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1.
Chesapeake Bay supports a diverse assemblage of marine and freshwater species of submersed aquatic vegetation (SAV) whose broad distributions are generally constrained by salinity. An annual aerial SAV monitoring program and a bi-monthly to monthly water quality monitoring program have been conducted throughout Chesapeake Bay since 1984. We performed an analysis of SAV abundance and up to 22 environmental variables potentially influencing SAV growth and abundance (1984–2006). Historically, SAV abundance has changed dramatically in Chesapeake Bay, and since 1984, when SAV abundance was at historic low levels, SAV has exhibited complex changes including long-term (decadal) increases and decreases, as well as some large, single-year changes. Chesapeake Bay SAV was grouped into three broad-scale community-types based on salinity regime, each with their own distinct group of species, and detailed analyses were conducted on these three community-types as well as on seven distinct case-study areas spanning the three salinity regimes. Different trends in SAV abundance were evident in the different salinity regimes. SAV abundance has (a) continually increased in the low-salinity region; (b) increased initially in the medium-salinity region, followed by fluctuating abundances; and (c) increased initially in the high-salinity region, followed by a subsequent decline. In all areas, consistent negative correlations between measures of SAV abundance and nitrogen loads or concentrations suggest that meadows are responsive to changes in inputs of nitrogen. For smaller case-study areas, different trends in SAV abundance were also noted including correlations to water clarity in high-salinity case-study areas, but nitrogen was highly correlated in all areas. Current maximum SAV coverage for almost all areas remain below restoration targets, indicating that SAV abundance and associated ecosystem services are currently limited by continued poor water quality, and specifically high nutrient concentrations, within Chesapeake Bay. The nutrient reductions noted in some tributaries, which were highly correlated to increases in SAV abundance, suggest management activities have already contributed to SAV increases in some areas, but the strong negative correlation throughout the Chesapeake Bay between nitrogen and SAV abundance also suggests that further nutrient reductions will be necessary for SAV to attain or exceed restoration targets throughout the bay.  相似文献   

2.
The decline of eelgrass (Zostera marina) in Chesapeake Bay in the 1960s and 1970s has been studied in the context of changes in water quality and habitat suitability; little effort has focused on the importance of reproductive ecology in understanding current and potential recovery of these populations. The spatial variability of seed-bank characteristics ofZ. marina in Chesapeake Bay was explored by a reproductive shoot and seed-bank sampling effort. Seed banks were sampled from 105 beds of submerged aquatic vegetation among 12 zones throughout the lower and middle Chesapeake Bay. Number of viable seeds was highly variable among and within zones, with seeds found in all but one zone and also found in cores not containing anyZ. marina shoots. Number of reproductive shoots was also highly variable among and within zones, with differences probably driven by different local environmental conditions. Bay-wide, viable seeds were found in more monospecificZ. marina cores than in mixed species or monospecificRuppia maritima cores suggesting local biological and environmental control on sexual reproduction. Lower densities of viable seeds in the middle Chesapeake Bay region reflect the lower abundance ofZ. marina in these regions and provide context for discussion of historical changes inZ. marina in Chesapeake Bay. While this study focused on a snap shot of the seed bank immediately after establishment, we highlight critical questions for future study that may be important for their conservation and restoration.  相似文献   

3.
A predictive model of submerged aquatic vegetation (SAV) biomass is coupled to a eutrophication model of Chesapeake Bay. Domain of the model includes the mainstem of the bay as well as tidal portions of major embayments and tributaries. Three SAV communities are modeled: ZOSTERA, RUPPIA, and FRESHWATER. The model successfully computes the spatial distribution and abundance of SAV for the period 1985–1994. Spatial distribution is primarily determined by computed light attenuation. Sensivitity analysis to reductions in nutrient and solids loads indicates nutrient controls will enhance abundance primarily in areas that presently support SAV. Restoration of SAV to areas in which it does not presently exist requires solids controls, alone or in combination with nutrient controls. For regions in which SAV populations exist at the refuge level or greater, improvements in SAV abundance are expected within 2 to 10 years of load reductions. For regions in which no refuge population exists, recovery time is unpredictable and will depend on propagule supply.  相似文献   

