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The effects of nutrients, trace elements, and trophic complexity on benthic photosynthesis and respriation were studied in the Paxtuxent River estuary near St. Leonard, Maryland. Experiments were conducted over three years (1995–1997) in mesocosms containing riverine sediment and water. The experimental design was 2×2×3 factorial with two levels of nutrients (ambient and + nutrients), two of trace elements (ambient and + trace elements) and three of trophic complexity (plankton, plankton + fish, and plankton + fish + benthos). Trace elements included arsenic (As), copper (Cu), and cadmium (Cd). The experiment was conducted three times in 1995 and 1997 and four times in 1996. In 1995 and 1996, sediments were muddy, while in the final year sediments were sandy. In mesocoms with sandy sediments, nutrient additions increased benthic photosynthesis overall, while trace element additions increased benthic photosynthesis in two of three experimental runs. Benthic photosynthesis in these mesocosms appeared to be related to nitrogen loading. Benthic respiration increased in nutrient and trace element amended mesocosms with sandy sediments, apparently in response to higher benthic photosynthesis. Increasing trophic complexity, particularly the presence of benthic organisms, also increased benthic respiration in mesocosms with sandy sediments. There were no effects of nutrient or trace element additions on benthic photosynthesis and respiration when the sediments were muddy. The lack of consistent responses to nutrient additions was surprising given that benthic respiration rates (and presumably nutrient regeneration) were similar in all three years, regardless of sediment type. Muddy, sediments did not mask, the effects of nutrient addition by supplying more nutrients to benthic microalgae than sandy sediments. In 1996, the presence of filter feeding bivalves increased the relative heterotrophy of sediments, measured as production: respiration. Consistent with increased heterotrophy, effluxes of ammonium and soluble reactive phosphorus from sediments were greater in mesocosms containing benthic organisms. Anthropogenically-induced changes in estuaries, such as loading of nutrients and trace elements or reduced trophic complexity, can have important effects on benthic processes and potentially pelagic processes through feedback mechanisms.  相似文献   
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A combination of field studies and mathematical modeling was used to examine the role of subtidal benthic algae in the eutrophication processes in two shallow estuarine systems. Field measurements indicated uptake by benthic algae retained ammonium and phosphate in the sediments when light at the sediment surface exceeded ≈150 μE m2 s?1. The measurements were used to calibrate a newly developed model of benthic algal activity. The benthic algal model was coupled with a hydrodynamic model, a eutrophication model, and a sediment diagenesis model. In the simulated ecosystem, benthic algae had a major influence in the intra-annual cycling of nitrogen and phosphorus. When nutrients were abundant in the water column (late winter and spring) they were transferred to the sediments through algal activity. Diagenesis released these nutrients to the water column in summer when nutrients were scarce. As a result of the nutrient transfer, annual primary production in the water column, in the presence of benthic algae, exceeded production in the absence of the algae.  相似文献   
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Due to the ecological importance of seagrasses and recent indications of disease and dieback, we have synthesized existing mapped survey information concerning the spatial and temporal distribution of seagrass beds (primarily eelgrass,Zostera marina) in Barnegat Bay, New Jersey. Mapped surveys from the 1960s, 1970s, 1980s, and 1990s were digitized and compiled in a geographic information system to facilitate analysis. Comparison of the earlier maps with the 1990s survey shows an overall decrease of approximately 2,000 to 3,000 ha in the area of seagrass beds. While there are indications of seagrass decline, due to the great difference in mapping methods used for each of the surveys, we are cautious in directly attributing the decrease in mapped eelgrass acreage to a large-scale dieback. We examined the extent to which light could be used to predict the distribution of seagrass in Barnegat Bay. Data on Secchi depth throughout the bay were combined with a modification of an existing model (Duarte 1991) of the relationship betweenZ. marina compensation depths and light attenuation coefficients to predict the distribution of seagrasses in Barnegat Bay. When compared with mapped seagrass distribution in the bay, the model correctly predicts seagrass presence-absence over two-thirds of the time. The majority of the model error is due to errors of commission, i.e., the model predicts seagrass occurrence where it was not observed to occur. Most of this commission error is located in specific geographic areas (i.e., southern third of Little Egg Harbor and the western shoreline of the bay).  相似文献   
