Denitrification rates measured along a salinity gradient in the eutrophic Neuse River estuary, North Carolina, USA

Denitrification rates along a salinity gradient in the eutrophic Neuse River Estuary, North Carolina, were quantified using membrane inlet mass spectrometry (MIMS) within short-term batch incubations. Denitrification rates within the system were highly variable, ranging from 0 to 275 μmol N m⁻² h⁻¹. Intrasite variability increased with salinity, but no significant differences were observed across the salinity gradient. Denitrification rates were positively correlated with sediment oxygen demand at the upstream sampling site where sediment organic carbon levels were lowest. This relationship was not observed in the more saline sampling sites. Denitrification rates were highest during winter. On an annual basis, denitrification accounted for 26% of the dissolved inorganic nitrogen and 12% of the total nitrogen supplied to the system.     

A resource-based framework for establishing freshwater inflow requirements for the Suwannee River estuary

The availability of methods for establishing freshwater inflow requirements for estuaries lags behind those for establishing flow requirements in riverine ecosystems. Some of the basic principles and approaches for establishing riverine flow requirements may be applicable to estuaries. An emerging approach for establishing freshwater inflow needs for the Suwannee River estuary involves maintaining a natural inflow regime (in terms of magnitude, frequency, duration, and timing of freshwater flows) and identifying important habitat targets to be protected. The salinity-river flow conditions needed to sustain the habitat targets in their existing condition are then identified. A variety of tools are employed, such as salinity metrics, biological metrics, limits of distribution of communities or habitats, and landscape-scale characteristics to define the salinity and corresponding flow ranges needed to protect and maintain the resource targets. With this information, combined with use of models to evaluate flow-salinity relationships and various withdrawal scenarios, river flow criteria can be set which address the freshwater inflow requirements to maintain these ranges. Subsequentmonitoring and research is undertaken to evaluate the effectiveness of the river flow criteria in protecting the estuarine resource targets. This information can be used to subsequently confirm, refine, or modity the flow criteria.     

Submersed vegetation as habitat for invertebrates in the Hudson River estuary

We sampled epiphytic and benthic macriinvertebrates in 20 beds of submersed vegetation throughout the Hudson River estuary to assess the importance of plant beds in providing habitat for macroinvertebrates and to determine which characteristics of plant beds affected the density and composition of macroinvertebrates. Macroinvertebrate densities in plant beds were 4–5 times higher, on average, than densities in unvegetated sediments in the Hudson. The macroinvertebrate community in plant beds was dominated by chironomid midges, oligochaete worms, hydroids, gastropods, and amphipods. Many species of macroinvertebrates were found chiefly on submersed plants, showing that plant beds are important in supporting biodiversity in the Hudson. Macroinvertebrates were most numerous in beds with high plant biomass and in the interiors of beds, whereas neither bed size nor position along the length of the estuary affected macroinvertebrate density. Community composition varied strongly with position along the river (freshwater versus brackish), habitat (epiphytic versus benthic), and position within the bed (edge versus interior). Plant biomass also influenced macroinvertebrate community composition, but bed area had relatively little influence.     

长江口主要阳离子随盐度变化的研究

以追踪混合水团的方式采样,并测定了长江河口混合区表层和底层中主要阳离子Mg2+、Ca2+、Na+和K+的浓度,讨论了长江口水体的pH分布特征和主要阳离子的混合特征.结果表明,长江口水体的pH状况受人为污染影响较小,河口混合区主要阳离子的浓度随着水体盐度的增大而增大,Ca/Mg比值随水体盐度的增大而迅速减小,且主要阳离子浓度相对于盐度的变化在表层和底层水体间没有显著的差别.在长江河口混合区主要阳离子的浓度及其比值随盐度的变化可用线性混合模型进行较好地预测和模拟.     