4.
Baltic clams (Macoma balthica) were the predominant food items of 323 canvasbacks (Aythya valisineria) collected throughout Chesapeake Bay during 1970–1979. Natural vegetation constituted 4% of the food volume. Widgeongrass (Ruppia maritima) and redhead grass (Potamogeton perfoliatus) constituted the greatest percent volume and frequency of occurrence among the plant species, whereas wild celery (Vallisneria americana) constituted only a trace of the food volume. These results contrast with historical records of food habits of canvasbacks in Chesapeake Bay. Canvasback population estimates during the 1970’s were examined to detect annual and seasonal changes in distribution. Linear regression analyses of winter canvasback populations in the bay showed a significant decline in the upper-bay and middle-bay populations, but no significant changes in the lower-bay and Potomac River populations. The changes in winter distribution and abundance of the canvasback appear related to changes in natural food availability, which is the result of altered environmental conditions.  相似文献   

5.
The decline of submersed aquatic vegetation (SAV) in tributaries of the Chesapeake Bay has been associated with increasing anthropogenic inputs, and restoration of the bay remains a major goal of the present multi-state “Bay Cleanup” effort. In order to determine SAV response to water quality, we quantified the water column parameters associated with success of transplants and natural regrowth over a three-year period along an estuarine gradient in the Choptank River, a major tributary on the eastern shore of Chesapeake Bay. The improvement in water quality due to low precipitation and low nonpoint source loadings during 1985–1988 provided a natural experiment in which SAV was able to persist upstream where it had not been for almost a decade. Mean water quality parameters were examined during the growing season (May–October) at 14 sites spanning the estuarine gradient and arrayed to show correspondence with the occurrence of SAV. Regrowth of SAV in the Choptank is associated with mean dissolved inorganic nitrogen <10 μM; mean dissolved phosphate <0.35 μM; mean suspended sediment <20 mg l?1; mean chlorophylla in the water column <15 μg l?1; and mean light attenuation coefficient (Kd) <2 m?1. These values correspond well with those derived in other parts of the Chesapeake, particularly in the lower bay, and may provide managers with values that can be used as target concentrations for nutrient reduction strategies where SAV is an issue.  相似文献   

6.
Long-term trends of waterfowl populations in Chesapeake Bay demonstrate the importance of shallow-water habitats for waterfowl species. Although recent increases in field feeding by geese and swans lessened the importance of shallow-water areas for these species, most duck species depend almost exclusively on shallow-water habitats. Many factors influenced the distribution and abundance of waterfowl in shallow-water habitats. Habitat degradation resulted in the decline in numbers of most duck species and a change in distribution of some species. Increased numbers of mallards (Anas platyrhynchos) in recent decades probably resulted from release programs conducted by the Maryland Department of Natural Resources and private individuals. Studies of food habits since 1885 showed a decline in submerged-aquatic vegetation in the diet of some species, such as the canvasback (Aythya valisineria), and an increase in the proportions of invertebrates in the diet. Diversity of food organisms for many waterfowl species has declined. Surveys of vegetation and invertebrates in the Chesapeake Bay generally reflect a degradation of shallow-water habitat. Human population increases in the Chesapeake Bay watershed directly and indirectly affected waterfowl distribution and abundance. The increase of exotic plant and invertebrate species in the bay, in most cases, benefited waterfowl populations. Increased contaminants have reduced the quality and quantity of habitat, although serious attempts to reverse this trend are underway. The use of shallow-water habitats by humans for fishing, hunting, boating, and other recreational and commercial uses reduced the use of shallow-water habitats by waterfowl. Humans can lessen the adverse influences on the valuable shallow-water habitats by restricting human population growth near these habitats and improving the water quality of the bay tributaries. Other affirmative actions that will improve these areas for waterfowl include greater restrictions on boat traffic in shallow-water habitats and establishing more sanctuaries in shallow-water areas that have complete protection from human disturbance. *** DIRECT SUPPORT *** A01BY074 00013  相似文献   

