Decadal changes in seagrass distribution and abundance in Florida Bay

The Florida Bay ecosystem has changed substantially in the past decade, and alterations in the seagrass communities have been particularly conspicuous. In 1987 large areas ofThalassia testudinum (turtlegrass) began dying rapidly in western Florida Bay. Although the rate has slowed considerably, die-off continues in many parts of the bay. Since 1991, seagrasses in Florida Bay have been subjected to decreased light availability due to widespread, persistent microalgal blooms and resuspended sediments. In light of these recent impacts, we determined the current status of Florida Bay seagrass communities. During the summer of 1994, seagrass species composition, shoot density, shoot morphometrics, and standing crop were measured at 107 stations. Seagrasses had been quantified at these same stations 10 yr earlier by Zieman et al. (1989).T. testudinum was the most widespread and abundant seagrass species in Florida Bay in both 1984 and 1994, and turtlegrass distribution changed little over the decade. On a baywide basis,T. testudinum density and biomass declined significantly between surveys; mean short-shoot density ofT. testudinum dropped by 22% and standing crop by 28% over the decade.T. testudinum decline was not homogeneous throughout Florida Bay; largest reductions in shoot density and biomass were located principally in the central and western bay. Percent loss ofT. testudinum standing crop in western Florida Bay in 1994 was considerably greater at the stations with the highest levels of standing crop in 1984 (126–215 g dry wt m⁻²) than at the stations with lower levels of biomass. While turtlegrass distribution remained consistent over time, both the distribution and abundance of two other seagrasses,Halodule wrightii andSyringodium filiforme, declined substantially between 1984 and 1994. Baywide,H. wrightii shoot density and standing crop declined by 92%, andS. filiforme density and standing crop declined by 93% and 88%, respectively, between surveys. Patterns of seagrass loss in Florida Bay between 1984 and 1994 suggest die-off and chronic light reductions were the most likely causes for decline. If die-off and persistent water-column turbidity continue in Florida Bay, the long-term future of seagrasses in the bay is uncertain.     

Habitat Affects Survival of Translocated Bay Scallops, Argopecten irradians concentricus (Say 1822), in Lower Chesapeake Bay

Bay scallop (Argopecten irradians) populations existed in Chesapeake Bay until 1933, when they declined dramatically due to a loss of seagrass habitat. Since then, there have been no documented populations within the Bay. However, some anecdotal observations of live bay scallops within the lower Bay suggest that restoration of the bay scallop is feasible. We therefore tested whether translocated adults of the southern bay scallop, Argopecten irradians concentricus, could survive during the reproductive season in vegetated and unvegetated habitats of the Lynnhaven River sub-estuary of lower Chesapeake Bay in the absence of predation. Manipulative field experiments evaluated survival of translocated, caged adult scallops in eelgrass Zostera marina, macroalgae Gracilaria spp., oyster shell, and rubble plots at three locations. After a 3-week experimental period, scallop survival was high in vegetated habitats, ranging from 98% in their preferred habitat, Z. marina, to 90% in Gracilaria spp. Survival in Z. marina was significantly higher than that in rubble (76%) and oyster shell (78%). These findings indicate that reproductive individuals can survive in vegetated habitats of lower Chesapeake Bay when protected from predators and that establishment of bay scallop populations within Chesapeake Bay may be viable.     

Changes in the abundance of the seagrassesZostera marina L. (eelgrass) andRuppia maritima L. (widgeongrass) in San Diego, California, following and El Niño Event

Changes in environmental conditions can be accompanied by shifts in the distribution and abundances of organisms. When physical factors become unsuitable for growth ofZostera marina (eelgrass), which is a dominant seagrass species in North America, other more ruderal seagrass species, includingRuppia maritima (widgeongrass), often increase in abundance or replace the dominant species. We report the proliferation of widgeongrass into eelgrass beds in Mission Bay and San Diego Bay in San Diego, California, during the 1997 to 1998 El Niño Southern Oscillation (ENSO). Widgeongrass persisted in these eelgrass beds at least one year after a return to non-ENSO conditions and an increase in eelgrass density. We suggest that a warming of the water in two bays in San Diego by 1.5–2.5°C could result, in a permanent shift in the local seagrass vegetation from eelgrass to widgeongrass. This shift, could, have substantial ecosystem-level ramifications.     