Seasonal changes in the nekton community of the Suwannee River estuary and the potential impacts of freshwater withdrawal

In Florida, issues related to alterations of estuarine salinity caused by freshwater withdrawal have recently gained increasing attention. We examined nekton community structure in the Suwannee River estuary (1997–2000) and investigated the relationship between environmental factors and the abundance of fisheries resources. We compared nekton community structure and environmental factors seasonally and annually using multidimensional scaling (MDS) ordination and cluster analysis and observed a strong seasonal pattern. This pattern was consistent among years and closely paralleled those for temperature and river discharge. Representative species for cold seasons includedLeiostomus xanthurus andLagodon rhomboides, and those for warm seasons includedMembras martinica andAnchoa hepsetus. Species that contributed most to the dissimilarity in community structures between wet and dry seasons were abundant and generally preferred lower salinity (e.g.,L. xanthurus, Eucinostomus spp., andMenidia spp.). A period of low freshwater inflow during the latter portion of our study coincided with both decreases and increases in the abundances of some dominant and some economically important species. We have established a baseline which will assist in measuring the effects of long-term changes in freshwater input on the nekton communities of the Suwannee River estuary, but our ability to predict these effects is still limited.     

Nutrient limitation of the macroalgaEnteromorpha intestinalis collected along a resource gradient in a highly eutrophic estuary

We conducted a laboratory experiment to quantify nutrient (nitrogen and phosphorus) limitation of macroalgae collected along a gradient in water column nutrient availability in Upper Newport Bay estuary, a relatively nutrient-rich system in southern California, United States. We collectedEnteromorpha intestinalis and water for use in the experiment from five sites ranging from the lower end of the estuary to the head. Initial algal tissue N and P concentrations and molar N∶P ratios—as well as water column NO₃ and total Kjeldahl nitrogen (TKN)—increased along a spatial gradient from the lower end toward the head. Water column soluble reactive phosphorus (SRP) varied among sites as well but did not follow a pattem of increasing from the seaward end toward the head. Algae from each site were assigned to one of four experimental treatments: control (C), nitrogen enrichment (+N), phosphorus enrichment (+P), and nitrogen and phosphorus enrichment (+N+P). Each week for 3 wk we replaced the water in each unit with the appropriate treatment water to mimic a poorly flushed estuary. After 3 wk, the degree of nutrient limitation ofE. intestinalis varied spatially with distance from the head of the estuary. Growth ofE. intestinalis collected from several sites increased with N enrichment alone and increased further when P was added in combination with N This indicated that N was limiting and that when N was sufficient, P became limiting. Sites from whichE. intestinalis exhibited nutrient limitation spanned the range of background water column NO₃ (12.9±0.4 to 55.2±2.1 μM) and SRP (0.8±0.0 to 2.9±0.2 μM) concentrations. Algae that were N limited had initial tissue N levels ranging from 1.18±0.03 to 2.81±0.08% dry weight and molar N∶P ratios ranging from 16.75±0.39 to 26.40±1.98.     

Seasonal and interannual dynamics of free-living bacterioplankton and microbially labile organic carbon along the salinity gradient of the Potomac River

Samples were collected at stations located in the mesohaline, oligohaline, and tidal fresh regions of the Potomac River, Maryland, between April 1998 and December 1999 to evaluate the seasonal distribution of bacterioplankton and microbially labile organic carbon (MLOC) in relation to hydrodynamic parameters (dissolved oxygen, salinity, and temperature). Bacterioplankton abundance (BA) averaged 13 × 10⁶ cells ml⁻¹ at all stations, a value that is higher than the average observed in many other temperate estuaries around the world, and were almost exclusively free-living. During the summer of 1998, BA often exceeded 30 × 10⁶ cells ml⁻¹ in the mesohaline region during periods of anoxia in subpycnocline waters. Dissolved MLOC typically accounted for 40% of total MLOC and on some occasions during summer and autumn accounted for 80%. A significant positive relationship between dissolved MLOC and BA was evident in the mesohaline Potomac River, the region where anoxia occurs each summer, but the regressions of particulate MLOC and chla on BA were not significant at this location. In the mesohaline Potomac River, BA regressed negatively and significantly on dissolved oxygen (r²=0.50, p<0.001). BA may be an important indicator of ecosystem health in this and other eutrophied estuaries, because of the relationships between BA, dissolved MLOC, and dissolved oxygen in the salinity stratified Potomac River and because free-living BA was elevated along the length of the river.     

