Submerged Aquatic Vegetation Beds Research Articles

Mapping Submerged Aquatic Vegetation along the Central Vietnamese Coast Using Multi-Source Remote Sensing

Submerged aquatic vegetation (SAV) in the Khanh Hoa (Vietnam) coastal area plays an important role in coastal communities and the marine ecosystem. However, SAV distribution varies widely, in terms of depth and substrate types, making it difficult to monitor using in-situ measurement. Remote sensing can help address this issue. High spatial resolution satellites, with more bands and higher radiometric sensitivity, have been launched recently, including the Vietnamese Natural Resources, Environment, and Disaster Monitoring Satellite (VNREDSat-1) (V1) sensor from Vietnam, launched in 2013. The objective of the study described here was to establish SAV distribution maps for South-Central Vietnam, particularly in the Khanh Hoa coastal area, using Sentinel-2 (S2), Landsat-8, and V1 imagery, and then to assess any changes to SAV over the last ten years, using selected historical data. The satellite top-of-atmosphere signals were initially converted to radiance, and then corrected for atmospheric effects. This treated signal was then used to classify Khanh Hoa coastal water substrates, and these classifications were evaluated using 101 in-situ measurements, collected in 2017 and 2018. The results showed that the three satellites could provide high accuracy, with Kappa coefficients above 0.84, with V1 achieving over 0.87. Our results showed that, from 2008 to 2018, SAV acreage in Khanh Hoa was reduced by 74.2%, while gains in new areas compensated for less than half of these losses. This is the first study to show the potential for using V1 and S2 data to assess the distribution status of SAV in Vietnam, and its outcomes will contribute to the conservation of SAV beds, and to the sustainable exploitation of aquatic resources in the Khanh Hoa coastal area.

Habitat Associations and Co‐Occurrence Patterns of Two Estuarine‐Dependent Predatory Fishes

AbstractEstuarine‐dependent fishes experience a wide range of environmental conditions, and most species exhibit distinct associations with particular habitats. However, similar species or multiple conspecifics often overlap spatiotemporally, which can result in ecological interactions that have consequences for behaviors that can shape the structure and function of ecosystems. We used a long‐term gill‐net data set (2001–2015) to investigate the habitat associations and co‐occurrence patterns of two estuarine‐dependent predatory fishes, Red Drum Sciaenops ocellatus and Spotted Seatrout Cynoscion nebulosus, in coastal Alabama, USA. Both species were associated with similar environmental conditions, primarily low dissolved oxygen and low salinity, especially when temperature was low. However, differences emerged between the species with respect to the effects of interacting environmental variables on their habitat use patterns, which were likely driven by physiological, biological, and ecological dissimilarities between them. Concerning their biogenic habitat use, extensive submerged aquatic vegetation (SAV) was an important habitat for both species, but Spotted Seatrout appeared to prefer high‐salinity SAV beds, while Red Drum associated with SAV regardless of salinity. Spotted Seatrout were associated with extensive emergent marsh edges, and the positive relationship between Red Drum and SAV was diminished when marsh edge was abundant. Co‐occurrence was observed primarily in habitats with which both species were associated, most frequently in shallow, prey‐rich marsh edges and high‐salinity seagrass beds. These observed habitat use patterns elucidate the subtle differences in resource use that allow these species to coexist and suggest potential areas where interactions between them may shape their roles as predators.

Submerged aquatic vegetation cover and complexity drive crustacean zooplankton community structure in a large fluvial lake: An in situ approach

Submerged aquatic vegetation (SAV) is considered an important driver of the zooplankton community, owing to the refuge offered by its structural complexity. However, non-destructive in situ approaches to assess quantitatively the features of SAV beds and their effects on zooplankton community are still lacking. This study aims to: 1) estimate SAV cover and complexity using subaquatic upward photographs (SUP) and 2) analyse the relationship between SAV variables and crustacean zooplankton composition and functional diversity (FD). SAV biomass and zooplankton were sampled at 52 stations in a large fluvial lake. Two metrics based on SUP were used to quantify SAV cover and complexity: 1) the percentage of the underwater landscape occupied by SAV (PLAND: SAV cover), and 2) its perimeter-area fractal dimension (PAFRAC: SAV complexity). Zooplankton composition and functional diversity were estimated based on crustacean species assemblages and functional traits. We used linear models to describe the relationships between SAV indices, environmental variables and zooplankton abundance, biomass and FD. SUP was an effective method to estimate in situ SAV cover and complexity. PLAND and PAFRAC were positively related to SAV biomass and zooplankton abundance, yielding a non-destructive assessment of the SAV-zooplankton relationships. Our study partially supports the habitat complexity-ecological niche availability hypothesis, as SAV cover and complexity influenced zooplankton functional groups and diversity indices. However, water transparency and depth also had important effects in interaction with PLAND and PAFRAC, and it remains difficult to fully disentangle the effects of SAV complexity from SAV cover on crustacean zooplankton community structure.

