Rhode River Estuary Research Articles

Identification of sulfidic materials in the Rhode River subestuary of Chesapeake Bay

Sulfide-containing soil materials can undergo a process known as sulfuricization if disturbed, triggering the production of sulfuric acid through the oxidation of Fe sulfides and causing environmental degradation. Several systems exist to classify these types of materials based on the level of environmental hazard that they may pose. Hypersulfidic materials undergo extreme acidification, hyposulfidic materials may undergo acidification to a lesser extent or not at all, and monosulfidic materials contain a more reactive form of Fe sulfide. The definitions for these terms vary, so a brief review of how these materials are described and classified both globally and in the Rhode River region is provided. Testing for these materials is costly and time consuming, with current methods sometimes taking 16weeks or longer to identify these materials. In subaqueous environments, where dredging and other marine construction activities may be delayed by requirements to obtain this information, better methods for the field identification of these materials would be of use to subaqueous soil surveyors. In this study, subaqueous soil materials from the Rhode River estuary were sampled, described, and divided into six categories based on morphologic properties: fluid muds, unconsolidated Holocene sandy materials, organic materials, buried A horizons, Tertiary materials with Fe oxide concentrations, and Tertiary materials without Fe oxide concentrations. These materials were then evaluated and classified as different types of sulfide-containing materials using current methods. Buried A horizons, organic materials, and Tertiary materials without Fe oxide concentrations are the most likely to be hypersulfidic materials, and therefore of the greatest environmental concern. Fluid muds, unconsolidated Holocene sandy materials, and Tertiary materials with Fe oxide concentrations are less likely to consist of hypersulfidic materials, but may still be of environmental concern as hyposulfidic materials or monosulfidic materials. Subaqueous soil surveyors can use these findings to help understand the relative environmental hazards posed by similar subaqueous soil materials in similar settings.

3000 Years of Human Subsistence and Estuarine Resource Exploitation on the Rhode River Estuary, Chesapeake Bay, Maryland

Abstract Chesapeake Bay is home to highly productive marine ecosystems that were a key part of Native American subsistence for millennia. Despite a number of archaeological projects focused on Chesapeake Bay prehistory, key questions remain about the nature of human use of the estuary through time and across space. Recent work at 7 shell middens on the Rhode River Estuary, MD, provides insight into human subsistence and estuarine res ource exploitation from ∼3200 years ago through the mid-19th century. This is an important diachronic sequence of coastal land use and subsistence for the Chesapeake and helps fill a gap in our understanding of coastal adaptations along North America's Atlantic Coast. Despite climate change, fluctuating sea levels, and the likely appearance of maize agriculture in the area ∼1000 years ago, Native American exploitation of oysters and estuarine resources remained fairly consistent across the Early to Late Woodland. These data stand in contrast to the mid-1800s assemblage, which...

Shell Middens, Cultural Chronologies, and Coastal Settlement on the Rhode River Sub‐Estuary of Chesapeake Bay, Maryland, USA

North America's Atlantic Coast has been a focus of human settlement and subsistence for millennia, but sea‐level rise, sedimentation, and other processes pose significant challenges for archaeological research. Radiocarbon dating of 31 shell middens near the Rhode River Estuary, Maryland provides an opportunity to evaluate human land use, settlement, and cultural chronologies on the Chesapeake Bay. Sixty calibrated radiocarbon dates on eastern oyster (Crassostrea virginica) shell and charcoal demonstrate that Native Americans, colonial, and historic peoples harvested oysters and other shellfish from at least 3200 years ago through the 19th century. The number of dated sites increases during the Late Woodland period after about 1000 cal yr B.P., a factor probably related to greater site visibility and preservation, as well as increased human exploitation of the watershed. Accumulation rates for five of the shell middens provide preliminary indications that some of the sites accumulated rapidly suggesting, along with other evidence, that many of the region's shell middens were logistical or perhaps seasonal camps. Our study demonstrates the importance of regional watershed surveys and radiocarbon dating programs to help build and refine cultural chronologies in coastal regions threatened by sea‐level rise and other processes.

