Submerged Aquatic Vegetation Coverage Research Articles

The Implicit Price of Aquatic Grasses

Submerged aquatic vegetation (SAV) in the Chesapeake Bay is well below half of its historic levels, largely due to excessive nutrient and sediment loads degrading water quality. SAV provides important ecosystem functions, many of which are beneficial to local residents. To understand the implicit value residents place on SAV and the ecosystem services it provides, we undertake a hedonic property value study using residential transactions in 11 Maryland counties adjacent to the Chesapeake Bay. These data are matched to high-resolution maps of SAV coverage. We pose a quasi-experimental comparison and examine how the prices of homes near the waterfront vary with the presence of SAV. On average, waterfront and near-waterfront homes within 200 meters of the shore sell at about a 6.5% premium when SAV is present. Applying these estimates to the 185,000-acre SAV attainment goal suggests total property value gains on the order of $436 million.

Does submerged aquatic vegetation shape zooplankton community structure and functional diversity? A test with a shallow fluvial lake system

Submerged aquatic vegetation (SAV) plays important roles in shallow lakes. In addition to its refuge effect for zooplankton, one key role of SAV is to provide diverse ecological niches to these organisms. The reduction of habitat complexity due to loss of SAV might thus have huge effects on zooplankton communities. The objective of this study was to investigate the relationship between SAV abundance and composition and zooplankton functional diversity and community structure. We used as model system the littoral zone of Lake St. Pierre (Quebec, Canada), a shallow fluvial lake experiencing dramatic changes in SAV cover. Our sampling protocol allowed us to analyse the relationship between SAV and zooplankton along a gradient of SAV abundance. We showed that SAV abundance explained 41% of the variation in the zooplankton community structure and 25% of the variation in zooplankton functional diversity. Our results also indicated that the presence of the benthic cyanobacterium Gloeotrichia sp. in SAV beds had a structuring effect within the dominant family of Chydoridae, being negatively correlated to the abundance of relatively large-bodied species. Our findings suggest that loss in SAV biomass and complexity can affect both community structure and functional diversity of zooplankton in shallow fluvial lakes.

The Effect of Submerged Aquatic Vegetation Expansion on a Declining Turbidity Trend in the Sacramento-San Joaquin River Delta

Submerged aquatic vegetation (SAV) has well-documented effects on water clarity. SAV beds can slow water movement and reduce bed shear stress, promoting sedimentation and reducing suspension. However, estuaries have multiple controls on turbidity that make it difficult to determine the effect of SAV on water clarity. In this study, we investigated the effect of primarily invasive SAV expansion on a concomitant decline in turbidity in the Sacramento-San Joaquin River Delta. The objective of this study was to separate the effects of decreasing sediment supply from the watershed from increasing SAV cover to determine the effect of SAV on the declining turbidity trend. SAV cover was determined by airborne hyperspectral remote sensing and turbidity data from long-term monitoring records. The turbidity trends were corrected for the declining sediment supply using suspended-sediment concentration data from a station immediately upstream of the Delta. We found a significant negative trend in turbidity from 1975 to 2008, and when we removed the sediment supply signal from the trend it was still significant and negative, indicating that a factor other than sediment supply was responsible for part of the turbidity decline. Turbidity monitoring stations with high rates of SAV expansion had steeper and more significant turbidity trends than those with low SAV cover. Our findings suggest that SAV is an important (but not sole) factor in the turbidity decline, and we estimate that 21–70 % of the total declining turbidity trend is due to SAV expansion.

Long‐term spatial dynamics in vegetated seascapes: fragmentation and habitat loss in a human‐impacted subtropical lagoon