Herbivory regulates the establishment of a native species of submerged aquatic vegetation (SAV) in a tidal estuary of the USA

Herbivores are a diverse group of fauna that shape the distribution and composition of plant communities. In some cases, herbivory may prevent the re-establishment of submerged aquatic vegetation (SAV), such as Vallisneria americana, into systems. The goal of this study was to investigate the role and nature of herbivory on V. americana transplants with camera and transect surveys of grazing intensity and with field and laboratory grazing experiments using a suspected herbivore, the blue crab, Callinectes sapidus. Camera surveys recorded C. sapidus clipping and consuming shoots of V. americana for the first time. Grazing intensity surveys in low-salinity regions of the lower Chesapeake Bay indicated that the majority of V. americana transplants (50–75%) were clipped off at their base within one week of planting. Field and laboratory experiments demonstrated that C. sapidus clips and consumes V. americana as well as other rapidly colonizing, non-native SAV. Analysis of the gut contents of C. sapidus caught in SAV beds in the Chesapeake Bay revealed that SAV comprised 16% of their stomach contents, suggesting low levels of C. sapidus herbivory occurred over a wide area. Callinectes sapidus is yet another animal documented to consume SAV for some portion of their diet. These results also suggest that herbivores or omnivores, including C. sapidus, can serve as bottlenecks to recovery of SAV, like V. americana, in some areas. Herbivores may not serve as bottlenecks in other environments or to other SAV with more rapid plant growth or higher recruitment levels that may overcome grazing pressure.

Effects of Nonnative Eurasian Watermilfoil, Myriophyllum spicatum, on Nekton Habitat Quality in a Louisiana Oligohaline Estuary

Submerged aquatic vegetation (SAV) provides critical habitat for estuarine nekton in the Gulf of Mexico, but habitat quality of SAV beds may change when nonnative species, such as Eurasian watermilfoil (Myriophyllum spicatum), become established. We compared the habitat value of Myriophyllum with another common native SAV (Ruppia maritima) by using field collections to document shifts in nekton community structure and a field experiment to compare growth rates of commercially important juvenile white shrimp (Litopenaeus setiferus). Similar communities were collected from both SAV species, though nekton abundance scaled with SAV biomass. The habitat quality provided by Myriophyllum for white shrimp appeared to meet or exceed that of Ruppia, with densities and growth rates of shrimp in Myriophyllum (2.2 ± 0.47 m−2, 1.0 ± 0.07 mm TL day−1, 25.7 ± 2.98 mg day−1) higher than in Ruppia (1.0 ± 0.36 m−2, 0.6 ± 0.09 mm TL day−1, 11.6 ± 2.04 mg day−1). Though differences were detected between SAV species, other factors, such as hypoxia and interspecific competition, likely contributed to the differences we documented between these habitat types in shrimp densities and growth. Our study provides an example of a nonnative habitat-forming species providing a viable alternative to native habitat. This result contradicts the paradigm of negative effects associated with nonnative species following their introduction into an ecosystem.

In Situ Effects of Shoreline Type and Watershed Land Use on Submerged Aquatic Vegetation Habitat Quality in the Chesapeake and Mid-Atlantic Coastal Bays

Submerged aquatic vegetation (SAV) is an ecologically and economically valuable component of coastal estuaries that acts as an early indicator of both degrading and improving water quality. This study aimed to determine if shoreline hardening, which is associated with increased population pressure and climate change, acts to degrade SAV habitat quality at the local scale. In situ comparisons of SAV beds adjacent to both natural and hardened shorelines in 24 subestuaries throughout the Chesapeake and Mid-Atlantic Coastal Bays indicated that shoreline hardening does impact adjacent SAV beds. Species diversity, evenness, and percent cover were significantly reduced in the presence of riprap revetment. A post hoc analysis also confirmed that SAV is locally affected by watershed land use associated with increased population pressure, though to a lesser degree than impacts observed from shoreline armoring. When observed over time, SAV recovery at the local level took approximately 3 to 4 years following storm impacts, and SAV adjacent to natural shorelines showed more resilience to storms than SAV adjacent to armored shorelines. The negative impacts of shoreline hardening and watershed development on SAV shown here will inform coastal zone management decisions as increasing coastal populations and sea level rise drive these practices.