Importance of water quality to nekton habitat use in a North Carolina Branch estuary

Nekton abundance and water quality were examined over 8 yr (1986–1993) in Isaac Creek, a small (2.5 km long), shallow (1–2 m), estuarine creek draining to Adams Creek (Neuse River system), North Carolina, United States. Water quality and nekton were sampled at 8 to 12 stations at 2–3 wk intervals from April to October (76 dates). The nekton assemblage, sampled by trawl, included 42 taxa but was dominated by 11 species (7 fish and 4 decapod crustaceans). Nekton and water quality (temperature, salinity, dissolved oxygen [DO], salinity gradient) data were grouped and analyzed by three (upper, middle, and lower) creek zones to determine if estimated abundance was correlated with water quality. Potentially stressful water quality conditions for salinity (<5 ppt), temperature (>30°C in morning), and DO (<2 mg 1⁻¹) mainly occurred in the upper and middle zones. The most frequent occurrence of potentially stressful conditions for salinity was in the spring and for dissolved oxygen and temperature in middle to late summer. The frequency of potentially stressful conditions increased during a 3-yr period following timber harvest of a large portion of the watershed. Canonical correspondence analysis (CCA) suggested little correlation between the top 11 species and water quality and indicated an assemblage with regular seasonal changes. Comparison of nekton use of the middle and upper zones of the creek for 3-yr pre- and post-harvest periods showed an increase in proportion of nekton caught in those zones, despite the higher frequency of potentially stressful water quality conditions. This observation suggests that a complex set of factors, including water quality, influence the pattern of nekton use in Isaac Creek.     

River profiles along the Himalayan arc as indicators of active tectonics

Longitudinal profiles along sixteen major transverse Himalayan rivers add important constraints to models of active continental subduction and its evolution. These profiles are characterized by a zone of relatively high gradient that cannot be associated with differential resistence to erosion in all cases. The base of the zone of increased gradients correlates with (1) the topographic front between the Lesser and High Himalayas, (2) the narrow belt of intermediate-magnitude thrust earthquakes, (3) the Main Central Thrust zone (MCT). These features define a small circle in the central portion of the Himalayan arc. These correlations suggest that the discontinuity in the river profiles and the other features are controlled by a major tectonic boundary between the rising High Himalayas and the Lesser Himalayas. No sharp increases in gradient are observed near the Main Boundary Thrust (MBT), except on a few rivers, such as the Jhelum or Kundar, where the MBT lies close to both the MCT and the seismic belt. Thus, it is unlikely that the MBT is a major tectonic boundary. The diversion of river courses along the MBT and around anticlines in the Sub Himalayas has probably been caused by aggradation near the rosion-deposition boundary, upstream of uplifts in the Mahabharat range and Sub Himalayas.A parallel is drawn between the Himalayas and New Guinea based on the hypothesis that continent-arc collision, of the type occurring in northern Australia, preceded continent-continent collision in the Himalayas. The present sedimentary/tectonic phase in New Guinea resembles the Subathu (Paleocene-Eocene) phase in the Himalayas. Incipient counterparts of the major Himalayan structures, including the MCT and the MBT, are recognized in New Guinea. The drainage patterns in the Himalayas and in New Guinea bear a similar relation to major structures. This suggests that (1) the tectonic evolution of the Himalayas has been rather uniform since early stages of collision, and (2) the Himalayan drainage was also formed at these early stages and is therefore antecedent to the rise of the High Himalayas.     