AbstractVegetated coastal seascapes exhibit dynamic spatial patterning, some of which is directly linked to human coastal activities. Human activities (e.g. coastal development) have modified freshwater flow to marine environments, resulting in significant changes to submerged aquatic vegetation (SAV) communities. Yet, very little is known about the spatially complex process of SAV habitat loss and fragmentation that affects ecosystem function. Using habitat mapping from aerial photography spanning 71 years (1938–2009) for Biscayne Bay (Florida, USA), we quantify both SAV habitat loss and fragmentation using a novel fragmentation index. To understand the influence of water management practices on SAV seascapes, habitat loss and fragmentation were compared between nearshore and offshore locations, as well as locations adjacent to and distant from canals that transport freshwater into the marine environment. Habitat loss and fragmentation were significantly higher along the shoreline compared with offshore seascapes. Nearshore habitats experienced a net loss of 3.31% of the total SAV mapped (2.57 km2) over the time series. While areas adjacent to canals had significantly higher SAV cover, they still experienced wide fluctuations in cover and fragmentation over time. All sites exhibited higher fragmentation in 2009 compared with 1938, with four sites exhibiting high fragmentation levels between the 1990s and 2000s. We demonstrate that freshwater inputs into coastal bays modify the amount of SAV and the fragmentation dynamics of SAV habitats. Spatial changes are greater close to shore and canals, indicating that these coastal developments have transformative impacts on vegetated habitats, with undetermined consequences for the provisioning of ecosystem goods and services.

Magnitude and Patterns of Change in Submerged Aquatic Vegetation of the Tidal Freshwater Hudson River

Three aerial photography inventories were used to examine change in submerged aquatic vegetation (SAV) in the tidal freshwater Hudson River over the interval 1997 to 2007. Overall, there was about a 30 % net decline in SAV coverage although there were also many individual areas of expansion. The invasive water chestnut (Trapa natans) did not change appreciably in net cover over the interval, and there was replacement of SAV by water chestnut along with slightly fewer cases of SAV replacing the exotic. A fine-scale (100 m by 100 m quadrats) analysis showed that about 30 % of quadrats that supported vegetation changed by more than 10 % in plant cover and overall SAV was quite dynamic. SAV in the Hudson is limited by light which is in turn controlled by suspended sediment. SAV was rarely found at depths >1 m below low water, and interannual differences in clarity affected the ability of SAV beds to maintain locally supersaturated levels of dissolved oxygen. We found that location within the River channel (proximity to shore) influenced the magnitude and variability in change in SAV between census periods. The physical nature of the adjacent shoreline also affected the magnitude of change with greater declines in cover in areas next to hard-engineered shore types. SAV in the Hudson is highly dynamic, apparently quite resilient, and the control of light by suspended sediment rather than phytoplankton growth offers a contrast to eutrophication-influenced changes in other estuaries. Management and protection of SAV habitat must recognize the highly variable nature of plant cover and that absence in any particular year does not preclude future appearance of submerged plants at that location.

Predicting submerged aquatic vegetation cover and occurrence in a Lake Superior estuary

Submerged aquatic vegetation (SAV) provides the biophysical basis for multiple ecosystem services in Great Lakes estuaries. Understanding sources of variation in SAV is necessary for sustainable management of SAV habitat. From data collected using hydroacoustic survey methods, we created predictive models for SAV in the St. Louis River Estuary (SLRE) of western Lake Superior. The dominant SAV species in most areas of the estuary was American wild celery (Vallisneria americana Michx.). Maximum depth of SAV in 2011 was approximately 2.1m. In regression tree models, most of the variation in SAV cover was explained by an autoregression (lag) term, depth, and a measure of exposure based on fetch. Logistic SAV occurrence models including water depth, exposure, bed slope, substrate fractal dimension, lag term, and interactions predicted the occurrence of SAV in three areas of the St. Louis River with 78–86% accuracy based on cross validation of a holdout dataset. Reduced models, excluding fractal dimension and the lag term, predicted SAV occurrence with 75–82% accuracy based on cross validation and with 68–85% accuracy for an independent SAV dataset collected using a different sampling method. In one area of the estuary, the probability of SAV occurrence was related to the interaction of depth and exposure. At more exposed sites, SAV was more likely to occur in shallow areas than at less exposed sites. Our predictive models show the range of depth, exposure, and bed slope favorable for SAV in the SLRE; information useful for planning shallow-water habitat restoration projects.

Multi-decade Responses of a Tidal Creek System to Nutrient Load Reductions: Mattawoman Creek, Maryland USA

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.

