Narragansett Bay Research Articles

The Declining Role of Organic Matter in New England Salt Marshes

The Northeast USA is experiencing severe impacts of a changing climate, including increased winter temperatures and accelerated relative sea level rise (RSLR). The sediment-poor, organic-rich nature of many Southern New England salt marshes makes them particularly vulnerable to these changes. In order to assess how marsh accretion has changed over time, we returned to Narragansett Bay, RI where salt marsh vertical accretion rates were documented almost 30 years ago. Using radionuclide tracers (210Pb and 137Cs), we observe no significant change in overall accretion rates (0.27–0.69 cm year−1) compared to historical averages (0.24–0.60 cm year−1), but we document a shift in how these marshes maintain elevation. Organic matter now plays a smaller role in contributing to vertical accretion across all study sites, declining by 22 % on average. We attribute this reduction to potentially higher decomposition rates fueled by higher water temperature. Inorganic matter also contributes less to accretion (declining by 44 % on average at marshes located more internal to the estuary), likely due to diminishing sediment supply in this region. With organic and inorganic solids accounting for less of the total accretion, several of the marshes are experiencing symptoms of swelling, with water and porespace contributing more towards accretion compared to historical values. Accretion rates (0.27–0.45 cm year−1) at these organic-rich (>40 % sediment organic matter) marshes are predominantly lower than the current (30 years) rate of RSLR (0.41 ± 0.07 cm year−1). These results, combined with the increased rate of RSLR and the hardened shorelines inhibiting landward migration, call into question the long-term survivability of these marshes.

Developing the First Operational Nutrient Observatory for Ecosystem, Climate, and Hazard Monitoring for NERACOOS

Abstract An integrated nutrient observatory is being developed within the Northeastern Regional Association of Coastal Ocean Observing Systems (NERACOOS), capable of monitoring nutrient dynamics year-round at temporal and spatial scales necessary to address critical needs of stakeholders throughout the Northeast region. Nutrient levels and fluxes drive total biological productivity throughout the region, from phytoplankton to commercially exploited fish stocks. Nitrate sensors (Satlantic SUNAs) are being installed on existing mooring assets in western Long Island Sound, Narragansett Bay (Prudence Island), Great Bay in New Hampshire, Massachusetts Bay, three sites along the coastal shelf of the Gulf of Maine (GOM), at five depths in Jordan Basin in the interior GOM, and at two depths in the GOM Northeast Channel. Phosphate and ammonium sensors (WET Labs Cycle-PO4 and Cycle-NH4) are also being deployed at the three nearshore sites. The measurements from these sensors will extend the current sparse, long-term records of nutrients from discretely collected samples in the Northeast region and will dramatically improve temporal resolution and continuity of the data for use in studying potential impacts of climate change. Nearshore measurements will be used by NERACOOS stakeholders to help assess, regulate, and mitigate the adverse impacts on water quality associated with excessive pollutant loadings. Measurements throughout the GOM will be used to assess basin-wide nutrient variability and to initialize harmful algal bloom (Alexandrium fundyense) forecast models.

Metatranscriptome analyses indicate resource partitioning between diatoms in the field

Diverse communities of marine phytoplankton carry out half of global primary production. The vast diversity of the phytoplankton has long perplexed ecologists because these organisms coexist in an isotropic environment while competing for the same basic resources (e.g., inorganic nutrients). Differential niche partitioning of resources is one hypothesis to explain this "paradox of the plankton," but it is difficult to quantify and track variation in phytoplankton metabolism in situ. Here, we use quantitative metatranscriptome analyses to examine pathways of nitrogen (N) and phosphorus (P) metabolism in diatoms that cooccur regularly in an estuary on the east coast of the United States (Narragansett Bay). Expression of known N and P metabolic pathways varied between diatoms, indicating apparent differences in resource utilization capacity that may prevent direct competition. Nutrient amendment incubations skewed N/P ratios, elucidating nutrient-responsive patterns of expression and facilitating a quantitative comparison between diatoms. The resource-responsive (RR) gene sets deviated in composition from the metabolic profile of the organism, being enriched in genes associated with N and P metabolism. Expression of the RR gene set varied over time and differed significantly between diatoms, resulting in opposite transcriptional responses to the same environment. Apparent differences in metabolic capacity and the expression of that capacity in the environment suggest that diatom-specific resource partitioning was occurring in Narragansett Bay. This high-resolution approach highlights the molecular underpinnings of diatom resource utilization and how cooccurring diatoms adjust their cellular physiology to partition their niche space.