Photobleaching of Dissolved Organic Material from a Tidal Marsh‐Estuarine System of the Chesapeake Bay†

Wetlands and tidal marshes in the Rhode River estuary of the Chesapeake Bay act as important sources of dissolved organic carbon and strongly absorbing dissolved organic matter (DOM) for adjacent estuarine waters. The effects of solar exposure on the photochemical degradation of colored DOM (CDOM) were examined for material derived from different sources (estuarine and freshwater parts of the Rhode River, sub-watershed stream, marshes) in this estuarine ecosystem. Consistent with changes in fluorescence emission, absorption loss upon exposure to different portions of the solar spectrum (i.e. different long-pass cut-off filters) occurred across the entire spectrum but the wavelength of maximum photobleaching decreased as the cut-off wavelength of the filter decreased. Our results illustrate that solar exposure can cause either an increase or a decrease in the CDOM absorption spectral slope, S(CDOM), depending on the spectral quality of irradiation and, thus, on the parameters (e.g. atmospheric composition, concentration of UV-absorbing water constituents) that affect the spectral characteristics of the light to which CDOM is exposed. We derived a simple spectral model for describing the effects of solar exposure on CDOM optical quality. The model accurately, and consistently, predicted the observed dependence of CDOM photobleaching on the spectral quality of solar exposure.

The clearance rate of microzooplankton as the key element for describing estimated non-linear dilution plots demonstrated by a model

The objective of this study was to determine whether the clearance rate of grazers (as an individual response) was sensitive enough to describe non-linear plots estimated by dilution experiments for measuring the instant grazing rate of microzooplankton. The study was based on an initial analysis of a non-linear feeding pattern based on the food concentration dependence of clearance rate of microzooplankton. In contrast to the traditional assumption of a linear functional response, I assumed that the microzooplankton functional response was non-linear and that the dependence of the clearance rate can be sub-divided into four intervals of food concentration (Sections I–IV) as follows: in Section I clearance rate is zero; Section II is a transitional interval in which the clearance rate increases from zero to a maximum value; in Section III, the clearance rate is maximal and constant, and in Section IV, the clearance rate decreases from its maximum value due to saturated ingestion rate. A set of derived differential equations describes the phytoplankton growth rate in each section, leading to the possibility of comparing predicted non-linear dilution plots with observed non-linear dilution data, using only the specific solutions for Sections III and IV. One should evaluate the quality of fit provided by the non-linear and linear models, rather than uncritically accepting only the linear model for observed non-linear dilution data, using calculated expected non-linear and linear dilution plots as alternative hypotheses. It can be demonstrated that the non-linear model provided a better fit to estimated non-linear dilution data from the Red Sea, Rhode River Estuary (USA) and Kiel Fjord (Germany) than the standard linear model. Published dilution experiments which had a non-linear shape were also selected as illustrative examples to demonstrate the superior fit of the non-linear model.

Dissolved Silicate Dynamics of the Rhode River Watershed and Estuary

We continuously measured dissolved silicate concentrations and fluxes discharged from various Rhode River subwatersheds for a period of 14 yr from 1984 to 1998 and for 15 mo in 1971–1972. We also measured dissolved silicate concentrations along a transect from the head of the tide in Rhode River estuary to Chesapeake Bay. The average concentration of dissolved silicate discharged from the Rhode River watershed was 10.8 mg Si l−1. There were consistent and significant differences in silicate concentrations discharged over time and space among subwatersheds. Mean annual silicate flux from the watershed was 26.6 kg Si ha−1 and 93% of this occurred during the winter and spring seasons. There were large interannual variations in silicate flux, due primarily to differences in precipitation and water discharge, rather than silicate concentration. Land use had little or no effect on silicate flux from various subwatersheds. Silicate concentrations discharged from a subset of subwatersheds in 1995–1996 were 25% to 35% lower than in a period with similar precipitation in 1971–1972. Mean annual concentrations of silicate discharged from nine subwatersheds have been declining about 1.5% yr−1 or by 0.21–0.26 mg Si l−1 yr−1 over the last 25 yr. Despite high average silicate fluxes from the watershed, at times the Rhode River estuary developed low dissolved silicate concentrations, which could have been limiting to the growth of diatoms. Examples were in the spring after a winter with low watershed discharge (as low as 0.019 mg Si l−1 in 1995) and after protracted drought (as low as 0.041 mg Si l−1 in 1993).