Variations in water clarity and bottom albedo in Florida Bay from 1985 to 1997

Following extensive seagrass die-offs of the late 1980s and early 1990s, Florida Bay reportedly had significant declines in water clarity due to turbidity and algal blooms. Scant information exists on the extent of the decline, as this bay was not investigated for water quality concerns before the die-offs and limited areas were sampled after the primary die-off. We use imagery from the Advanced Very High Resolution Radiometer (AVHRR) to examine water clarity in Florida Bay for the period 1985 to 1997. The AVHRR provides data on nominal water reflectance and estimated light attenuation, which are used here to describe turbidity conditions in the bay on a seasonal basis. In situ observations on changes in seagrass abundance within the bay, combined with the satellite data, provide additional insights into losses of seagrass. The imagery shows an extensive region to the west of Florida Bay having increased reflectance and light attenuation in both winter and summer begining in winter of 1988. These increases are consistent with a change from dense seagrass to sparse or negligible cover. Approximately 200 km² of these offshore seagrasses may have been lost during the primary die-off (1988 through 1991), significantly more than in the bay. The imagery shows the distribution and timing of increased turbidity that followed the die-offs in the northwestern regions of the bay, exemplified in Rankin Lake and Johnson Key Basin, and indicates that about 200 km² of dense seagrass may have been lost or severely degraded within the bay from the start of the die-off. The decline in water clarity has continued in the northwestern bay since 1991. The area west of the Everglades National Park boundaries has shown decreases in both winter turbidity and summer reflectances, suggestive of partial seagrass recovery. Areas of low reflectance associated with a majorSyringodium filiforme seagrass meadow north of Marathon (Vaca Key, in the Florida Keys) appear to have expanded westward toward Big Pine Key, indicating changes in the bottom cover from before the die-off. The southern and eastern sections of the Bay have not shown significant changes in water clarity or bottom albedo throughout the entire time period.     

Patterns of seagrass community response to local shoreline development

Three quarters of the global human population will live in coastal areas in the coming decades and will continue to develop these areas as population density increases. Anthropogenic stressors from this coastal development may lead to fragmented habitats, altered food webs, changes in sediment characteristics, and loss of near-shore vegetated habitats. Seagrass systems are important vegetated estuarine habitats that are vulnerable to anthropogenic stressors, but provide valuable ecosystem functions. Key to maintaining these habitats that filter water, stabilize sediments, and provide refuge to juvenile animals is an understanding of the impacts of local coastal development. To assess development impacts in seagrass communities, we surveyed 20 seagrass beds in lower Chesapeake Bay, VA. We sampled primary producers, consumers, water quality, and sediment characteristics in seagrass beds, and characterized development along the adjacent shoreline using land cover data. Overall, we could not detect effects of local coastal development on these seagrass communities. Seagrass biomass varied only between sites, and was positively correlated with sediment organic matter. Epiphytic algal biomass and epibiont (epifauna and epiphyte) community composition varied between western and eastern regions of the bay. But, neither eelgrass (Zostera marina) leaf nitrogen (a proxy for integrated nitrogen loading), crustacean grazer biomass, epifaunal predator abundance, nor fish and crab abundance differed significantly among sites or regions. Overall, factors operating on different scales appear to drive primary producers, seagrass-associated faunal communities, and sediment properties in these important submerged vegetated habitats in lower Chesapeake Bay.     