7.
A procedure was developed using aboveground field biomass measurements of Chesapeake Bay submersed aquatic vegetation (SAV), yearly species identification surveys, annual photographic mapping at 1∶24,000 scale, and geographic information system (GIS) analyses to determine the SAV community type, biomass, and area of each mapped SAV bed in the bay and its tidal tributaries for the period of 1985 through 1996. Using species identifications provided through over 10,000 SAV ground survey observations, the 17 most abundant SAV species found in the bay were clustered into four species associations: ZOSTERA, RUPPIA, POTAMOGETON, and FRESHWATER MIXED. Monthly aboveground biomass values were then assigned to each bed or bed section based upon monthly biomass models developed for each community. High salinity communities (ZOSTERA) were found to dominate total bay SAV aboveground biomass during winter, spring, and summer. Lower salinity communities (RUPPIA, POTAMOGETON, and FRESHWATER MIXED) dominated in the fall. In 1996, total bay SAV standing stock was nearly 22,800 metric tons at annual maximum biomass in July encompassing an area of approximately 25,670 hectares. Minimum biomass in December and January of that year was less than 5,000 metric tons. SAV annual maximum biomass increased baywide from lows of less than 15,000 metric tons in 1985 and 1986 to nearly 25,000 metric tons during the 1991 to 1993 period, while area increased from approximately 20,000 to nearly 30,000 hectares during that same period. Year-to-year comparisons of maximum annual community abundance from 1985 to 1996 indicated that regrowth of SAV in the Chesapeake Bay from 1985–1993 occurred principally in the ZOSTERA community, with 85% of the baywide increase in biomass and 71% of the increase in are a occurring in that community. Maximum biomass of FRESHWATER MIXED SAV beds also increased from a low of 3,200 metric tons in 1985 to a high of 6,650 metric tons in 1993, while maximum biomass of both RUPPIA and POTAMOGETON beds fluctuated between 2,450 and 4,600 metric tons and 60 and 600 metric tons, respectively, during that same period with net declines of 7% and 43%, respectively, between 1985 and 1996. During the July period of annual, baywide, maximum SAV biomass, SAV beds in the Chesapeake Bay typically averaged approximately 0.86 metric tons of aboveground dry mass per hectare of bed area.  相似文献   

8.
We developed an algorithm for calculating habitat suitability for seagrasses and related submerged aquatic vegetation (SAV) at coastal sites where monitoring data are available for five water quality variables that govern light availability at the leaf surface. We developed independent estimates of the minimum light required for SAV survival both as a percentage of surface light passing though the water column to the depth of SAV growth (PLW min) and as a percentage of light reaching reaching leaves through the epiphyte layer (PLL min). Value were computed by applying, as inputs to this algorithm, statistically dervived values for water quality variables that correspond to thresholds for SAV presence in Chesapeake Bay. These estimates ofPLW min andPLL min compared well with the values established from a literature review. Calcultations account for tidal range, and total light attenuation is partitioned into water column and epiphyte contributions. Water column attenuation is further partitioned into effects of chlorophylla (chla), total suspended solids (TSS) and other substances. We used this algorithm to predict potential SAV presence throughout the Bay where calculated light available at plant leaves exceededPLL min. Predictions closely matched results of aerial photographic monitoring surveys of SAV distribution. Correspondence between predictions and observations was particularly strong in the mesohaline and polythaline regions, which contain 75–80% of all potential SAV sites in this estuary. The method also allows for independent assessment of effects of physical and chemical factors other than light in limiting SAV growth and survival. Although this algorithm was developed with data from Chesapeake Bay, its general structure allows it to be calibrated and used as a quantitative tool for applying water quality data to define suitability of specific sites as habitats for SAV survival in diverse coastal environments worldwide.  相似文献   

