 m−2 at the riverine site. Despite large macroalgal blooms at the marine site, eelgrass biomass exhibited no seasonal or interannual declines. Through experimental manipulations, we found that pulsed additions of macroalgae biomass (+4,000 mL) did not affect eelgrass in marine areas, but it had negative effects in riverine areas. In upwelling-influenced estuaries, the negative effects of macroalgal blooms are context dependent, affecting the management of seagrass habitats subject to nutrient inputs from both land and sea.

Influence of organic matter source and diagenetic state on photochemical release of dissolved organic matter and nutrients from resuspendable estuarine sediments

Presently there is a paucity of knowledge regarding the influence of sedimentary organic matter (SOM) source and diagenetic state on photolytic fluxes of dissolved organic matter and nutrients from sediments likely to be resuspended during high energy events such as storms. Here we present data on the photochemical release of dissolved organic carbon (DOC), total dissolved nitrogen (TDN), dissolved organic nitrogen (DON), NH 4 +, and PO 4 3− from the fine fraction of sediments obtained along an estuarine gradient, coupled with analyses of biogeochemical and molecular indicators of SOM source. Photochemical release of DOC and PO 4 3− was significantly higher than dark controls at all sites. DOC photorelease from fine (< ca. 10–20 μm) sediments was ten times higher than previous studies that used bulk sediment in suspensions. The magnitude, stoichiometry, and speciation of nutrients and dissolved organic matter released during sediment photolysis appear to be influenced by a combination of SOM source and diagenetic processing. Release of DOC and PO 4 3− was higher at upstream sites, where sediments were dominated by relatively unaltered terrestrially-derived SOM. In contrast, TDN, DON, and NH 4 + releases were higher at downstream sites, which exhibited more degraded organic matter of both terrestrial and aquatic origin. The ratio of inorganic nutrients released was low at all sites (NH 4 +: PO 4 3− ~ 1–7) suggesting that SOM photolysis may be relatively more important as a source of inorganic P than N. Our results also demonstrate that SOM is broadly photolabile and that previous studies may underestimate photolytic release of DOC from the fine fraction of sediments.

In situ experimental decomposition studies in estuaries: A comparison of Phragmites australis and Fucus vesiculosus

The decomposition rates of Phragmites australis and Fucus vesiculosus were experimentally determined in an estuarine system using the leaf-bag technique. The study was conducted in fifteen sites arranged in five areas, extending from freshwater, outside the tidal range, to the marine environment, near the mouth of the estuary. The leaf-bags (5 mm mesh), were set up with 3.0 g of dried substrate, submerged in the experimental sites at day 0 and collected at days 3, 7, 15, 30 and 60, to follow biomass loss. The biomass loss through the leaching phase (day 3) was about 16% for Phragmites australis and 33% for Fucus vesiculosus and was independent of salinity for both substrates. The difference in the remaining biomass between the two species increased with time and the decomposition rates differed along the salinity gradient. For F. vesiculosus, the decomposition rate was highest near the mouth of the estuary, corresponding to the preferential distribution area of the algae, and decreased towards freshwater. For Phragmites australis, the fastest decay was observed in the mid estuary, where Phragmites australis occurs naturally, confirming previous studies. The decomposition rates measured at different time intervals (0–15, 0–30 and 0–60 days) were always higher for the algae and decreased with time for both species. These results indicate that the use of decomposition rates as a measure of ecosystem integrity or quality status in transitional waters will not be straightforward and must take into account, among others, the test species, the study area positioning along the estuarine gradient, and the time interval for the calculation of the decomposition rate.

Effect of organic matter source and salinity on dissolved organic matter isolation via ultrafiltration and solid phase extraction

In this study, samples were taken from three contrasting freshwater sources and amended with salt in order to determine the influence of salinity and dissolved organic matter (DOM) composition on DOM recovery via ultrafiltration and solid phase extraction (SPE) with C18 disks. Salt addition caused variable recovery of DOM when using C18 SPE, and ultraviolet–visible spectroscopic characterization of the extracted material showed spectral responses that varied among sample sources. In contrast, increasing sample salinity from 0 to 30 ppt consistently caused a 15–25% reduction in the amount of high molecular weight DOM isolated by ultrafiltration for both dissolved organic carbon (DOC) and chromophoric DOM (CDOM), regardless of DOM composition. We hypothesize that a change in conformation (such as coiling or disaggregation) of DOM molecules occurs in the presence of salt, allowing them to pass through the ultrafiltration membrane and thereby decreasing the DOM retained by ultrafiltration. These results are important because they demonstrate that changes in salinity can influence DOM recovery in estuaries. Interpretation of DOM characteristics along estuarine gradients needs to account for potential artifacts introduced by sample isolation techniques.

Isotopic Determination of Food Web Origins in Restoring and Ancient Estuarine Wetlands of the San Francisco Bay and Delta

We compared the extent to which ancient and restoring wetlands in three estuary regions of San Francisco Bay support estuarine ecosystems through food web contributions. In comparison to mature marshes, we hypothesized that food webs of increasingly younger restoration sites would display increased dependency upon allochthonous subsidies due to nominal internal production. Using multiple stable isotopes (δ13C, δ15N, δ34S) in a mixing model, we traced links among primary producers and estuarine consumers. Results indicate that food webs of estuarine marshes are heavily dependent upon autochthonous marsh materials (76 ± 17%), even within the youngest restoration marshes (11 years). Nearly all sampled organisms relied upon autochthonous marsh materials, with the exception of Neomysis kadiakensis, a mysid shrimp, which derived the majority of its support from freshwater-produced phytoplankton. Marsh-derived organic matter (OM) support was consistent both temporally throughout the year and spatially along the three estuary regions, but evidence suggests that the specific type of OM supporting estuarine consumers depends on position along the estuarine gradient and on seasonal shifts in freshwater flow. These results indicate that wetland restoration rapidly provides important contributions to marsh consumers and potentially bolsters food web linkages in shallow-water ecosystems.