海岸带地理特征对沉水植被丰度的影响

Submerged aquatic vegetation(SAV)consists of a taxonomically diverse group of plants that lives entirely beneath the water surface.SAV provides habitat and supplies food for aquatic life,absorbs excess nutrients,and helps purify the water.Since the 1960s,SAV coverage has experienced dramatic decline worldwide due to serious deterioration of water quality in coastal ecosystems. In recent years,landscape analyses have been used to predict direct or indirect effects of geographic characteristics on aquatic organisms.In this study,we analyzed the effects of coastal geographical characteristics(such as watershed land use types,sub-estuary mouth width,shoreline fractal dimension,ratio of watershed area/sub-estuary area,high resolution average wave height and estuary sediment,etc.) on the spatial variation of SAV abundance based on the long-term dataset(1984—2009) from 99 sub-estuaries and their linked coastal sub-watersheds of the Chesapeake Bay in the United States.Coastal watershed land use is an important factor influencing inputs of nutrient and sediment to its associated sub-estuary,and thus indirectly affects SAV abundance.Our results showed that sub-estuaries with different dominant land covers in each salinity regime had significantly different variation in SAV abundance.Abundance of different SAV species varies spatially among different salinity regimes in shallow water area.In each salinity regime,SAV abundance in sub-estuaries declined with their linked coastal dominant watershed land use type in the following order: mixed-land developed land or forested land agricultural land in fresh water(TF) regime;mixed-land or agricultural land developed land forested land in oligohaline(OH) regime;forested land or mixed-land developed land or agricultural land in mesohaline(MH) regime;mixed-land or forested land developed land agricultural land in polyhaline(PH) regime.Sub-estuaries with different salinity regimes for each dominant land cover also had significantly different levels of SAV abundance.In each dominant watershed land use type,SAV abundance in sub-estuaries declined with salinity regime in the following order: no significant difference among four salinity regimes for developed land;OH MH TF or PH for agricultural land;OH or PH TF or MH for mixed-land;MH or PH TF or OH for forested land.Relative weaker linear correlation was found between a single geographical characteristics and SAV abundance(-0.157≤r≤0.442),but SAV abundance had significantly positive correlations with a few estuary characteristics,such as sub-estuary mouth width(r=0.442,P0.001),shoreline fractal dimension(r=0.290,P=0.007) and average wave height(r=0.306,P=0.002).A wider sub-estuary mouth could lead more sea water to diluting pollutants in shorter time and thus favors SAV growth.A higher shoreline fractal dimension value represents more complex estuary structure and small tributaries in each sub-estuary that favors SAV growth as well.Higher wave height could positively dilute pollutants or negatively erode SAV beds in a sub-estuary.The integrated impact on SAV abundance was positive.Using the classification and regression tree(CART) model,we predicted that ratio of watershed area/sub-estuary area had the greatest impact on SAV abundance that appeared at the highest level of the tree,followed by shoreline fractal dimension,salinity regime,and average wave height.The four geographical variables explained 63% of the total spatial variation in SAV abundance across the Chesapeake Bay. Our findings imply that variation in the coastal geographical characteristics can indirectly affect spatial SAV abundance.Therefore,geographical characteristics in both sub-estuaries and their associated watersheds should be considered in making strategy of SAV recovery or protection policy at the regional scale in coastal area.

Submerged aquatic vegetation in the Bay of Quinte: Response to decreased phosphorous loading and Zebra Mussel invasion

Originally mesotrophic, the Bay of Quinte ecosystem has experienced eutrophication since the 1940s, which resulted in the decline of once-lush submerged aquatic vegetation (SAV) beds in the upper bay by the mid-1960s. Since 1972, twelve SAV surveys have been conducted along ten index transects, recording:% cover, distance SAV beds extended from shore (extent), maximum depth of colonization (Zc), species composition, and, in later years, wet plant biomass. Offshore secchi depth and ϵpar, (the vertical light extinction rate (m−1) for photosynthetically active radiation), were also recorded either weekly or bi-weekly during the growing season since 1972. During this time, two major changes occurred within the bay: the reduction in point-source phosphorus (P-control) loadings in 1978 and the 1993 invasion by Dreissenid Mussel. SAV response to these changes varied temporally and spatially, with the shallow upper bay showing the greatest response, particularly after Zebra Mussels establishment. In the upper bay, mean secchi depth increased by 8% from 1.2 m prior to P-control (pre-P), to 1.3 m after P-control (post-P) and further increased by 46% to 1.9 m after Dreissena establishment (post-D). Upper bay SAV responded to these invasive species with increases in the means of three variables: Zc from 1.6 to 3.5 m, extent from 114 m to 417 m and wet biomass from 50 g m−2 to 962 g m−2. SAV in the middle and lower bays were in better condition in 1972, with pre-P cover in excess of 50% and Zc of 2.6 and 3.7 m, respectively. SAV cover did increase in the post-D (1994 to 2007) period by approximately 25% and Zc increased to 3.7 and 6.5 m, but the narrow fringing strip of shallower water along the shore in these two deeper bays limited substantial increases in bed extent. Both water clarity and basin morphometry strongly influenced SAV distribution and abundance within the Bay of Quinte.