A Marine Autonomous Surface Craft for Long-Duration, Spatially Explicit, Multidisciplinary Water Column Sampling in Coastal and Estuarine Systems

AbstractThe Surveying Coastal Ocean Autonomous Profiler (SCOAP) is a large catamaran marine autonomous surface craft (MASC) for unattended weeks-long, spatially explicit, multidisciplinary oceanographic water column profile sampling in coastal/estuarine waterbodies. Material transport rates/pathways, crucial to understanding these ecosystems, are typically poorly known. SCOAP addresses demanding spatiotemporal sampling needs and operational challenges (strong currents, open coastal sea states, complex bathymetry, heavy vessel traffic). Its large size (11-m length, 5-m beam) provides seaworthiness/stability. The average speed of 2.5 m s−1 meets the representative goal to traverse an 18-km transect, sampling 10 min at each of 10 stations 2 km apart, nominally 4 times daily. Efficient hulls and a diesel–electric energy system can provide the needed endurance. The U.S. Coast Guard guidelines are followed: lighting, code flags, the Automatic Identification System (AIS), and collision avoidance regulations (COLREGs)-based collision avoidance (CA) by onboard autonomy software. Large energy reserves obviate low-power optimization of sensors, enabling truly multidisciplinary sampling, and provide on-demand propulsion for effective CA. Vessel stability facilitates high-quality current profile observations and will aid engineering/operation of the planned winched profiling system, performance of an anticipated radar system to detect/track non-AIS vessels, and potential research-quality meteorological sensor operation. A Narragansett Bay test deployment, attended by an escort vessel, met design goals; an unattended open coastal deployment is planned for Rhode Island Sound. Scientific and operational strengths of large catamaran MASCs suggest they could be an important cost-effective complement to other sampling platforms (e.g., improved spatiotemporal coverage and resolution, extending farther inshore, with a broader range of sensors, compared to underwater gliders) in coastal/estuarine waters.

Relationships of Modeled Nitrogen Loads with Marsh Fish in the Narragansett Bay Estuary, Rhode Island

The human population and associated watershed development has risen steadily since the 1850s in Rhode Island. With these increases, human-derived wastewater has also risen dramatically, resulting in increasing nitrogen loads to estuarine systems. In this study, we examined relationships of modeled watershed nitrogen loads of 6 coastal subwatersheds

Developing a wintering waterfowl community baseline for environmental monitoring of Narragansett Bay, Rhode Island

In 2004, the Atlantic Ecology Division of the US Environmental Protection Agency’s Office of Research and Development began an annual winter waterfowl survey of Rhode Island’s Narragansett Bay. Herein, we explore the survey data gathered from 2004 to 2011 in order to establish a benchmark understanding of the Narragansett Bay’s waterfowl communities and to establish a statistical framework for future environmental monitoring. Abundance and diversity of Narragansett Bay’s wintering waterfowl were relatively stable during the initial years of this survey, except in 2010 when there was a large spike in abundance and a reciprocal fall in diversity. There was no significant change in ranked abundance of most waterfowl species, with only Bufflehead (Bucephala albeola) and Hooded Merganser (Lophodytes cucllatus) showing a slight yet significant upward trend during the course of our survey period. Nonmetric multidimensional scaling was used to examine the community structure of wintering waterfowl in Narragansett Bay. The results of the Nonmetric multidimensional scaling indicate that there is a spatial structure to the waterfowl communities of Narragansett Bay and this structure has remained relatively stable since the survey began. Our Nonmetric multidimensional scaling analysis helps to solidify what is known anecdotally about Narragansett Bay’s waterfowl ecology, and provides a formalized benchmark for long-term monitoring of Narragansett Bay’s waterfowl communities. Birds, including waterfowl, are preferred bioindicators and we propose using our multivariate approach to monitor the future health of Narragansett Bay.