Eelgrass (<Emphasis Type="Italic">Zostera marina</Emphasis> L.) in the Chesapeake Bay Region of Mid-Atlantic Coast of the USA: Challenges in Conservation and Restoration

Decreases in seagrass abundance reported from numerous locations around the world suggest that seagrass are facing a global crisis. Declining water quality has been identified as the leading cause for most losses. Increased public awareness is leading to expanded efforts for conservation and restoration. Here, we report on abundance patterns and environmental issues facing eelgrass (Zostera marina), the dominant seagrass species in the Chesapeake Bay region in the mid-Atlantic coast of the USA, and describe efforts to promote its protection and restoration. Eelgrass beds in Chesapeake Bay and Chincoteague Bay, which had started to recover from earlier diebacks, have shown a downward trend in the last 5–10 years, while eelgrass beds in the Virginia coastal bays have substantially increased in abundance during this same time period. Declining water quality appears to be the primary reason for the decreased abundance, but a recent baywide dieback in 2005 was associated with higher than usual summer water temperatures along with poor water clarity. The success of eelgrass in the Virginia coastal bays has been attributed, in part, to slightly cooler water due to their proximity to the Atlantic Ocean. A number of policies and regulations have been adopted in this region since 1983 aimed at protecting and restoring both habitat and water quality. Eelgrass abundance is now one of the criteria for assessing attainment of water clarity goals in this region. Numerous transplant projects have been aimed at restoring eelgrass but most have not succeeded beyond 1 to 2 years. A notable exception is the large-scale restoration effort in the Virginia coastal bays, where seeds distributed beginning in 2001 has initiated an expanding recovery process. Our research on eelgrass abundance patterns in the Chesapeake Bay region and the processes contributing to these patterns have provided a scientific background for management strategies for the protection and restoration of eelgrass and insights into the causes of success and failure of restoration efforts that may have applications to other seagrass systems.     

Dynamic simulation of littoral zone habitats in lower Chesapeake Bay. I. Ecosystem characterization related to model development

The fringing environments of lower Chesapeake Bay include sandy shoals, seagrass meadows, intertidal mud flats, and marshes. A characterization of a fringing ecosystem was conducted to provide initialization and calibration data for the development of a simulation model. The model simulates primary production and material exchange in the littoral zone of lower Chesapeake Bay. Carbon (C) and nitrogen (N) properties of water and sediments from sand, seagrass, intertidal silt-mud, and intertidal marsh habitats of the Goodwin Islands (located within the Chesapeake Bay National Estuarine Research Reserve in Virginia, CBNERR-VA) were determined seasonally. Spatial and temporal differences in sediment microalgal biomass among the habitats were assessed along with annual variations in the distribution and abundance ofZostera marina L. andSpartina alterniflora Loisel. Phytoplankton biomass displayed some seasonality related to riverine discharge, but sediment microalgal biomass did not vary spatially or seasonally. Macrophytes in both subtidal and intertidal habitats exhibited seasonal biomass patterns that were consistent with other Atlantic estuarine ecosystems. Marsh sediment organic carbon and inorganic nitrogen differed significantly from that of the sand, seagrass, and silt habitats. The only biogeochemical variable that exhibited seasonality was low marsh NH₄ ⁺. The subtidal sediments were consistent temporally in their carbon and nitrogen content despite seasonal changes in seagrass abundance. Eelgrass has a comparatively low C:N ratio and is a potential N sink for the ecosystem. Changes in the composition or size of the vegetated habitats could have a dramatic influence over resource partitioning within the ecosystem. A spatial database (or geographic information system, GIS) of the Goodwin Islands site has been initiated to track long-term spatial habitat features and integrate model output and field data. This ecosystem characterization was conducted as part of efforts to link field data, geographic information, and the dynamic simulation of multiple habitats. The goal of these efforts is to examine ecological structure, function, and change in fringing environments of lower Chesapeake Bay.     