Quantifying environmental reservoirs of fecal indicator bacteria associated with sediment and submerged aquatic vegetation

Elevated concentrations of fecal indicator bacteria (FIB) in aquatic sediments and vegetation have prompted concern that environmental reservoirs of FIB disrupt the correlation between indicator organisms, pathogens and human health risks. FIB numbers, however, are typically normalized to volume of water or mass of substrate. Because these reservoirs tend to differ greatly in magnitude within and between water bodies, direct comparison between water column and benthic population sizes can be problematic. Normalization to a set volume of water or mass of substrate, e.g. cfu (100 ml)(-1) or cfu(100 g)(-1), can give a false picture of the relative contributions of various reservoirs to FIB numbers across the ecosystem, and of the potential for FIBs to trigger health advisories as they pass from one reservoir to another. Here, we normalized enterococci concentrations from water, sediment and submerged aquatic vegetation (SAV) to land surface area (m(2) ) to compare their relative importance in the entire system. SAV-associated enterococci comprised only 0-18% of the entire population, even though they displayed the highest concentrations of enterococci per unit mass. The largest proportion of the enterococci population was in the water column (4-77%) or sediments (20-95%), depending on the volume of each substrate available at a site and FIB concentrations within them. Models indicated that large shifts in the relative size of FIB populations in each substrate can result from changes in per cent SAV cover, water depth and depth of sediment colonization. It follows that high concentrations of FIB in sediments or SAV do not necessarily signify large environmental reservoirs of FIB that can affect the water column. Comprehensive analyses that include FIB measurements from water, SAV and sediment normalized to land surface area offer a more balanced perspective on total FIB numbers contained in various matrices of an aquatic system.

Long-term reductions in anthropogenic nutrients link to improvements in Chesapeake Bay habitat

Great effort continues to focus on ecosystem restoration and reduction of nutrient inputs thought to be responsible, in part, for declines in estuary habitats worldwide. The ability of environmental policy to address restoration is limited, in part, by uncertainty in the relationships between costly restoration and benefits. Here, we present results from an 18-y field investigation (1990-2007) of submerged aquatic vegetation (SAV) community dynamics and water quality in the Potomac River, a major tributary of the Chesapeake Bay. River and anthropogenic discharges lower water clarity by introducing nutrients that stimulate phytoplankton and epiphyte growth as well as suspended sediments. Efforts to restore the Chesapeake Bay are often viewed as failing. Overall nutrient reduction and SAV restoration goals have not been met. In the Potomac River, however, reduced in situ nutrients, wastewater-treatment effluent nitrogen, and total suspended solids were significantly correlated to increased SAV abundance and diversity. Species composition and relative abundance also correlated with nutrient and water-quality conditions, indicating declining fitness of exotic species relative to native species during restoration. Our results suggest that environmental policies that reduce anthropogenic nutrient inputs do result in improved habitat quality, with increased diversity and native species abundances. The results also help elucidate why SAV cover has improved only in some areas of the Chesapeake Bay.

Benthic Macrophyte Distribution and Abundance in Estuarine Mangrove Lakes and Estuaries: Relationships to Environmental Variables

Annual mean salinity, light availability, and sediment depth to bedrock structured the submerged aquatic vegetation (SAV) communities in subtropical mangrove-lined estuaries. Three distinct SAV communities (i.e., Chara group, Halodule group, and Low SAV coverage group) were identified along the Everglades–Florida Bay ecotone and related to water quality using a discriminant function model that predicted the type of plant community at a given site from salinity, light availability, and sediment depth to bedrock. Mean salinity alone was able to correctly classify 78% of the sites and reliably separated the Chara group from the Halodule group. The addition of light availability and sediment depth to bedrock increased model accuracy to 90% and further distinguished the Chara group from the Halodule group. Light availability was uniquely valuable in separating the Chara group from the Low SAV coverage group. Regression analyses identified significant relationships between phosphorus concentration, phytoplankton abundance, and light availability and suggest that a decline in water transparency, associated with increasing salinity, may have also contributed to the historical decline of Chara communities in the region. This investigation applies relationships between environmental variables and SAV distribution and provides a case study into the application of these general principals to ecosystem management.