Whole truths vs. half truths – And a search for clarity in long-term water temperature records

There is widespread acceptance among the scientific community that human activities are the primary cause of present day climate change. But, how a changing climate impacts ecosystems is still a source of confusion to the public. Some of this confusion is associated with a lack of clear communication among journalists and scientists, particularly when it comes to addressing variability and uncertainty in ecological datasets. Here we use long-term surface water temperature data sets from Narragansett Bay and a recent misunderstanding of long-term temperature data that occurred on the national stage as a case study. Specifically, we re-evaluate and update the record and examine the variability inherent in long-term data sets. We found that despite high year to year variations the surface waters of Narragansett Bay have increased between 1.4 °C to 1.6 °C total rise over the last fifty years. Winter warming has been especially high over this time period, increasing between 1.6 °C to 2.0 °C. Finally, we identify the need for scientists, politicians, and journalists to appropriately address data variability and we argue for increased communication among these groups.

Submarine groundwater discharge of rare earth elements to a tidally-mixed estuary in Southern Rhode Island

Rare earth element (REE) concentrations were analyzed in surface water and submarine groundwater within the Pettaquamscutt Estuary, located on the western edge of Narragansett Bay in Rhode Island. These water samples were collected along the salinity gradient of the estuary. Rare earth element concentrations in the majority of the groundwater samples are substantially higher than their concentrations in the surface waters. In particular, Nd concentrations in groundwater range from 0.43nmolkg−1 up to 198nmolkg−1 (mean±SD=42.1±87.2nmolkg−1), whereas Nd concentrations range between 259pmolkg−1 and 649pmolkg−1 (mean±SD=421±149pmolkg−1) in surface waters from the estuary, which is, on average, 100 fold lower than Nd in the groundwaters. Groundwater samples all exhibit broadly similar middle REE (MREE) enriched shale-normalized REE patterns, despite the wide variation in pH of these natural waters (4.87≤pH≤8.13). The similarity of the shale-normalized REE patterns across the observed pH range suggests that weathering of accessory minerals, such as apatite, and/or precipitation of LREE enriched secondary phosphate minerals controls groundwater REE concentrations and fractionation patterns. More specifically, geochemical mixing models suggest that the REE fractionation patterns of the surface waters may be controlled by REE phosphate mineral precipitation during the mixing of groundwater and stream water with incoming water from the Rhode Island Sound. The estimated SGD (Submarine Groundwater Discharge) of Nd to the Pettaquamscutt Estuary is 26±11mmolNdday−1, which is in reasonable agreement with the Nd flux of the primary surface water source to the estuary, the Gilbert Stuart Stream (i.e., 36mmolday−1), and of the same order of magnitude for a site in Florida.

Recruitment, abundance, and predation on the blue mussel (Mytilus edulis) on northeastern estuarine rocky shores

We report on patterns of abundance, recruitment, and predation on the blue mussel (Mytilus edulis) in three human‐dominated estuaries in the northeastern United States. Through replicate field experiments and observational studies at multiple sites nested within each of the three estuaries, we investigated the relative influences of local and regional scale variation in select bottom‐up and top‐down factors on blue mussel populations on wave‐protected rocky shores. The most striking result was the decoupling between adult abundance and recruitment: mussel recruitment rates were highest in the most northern estuary, Casco Bay, while adult abundances were highest in the most southern estuary, Long Island Sound. We detected evidence of top‐down forcing on adult abundance by consumers in the two more southern estuaries, Narragansett Bay and Long Island Sound, but not in Casco Bay. Finally, we observed some indications of bottom‐up forcing on mussel abundance and recruitment at the within‐estuary scale, but these signals were not consistent among estuaries or across the responses measured (e.g., adult abundances and recruitment rates). Our results support previous work demonstrating the importance of both top‐down and bottom‐up influences on rocky shore populations, and also highlight how future research—particularly integrating studies of the different ontogenetic stages of mussels—could further advance understanding of biological population dynamics in this and other systems.