Epiphyte-grazer relationships in seagrass meadows: Consequences for seagrass growth and production

Studies of seagrass meadows have shown that the production of algal epiphytes attached to seagrass blades approaches 20% of the seagrass production and that epiphytes are more important as food for associated fauna than are the more refractory seagrass blades. Since epiphytes may compete with seagrasses for light and water column nutrients, excessive epiphytic fouling could have serious consequences for seagrass growth. We summarize much of the literature on epiphytegrazer relationships in seagrass meadows within the context of seagrass growth and production. We also provide insights from mathematical modeling simulations of these relationships for a Chesapeake BayZostera marina meadow. Finally we focus on future research needs for more completely understanding the influences that epiphyte grazers have on seagrass production.     

Variations in stable carbon isotope ratio of the copepod Acartia tonsa during the onset of the Texas brown tide

The Laguna Madre of South Texas is a shallow coastal lagoon whose dominant primary producers shifted from seagrasses to phytoplankton with the onset of the Texas brown tide, which persisted from 1990 through 1997. Acartia tonsa is the dominant component of the mesozooplankton and forms an important link in both the phytoplankton and detritus-based pelagic food webs. Stable carbon isotope ratios of A. tonsa, as well as the two major primary producers: phytoplankton (as particulate organic carbon) and seagrasses, were measured from March 1989 to October 1991. Zooplankton samples were collected at four locations in the Laguna Madre: two in shallow water (c. 1 m) over seagrass beds and two in slightly deeper water (c. 2–3 m) over a muddy bottom in a secondary bay without seagrasses. We found seasonal trends in the isotopic composition of A. tonsa collected within both habitats as well as distinct differences between the average {ie995-1} values of individuals collected in the two regions. Isotopic ratios of animals collected during the summer months were generally 4–8‰ enriched in ¹³C compared with those collected in the winter, at all stations. A. tonsa collected over seagrass beds were 2–5‰ more enriched in ¹³C than those collected over muddy bottoms. These observations suggest carbon derived from seagrasses can be an important source of nutrition for these copepods in summer, especially for copepods living over seagrass beds. The effects of the persistent brown tide decreased the contribution of seagrasses as a carbon source for A. tonsa during the summer of 1991. The pathway by which seagrass carbon enters the diet of A. tonsa is unclear, but the two pathways considered most likely are through copepods feeding on microzooplankton that have fed on bacteria nourished on seagrass carbon, or by copepods feeding directly on particles of seagrass detritus.     

Modeling seagrass density and distribution in response to changes in turbidity stemming from bivalve filtration and seagrass sediment stabilization

In many areas of the North American mid-Atlantic coast, seagrass beds are either in decline or have disappeared due, in part, to high turbidity that reduces the light reaching the plant surface. Because of this reduction in the areal extent of seagrass beds there has been a concomitant diminishment in dampening of water movement (waves and currents) and sediment stabilization. Due to ongoing declines in stocks of suspension-feeding eastern oysters (Crassostrea virginica) in the same region, their feeding activity, which normally serves to improve water clarity, has been sharply reduced. We developed and parameterized a simple model to calculate how changes in the balance between sediment sources (wave-induced resuspension) and sinks (bivalve filtration, sedimentation within seagrass beds) regulate turbidity. Changes in turbidity were used to predict the light available for seagrass photosynthesis and the amount of carbon available for shoot growth. We parameterized this model using published observations and data collected specifically for this purpose. The model predicted that when sediments were resuspended, the presence of even quite modest levels of eastern oysters (25 g dry tissue weight m^?2) distributed uniformly throughout the modeled domain, reduced suspended sediment concentrations by nearly an order of magnitude. This increased water clarity, the depth to which seagrasses were predicted to grow. Because hard clams (Mercenaria mercenaria) had a much lower weight-specific filtration rate than eastern oysters; their influence on reducing turbidity was much less than oysters. Seagrasses, once established with sufficiently high densities (>1,000 shoots m^?2), damped waves, thereby reducing sediment resuspension and improving light conditions. This stabilizing effect was minor compared to the influence of uniformly distributed eastern oysters on water clarity. Our model predicted that restoration of eastern oysters has the potential to reduce turbidity in shallow estuaries, such as Chesapeake Bay, and facilitate ongoing efforts to restore seagrasses. This model included several simplifiying assumptions, including that oysters were uniformly distributed rather than aggregated into offshore reefs and that oyster feces were not resuspended.     