Dilution effects on microzooplankton in dilution grazing experiments

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 200:127-139 (2000) - doi:10.3354/meps200127 Dilution effects on microzooplankton in dilution grazing experiments J. R. Dolan1,2,*, C. L. Gallegos1, A. Moigis1,** 1Smithsonian Environmental Research Center, PO Box 28, Edgewater, Maryland 21037, USA 2Marine Microbial Ecology Group, CNRS ESA 7076, Université Paris VI, Station Zoologique, BP 28, 06230 Villefranche-sur-Mer, France *E-mail: dolan@obs-vlfr.fr **Present address: Ulmenstrasse 73, 24306 Ploen, Germany ABSTRACT: In dilution experiments, filtered seawater is used to create a gradient of grazing pressure on phytoplankton. Microzooplankton grazing is estimated by examining phytoplankton growth within the gradient. However, the dilution series also represents a resource gradient for microzooplankton. Here we report the effects of dilution on grazers. In 2 standard dilution experiments, using communities from the eutrophic (chlorophyll a = 12 to 15 µg l-1) Rhode River Estuary, we examined the effects of dilution on different groups of microzooplankters: rotifers, tintinnid ciliates, oligotrich ciliates, predacious ciliates, and Mesodinium rubrum. Apparent growth rates of tintinnids and oligotrichs varied with prey concentration, decreasing with the dilution factor from about +0.5 d-1 in undiluted whole water to about -1 d-1 in the 5% whole water, closely resembling numerical response curves. Among tintinnids, there was an increase in the relative abundance of larger tintinnids in the time 24 h samples of dilute treatments compared to the less dilute treatments. No consistent dilution effect was shown by rotifers or predacious ciliates. The growth rates of the photosynthetic ciliate M. rubrum increased with dilution, resembling the typical pattern of chlorophyll a and autotrophic nanoplankton. Grazer growth in undiluted waters and grazer mortality in dilute water may be common and result in uncertainty in measured grazing rates. We urge that grazers be examined in grazing experiments not only to assess possible artifacts in grazing rate estimates, but also to provide information, beyond a simple grazing rate, on the grazer populations. KEY WORDS: Ciliate · Oligotrich · Tintinnid · Phytoplankton · Nanoplankton · Rotifer · Flagellate Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 200. Publication date: July 14, 2000 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 2000 Inter-Research.

Seasonal progression of factors limiting phytoplankton pigment biomass in the Rhode River estuary, Maryland (USA). I. Controls on phytoplankton growth

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 161:185-198 (1997) - doi:10.3354/meps161185 Seasonal progression of factors limiting phytoplankton pigment biomass in the Rhode River estuary, Maryland (USA). I. Controls on phytoplankton growth C. L. Gallegos*, T. E. Jordan Smithsonian Environmental Research Center, PO Box 28, Edgewater, Maryland 21037, USA *E-mail: gallegos@serc.si.edu Experiments to measure phytoplankton growth rates and microzooplankton grazing rates, both with and without additions of nutrients, were conducted over a 5 yr span to determine the relative roles of physical factors, nutrients, and microzooplankton grazing in regulating phytoplankton pigment biomass in the Rhode River estuary (Maryland, USA). Microzooplankton grazing rate was overall only weakly correlated with phytoplankton growth rate, although fluctuations in the rates appeared to parallel one another for restricted periods. Grazing was about 73% of growth rate at ambient nutrients during 1995, a year in which the two were correlated. In winter and early spring, phytoplankton growth rates were low and unaffected by nutrient additions. In late spring, stimulation of growth by nutrient addition was more common, but microzooplankton grazing was partially effective in relieving nutrient limitation. Growth enhancement by nutrient addition was most common in summer, and microzooplankton grazing was less effective at relieving nutrient limitation. P or N addition stimulated phytoplankton growth in late spring and autumn, but only N addition stimulated growth in summer. Temperature constrained upper bounds of observed growth rates throughout the year, but many observed growth rates were far below the temperature-dependent maximum, even with nutrients added. Considered as a single group, temperature-normalized growth rates were poorly related to in situ daily photon flux; but the rates appeared to fall into species-dependent higher and lower groups. The higher growth rates were statistically associated with the diatoms Thalassiosira pseudonana, Thalassiosira sp., and Nitzschia longissima. The lower growth rates were associated with the dinoflagellates Gyrodinium uncatenum and Prorocentrum minimum. Results suggest that interpretation of bioassay experiments requires consideration of both grazing effects and species composition of the phytoplankton community. Nutrient stimulation of some slow growing assemblages may be difficult to detect in 1 d incubation experiments. Phytoplankton · Estuary · Growth · Nutrients · Grazing · Light · Temperature Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 161. Publication date: December 31, 1997 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1997 Inter-Research.