9.
Conserving and restoring submerged aquatic vegetation (SAV) are key management goals for estuaries worldwide because SAV integrates many aspects of water quality and provides a wide range of ecosystem services. Management strategies are typically focused on aggregated abundance of several SAV species, because species cannot be easily distinguished in remotely sensed data. Human land use and shoreline alteration have been shown to negatively impact SAV abundance, but the effects have varied with study, spatial scale, and location. The differences in reported effects may be partly due to the focus on abundance, which overlooks within-community and among-community dynamics that generate total SAV abundance. We analyzed long-term SAV aerial survey data (1984–2009) and ground observations of community composition (1984–2012) in subestuaries of Chesapeake Bay to integrate variations in abundance with differences in community composition. We identified five communities (mixed freshwater, milfoil-Zannichellia, mixed mesohaline, Zannichellia, and Ruppia-Zostera). Temporal variations in SAV abundance were more strongly related to community identity than to terrestrial stressors, and responses to stressors differed among communities and among species. In one fifth of the subestuaries, the community identity changed during the study, and the probability of such a change was positively related to the prevalence of riprapped shoreline in the subestuary. Mixed freshwater communities had the highest rates of recovery, and this may have been driven by Hydrilla verticillata, which was the single best predictor of SAV recovery rate. Additional species-specific and community-specific research will likely yield better understanding of the factors affecting community identity and SAV abundance, more accurate predictive models, and more effective management strategies.  相似文献   

10.
A two-year trawling and gill-netting study of vegetated and unvegetated bottoms near Parson’s Island, Maryland and near the mouth of the York River, Virginia was carried out to assess the nursery function of submerged vegetation for populations of fishes and decapod crustaceans in the Chesapeake Bay. Results revealed that vegetated bottoms supported substantially larger numbers of decapods, but not fishes, than unvegetated substrates. The lower Bay grassbed was an important nursery area for juvenile blue crabs, although neither of the grassbeds functioned as a nursery for commercially or recreationally valuable fishes. Our results suggest that: (1) further decreases in lower Bay Seagrass biomass would result in reduced numbers of adult blue crabs, but should not substantially affect populations of valuable fish species; (2) additional decreases in Upper Bay submerged vegetation should not produce dramatic change in the population sizes of either adult blue crabs or fishes.  相似文献   

11.
Estuarine seagrass ecosystems provide important habitat for fish and invertebrates and changes in these systems may alter their ability to support fish. The response of fish assemblages to alteration of eelgrass (Zostera marina) ecosystems in two ecoregions of the Mid-Atlantic Bight (Buzzards Bay and Chesapeake Bay) was evaluated by sampling historical eelgrass sites that currently span a broad range of stress and habitat quality. In two widely separated ecoregions with very different fish faunas, degradation and loss of submerged aquatic vegetation (SAV) habitat has lead to declines in fish standing stock and species richness. The abundance, biomass, and species richness of the fish assemblage were significantly higher at sites that have high levels of eelgrass habitat complexity (biomass >100 wet g m?2; density <100 shotts m?2) compared to sites that have reduced eelgrass (biomass <100 wet g m?2; density <100 shoots m?2) or that have completely lost eelgrass. Abundance, biomass, and species richness at reduced eelgrass complexity sites also were more variable than at high eelgrass complexity habitats. Low SAV complexity sites had higher proportions of pelagic species that are not dependent on benthic habitat structure for feeding or refuge. Most species had greater abundance and were found more frequently at sites that have eelgrass. The replacement of SAV habitats by benthic macroalgae, which occurred in Buzzards Bay but not Chesapeake Bay, did not provide an equivalent habitat to seagrass. Nutrient enrichment-related degradation of eelgrass habitat has diminished the overall capacity of estuaries to support fish populations.  相似文献   

12.
Submersed aquatic vegetation (SAV) have been a prominent feature on the Susquehanna Flats, the shallow, subaqueous delta of the Susquehanna River, Maryland. SAV were absent from the Flats between 1972 and 2000, but have since recovered. While it is well established that SAV can improve water quality by promoting sediment and nutrient retention, it is not well understood how SAV on the Flats modulate sediment input from the Susquehanna River into the Upper Chesapeake Bay over different timescales. This study evaluates sedimentation on the Flats over seasonal to decadal timescales, using naturally occurring radioisotopes (7Be, 210Pb) within the context of SAV biomass and Flats geomorphology. Results indicate that sedimentation on the Flats is both spatially and temporally variable. Although this variability cannot be explained by relationships with grain size and SAV biomass, river discharge, sediment supply, and geometry over the SAV bed likely control sedimentation in this system. Decadal-scale sedimentation is influenced by both flood events and changes in SAV biomass abundance. Average annual sediment accumulation was higher when SAV were present than when SAV were absent. SAV bed area was strongly correlated with average annual accumulation rate. These results suggest that a positive feedback between SAV abundance and accumulation rate exists; however, sediment supply and transport pathways are also important factors.  相似文献   