Mapping large-scale distribution of submerged aquatic vegetation coverage using remote sensing

This study investigated the potential of using remote sensing for mapping and monitoring of submerged aquatic vegetation (SAV) on a large scale. The spectral characteristics of SAV with varied coverage were measured using a portable spectroradiometer on a man-made lake at the Chongming International Wetland Park, Shanghai, China. A good relationship between the coverage of SAV and their field spectral characteristics was established and the reflectance of SAV increased with its increasing coverage. A regression analysis was then carried out between the coverage and the reflectance at the wavelengths of four QuickBird bands. After making an atmospheric correction from a synchronous QuickBird image for the study site, the image digital number (DN) was converted into the ground reflectance. The reflectance image was then deduced into a distribution map of SAV coverage by using the results of the regression functions between the coverage of SAV and the reflectance rate measured in situ. An accuracy assessment indicated that this approach could be used to quickly monitor the distribution and growth situation of SAV. The implications of this observation, in terms of the ability of remote sensing to estimate and monitor the distribution and dynamics of SAV on a large scale are discussed.

Effects of watershed and estuarine characteristics on the abundance of submerged aquatic vegetation in Chesapeake Bay subestuaries

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 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.

Quantification of Historical Changes of Submerged Aquatic Vegetation Cover in Two Bays of Lake Ontario with Three Complementary Methods

Submerged aquatic vegetation (SAV) distribution and coverage were quantified in two bays of Lake Ontario in 1972, 1980 (1982), and 1999–2002, using a combination of aerial photograph interpretation (API), hydroacoustics, and rake sampling. The three methods gave similar estimates of SAV presence in 2002, supporting our use of API for quantifying SAV changes across decades in bays of a large lake. The SAV coverage in Sodus Bay increased by 5% between 1972 and 1980 and by 35% between 1980 and 1999–2002 whereas the maximum depth of SAV colonization extended from 5.5 to 6.4 m during this period. In Chaumont Bay, the SAV coverage tripled while its maximum depth of occurrence increased from 5.1 to 6.1 m from 1982 to 2002. Although the difference in SAV coverage between 1972 and 1980 was not larger than the difference between consecutive years in the 2000s, the large increase in SAV coverage between the 1980s and 2000s represents a major ecosystem change in these bays. This change was likely caused by increased water clarity in Lake Ontario, which could be associated with the implementation of the Great Lakes Water Quality Agreement (GLWQA) and the dreissenid mussel invasion. Although other factors such as water level, wave exposure, bottom slope, and sediment nutrients may be important, they have not changed in a fashion that would predict local increases of SAV coverage.

Assessment of changes in the seagrass-dominated submerged vegetation of tropical Chwaka Bay (Zanzibar) using satellite remote sensing

Spatial and temporal dynamics of submerged aquatic vegetation (SAV) cover were studied in the relatively pristine and seagrass-dominated area of Chwaka Bay, Zanzibar (Tanzania) by using satellite remote sensing. Through complementary field work the potential of the technique for change detection was verified. The general changes in SAV cover were examined using Landsat images from 1986, 1987, 1998, 2001 and 2003. Two of these images, from 1987 (Landsat TM) and 2003 (Landsat ETM+), were specifically analysed to create a map of the change in SAV cover. Overall, the general distribution of SAV stayed fairly stable over the period investigated, but the result also showed regions where significant alterations, both losses and gains, had occurred between the two years. Based on our findings and anecdotal information from local fishermen and seaweed farmers potential causative factors are discussed. It was concluded that a repeated mapping with satellite remote sensing is a suitable tool to monitor changes of seagrass and seaweed distribution in shallow tropical environments.