Seasonal progression of factors limiting phytoplankton pigment biomass in the Rhode River estuary, Maryland (USA). II. Modeling N versus P limitation

Seasonal progression of factors limiting phytoplankton pigment biomass in the Rhode River estuary, Maryland (USA). II. Modeling N versus P limitation

Interannual variability in spring bloom timing and magnitude in the Rhode River, Maryland, USA:observations and modeling

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 154:27-40 (1997) - doi:10.3354/meps154027 Interannual variability in spring bloom timing and magnitude in the Rhode River, Maryland, USA: observations and modeling Gallegos CL, Jordan TE, Correll DL The long-term average seasonal distribution of chlorophyll concentration in a central reach of the Rhode River estuary (Maryland, USA) has a peak in the spring of about 90 mg m-3. Here we examine interannual variability in chlorophyll and nutrient concentrations in recent years. Years are classified as having an average bloom, extraordinary bloom, or no bloom, and patterns of nutrient concentration are described for each category. To determine processes associated with the different categories of behavior, we performed a generalized sensitivity analysis of a model of nutrient-limited phytoplankton net growth which was previously shown to simulate the chlorophyll concentration and the observed pattern of shift from P to N limitation in an average year. Model parameters controlling the delivery of N and P by the environment and the biological utilization of nutrients by the phytoplankton were drawn from random distributions based on literature reports or, where available, data from the Rhode River. Model output for each parameter set was classified according to which, if any, of the classes of bloom it exhibited. Environmental processes most responsible for the variability in predicted spring blooms were the rate of phosphorus release from the sediment, the timing of nitrate depletion at the seaward boundary, and the maximal nitrate concentration at the boundary. Simulation of bloom failure was most strongly associated with low rates of phosphorus release from the sediment. Phytoplankton physiological parameters most responsible for variability in categories of behavior were the nitrogen:chlorophyll conversion factor, and the maximum internal storage capacity for phosphorus. Blooms of extraordinary magnitude were associated with low nitrogen:chlorophyll ratios and high storage capacity for phosphorus. Results help identify key processes and parameters in need of further understanding as well as possible points for managerial intervention. Phytoplankton · Spring bloom · Estuarine · Nitrogen · Phosphorus Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 154. Publication date: July 31, 1997 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1997 Inter-Research.

Nutrient Flux in a Landscape: Effects of Coastal Land Use and Terrestrial Community Mosaic on Nutrient Transport to Coastal Waters