Does the relative value of submerged aquatic vegetation for penaeid shrimps vary with proximity to a tidal inlet? Preliminary evidence from a subtropical coastal lagoon

The value of submerged aquatic vegetation (SAV) as a nursery habitat for penaeid shrimp has been recognised previously; however, the importance of SAV in terms of its distance from a tidal inlet (a site through which shrimp postlarvae migrate into coastal lagoons) has not been evaluated. In the present study, the effect of proximity to a tidal inlet on the relative importance of SAV beds for Farfantepenaeus shrimp was evaluated in the Laguna Madre of Tamaulipas (Mexico). Sampling was performed monthly from February to December 2005, diurnally and nocturnally, in two SAV bed areas, one 25km (distant) and the other 1km (nearby) from the inlet. Densities of shrimp (F. aztecus, F. duorarum and F. brasiliensis) were consistently higher in the nearby SAV bed during both the day and night. Water temperature was the most important explanatory variable of shrimp abundance in both SAV beds. The temperature–abundance relationship was negative in all cases, except in the distant SAV bed at night, when shrimp abundance was positively related to dissolved oxygen and salinity. The nearby SAV bed had higher recruit abundance and supported a greater numbers of juveniles and subadults. In contrast, shrimp abundance in the distant SAV bed was apparently limited by recruitment.

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.

The Relationship Between Shoreline Armoring and Adjacent Submerged Aquatic Vegetation in Chesapeake Bay and Nearby Atlantic Coastal Bays

Shoreline armoring is an ancient and globally used engineering strategy to prevent shoreline erosion along marine, estuarine, and freshwater coastlines. Armoring alters the land water interface and has the potential to affect nearshore submerged aquatic vegetation (SAV) by changing nearshore hydrology, morphology, water clarity, and sediment composition. We quantified the relationships between the condition (bulkhead, riprap, or natural) of individual shoreline segments and three measures of directly adjacent SAV (the area of potential SAV habitat, the area occupied by SAV, and the proportion of potential habitat area that was occupied) in the Chesapeake Bay and nearby Atlantic coastal bays. Bulkhead had negative relationships with SAV in the polyhaline and mesohaline zones. Salinity and watershed land cover significantly modified the effect of shoreline armoring on nearshore SAV beds, and the effects of armoring were strongest in polyhaline subestuaries with forested watersheds. In high salinity systems, distance from shore modified the relationship between shoreline and SAV. The negative relationship between bulkhead and SAV was greater further off shore. By using individual shoreline segments as the study units, our analysis separated the effects of armoring and land cover, which were confounded in previous analyses that quantified average armoring and SAV abundance for much larger study units (subestuaries). Our findings suggest that redesigning or removing shoreline armoring structures may benefit nearshore SAV in some settings. Because armoring is ubiquitous, such information can inform efforts to reverse the global decline in SAV and the loss of the ecosystem services that SAV provides.

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.

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.

Fish abundances in shoreline habitats and submerged aquatic vegetation in a tidal freshwater embayment of the Potomac River

Submerged aquatic vegetation (SAV) is considered an important habitat for juvenile and small forage fish species, but many long-term recruitment surveys do not effectively monitor fish communities in SAV. To better understand the impact of recent large increases of SAV on the fish community in tidal freshwater reaches of the Potomac River, we compared traditional seine sampling from shore with drop ring sampling of SAV beds (primarily Hydrilla) in a shallow water (depths, <1.5m) embayment, Gunston Cove. To accomplish this, we developed species-specific catch efficiency values for the seine gear and calculated area-based density in both shoreline and SAV habitats in late summer of three different years (2007, 2008, and 2009). For the dominant species (Fundulus diaphanus, Lepomis macrochirus, Etheostoma olmstedi, Morone americana, Lepomis gibbosus, and Fundulus heteroclitus), density was nearly always higher in SAV, but overall, species richness was highest in shoreline habitats sampled with seines. Although historical monitoring of fish in Gunston Cove (and throughout Chesapeake Bay) is based upon seine sampling (and trawl sampling in deeper areas), the high densities of fish and larger areal extent of SAV indicated that complementary sampling of SAV habitats would produce more accurate trends in abundances of common species. Because drop ring samples cover much less area than seines and may miss rare species, a combination of methods that includes seine sampling is needed for biodiversity assessment. The resurgence of SAV in tidal freshwater signifies improving water quality, and methods we evaluated here support improved inferences about population trends and fish community structure as indicators of ecosystem condition.