Quantitative assessment of benthic food resources for juvenile Gulf sturgeon,Acipenser oxyrinchus desotoi in the Suwannee River estuary, Florida, USA

Gulf sturgeon,Acipenser oxyrinchus desotoi, forage extensively in the Suwannee River estuary following emigration out of the Suwannee River, Florida. While in the estuary, juvenile Gulf sturgeon primarily feed on benthic infauna. In June–July 2002 and February–April 2003, random sites within the estuary were sampled for benthic macrofauna (2002 n = 156; 2003 n = 103). A mean abundance of 2,562 ind m⁻² (SE ± 204) was found in the summer, with significantly reduced macrofaunal abundance in the winter (mean density of 1,044 ind m⁻², SE ± 117). Benthic biomass was significantly higher in the summer with an average summer sample dry weight of 5.92 g m⁻² (SE ± 0.82) compared to 3.91 g m⁻² (SE ± 0.67) in the winter. Amphipods and polychaetes were the dominant taxa collected during both sampling periods. Three different estimates of food availability were examined taking into account principal food item information and biomass estimates. All three estimates provided a slightly different view of potential resources but were consistent in indicating that food resource values for juvenile Gulf sturgeon are spatially heterogeneous within the Suwannee River estuary.     

Water quality and phytoplankton as indicators of hurricane impacts on a large estuarine ecosystem

Three sequential hurricanes in the fall of 1999 provided the impetus for assessing multi-annual effects on water quality and phytoplankton dynamics in southwestern Pamlico Sound, North Carolina. Two and a half years of post-hurricane data were examined for short- and long-term impacts from the storms and >100 year flooding. Salinity decreased dramatically and did not recover until May 2000. Inorganic nitrogen and phosphorus concentrations were briefly elevated during the flooding, but later returned to background levels. Dissolved organic carbon concentrations declined through the whole study period, but did not appear to peak as was observed in the Neuse River estuary, a key tributary of the Sound. Light attenuation was highest in the fall to spring following the storms and was best correlated with chlorophylla concentrations. Phytoplankton biomass (chla) increased and remained elevated until late spring 2000 when concentrations returned to pre-storm levels and then cycled seasonally. Phytoplankton community composition varied throughout the study, reflecting the complex interaction between physiological optimal and combinations of salinity, residence time, nutrient availability, and possibly grazing activity. Floodwater advection or dilution from upstream maxima may have controlled the spatial heterogeneity in total and group-specific biomass. The storms produced areas of shortterm hypoxia, but hypoxic events continued during the following two summers, correlating strongly with water column stratification. Nitrogen loading to the southwestern sound was inferred from network analysis of previous nitrogen cycling studies in the Neuse River estuary. Based on these analyses, nutrient cycling and removal in the sub-estuaries would be decreased under high flow conditions, confirming observations from other estuaries. The inferred nitrogen load from the flood was 2–3 times the normal loading to the Sound; this estimate was supported by the substantial algal bloom. After 8-mos, the salinity and chla data indicated the Sound had returned to pre-hurricane conditions, yet phytoplankton community compositional changes continued through the multi-year study period. This is an example of long-term aspects of estuarine recovery that should be considered in the context of a predicted 10–40 yr period of elevated tropical storm activity in the western Atlantic Basin.     

Biodiversity and ecosystem functions in wetlands: A case study in the estuary of the Seine River, France

The integrity of estuarine wetlands is maintained by physical connections between river and sea to floodplain. Their ecological importance can be assessed through plant biodiversity and such ecosystem functions as primary productivity and nitrate removal capacity. Multivariate analysis were used to establish a hierarchy of environmental factors related to the vegetation structure and diversity. Four different measures of plant diversity (both structural and functional) were made on a Seine River wetland. Key functions of estuarine floodplain (productivity and denitrification capacity) were either measured directly or assessed using remotely sensed data. The richest plant communities correspond to mesophilous grasslands which have an intermediate position between natural and anthropogenic disturbance regimes. These species assemblages occur in ecosystems presenting both a regular productivity in time and space and the highest denitrification potentiality.     