13.
The seasonal abundance and spatial distribution of eggs and early larvae of the bay anchovy,Anchoa mitchilli, and the weakfish,Cynoscion regalis, were determined from plankton collections taken during 1971–1976 in the lower Chesapeake Bay. Eggs and larvae of the bay anchovy,Anchoa mitchilli, dominated the ichthyoplankton, making up 96% of the total eggs and 88% of all larvae taken. A comparison of egg and larval densities from the lower Chesapeake Bay to existing data from other East Coast estuaries suggested that Chesapeake Bay is a major center of spawning activity for this species.Anchoa mitchilli spawning commenced in May when mean water column temperatures approached 17°C and abruptly ceased after August. Eggs and early larvae presented a continuous distribution throughout the study area during these months. Eggs and larvae of several sciaenid species, especiallyC. regalis, ranked second in numerical abundance. Larval weakfish were consistently taken in late summer of each sampling year but peak abundance and distribution was observed in August 1971. Sciaenid eggs exhibited a distinct polyhaline distribution with greatest concentrations observed at the Chesapeake Bay entrance or along the Bay eastern margin. Analysis of sciaenid egg morphometry and larval occurrence suggested spawning activity of at least four species. Additional important species represented by eggs and/or larvae in the lower Chesapeake Bay wereHypsoblennius hentzi, Gobiosoma ginsburgi, Trinectes maculatus, Symphurus plagiusa andParalichthys dentatus with the remaining species occurring infrequently.  相似文献   

14.
A model of the spectral diffuse attenuation coefficient of downwelling irradiance was constructed for Chincoteague Bay, Maryland, and the Rhode River, Maryland. The model is written in terms of absorption spectra of dissolved yellow substance, the chlorophyll-specific absorption of phytoplankton, and absorption and scattering by particulate matter (expressed as turbidity). Based on published light requirements for submersed aquatic vegetation (SAV) in Chesapeake Bay, the model is used to calculate the range of water-quality conditions that permit survival of SAV at various depths. Because the model is spectrally based, it can be used to calculate the attenuation of either photosynthetically active radiation (PAR, equally weighted quanta from 400 nm to 700 nm) or photosyntheticallyusable radiation (PUR, the integral of the quantum spectrum weighted by the pigment absorption spectrum of SAV). PUR is a more accurate measurement of light that can be absorbed by SAV and it is more strongly affected by phytoplankton chlorophyll in the water column than is PAR. For estuaries in which light attenuation is dominated by turbidity and chlorophyll, the model delimits regions in which turbidity alone (chlorophyll <10 μg 1?1), chlorophyll alone (turbidity <1 NTU) or both factors (chlorophyll >10 μg 1?1, turbidity >1 NTU) must be reduced to improve survival depths for SAV.  相似文献   

15.
Restoration of the Chesapeake Bay ecosystem has been a priority for residents and governments of the bay watershed for the past decade. One obstacle in the efforts to “save the bay” has been continuing nutrient enrichment from agricultural and sewer runoff. The attainability of a mandated 40% nutrient reduction goal has yet to be seen. Furthermore, disappearance of certain organisms may have had an adverse effect on the resilience of the ecosystem. The Eastern oyster (Crassostrea virginica), once abundant in Chesapeake Bay, was a vital part of the food web, processing excess phytoplankton and depositing materials on the bottom. Over harvesting and disease have decimated the native oyster population. The introduction of an exotic species, the Japanese oyster (Crassostrea gigas), may be a way to reestablish a robust oyster community in the bay. The literature on the role of bivalve molluscs in estuarine ecosystems shows that they are an essential part of healthy estuaries around the world. A comparison ofC. virginica andC. gigas in terms of temperature and salinity tolerance and resistance to disease shows thatC. virginica is ideally adapted to conditions in Chesapeake Bay, but it is unable to stave off the endemic diseases, whereasC. gigas is adapted to conditions in the lower bay only but is much less susceptible to the same diseases. We conclude that the potential introduction ofC. gigas to Chesapeake Bay would be limited by the Japanese species’ physiological requirements but that the revitalization of a bivalve population is imperative to the restoration of ecosystem function.  相似文献   