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Natural Hazards - The main objective of this article is to study the robustness of road networks to extreme flooding events that can negatively affect entire regional systems in a relatively...  相似文献   
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Methods were developed for determining rates of denitrification in coastal marine sediments by measuring the production of N2 from undisturbed cores incubated in gas-tight chambers. Denitrification rates at summer temperatures (23°C) in sediment cores from Narragansett Bay, Rhode Island, were about 50μmol N2m?2 hr?1. This nitrogen flux is equal to approximately one-half of the NH+4flux from the sediments at this temperature and is of the magnitude necessary to account for the anomalously low N/P and anomalously high O/N ratios often reported for benthic nutrient fluxes. The loss of fixed nitrogen as N2 during the benthic remineralization of organic matter, coupled with the importance of benthic remineralization processes in shallow coastal waters may help to explain why the availability of fixed nitrogen is a major factor limiting primary production in these areas. Narragansett Bay sediments are also a source of N2O, but the amount of nitrogen involved was only about 0.2 μmol m?2 hr?1 at 23°C.  相似文献   
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Routine determination of dissolved organic nitrogen (DON) is performed in numerous laboratories around the world using one of three families of methods: UV oxidation (UV), persulfate oxidation (PO), or high temperature combustion (HTC). Essentially all routine methods measure total dissolved nitrogen (TDN) and calculate DON by subtracting the dissolved inorganic nitrogen (DIN). While there is currently no strong suggestion that any of these methods is inadequate, there are continuing suspicions of slight inaccuracy by UV methods.This is a report of a broad community methods comparison where 29 sets (7 UV, 13 PO, and 9 HTC) of TDN analyses were performed on five samples with varying TDN and DIN concentrations. Analyses were done in a “blind” procedure with results sent to the first author. With editing out one set of extreme outliers (representing 5 out of 145 ampoules analyzed), the community comparability for analyzing the TDN samples was in the 8–28% range (coefficient of variation representing one standard deviation for the five individual samples by 28 analyses). When DIN concentrations were subtracted uniformly (single DIN value for each sample), the comparability was obviously worse (19–46% cv). This comparison represents a larger and more diverse set of analyses, but the overall comparability is only marginally better than that of the Seattle workshop of a decade ago. Grouping methods, little difference was seen other than inconclusive evidence that the UV methods gave TDN values for several of the samples higher than HTC methods. Since there was much scatter for each of the groups of methods and for all analyses when grouped, it is thought that more uniformity in procedures is probably needed. An important unplanned observation is that variability in DIN analyses (used in determining the final analyte in most UV and PO methods) is essentially as large as the variability in the TDN analyses.This exercise should not be viewed as a qualification exercise for the analysts, but should instead be considered a broad preliminary test of the comparison of the families of methods being used in various laboratories around the world. Based on many independent analyses here, none of the routinely used methods appears to be grossly inaccurate, thus, most routine TDN analyses being reported in the literature are apparently accurate. However, it is not reassuring that the ability of the international community to determine DON in deep oceanic waters continues to be poor. It is suggested that as an outgrowth of this paper, analysts using UV and PO methods experiment and look more carefully at the completeness of DIN conversion to the final analyte and also at the accuracy of their analysis of the final analyte. HTC methods appear to be relatively easy and convenient and have potential for routine adoption. Several of the authors of this paper are currently working together on an interlaboratory comparison on HTC methodology.  相似文献   