Long-term interdisciplinary studies of the Rhode River estuary and its watershed in the mid-Atlantic coastal plain of North America have measured fluxes of nitrogen and phosphorus fractions through the hydrologically-linked ecosystems of this landscape. These ecosystems are upland forest, cropland, and pasture; streamside riparian forests; floodplain swamps; tidal brackish marshes and mudflats; and an estuarine embayment. Croplands discharged far more nitrogen per hectare in runoff than did forests and pastures. However, riparian deciduous hardwood forest bordering the cropland removed over 80 percent of the nitrate and total phosphorus in overland flows and about 85 percent of the nitrate in shallow groundwater drainage from cropland. Nevertheless, nutrient discharges from riparian forests downslope from croplands still exceeded discharges from pastures and other forests. The atomic ratio of nitrogen to phosphorus discharged from the watersheds into the estuary was about 9 for total nutrients and 6 for inorganic nutrient fractions. Such a low N:P ratio would promote nitrogen rather than phosphorus limitation of phytoplankton growth in the estuary. Estuarine tidal marshes trapped particulate nutrients and released dissolved nutrients. Subtidal mudflats in the upper estuary trapped particulate P, released dissolved phosphate, and consumed nitrate. This resulted in a decrease in the ratio of dissolved inorganic N:P in the estuary. However, the upper estuary was a major sink for total phosphorus due to sediment accretion in the subtidal area. Bulk precipitation accounted for 31 percent of the total nongaseous nitrogen influx to the landscape, while farming accounted for 69 percent. Forty-six percent of the total non-gaseous nitrogen influx was removed as farm products, 53 percent either accumulated in the watershed or was lost in gaseous forms, and 1 percent entered the Rhode River. Of the total phosphorus influx to the landscape, 7 percent was from bulk precipitation and 93 percent was from farming. Forty-five percent of the total phosphorus influx was removed as farm products, 48 percent accumulated in the watershed, and 7 percent entered the Rhode River. These nitrogen and phosphorus discharges into the Rhode River, although a small fraction of total loadings to the watershed, were large enough to cause seriously overenriched conditions in the upper estuary.

ESTIMATING MARYLAND CRITICAL AREA ACVS IMPACT ON FUTURE NONPOINT POLLUTION ALONG THE RHODE RWER ESTUARY1

ABSTRACT: This paper presents the results of an investigation of the effects of the Maryland Critical Area Act on generation of non‐point source loads of phosphorus, nitrogen, and sediment to the Rhode River estuary. The Simple Method model, the Marcus and Kearney regression model, and the CREAMS model were used to estimate annual loads under: (1) present conditions, (2) maximum land use development allowable under the Act, and (3) two sets of future land use conditions that might occur if the Act were not in place. Results indicate that the Critical Area Act can reduce the present generation of nonpoint nutrient and sediment loadings 20–30 percent from the regulated area. These reductions can occur while preserving agricultural lands and allowing limited residential and urban development. The decrease in nutrient loadings is primarily dependent upon implementation and enforcement of agricultural best management practices (BMPs). The BMPs could reduce present agricultural nutrient loadings by 90 percent to a level comparable to loadings from residential areas. The estimated effectiveness of the Critical Area Act is even greater when compared to potential future nutrient loadings if development in the area remains unregulated. Unrestricted residential and urban development could increase nutrient loadings by 200 percent to 1000 percent as compared to controlled development under Critical Area Act guidelines. The Critical Area Act primarily prevents these future increases by severely limiting woodland cutting, with lesser results obtained by requiring urban BMPs.

Trophic role of planktonic rotifers in the Rhode River Estuary, spring-summer 1991

Trophic role of planktonic rotifers in the Rhode River Estuary, spring-summer 1991

Nutrients and chlorophyll at the interface of a watershed and an estuary

We investigated the fates of nutrients entering the Rhode River estuary from its watershed and from atmospheric deposition. Production or consumption of materials in the upper estuary was calculated from a mixing model with chloride as a conservative tracer. The upper estuary produced chlorophyll and dissolved PO43− (DPO43−) but consumed particulate PO43− (PPO43−), total inorganic N, dissolved organic N, and particulate organic C. These net fluxes were influenced more by shallow, open‐water areas than by the tidal marshes which cover two‐thirds of the area of the upper estuary. Ratios of chlorophyll to organic C, N, and P suggest that most of the suspended particulate organic matter in the upper estuary was produced by phytoplankton rather than derived from watershed inputs. The consumption of nitrate due to phytoplankton production and the production of DPO43− due to release from particulate P after deposition in sediments resulted in low inorganic N : P ratios, contrasting sharply with the lower estuary and adjacent Chesapeake Bay. Dissolved inorganic N and P entered the upper estuary from the watershed at an atomic ratio of 26 but left the upper estuary at a ratio of 2.7. The release of DPO43− from watershed‐derived sediments may be a common feature among estuaries and could promote N limitation of primary production in estuarine and coastal waters.