Sea-Level Rise Effects on Hurricane-Induced Salinity Transport in Apalachicola Bay

Huang, W.; Hagen, S.C.; Bacopoulos, P., and Teng, F., 2014. Sea-level rise effects on hurricane-induced salinity transport in Apalachicola Bay.Salinity is an important indicator for estuarine ecosystem. Estuarine salinity can be affected by hurricane and sea-level rises. In this study, hydrodynamic modeling study has been conducted to investigate the effects of sea-level rise on hurricane-induced salinity in Apalachicola Bay. By using the dataset for the Hurricane Dennis occurred in July, 2005, model simulations were conducted under different sea-level rise scenarios. Results from model simulations show the effects of sea-level rise on the estuarine salinity transport during different phases of the storm surge. Generally, the increase of water level by either storm surge or sea-level rise results in the intrusion of majority saline sea water from the east to the west through East Pass. Salinity at two oyster bars responds to the storm surge and sea-level rise differently because Cat Point is located in the east and Dry Bar is in the west of the river mouth. In Cat Point, sea-level rise can cause substantial increase of salinity because it is located between the river mouth and East Pass. Salinity at the peak of the storm surge reaches 30 ppt even without sea-level rise. While salinity at the end of the storm surge reduces to about 20 ppt under no sea-level rise condition at Cat Point, it substantially increases to 30 ppt in response to a sea-level rise of 0.2 m. However, in Dry Bar, salinity is less sensitive to the sea-level rise and the storm surge. At the peak of the storm surge, salinity in Dry Bar is 30 ppt, 28 ppt, 30 ppt., under SLR 0.2 m, 0.5 m, and 1.2 m, respectively. However, near the end of the storm surge, salinity is 22 ppt, 22 ppt, and 27 ppt under 0.2 m, 0.5 m, and 1.2 m SLR conditions, respectively. This indicates that, after the storm surge, salinity in Dry Bar can recover to the normal range (below 26 ppt) if sea-level rise is less or equal to 0.5 m.

Seasonal Abundance of the CtenophoreMnemiopsis leidyiin Relation to Water Temperature and Other Zooplankton in the Thames River Estuary, Connecticut

I examined seasonal patterns in the abundance of the ctenophore Mnemiopsis leidyi (Atlantic Comb Jelly) to determine whether unusually early ctenophore blooms occur outside the range in which they were originally reported (Narragansett Bay, RI) and whether ctenophore abundance is correlated with sea surface temperature (SST) or with declines in zooplankton prey. Sampling was conducted from 23 April to 16 November 2004 in the Thames River Estuary, CT. Adult (≥1 cm) ctenophores first appeared in the estuary on 20 May 2004, with a seasonal peak in mean (± SD) density (9.9 ± 2.5 individuals m-3, n = 7 stations) and biovolume (26.6 ± 9.9 ml m-3, n = 7 stations) on 9 July 2004. Adult Atlantic Comb Jelly abundance was positively correlated with SST, but only during the spring and early summer. Food availability also likely played an important role in ctenophore-population growth early in the year. However, I observed a delay between copepod density and ctenophore biovolume, possibly due to unusually cold winter temperatures that reduced ctenophore survivorship in the months preceding the study. Ctenophore diets in 2004 consisted primarily of copepods. I observed an inverse correlation between ctenophore biovolume and copepod density, but my data suggest that adult Atlantic Comb Jellies consumed only 0.0–2.2% of the copepod standing stock per day. Overall, my results are consistent with the hypothesis that early summer ctenophore blooms occured over a relatively large spatial scale; however, trophic impacts in the Thames River Estuary in 2004 appeared to be relatively low.