A comparison of early juvenile red drum densities among various habitat types in Galveston Bay, Texas

Seagrass meadows are often cited as important nursery areas for newly settled red drum even though many estuaries, such as Galveston Bay, Texas, support large numbers of red drum and have limited seagrass cover, suggesting the use of alternate nursery areas. We examined patterns of habitat use for newly settled red drum at six sampling areas in Galveston Bay; two areas had seagrass beds and four areas had no seagrass. We measured densities in different habitat types using epibenthic sleds and enclosure samplers. Peak recruitment of young red drum to the estuary occurred during September through December. Highest densities of new settlers were found in seagrass meadows (primarilyHalodule wrightii), but when seagrass was absent, the highest densities of red drum occurred along theSpartina alterniflora marsh edge interface. Densities were relatively low on nonvegetated bottom away from the marsh edge. We also examined density patterns in other habitat types at selected sampling areas and found no red drum within marsh vegetation away from the marsh edge interface (5 and 10 m into the marsh interior). Oyster reefCrassostrea virginica was sampled using lift nets, and we found no red drum using this habitat, although adjacent seagrass and marsh interface habitats were used. Even though red drum densities in marsh edge were low relative to seagrass, the large areal extent of marshes in the bay complex probably makes marsh edge the most important nursery habitat for red drum in Galveston Bay.     

Distribution and abundance of submerged aquatic vegetation in Chesapeake Bay: An historical perspective

An historical summary of the distribution and abundance of submerged aquatic vegetation (SAV) in the Chesapeake Bay is presented. Evidence suggests that SAV has generally been common throughout the bay over the last several hundred years with several fluctuations in abundance. The decline ofZostera marina (eelgrass) in the 1930’s and the rapid expansion ofMyriophyllum spicatum (watermilfoil) in the late 1950’s and early 1960’s were two significant events involving a single species. Since 1965, however, there has been a significant reduction of all species in most sections of the bay. Declines were first observed in the Patuxent, Potomac and sections of other rivers in the Maryland portion of the Bay between 1965 and 1970. Dramatic reductions were observed over the entire length of the bay from 1970 to 1975. Particularly severe losses were observed at the head of the bay around Susquehanna Flats as well as in numerous rivers along Maryland’s eastern and western shores. Changes in the lower, Virginia portion of the bay occurred primarily in the western tributaries. Greatest losses of vegetation occurred in the years following Tropical Storm Agnes in 1972. Since 1975 little regrowth has been observed in the Chesapeake Bay. Other areas along the Atlantic Coast of the U.S. during the same period have experienced no similar widespread decline. It thus appears that the factors affecting the recent changes in distribution and abundance of submerged vegetation in the bay are regional in nature. Causes for this decline may be related to changes in water quality, primarily increased eutrophication and turbidity.     

Setting seagrass depth, coverage, and light targets for the Indian River Lagoon system, Florida