珠江河口水环境时空变异对河口生态系统的影响

分析近20年来珠江河口水环境变异的特征,探讨珠江河口水环境与水生物相互联系和相互作用的关系。结果表明,珠江河口形态、地貌、水文情势和入河污染物等变异改变了珠江河口水生物的栖息条件,削弱了河口生态系统的自动调节修复能力和稳定性,对河口生态系统的物质循环、能量流动和发育演化平衡构成了重大影响。     

Historical ecology of a central California estuary: 150 years of habitat change

We investigated the historical ecology of Elkhorn Slough, a 1,200 ha tidal wetland system in central California. The goal of this study was to identify patterns of change in the extent and distribution of wetland habitats during a 150-yr period and to investigate the causes of these changes. Using a geographic information system (GIS), we interpreted historic maps, charts, and aerial photographs. We created a series of summary maps to illustrate and quantify changes in tidal flow and habitat types at six representative historical periods. With the aid of custom software tools, we performed semi-automated spatial analysis of historic aerial photographs to quantify changes in marsh cover at fixed quadrats and tidal creek width at fixed cross sections. Our multiscale analysis documents dramatic shifts in the distribution of habitat types resulting from anthropogenic modifications to the hydrology of the slough. More than half of the marshlands were diked, and more than two thirds have either degraded or been converted to other habitat types. The construction of an artificial mouth abruptly transformed the wetland system from depositional to highly erosional, enlarging channels, widening creeks, and converting marsh to intertidal mudflat or open water. Increased tidal amplitude and velocity are the likely causes. In recent decades, levee failure and intentional breaching have restored the acreage under tidal influence to nearly historic levels, but recolonization of former wetlands by salt marsh vegetation has been minimal. Degraded former marshland and unvegetated mudflat are now the dominant habitat types at Elkhorn Slough. The rate of habitat change remains high, suggesting that a new equilibrium may not be reached for many decades. This study can help tidal wetland managers identify patterns and mechanisms of habitat change and set appropriate conservation and restoration goals.     

Biochemical, chromatographic and spectroscopic analyses of Suwannee River fulvic acid-Evidence for the presence of a DNA-like material

Naturally occurring organic compounds are complicated materials that are hard to fractionate into individual components. Organic materials from lakes, rivers and oceans often contain biological molecules related to proteins and DNA, and have global environmental and ecological importance. In this study, we used a combination of biochemical, chromatographic and spectroscopic methods to fractionate and characterize Suwannee River fulvic acid （SRFA） to determine if it contains proteins and DNA or DNA-like materials. A stock solution of SRFA （50 mg/mL） was prepared in double distilled deionized water. An initial biochemical analysis indicated that SRFA contained approximately 2% protein by weight, as determined by Bradford method using bovine serum albumin as standard. In order to determine the presence of DNA-like materials, a portion of the SRFA sample was extracted for DNA using chloroform/phenol extraction and ethanol precipitation method. When ethanol precipitated fraction was separated by agarose gel electrophoresis along with known DNA markers, a diffused ehthidium bromide staining was observed throughout the lane containing SRFA sample with a strong staining observed at the dye front, suggesting the presence of DNA or ＂DNA-like＂ molecules of various sizes in SRFA. To further fractionate, the DNA-like material on the gel was cut into seven fractions, each of which was extracted into aqueous phase. These fractions, along with the fractions obtained similarly from the original SRFA and an interface material during chloroform/ phenol extraction, were analyzed using UV-vis and fluorescence spectrometers, and LC/UV/MS. Comparisons of the absorption, fluorescence and mass spectra of various fractions revealed significant differences in their spectral properties. The DNA extracted materials showed stronger absorption in the DNA range of -260 nm than the original SRFA and interface material.     