ISOLATION AND GROWTH OF DINOFLAGELLATE, Scrippsiella sp. AND DIATOM, Melosira cf. moniliformis IN CONTROLLED CONDITIONS

The growth of the dinoflagellate, Scrippsiella sp. from Narragansett Bay, USA and the chain-forming benthic diatom, Melosira cf. moniliformis from Kendari Bay, Indonesia was evaluated under optimized laboratory conditions to investigate potential new candidates for shrimp aquaculture hatchery feeds. Isolation of microalgae was performed using capillary pipets in f/8-Si for Scrippsiella sp. and f/2 for M. cf. moniliformis. Isolated cells were placed in an incubator with a photoperiod of 12:12 hour (light : dark cycle), at light intensities of 62-89 μmol photons.m-2.s and at temperature of 20oC. Microalgae cells were cultured in 150-mL Erlenmeyer flasks containing 100 mL of f/2-Si medium for Scrippsiella spin triplicates and in 50-mL culture tubes containing 20 mL of f/2 medium for M. cf. moniliformis in four replicates. The cells in culture flasks were used in cell counting experiments while those in tubes were for fluorometer trials. Growth evaluation was conducted every 2-3 days. The diatom, M. cf. moniliformis was in logarithmic phase when observed at 2 to 7 days after inoculation and showed a high growth rate (μ = 0.52 day-1) and high division rate (k = 0.76 day-1, 1 division every 1.3 days). Logarithmic phase of Scrippsiella in culture flasks was started on day 7 to 30 (μ = 0.17 day-1 and k = 0.25 day-1 or 1 division every 4 days). In culture tubes, Scrippsiella sp. reached logarithmic phase at day 21 to 47 (μ = 0.12 day-1 and k = 0.18 day-1, 1 division every 5.65 days). This study indicates that M. cf. moniliformis can be used for aquaculture hatcheries feed but further study for the nutrition composition is needed. Scrippsiella sp. is potentially toxic for aquaculture at high densities, therefore they should be assessed carefully if used for aquaculture feeds.

Salt marsh tidal exchange increases residence time of silica in estuaries

We present flux measurements of dissolved silicon (DSi), biogenic Si (BSi), and dissolved inorganic nitrogen (DIN) and phosphorus (DIP) on a temperate salt marsh in Narragansett Bay, Rhode Island, over neap, mid, and spring tide cycles during the spring and the summer seasons to determine the stoichiometry of marsh tidal export and the potential effect on Si availability in the receiving estuary. During the spring, when incoming waters were depleted in DSi (average 1.7 µmol L−1), the marsh was a net sink of BSi (132 mol h−1) and a source of DSi (31 mol h−1) to the estuary. During this period, the DIN : DSi ratio of ebbing water was more than five times lower than that of flood water. Together, these data indicate that marsh nutrient fluxes “outwell” nutrients in ratios that support spring diatom growth in Narragansett Bay. However, during the entire sampling period the marsh served as a net sink for both DSi and BSi, importing, on average, 14 mol DSi h−1 and 86 mol BSi h−1. We hypothesize that the net import and retention of Si by salt marshes provides a previously overlooked ecosystem service of increasing the residence time of Si in estuarine systems. In the absence of salt marshes, we calculate that 5.1 × 104 kmol of Si would be exported from the estuary during the growing season, decreasing the availability of Si in the system and having direct repercussions for phytoplankton species composition in nearby estuarine waters.

Overwintering strategies of bloom-forming Ulva species in Narragansett Bay, Rhode Island, USA

Abstract Temperate coastal estuaries worldwide, such as Narragansett Bay, Rhode Island, are influenced by seasonal macroalgal blooms (e.g., Ulva) during warm months, whereas bloom-forming macroalgae are rarely encountered during winter. We assessed the ability of distromatic Ulva to overwinter through fragments, recruits, and/or microscopic propagules. We documented (a) small tissue fragments in sediment cores and the water column, (b) recruits and microscopic propagules on field-based settlement tiles, and (c) production of reproductive propagules, throughout the winter months. Laboratory culturing experiments indicated that both fragments and propagules are viable. Our data indicate that bloom-forming overwintering Ulva simultaneously use multiple reproductive strategies.