Trophic coupling of rotifers, microflagellates, and bacteria during fall months in the Rhode River Estuary

Trophic coupling of rotifers, microflagellates, and bacteria during fall months in the Rhode River Estuary

Flux of particulate matter in the tidal marshes and subtidal shallows of the Rhode River Estuary

Flux of particulate matter in the tidal marshes and subtidal shallows of the Rhode River Estuary

Flux of Particulate Matter in the Tidal Marshes and Subtidal Shallows of the Rhode River Estuary

There was a net influx of suspended particulate matter to the uppermost part of the Rhode River estuary during the several years of this study. Most of the influx was due to episodic discharges of suspended sediment from the watershed during heavy rains. In contrast, tidal exchange of particulate matter was not related to rainstorms. Sediment composition data and historical records indicate that marsh accretion accounts for only 13% of the sediment trapping although marshes occupy 60% of the study area. Influx of particulate matter to the marshes is directly related to the amount of time they are submerged during tidal cycles.

Response of Underwater Light Transmittance in the Rhode River Estuary to Changes in Water-Quality Parameters

A year-long study of incident and underwater light transmittance (400–800 nm) in the Rhode River, Maryland, a tidal tributary to Chesapeake Bay, indicated that light transmittance responded in both intensity and spectral quality to changes in the amount and type of dissolved and suspended materials in the water. At times of relatively clear water, transmittance was similar to that previously reported in the literature for coastal waters. With high concentrations of suspended and dissolved materials in the water, attenuation of irradiance was high in the upper part of the water column and different for the various wave bands, depending on the type of material present. At such times, attenuation was higher in the upper part of the water column under sunny, clear skies than on cloudy days. We believe this to be due to higher concentrations of pigments and suspended particles in the water on sunny days, increasing the scattering and adsorption. A second factor was a lower average cosine on cloudy days, decreasing the effect of scattering on the average path length per meter of depth. High attenuation coefficients in the middle of the spectrum are attributed to accessory pigments. Regression of the diffuse attenuation coefficient on eight water-quality parameters explained up to 93% of the variance in the attenuation coefficient. Chlorophylls a and c and mineral suspensates were the three most important variables for data taken under clear skies. In contrast, under cloudy skies, the three most important variables were different for different wavelengths. Models of irradiance attenuation in turbid estuarine waters require the use of more variables than models for open ocean waters.

Distribution and activity of bacteria in the headwaters of the Rhode River Estuary, Maryland, USA

A transect along the axis of the headwaters of a tidal estuary was sampled for microbial, nutrient, and physical parameters. Chlorophylla averaged 42μg 1(-1) and phytoplankton comprised an estimated 80% of the total microbial biomass as determined by adenosine triphosphate (ATP). Bacterial concentrations ranged from 0.3-53.9×10(6) cells ml(-1) and comprised about 4% of the total living microbial biomass. Bacterial production, determined by(3)H-methyl-thymidine incorporation was about 0.05-2.09× 10(9) cells 1(-1) h(-1), with specific growth rates of 0.26-1.69 d(-1). Most bacterial production was retained on 0.2μm pore size filters, but passed through 1.0μm filters. Significant positive correlations were found between all biomass measures and most nutrient measures with the exception of dissolved inorganic nitrogen nutrients where correlations were negative. Seasonal variability was evident in all parameters and variability among the stations was evident in most. The results suggest that bacterial production requires a significant carbon input, likely derived from autotrophic production, and that microbial trophic interactions are important.

Nutrient flux in the Rhode River: Tidal exchange of nutrients by brackish marshes

Tidal exchanges of nitrogen, phosphorus, and organic carbon by a high and a low elevation marsh in the Rhode River estuary were measured throughout the year. Both marshes tended to import particulate matter and export dissolved matter, although they differed in the fluxes of certain nutrients. Compared with tidal exchanges, bulk precipitation was a major source of ammonia and nitrate and a minor source of other nutrients. There was a net retention of nutrients by the portion of the Rhode River that included both marshes and a mudflat. However, the marshes accounted for only 10% of the phosphorus retention and 1% of the nitrogen retention while they released organic carbon amounting to 20% of the retention. This suggests that the mudflat acted as a sink for nutrients. The primary role of the marshes seems to be transformation of particulate to dissolved nutrients rather than nutrient retention or release.