Physiological responses of transplants of the freshwater angiosperm Vallisneria americana along a salinity gradient in the Caloosahatchee Estuary (Southwestern Florida)

Fluctuations in freshwater input may affect the physiology and survival of submerged aquatic vegetation (SAV) occurring in oligoaline to mesohaline estuarine regions. Controls on the distribution of the freshwater angiosperm Vallisneria americana, were investigated by transplanting ramets. Pots (3.8-1) containing ramets were distributed among four sites (upstream site [least saline], donor site, near downstream site, and far downstream site [most saline]) in the Caloosahatchee Estuary (Southwest Florida) during wet (May–August) and dry (October–February) seasons. During 2–4 mo of each season, physiological indicators were monitored, including photosynthesis, glutamine synthetase activity, and protein content in shoots, and carbohydrates and total nitrogen and carbon in shoot and subterranean tissues. Where the physical environment (light or salinity) was suboptimal, all physiological indices, except photosynthetic rate, showed similar stress responses, which ranged from a slow decline to a rapid drop in physiological function. Levels of soluble carbohydrates decreased in response to unfavorable conditions more rapidly than did insoluble carbohydrates. Shoot protein of V. americana declined prior to transplant death, suggesting that measuring protein content may provide a rapid assessment of physiological health. V. americana transplants at the low-salinity upstream site died during both wet and dry season experiments, likely in response to light limitation and/or partial burial by sediments. At the far downstream site, death occurred within 2–4 wk, and was attributable to elevated salinities (>ca. 15‰). Comparison of physiological responses with salinity and light regimes at the donor and near downstream sites suggest that light may ameliorate salinity stress. This study demonstrates that V. americana, nominally classed as a freshwater macrophyte, is capable of a remarkable degree of halotolerance.     

Patterns of mangrove forest structure and soil nutrient dynamics along the Shark River estuary, Florida

The basal area and productivity of managrove wetlands are described in relation to selected soil properties to understand the general pattern of optimum forest stature at the mouth of estuaries in the Everglades, such as the Shark River Slough, Florida (U.S.). The basal area of mangroves decreases from 40.4 m² ha⁻¹ and 39.7 m² ha⁻¹ at two stations 1.8 km and 4.1 km from the estuary mouth to 20.7 m² ha⁻¹ and 19.6 m² ha⁻¹ at two sites 9.9 km and 18.2 km from the mouth, respectively. The gradient in basal area at these four sites is mostly the result of approximately 34 yr of growth since Hurricane Donna. Wood productivity is higher in the lower estuary (10.7 Mg ha⁻¹ yr⁻¹ and 12.0 Mg ha⁻¹ yr⁻¹) than in the upper estuary (3.2 Mg ha⁻¹ yr⁻¹ and 4.2 Mg ha⁻¹ yr⁻¹). Porewater salinity among these four mangrove sites during seasonal sampling in 1994 and 1995 ranged from 1.6 g kg⁻¹ to 33.5 g kg⁻¹, while sulfide was generally<0.15 mM at all sites. These soil values indicate that abiotic stress cannot explain the decrease in forest structure along this estuarine gradient. Concentrations of nitrogen (N) and phosphorus (P) are more closely related to patterns of forest development, with higher soil fertility at the mouth of the estuary as indicated by higher concentrations of extractable ammonium, total soil P, and available P, along with higher ammonium production rates. The more fertile sites of the lower estuary are dominated by Laguncularia racemosa, whereas the less fertile sites in the intermediate and upper estuary are dominated by Rhizophora mangle. Relative N mineralization per unit of total N is higher in the lower estuary and is related positively to concentrations of available P, indicating the importance of turnover rates and nutrient interactions to soil fertility. Concentrations of Ca-bound P per volume soil in the lower estuary is 40-fold higher than in the upper estuary, and along with an increase in residual P in the upper estuary, indicate a shift from mineral to organic P along the estuarine gradient. Mineral inputs to the mouth of Shark River estuary from the Gulf of Mexico (rather than upland inputs) apparently control the patterns of mangrove structure and productivity.     