16.
Watershed land use can affect submerged aquatic vegetation (SAV) by elevating nutrient and sediment loading to estuaries. We analyzed the effects of watershed use and estuarine characteristics on the spatial variation of SAV abundance among 101 shallow subestuaries of Chesapeake Bay during 1984–2003. Areas of these subestuaries range from 0.1 to 101 km2, and their associated local watershed areas range from 6 to 1664 km2. Watershed land cover ranges from 6% to 81% forest, 1% to 64% cropland, 2% to 38% grassland, and 0.3% to 89% developed land. Landscape analyses were applied to develop a number of subestuary metrics (such as subestuary area, mouth width, elongation ratio, fractal dimension of shoreline, and the ratio of local watershed area to subestuary area) and watershed metrics (such as watershed area). Using mapped data from aerial SAV surveys, we calculated SAV coverage for each subestuary in each year during 1984–2003 as a proportion of potential SAV habitat (the area < 2 m deep). The variation in SAV abundance among subestuaries was strongly linked with subestuary and watershed characteristics. A regression tree model indicated that 60% of the variance in SAV abundance could be explained by subestuary fractal dimension, mean tidal range, local watershed dominant land cover, watershed to subestuary area ratio, and mean wave height. Similar explanatory powers were found in wet and dry years, but different independent variables were used. Repeated measures ANOVA with multiple-mean comparison showed that SAV abundance declined with the dominant watershed land cover in the order: forested, mixed-undisturbed, or mixed-developed > mixed-agricultural > agricultural > developed. Change-point analyses indicated strong threshold responses of SAV abundance to point source total nitrogen and phosphorus inputs, the ratio of local watershed area to subestuary area, and septic system density in the local watershed.  相似文献   

17.
Aerial surveys were conducted in the lower Chesapeake Bay during 1986–1989 to estimate abundance and examine the distribution of the cownose ray,Rhinoptera bonasus, during its seasonal residence, May–October. Most of the survey effort was concentrated in the lower and mid-bay regions. Cownose rays appeared uniformly distributed across the bay during mid-summer, but were more abundant in the eastern portion of the bay during migration. North-south distribution varied and reflected the general seasonal migration pattern. Mean abundance increased stepwise monthly from June through September and declined dramatically in October with their emigration from the bay. Abundance estimates from individual surveys varied. The greatest range of individual survey abundance estimates occurred in September (0–3.7×107 cownose rays0 due to high variation in school size and abundance between surveys. Monthly mean cownose ray abundance ranged from 0 in May and November to an estimated maximum of 9.3×106 individuals in September. The magnitude of the population suggests that the cownose ray plays an important role in the trophic dynamics of the Chesapeake Bay ecosystem. The historical data were insufficient to determine whether the population has increased, but these surveys provided the baseline data which would allow future investigation of cownose ray population dynamics in lower Chesapeake Bay.  相似文献   