Coastal ocean acidification: The other eutrophication problem

Increased nutrient loading into estuaries causes the accumulation of algal biomass, and microbial degradation of this organic matter decreases oxygen levels and contributes towards hypoxia. A second, often overlooked consequence of microbial degradation of organic matter is the production of carbon dioxide (CO2) and a lowering of seawater pH. To assess the potential for acidification in eutrophic estuaries, the levels of dissolved oxygen (DO), pH, the partial pressure of carbon dioxide (pCO2), and the saturation state for aragonite (Ωaragonite) were horizontally and vertically assessed during the onset, peak, and demise of low oxygen conditions in systems across the northeast US including Narragansett Bay (RI), Long Island Sound (CT–NY), Jamaica Bay (NY), and Hempstead Bay (NY). Low pH conditions (<7.4) were detected in all systems during summer and fall months concurrent with the decline in DO concentrations. While hypoxic waters and/or regions in close proximity to sewage discharge had extremely high levels of pCO2, (>3000 μatm), were acidic pH (<7.0), and were undersaturated with regard to aragonite (Ωaragonite < 1), even near-normoxic but eutrophic regions of these estuaries were often relatively acidified (pH < 7.7) during late summer and/or early fall. The close spatial and temporal correspondence between DO and pH and the occurrence of extremes in these conditions in regions with the most intense nutrient loading indicated that they were primarily driven by microbial respiration. Given that coastal acidification is promoted by nutrient-enhanced organic matter loading and reaches levels that have previously been shown to negatively impact the growth and survival of marine organisms, it may be considered an additional symptom of eutrophication that warrants managerial attention.

Environmental controls of anammox and denitrification in southern New England estuarine and shelf sediments

We measured denitrification and anammox potential rates in homogenized sediments over two annual cycles from two estuarine sites (Providence River estuary and Narragansett Bay) and two continental shelf sites (Block Island and Rhode Island sounds). Denitrification varied both spatially and seasonally, but anammox only varied spatially. Denitrification ranged from 2.5 to 112 nmol N h−1 mL−1 sediment, and anammox from 0 to 8.7 nmol N h−1 mL−1 sediment, with the contribution of anammox to N2 fluxes (ra) ranging from undetectable to 4% in estuarine sites and 8–42% on the shelf. Oxygen microprofiles were analyzed in intact cores and concentrations of nitrate, nitrite, and ammonium were measured in the sediment slice corresponding to the depth layer in which potential rates were measured. Denitrification rates correlated positively with diffusive O2 consumption and temperature and potential anammox rates correlated positively with pore‐water concentrations. Anammox was positively correlated with denitrification in shelf sediments where anammox was regularly detected, but not in estuarine sediments. ra was negatively correlated with diffusive O2 consumption and positively correlated with pore‐water nitrate. Both organic matter and nitrate were important factors regulating the contribution of anammox to N2 production. As a whole, our results demonstrate that incorporating processes that control these two factors into models of N cycling, rather than focusing on organic matter availability alone, may improve predictions of the relative importance of anammox to N2 production in marine sediments.

Detectability of navigation obstacles with forward looking sonar in presence of boat wakes: Observations in Narragansett Bay

A Forward Looking Sonar (FLS) is designed to detect obstacles ahead of a vessel. Detection performance of an FLS is affected by several environmental factors surrounding the sonar. Wakes generated by nearby vessels degrade the detection capability of an FLS. With more than 40 000 registered boats, Rhode Island's Narragansett Bay is a good example of a zone with heavy boat traffic where the performance of an FLS may be effected. FarSounder has collected vast amounts of data while testing and demonstrating its FLS sonars in Narragansett Bay. Using that data, this paper presents observations of FLS performance with and without boat wakes. In order to explain the sonar performance, a probability of wake presence in sonar field of view is estimated based on shipping density, wake size, and lifetime.