Trends in phosphatase activity along a successional gradient of tidal freshwater marshes on the Cooper River South Carolina

Phosphatase activity was measured in sediments from tidal freshwater habitats adjacent to the Cooper River in South Carolina representing different stages of ecological succession. It was found that sediment (0–5 cm) acid phosphatase activity, alkaline phosphatase activity and phosphodiesterase activity increased with increasing successional stage and phytomass. Acid phosphatase activity in creased from 7.5±1.2 (±1 SD) in subtidal sediment from a shallow open water habitat without vegetation to 61.2±4.9 μmol g⁻¹ hr⁻¹ (μmol of p-nitrophenol released per gram of dry sediment per hour) in intertidal sediments colonized by emergent macrophytes, while alkaline phosphatase activity increased from 2.1±0.1 to 19.01±1.5 μmol g⁻¹ hr⁻¹. Phosphodiesterase activity increased from 1.8±0.1 to 20.2±2.0 μmol g⁻¹ hr⁻¹ along the same gradient. Acid phosphatase activity was highly correlated (R²=0.92, P<0.001) with the organic matter content of the sediment. A study of phosphatase kinetics showed that V_max of all phosphatases also increased along the successional gradient. Trends in phosphatase activity and V_max correlated positively with plant biomass and negatively with concentrations of soluble reactive phosphorus in porewater, sediment extractable phosphorus, and total phosphourus. The porewater N∶P atom ratio decreased along the succession gradient from 25.3 in an early stage, open water community to 13.0 in a community dominated by emergent vegetation. The data also show that the distribution of the forms of phosphorus changed with successional stage. The change in distribution and the increased biological demand for phosphorus that paralleled succession were mediated by the activity of phosphatase enzymes.     

Nutrient inputs to the Choptank River estuary: Implications for watershed management

Degraded water quality due to water column availability of nitrogen and phosphorus to algal species has been identified as the primary cause of the decline of submersed aquatic vegetation in Chesapeake Bay and its subestuaries. Determining the relative impacts of various nutrient delivery pathways on estuarine water quality is critical for developing effective strategies for reducing anthropogenic nutrient inputs to estuarine waters. This study investigated temporal and spatial patterns of nutrient inputs along an 80-km transect in the Choptank River, a coastal plain tributary and subestuary of Chesapeake Bay, from 1986 through 1991. The study period encompassed a wide range in freshwater discharge conditions that resulted in major changes in estuarine water quality. Watershed nitrogen loads to the Choptank River estuary are dominated by diffuse-source inputs, and are highly correlated to freshwater discharge volume. in years of below-average freshwater discharge, reduced nitrogen availability results in improved water quality throughout most of the Choptank River. Diffuse-source inputs are highly enriched in nitrogen relative to phosphorus, but point-source inputs of phosphorus from sewage treatment plants in the upper estuary reduce this imbalance, particularly during summer periods of low freshwater discharge. Diffuse-source nitrogen inputs result primarily from the discharge of groundwater contaminated by nitrate. Contamination is attributable to agricultural practices in the drainage basin where agricultural land use predominates. Groundwater discharge provides base flow to perennial streams in the upper regions of the watershed and seeps directly into tidal waters. Diffuse-source phosphorus inputs are highly episodic, occurring primarily via overland flow during storm events. Major reductions in diffuse-source nitrogen inputs under current landuse conditions will require modification of agricultural practices in the drainage basin to reduce entry rates of nitrate into shallow groundwater. Rates of subsurface nitrate delivery to tidal waters are generally lower from poorly-drained versus well-drained regions of the watershed, suggesting greater potential reductions of diffuse-source nitrogen loads per unit effort in the well-drained region of the watershed. Reductions in diffuse-source phosphorus loads will require long-term management of phosphorus levels in upper soil horizons. *** DIRECT SUPPORT *** A01BY074 00021     

Parasites of eastern oysters from subtidal reefs in a Louisiana estuary with a note on their use as indicators of water quality

A 12-month parasitological study in-volved examination of 1,242 eastern oysters,Crassostrea virginica, from two hydrologically different subtidal reefs in Calcasieu Lake, Louisiana. No evidence of morbidity attributed to infection with the protozoanPerkinsus marinus was detected among oysters at either site. Most oysters from the higher salinity site had shells infested with boring sponges,Cliona truitti. Oysters from both sites harbored infections with sporocysts and cercariae of the digenetic trematode,Bucephalus cuculus. However, average monthly incidence of infection was less than 0.5%.