Seagrass protection and restoration in Florida’s Indian River Lagoon system (IRLS) is a mutual goal of state and federal programs. These programs require, the establishment of management targets indicative of seagrass recovery and health. We used three metrics related to seagrass distribution: areal coverage, depth limit, and light requirement. In order to account for the IRLS’s spatial heterogeneity and temporal variability, we developed coverage and depth limit targets for each of its 19 segments. Our method consisted of two steps: mapping the union of seagrass coverages from all availabe mapping years (1943, 1986, 1989, 1992, 1994, 1996, and 1999) to delineate wherever seagrass had been mapped and determining the distribution of depth limits based on 5,615 depth measurements collected on or very near the deep-edge boundary of the union coverage. The frequency distribution of depth limits derived from the union coverage, along with the median (50th percentile) and maximum (95th percentile) depth limits, serve as the seagrass depth targets for each segment. The median and maximum depth targets for the IRLS vary among segments from 0.8 to 1.8 and 1.2 to 2.8 m, respectively.Halodule wrightii is typically the dominant seagrass species at the deep-edge of IRLS grass beds. We set light requirement targets by using a 10-yr record of light data (1990–1999) and the union coverage depth limit distributions from the most temporally stable seagrass segments. The average annual light requirement, based on the medians of the depth limit distributions, is 33 ± 17% of the subsurface light. The minimum annual light requirement, based on of the 95th percentile of the depth distributions, is 20 ± 14%; the minimum growing season light requirement (March to mid September) is essentially the same (20 ± 13%). Variation in depth limits and light requirements, is probably due to factors other than light that influence the depth limit of seagrasses (e.g., competition, physical disturbance). The methods used in this study are robust when applied to large or long-term data sets and can be applied to other estuaries where grass beds are routinely monitored and mapped.     

Comparison of restored and natural seagrass beds near Corpus Christi,Texas

Structural equivalence between seagrass restoration sites and adjacent natural seagrass beds on the mid Texas coast was assessed six times between April 1995 and May 1997. Throw traps and corers were used for quantitative sampling. Restoration sites were 2.7 to 6.6 yr old when first sampled and 3.7 to 8.2 yr old when last sampled. There were few significant differences in water column, seagrass, or sediment characteristics, in fish and decapod (nekton) densities, or in nekton and benthos community compositions between restored and natural seagrass habitats at any time during the study period. Differences in densities of dominant benthic invertebrates were regularly observed, with greater densities of more taxa observed in natural seagrasses than in restored beds. Densities of Class Oligochaeta and the polychaetePrionospio heterobranchiata are proposed as potential indicators of structural equivalence in restored seagrasses. This study indicates that seagrass restorations in the vicinity of Corpus Christi, Texas, exhibit minimal quantitative differences in community structure (except for benthos) relative to adjacent natural seagrass beds after 3 to 5 yr.     

The Optical Properties of Greater Florida Bay: Implications for Seagrass Abundance

Water column optical properties of Greater Florida Bay were investigated in the context of their impacts on seagrass distribution. Scattering played an important role in light attenuation throughout the shallow water system. The northwest region was characterized by an absence of seagrasses and the highest scattering by particles, mostly from resuspended carbonate sediments. Higher seagrass densities were observed in the open waters just north of the Florida Keys, where absorption coefficients were dominated by colored dissolved organic material and scattering was lower than in the northwest region. Patchy dense seagrass meadows were observed in the clear waters south of the Keys where scattering and absorption were low and contributed equally to light attenuation. In general, seagrasses were observed in areas where >7.5% of surface irradiance reached the plants and where optical properties were not dominated by scattering. Although the prevention of eutrophication and nuisance algal blooms may be necessary for preserving seagrass meadows in this system, our observations and model calculations indicate that nutrient control alone may be insufficient to permit seagrass recolonization if optical properties are dominated by particulate scattering from resuspended sediments.     

Seagrass die-off in Florida Bay: Long-term trends in abundance and growth of turtle grass,Thalassia testudinum