Turbulent Wake of a Bridge Pier in a Tidal Current

The evolution of a large Reynolds number O(10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> ) wake was investigated using data from a ship-mounted acoustic Doppler current profiler (ADCP). The ADCP was a custom configured 600-kHz Teleydyne RD Instruments Workhorse augmented to have a fifth beam that was nominally vertical. The test was conducted in Narragansett Bay, RI, USA, using a large support pier of the Newport Bridge as the turbulent source in the bay's tidal flow. The turbulence was manifested as a vigorously fluctuating wake in the lee of the bridge pier. A series of wake crossings between x = 10-100 D were made during the ebb of the tidal cycle, where x and D are, respectively, the downstream distance and bridge pier diameter. The data set serves as one of a handful where wake evolution has been documented for a large Reynolds number system, and is unique in that high-horizontal resolution vertical velocity profiles were taken with the vertical fifth beam of the ADCP. A range of scales was observed in the vertical velocity field within the wake, including small-scale turbulence O(2-10 m) and large vortex structures O(20-40 m). When cast in terms of nondimensional wake age ( N τ), where N and τ are, respectively, the buoyancy frequency and wake age, the decay of root mean square vertical velocity fluctuations was observed to be proportional to (N τ) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-0.82</sup> , faster than the decay of an unstratified 2-D plane wake.

Carbon stable isotopes as indicators of coastal eutrophication.

Coastal ecologists and managers have frequently used nitrogen stable isotopes (delta15N) to trace and monitor sources of anthropogenic nitrogen (N) in coastal ecosystems. However, the interpretation of delta15N data can often be challenging, as the isotope values fractionate substantially due to preferential retention and uptake by biota. There is a growing body of evidence that carbon isotopes may be a useful alternative indicator for eutrophication, as they may be sensitive to changes in primary production that result from anthropogenic nutrient inputs. We provide three examples of systems where delta13C values sensitively track phytoplankton production. First, earlier (1980s) mesocosm work established positive relationships between delta13C and dissolved inorganic nitrogen and dissolved silica concentrations. Consistent with these findings, a contemporary mesocosm experiment designed to replicate a temperate intertidal salt marsh environment also demonstrated that the system receiving supplementary nutrient additions had higher nutrient concentrations, higher chlorophyll concentrations, and higher delta13C values. This trend was particularly pronounced during the growing season, with differences less evident during senescence. And finally, these results were replicated in the open waters of Narragansett Bay, Rhode Island, USA, during a spring phytoplankton bloom. These three examples, taken together with the pre-existing body of literature, suggest that, at least in autotrophic, phytoplankton-dominated systems, delta13C values can be a useful and sensitive indicator of eutrophication.

Profiling gene expression to distinguish the likely active diazotrophs from a sea of genetic potential in marine sediments

Nitrogen (N) cycling microbial communities in marine sediments are extremely diverse, and it is unknown whether this diversity reflects extensive functional redundancy. Sedimentary denitrifiers remove significant amounts of N from the coastal ocean and diazotrophs are typically regarded as inconsequential. Recently, N fixation has been shown to be a potentially important source of N in estuarine and continental shelf sediments. Analysis of expressed genes for nitrite reductase (nirS) and a nitrogenase subunit (nifH) was used to identify the likely active denitrifiers and nitrogen fixers in surface sediments from different seasons in Narragansett Bay (Rhode Island, USA). The overall diversity of diazotrophs expressing nifH decreased along the estuarine gradient from the estuarine head to an offshore continental shelf site. Two groups of sequences related to anaerobic sulphur/iron reducers and sulphate reducers dominated libraries of expressed nifH genes. Quantitative polymerase chain reaction (qPCR) and quantitative reverse transcription polymerase chain reaction (qRT-PCR) data shows the highest abundance of both groups at a mid bay site, and the highest nifH expression at the head of the estuary, regardless of season. Several potential environmental factors, including water temperature, oxygen concentration and metal contamination, may influence the abundance and nifH expression of these two bacterial groups.