Distribution ofFarfantepenaeus aztecusandF. duorarumon submerged aquatic vegetation habitats along a subtropical coastal lagoon (Laguna Madre, Mexico)

The spatial distribution ofFarfantepenaeusshrimp was analysed in the Laguna Madre of Tamaulipas, Mexico. Sampling was carried out on submerged aquatic vegetation (SAV) habitats at five sites located along the coastal lagoon. Two nocturnal surveys were conducted during winter in 2005 (January–February), collecting a total of 3268 shrimp individuals. SAV beds were composed of a mixture of drift algae (mainlyDigenia simplex), attached algae (mainlyPenicillus capitatusandUdotea occidentalis) and seagrass (mainlyHalodule wrightii).Farfantepenaeus aztecuswas more abundant (39.5%) thanF. duorarum(36.8%), and the remaining 23.7% corresponding to small unidentifiedFarfantepenaeusspp. were classified as recruits. Abundance ofF. aztecuswas significantly higher at sites 2 and 4, whereasF. duorarumdid not show significant distribution differences along the Laguna Madre. Recruits, juveniles and total shrimp tended to decrease significantly at the northern part of the lagoon (site 1), where substrate was dominated by drifting algae and seagrasses were scarce or absent. The abundance of shrimp was positively related to seagrass biomass and/or water temperature, whereas there was a negligible or negative relationship with algal biomass. With the exception of subadults, a significant positive linear relationship between seagrass and shrimp abundance was fitted, indicating an increase in number of individuals of both species with increasing seagrass biomass. This suggests that seagrass is the most important component of SAV beds influencing the abundance ofF. aztecusandF. duorarumalong this hypersaline coastal lagoon.

Mute Swans’ Impact on Submerged Aquatic Vegetation in Chesapeake Bay

Abstract: Mute swans (Cygnus olor) are poorly studied despite their potential to impact submerged aquatic vegetation (SAV). We measured vegetation characteristics (i.e., percent cover, shoot density, and canopy ht) of SAV beds in controls (unfenced), 2‐year exclosures, and 1‐year exclosures at 18 sites in the Chesapeake Bay, Maryland, USA, to quantify the impact of herbivory by mute swans on SAV during 2003 and 2004. Mute swan herbivory had a substantial adverse impact on percent cover, shoot density, and canopy height of SAV. At the end of the study mean percent cover, shoot density, and canopy height in the controls were lower by 79%, 76%, and 40%, respectively, as compared to those in 2‐year exclosures. During 2004, percent cover, shoot density, and canopy height increased by 26%, 15%, and 22%, respectively, between early and late seasons of SAV growth in exclosures, but they decreased by 36%, 41%, and 18%, respectively, in the controls. Paired mute swans predominantly occupied 6 of 7 moderate‐depth sites (0.76‐0.99 m), and these sites experienced less (i.e., 32‐75%) SAV reduction. All (n = 7) shallow water sites (0.50‐0.75 m) were predominantly occupied by mute swan flocks, and percent cover reduction of SAV was as high as 75‐100% at these sites. Mute swan flocks also predominantly occupied 3 of the 5 deep‐water sites (≥1 m) and 1 of 7 moderate‐depth sites, wherein we recorded considerable (i.e., 77‐93%) SAV reduction. Thus, considering that flocks are more detrimental to SAV as compared to paired mute swans, we recommend that initial emphasis primarily be placed on controlling mute swans in flocks and secondarily on pairs.

Temporal Changes of Populations and Trophic Relationships of Wintering Diving Ducks in Chesapeake Bay