Beginning in late 1987 Florida Bay experienced a large and unprecedented die-off ofThalassia testudinum. The die-off occurred only in stands of denseT. testudinum. We initiated an experimental monitoring effort in 1989 to attempt to ascertain the cause of this die-off phenomenon. From 1989 to 1995 the abundance and productivity ofT. testudinum was measured at five stations associated with the seagrass die-off and three stations where no die-off had occurred (including one on the seaside of Key Largo, outside of Florida Bay). Early in the study the salinity was very high, exceeding 46 psu, but it has decreased to 29–38 psu in recent years. Seagrass standing crop and either short-shoot density or mass per short shoot declined at nearly all stations, including the stations without die-off (unaffected stations). Over the course of the study, areal productivity declined at three die-off stations; but mass-specific productivity increased at all die-off stations and one unaffected station. Seasonality was pronounced; detrended standardized residuals showed responses for all of the seagrass parameters to be greater than the yearly mean in spring and summer and less than the mean in fall and winter. Detrended residuals also showed decreased productivity to be correlated with increased salinities in the summer despite a long-term record of declining salinities. We propose a conceptual model of the seagrass die-off phenomenon. We document that salinity does contribute to stress onT. testudinum in Florida Bay, but salinity is believed to be only one contributing factor to the loss of seagrasses. The documented increase in the mass-specific productivity ofT. testudinum over the period 1989–1995 suggests seagrasses are growing rapidly in Florida Bay by 1995; we predict that the loss ofT. testudinum may be slowing down and that recovery is possible.     

Seed bank patterns in Chesapeake Bay eelgrass (Zostera marina L.): A bay-wide perspective

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.     

Historical Comparison of Fish Community Structure in Lower Chesapeake Bay Seagrass Habitats

Seagrass beds provide important habitat for fishes and invertebrates in many regions around the world. Accordingly, changes in seagrass coverage may affect fish communities and/or populations, given that many species utilize these habitats during vulnerable early life history stages. In lower Chesapeake Bay, seagrass distribution has contracted appreciably over recent decades due to decreased water clarity and increased water temperature; however, effects of changing vegetated habitat on fish community structure have not been well documented. We compared fish community composition data collected at similar seagrass sites from 1976–1977 and 2009–2011 to investigate potential changes in species richness, community composition, and relative abundance within these habitats. While seagrass coverage at the specific study sites did not vary considerably between time periods, contemporary species richness was lower and multivariate analysis showed that assemblages differed between the two datasets. The majority of sampled species were common to both datasets but several species were exclusive to only one dataset. For some species, relative abundances were similar between the two datasets, while for others, there were notable differences without directional uniformity. Spot (Leiostomus xanthurus) and northern pipefish (Syngnathus fuscus) were considerably less abundant in the contemporary dataset, while dusky pipefish (Syngnathus floridae) was more abundant. Observed changes in community structure may be more attributable to higher overall bay water temperature in recent years and other anthropogenic influences than to changes in seagrass coverage at our study sites.     

Isotopic and Elemental Composition of Marine Macrophytes as Biotracers of Nutrient Recycling Within a Coastal Lagoon in Baja California,Mexico

Nutrient sources of San Quintin Bay, a coastal lagoon affected by coastal upwelling off Baja California (Mexico), were traced using generalized additive (mixed) models (GAMM) to the stable nitrogen isotopic composition, C:N and N content of two co-occurring macrophytes (the macroalgae Ulva spp. and the seagrass Zostera marina). The geochemical tracers followed a spatial trend that partly responded to the long-term nutrient gradient from the ocean towards the interior of the bay. N content in Z. marina and Ulva spp. decreased linearly (while C:N increased) towards the middle section of the bay to concentration levels that indicate potential N limitation for growth. Concurrently midway into the bay (6–9 km), the δ¹⁵N of both macrophytes showed a gradual enrichment in ¹⁵N reflecting progressive denitrification. The spatial pattern of δ¹⁵N and the decrease in C:N of the macrophytes towards the innermost section of the bay indicated an additional nonoceanic source of dissolved nitrogen in this zone. The similarity of the δ¹⁵N pattern of Z. marina and Ulva spp. implies that their δ¹⁵N composition is mainly controlled by the availability of N, in spite of the physiological differences between taxa. A better fit of GAMM to N content and C:N was obtained for Z. marina than for Ulva spp. indicating that the former delineate more steadily and smoothly the influence of upwelling along the spatial gradient. Nonetheless, Ulva spp. may be analyzed in combination with Z. marina to characterize the environmental conditions at the time of sampling.     

Analysis of the abundance of submersed aquatic vegetation communities in the Chesapeake Bay

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.