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Ambient exchangeable ammonium concentrations in freshwater sediments are generally considerably greater than those reported for marine sediments. Laboratory measurements indicate that competition for cation exchange sites by ions in seawater is a factor responsible for the lower exchangeable ammonium concentrations in marine sediments. Exchangeable ammonium concentrations were 3- to 6-fold higher when river and estuarine sediments were incubated with fresh water relative to the same sediments incubated with salt water (%.-23). A model was developed to explore the implications for benthic nitrogen cycling of this salinity effect on exchangeable ammonium concentrations. Ammonium diffusion, exchangeable and dissolved ammonium concentrations, and nitrification rates were components of the model formulation. The model output suggests that higher exchangeable ammonium concentrations predicted in fresh water relative to marine sediments can markedly increase the fraction of the ammonium produced in sediments that is nitrified (and subsequently denitrified). These results are consistent with field and experimental laboratory data which indicate that a larger percentage of net ammonium production in aerobic freshwater sediments is nitrified and denitrified (80–100%) relative to marine sediments (40–60%).  相似文献   
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In sediments with oxidized surface layers, the percentage of mineralized nitrogen that is nitrified/denitrified, compared with that released directly as ammonium, appears to be affected by the presence of sea salts. In estuarine systems, a significant portion of the nitrogen is released as ammonium, whereas in freshwater systems, most of the mineralized nitrogen is often released from the sediments as nitrogen gas. We hypothesized that this discrepancy is caused by differential competition between physical diffusion and nitrification/denitrification in the two systems. The vertical migration (by Fickian diffusion) of ammonium out of the oxic layer may be hindered by cation exchange (or sorption) interactions with sediment particles to a greater extent in fresh water than in estuarine systems. The resulting relatively long residence time, and potentially high levels of particle-bound ammonium in the freshwater sediments, would favor nitrification as the major ammonium removal process. By contrast, ion pair formation of ammonium with seawater anions and blockage of sediment cation exchange sites with seawater cations may allow a sizable fraction of the ammonium to diffuse out of estuarine sediments before it is nitrified. A salt effect, consistent with this hypothesis, has been demonstrated in experimental systems by changing the ionic composition of water flowing above intact cores of freshwater and estuarine sediments. Steady-state ammonium release from Lake Michigan sediments was substantially enhanced in the presence of 30% seawater over that in the presence of lake water alone. Likewise, steady-state ammonium release, from Ochlockonee River and Bay sediments (Florida) and from Toms River and Barnegat Bay sediments (New Jersey), was usually higher in the presence of diluted synthetic seawater than it was in the presence of fresh water.  相似文献   
10.
Responses of autotrophic and heterotrophic processes to nutrients and trace elements were examined in a series of experimental estuarine food webs of increasing trophic complexity using twenty 1-m3 mesocosms. Nutrients (nitrogen and phosphorus) and trace elements (a mix of arsenic, copper, cadmium) were added alone and in combination during four experimental runs spanning from spring 1997 to spring 1998. Diel changes in dissolved oxygen were used to examine whole system gross primary production (WS-GPP), respiration (WS-RESP), and net ecosystem metabolism (NEM). Nutrient and trace element additions had the greatest effect on WS-GPP, WS-RESP, and NEM; trophic complexity did not significantly affect any of these parameters (p>0.3). Effects of trophic complexity were detected in nutrient tanks where bivalves significantly (p=0.03) reduced WS-GPP. Nutrient additions significantly enhanced WS-GPP and to a lesser extent WS-RESP during most mesocosm runs. The system shifted from net heterotrophy (−17.2±1.8 mmol C m−3 d−1) in the controls to net autotrophy (29.1±7.6 mmol C m−3 d−1) in the nutrient tanks. The addition of trace elements alone did not affect WS-GPP and WS-RESP to the same extent as nutrients, and their effects were more variable. Additions of trace elements alone consistently made the system more net heterotrophic (−24.9±1.4 mmol C m−3 d−1) than the controls. When trace elements were added in combination with nutrients, the nutrient-enriched system became less autotrophic (1.6±3.1 mmol C m−3 d−1). The effects of trace elements on NEM occurred primarily through reductions in WS-GPP rather than increases in WS-RESP. Our results suggest that autotrophic and heterotrophic processes respond differently to these stressors.  相似文献   
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