Population and trophic relationships among diving ducks in Chesapeake Bay are diverse and complex as they include five species of bay ducks (Aythya spp.), nine species of seaducks (Tribe Mergini), and the Ruddy Duck (Oxyura jamaicensis). Here we considered the relationships between population changes and diet over the past half century to assess the importance of prey changes to wintering waterfowl in the Bay. Food habits of 643 diving ducks collected from Chesapeake Bay during 1999-2006 were determined by analyses of their gullet (esophagus and proventriculus) and gizzard contents and compared to historical data (1885-1979) of 1,541 diving ducks. Aerial waterfowl surveys, in general, suggest that six species of seaducks were more commonly located in the meso- to polyhaline areas of the Bay, whereas five species of bay ducks and Ruddy Ducks were in the oligo- to mesohaline areas. Seaducks fed on a molluscan diet of Hooked Mussel (Ischadium recurvum), Amethyst Gemclam (Gemma gemma), and Dwarf Surfclam (Mulinia lateralis). Bay ducks and Ruddy Ducks fed more on Baltic Macoma (Macoma balthica), the adventive Atlantic Rangia (Rangia cuneata), and submerged aquatic vegetation (SAV). Mergansers were found over the widest salinity range in the Bay, probably because of their piscivorous diet. Each diving duck species appears to fill a unique foraging niche, although there is much overlap of selected prey. When current food habits are compared to historic data, only the Canvasback (Aythya valisineria) has had major diet changes, although SAV now accounts for less food volume for all diving duck species, except the Redhead (Aythya americana). Understanding the trophic-habitat relationships of diving ducks in coastal wintering areas will give managers a better understanding of the ecological effects of future environmental changes. Intensive restoration efforts on SAV and oyster beds should greatly benefit diving duck populations.

Spatial distribution and structure along ecological gradients: penaeid shrimps in a tropical estuarine habitat of Mexico

The spatial structure of salinity, submerged aquatic vegetation (SAV) and 4 shrimp (Far- fantepenaeus spp.) populations along an estuarine habitat was evaluated seasonally, based on data gathered every 1 km along a 1-dimensional transect in Celestun Lagoon, Mexico. Geostatistical tech- niques were used to evaluate autocorrelation among data points along the transect, and to predict their spatial distribution by kriging. The estuarine habitat was characterized by persistent salinity gra- dients, decreasing from the seaward (1 km) to the inner (18 km) zone of the lagoon. SAV biomass was almost always spatially structured and best described by spherical variograms. Spatial partitioning be- tween SAV species was evident: the seagrasses Halodule wrightii and Ruppia maritima dominated the seaward and central zones of the lagoon, respectively, whereas the green alga Chara fibrosa was con- strained to the inner zone. Shrimp abundance also presented a marked spatial structure best described by spherical variograms. Spatial structure of shrimps, discriminated by species (F. aztecus, F. brasilien- sis, F. duorarum and F. notialis) and population components (recruits, juveniles, subadults) was most evident in the Nortes season (i.e. the season of strong northerly winds) and poorly defined in the Dry season. Spatial partitioning among species and population components was not detectable. Total shrimps and recruits were generally most abundant close to the mouth of the lagoon, although juvenile and subadult abundance peaked farther from the mouth than recruits. Distance from the mouth was the most important factor determining the spatial distribution of shrimps along the estuarine habitat. At local scales, abundance and type of SAV beds were crucial factors determining the abundance and spatial distribution of shrimps. Analysis at different spatial scales allowed us to identify the importance of spatial structure in biotic and abiotic variables of this estuarine habitat.

Shallow water as a refuge habitat for fish and crustaceans in non-vegetated estuaries: an example from Chesapeake Bay

Abundances and size-frequency distributions of common epibenth~c flsh and crustaceans were compared among 3 depth zones (1-35, 35-70, 71-95 cm) of the Rhode River, a subestuary of Chesapeake Bay, USA. In the absence of submerged aquatic vegetation (SAV), interand intraspccific size segregation occurred by depth from May to October, 1989-1992. Small species (Palaemonetes pugjo, Crangon septernspjnosa, Fundulus heteroclitus, F majaljs, Rhithropanope~ls harrisii, Apeltes quadracus, Gobiosorna boscj) were most abundant at water depths 70 cm). In field experiments, mortality of tethered P pugio (30 to 35 mm), small E heteroclitus (40 to 50 mm), and small C. sapjdus (30 to 70 mm) increased significantly with depth. Wc hypothesize that predation risk was size-dependent, creating the observed intraand interspecific size differences among depth zones. For C. sep.temspinosa, burial may modify this size-dc>pendency and create the unusual absence of intraspecific size increase with depth. Historically, f? pugio and Fundulus spp. (and other small species) were not restricted to shallow (<70 cm) waters and were abundant in deeper SAV beds, which provided a structural refuge from predators. Since the recent demise of SAV in Chesapeake Bay, our results indicate many small species have shifted their distributions and now utilize primarily shallow water as an alternate refuge habitat.