18.
The exotic freshwater clam speciesCorbicula fluminea (Asiatic clam) was first reported in the tidal freshwater Potomac estuary near Washington, D.C., in 1977, and was found in benthic surveys, conducted in 1978, 1982, 1984, 1986, and 1992. In 1981 a tripling of water clarity was reported in the region of the clam beds, followed in 1983 by reapperance of submerged aquatic vegetation (SAV) absent for 50 yr. Submerged aquatic vegetation (SAV) has been surveyed and mapped over the entire Potomac estuary region in almost every year from 1976 to 1993 by aerial photography, as part of the United States Environmental Protection Agency's Chesapeake Bay program. Fish surveys in 1986 found populations increased up to 7× in beds of SAV. Starting in 1984, the Washington, D.C. Christmas Bird Census reported significant increases in several aquatic bird populations both nonmigratory and migratory. An extensive benthic survey in September 1986 estimated a spring-summer population of 8.7×106 kg Asiatic clams (wet weight including shell) in the 5-km region of the Potomac below Washington, D.C. This population was calculated as having the capacity to filter one-third to all of the water in this region of the estuary daily, depending on river flow. The 1986 clam population was smaller than that of 1984 and the 1992 population was 25% of that in 1986. Since 1986, SAV acreage has been decreasing in this area of the Potomac. Aquatic bird populations have declined. Yearly nuisance algae (Microcystis) blooms, which had been absent since 1983, reappeared in 1993. This paper presents evidence to support the theory the invasive Asiatic clam population in the 10 km below Washington, D.C., was responsible for SAV resurgence through filtration affecting turbidity. It suggests the clam populations triggered system-level changes in biota, including increase and decrease in local Potomac estuary populations (SAV, bird, fish, algae) over 10 yr, from 1983 to 1993. Major changes in the Asiatic clam population took place approximately 2 yr before parallel changes in SAV acreage were observed.  相似文献   

19.
We developed a synthesis using diverse monitoring and modeling data for Mattawoman Creek, Maryland, USA to examine responses of this tidal freshwater tributary of the Potomac River estuary to a sharp reduction in point-source nutrient loading rate. Oligotrophication of these systems is not well understood; questions concerning recovery pathways, threshold responses, and lag times remain to be clarified and eventually generalized for application to other systems. Prior to load reductions Mattawoman Creek was eutrophic with poor water clarity (Secchi depth <0.5 m), no submerged aquatic vegetation (SAV), and large algal stocks (50–100 μg L?1 chlorophyll-a). A substantial modification to a wastewater treatment plant reduced annual average nitrogen (N) loads from 30 to 12 g N m?2 year?1 and phosphorus (P) loads from 3.7 to 1.6 g P m?2 year?1. Load reductions for both N and P were initiated in 1991 and completed by 1995. There was no trend in diffuse N and P loads between 1985 and 2010. Following nutrient load reduction, NO2?+?NO3 and chlorophyll-a decreased and Secchi depth and SAV coverage and density increased with initial response lag times of one, four, three, one, and one year, respectively. A preliminary N budget was developed and indicated the following: diffuse sources currently dominate N inputs, estimates of long-term burial and denitrification were not large enough to balance the budget, sediment recycling of NH4 was the single largest term in the budget, SAV uptake of N from sediments and water provided a modest seasonal-scale N sink, and the creek system acted as an N sink for imported Potomac River nitrogen. Finally, using a comparative approach utilizing data from other shallow, low-salinity Chesapeake Bay ecosystems, strong relationships were found between N loading and algal biomass and between algal biomass and water clarity, two key water quality variables used as indices of restoration in Chesapeake Bay.  相似文献   

20.
The loss of submerged aquatic vegetation (SAV) from the Patuxent estuary during the latter part of the 20th century was explored using diverse data sets that included historic SAV coverage and distribution data, SAV ground truth observations, water clarity and nutrient loading data, and epiphyte light attenuation measurements. Analysis of aerial photography from 1952 showed that SAV was abundant and widely distributed along the entire mesohaline region of the estuary; by the late 1960s rapid declines in SAV took place following large increases in nutrient loading to the estuary. An examination of water clarity and epiphyte data suggest that the processes that led to the loss of SAV varied in strength along the axis of the estuary. In the upper mesohaline region, Secchi depths were consistently less than established mesohaline SAV habitat requirements at 1-m water depth, suggesting that water clarity was responsible for SAV decline. In the lower mesohaline region, where water clarity was consistently above SAV requirements, high epiphyte fouling rates significantly reduced light available to SAV. Experimental results show that epiphyte fouling had the capacity to reduce available light to SAV blades from 30% to 7% of surface light within a week, and likely contributed to the local decline and near total loss of SAV during the late 1960s and early 1970s. The prognosis for near-term SAV recovery within the mesohaline portion of the estuary seems unlikely given existing water quality conditions.  相似文献   

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