Effects of Exotic Submerged Aquatic Vegetation on Waterfowl in the Mobile-Tensaw Delta

Surveys conducted in the Mobile-Tensaw Delta, located in the Northern Gulf of Mexico, have documented a 96% decline in waterfowl populations from over 100,000 birds in 1939 to around 4,000 birds in 1999. Coincident with this decline has been the introduction and spread of nonnative Eurasian watermilfoil (Myriophyllum spicatum). Six surveys have documented the replacement of native wild celery (Vallisneria americana), the perceived preferred food for waterfowl, by M. spicatum as the dominant species of submerged aquatic vegetation (SAV) in this setting. Simple comparisons of SAV coverage and waterfowl surveys indicate that declines in waterfowl populations are not strongly related to invasion of M. spicatum. Stable isotope analysis of three species of waterfowl (Anas strepera, Anas fulvigula, and Aix sponsa) and their food sources show these waterfowl feed on both wild celery and milfoil. Isotopic signatures of animals living on these SAV were also in waterfowl tissues. Based on these two lines of evidence, it is unlikely that the invasion of milfoil, by itself, is responsible for waterfowl declines in this delta.

Macrofaunal distributions and habitat change following winter–spring releases of freshwater into the Breton Sound estuary, Louisiana (USA)

We examined the effect of freshwater inflows on the aquatic environment and macrofauna in the intermediate and brackish zones of the Breton Sound estuary. Following water releases from the Caernarvon Freshwater Diversion Structure in winter 2000 and spring 2001, we compared environmental conditions and the abundance and distribution of nekton in May 2001 between the inflow area, which receives freshwater directly from the structure, and a nearby reference area. We used these data and stable isotope analyses for C, N, and S in brown shrimp Farfantepenaeus aztecus and two species of grass shrimp ( Palaemonetes paludosus and Palaemonetes intermedius) to test four null hypotheses: (1) water quality and SAV (submerged aquatic vegetation) coverage were similar between the inflow and reference areas, (2) macrofaunal abundance and biomass were similar between the two areas, (3) stable isotopic values of brown shrimp and grass shrimp were similar between areas, habitat types, and species, and (4) brown shrimp distributions were unaffected by river inputs. Freshwater from the structure clearly influenced the estuarine environment within the inflow area. Releases from the Caernarvon structure freshened the inflow area as intended and increased SAV and daytime dissolved oxygen concentrations. The response by macrofauna to these increased freshwater flows and habitat changes involved mostly changes in density and biomass rather than shifts in species composition. Although we detected no strong effect of the freshwater diversion on brown shrimp abundance or size in the inflow area, results of the sulfur stable isotope analysis indicated that brown shrimp collected in the inflow area had been growing in higher salinity waters, possibly following downstream displacement by the diversion. Species that would benefit most from continued freshwater diversions are likely to be those species that both use SAV as nursery habitat and thrive in a low-salinity environment. Nutrients carried by water from the structure were incorporated into the estuarine food web, and these nutrient inputs, together with an increase in SAV habitat, may enhance overall secondary productivity in the inflow area.

An evaluation of glass prisms in boat docks to reduce shading of submerged aquatic vegetation in the lower St. Johns River, Florida

The effectiveness of glass prisms in boat docks was assessed to determine if shading impacts to submerged aquatic vegetation (SAV), primarilyVallisneria americana, were reduced. Six experimental docks, three with prisms and three without prisms, were constructed in the lower St. Johns River, Florida. SAV percent cover and photosynthetically active radiation (PAR) were monitored under each dock and in an adjacent control area with no experimental docks. Subsurface PAR during the growing season of the first year of the study was not significantly greater beneath docks having prisms than beneath docks without prisms. Postconstruction SAV monitoring (February 2000 to May 2002) revealed no significant differences in SAV percent cover between dock treatments, although coverage declined in both dock treatments and the control area. Declining water quality conditions at the study site clearly impacted the health of the SAV habitat as indicated by the decline in SAV coverage in the control area initially in the study. Given the subsequent resurgence of SAV in the control area, the additional light transmitted through the prisms did not appear to be biologically significant or adequate to counteract effects from larger-scale environmental stressors.

Sumberged aquatic vegetation in the mesohaline region of the Patuxent estuary: Past, present, and future status

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.