Roger I. E. Newell

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.

Influence of Eastern Oysters on Nitrogen and Phosphorus Regeneration in Chesapeake Bay, USA

Suspension-feeding bivalves couple pelagic and benthic processes because they consume seston from the water column, and their biodeposits (feces and pseudofeces) settle on the sediment surface. Abundant stocks of bivalves can exert grazer control on phytoplankton, and this results in some nitrogen and phosphorus being regenerated to the water column as excreta and via microbial decomposition of biodeposits. Bivalve biodeposition, however, enhances net ecosystem losses of N and P via sediment burial and bacterially mediated, coupled nitrification-denitrification. Bivalve feeding also reduces turbidity and thereby increases light available for microphytobenthos. Although microphytobenthos may compete with nitrifying bacteria for N, potentially reducing coupled nitrification-denitrification, they retain N and P within sediments, further reducing net regeneration to the water column.

Seasonal utilization of different seston carbon sources by the ribbed mussel, Geukensia demissa (Dillwyn) in a mid-Atlantic salt marsh

Seston in salt marshes contains a temporally and spatially complex mixture of natural microparticulate organic material, including phytoplankton, vascular plant detritus, bacteria, heterotrophic nanoflagellates and benthic diatoms. Quantitative information is available concerning how suspension-feeding consumers, such as the ribbed mussel, Geukensia demissa (Dillwyn), utilize some of these components to satisfy their carbon demands. Despite this information there is still a limited understanding of how the relative nutritive contribution of these different dietary items may shift during the year associated with variations in both seston composition and the mussels physiological condition. To investigate if the mussels ability to use specific constituents of natural seston varies seasonally, we ran a series of pulse-chase 14C feeding experiments under ambient conditions in March, May, August and November 1996. Phytoplankton, cellulosic detritus, bacteria, heterotrophic nanoflagellates and benthic diatoms were radiolabeled and supplemented in small amounts to natural marsh water for feeding to mussels. The fate of 14C in mussel tissues, feces, respiration and excretion was quantified and contrasted among the different diet types and seasons. Microcapsules containing radiolabeled carbohydrate and protein were used as standards to differentiate possible between-experiment variations in seston composition from seasonal changes in the mussels feeding and digestive physiology. Mussel clearance rates for all diets were highest in summer and autumn and lowest in winter and spring. In contrast, seasonal shifts in digestive physiology were only found for certain diets. The seasonal range of assimilation efficiencies for microcapsule standards (18-29%) and field-collected microheterotrophs (bacteria 76-93% and heterotrophic nanoflagellates 87-94%) did not differ significantly during the year, whereas summer and autumn assimilation efficiencies for cellulosic detritus (22-24%), phytoplankton (71-79%) and benthic diatoms (89-93%) were up to twofold greater than those in winter and spring (13%, 40-59% and 45-81%, respectively). We conclude that the digestive physiology (e.g., digestive enzyme production) of mussels responds to shifts in dietary components during the year.

Assessment of recent habitat conditions of eastern oyster Crassostrea virginica bars in Mesohaline Chesapeake Bay

Abstract Eastern oysters Crassostrea virginica in Chesapeake Bay, USA, have been subjected to intense harvest pressure since about 1850 and to disease epizootics since the 1950s. Combined, these processes have degraded the once extensive eastern oyster bottom; the remaining oyster shells, which are required as substrate for successful larval oyster settlement, are thus rendered increasingly susceptible to siltation. Between 1999 and 2001, we used an acoustic seabed classification system and underwater videography to assess oyster habitat conditions throughout Marylands portion of Chesapeake Bay relative to eastern oyster recruitment and habitat restoration activities. We performed 16 surveys that covered a total of 39 km2 of bottom that were classified in 1911 as supporting productive oyster populations. Over 90% of this area has degraded from productive oyster bottom to mud, sand, or heavily sedimented oyster shell. Seventy percent of the locations we identified as containing unsedimented shell came fro...

Trophic Complexity Between Producers and Invertebrate Consumers in Salt Marshes

Salt marshes on the Atlantic coast of North America are characterized by having a high biomass of smooth cordgrass, Spartina alterniflora. Because of the refractory nature of the lignocellulosic structure of this angiosperm, invertebrates utilize C from these plants with very low efficiency, if at all. This is true for both living cordgrass and post-senescent plant detritus. To balance their C demands, invertebrate consumers living in salt marshes must utilize a wide variety of other resources, including microheterotrophs (bacteria and bacterivorous flagellates) either associated with detritus or free in the water column, fungi colonizing decaying vascular plants, surface-associated algae (e.g., microphytobenthic diatoms and cyanobacteria, epiphytes, surface film algae) and phytoplankton. This high degree of trophic complexity is likely to be an important source of community stability. As an example, we estimate that ribbed mussels, Geukensia demissa, in a Delaware marsh must rely on a variety of different food resources since no single food type can meet their nutritional demands for either C or N. To balance their C demands, mussels appear to rely mainly on microheterotrophs, followed by phytoplankton > microphytobenthos > cellulosic detritus. Non-detrital foods are even more important for maintaining positive N balance in G. demissa. Previous and emerging evidence from other studies suggests that other important marsh consumers have a similar general diet. Although cordgrass may dominate overall rates of primary production and detritus from cordgrass contributes significantly to secondary production, we challenge the paradigm that salt marshes have a “detritus-based food web.” Further research is needed to deduce the importance of microphytobenthos and microheterotrophs as sources of C and N for dominant animal consumers in these marsh systems.

Evaluating ecosystem response to oyster restoration and nutrient load reduction with a multispecies bioenergetics model

Many of the worlds coastal ecosystems are impacted by multiple stressors each of which may be subject to different management strategies that may have overlapping or even conflicting objectives. Consequently, management results may be indirect and difficult to predict or observe. We developed a network simulation model intended specifically to examine ecosystem-level responses to management and applied this model to a comparison of nutrient load reduction and restoration of highly reduced stocks of bivalve suspension feeders (eastern oyster, Crassostrea virginica) in an estuarine ecosystem (Chesapeake Bay, USA). Model results suggest that a 50% reduction in nutrient inputs from the watershed will result in lower phytoplankton production in the spring and reduced delivery of organic material to the benthos that will limit spring and summer pelagic secondary production. The model predicts that low levels of oyster restoration will have no effect in the spring but does result in a reduction in phytoplankton standing stocks in the summer. Both actions have a negative effect on pelagic secondary production, but the predicted effect of oyster restoration is larger. The lower effect of oysters on phytoplankton is due to size-based differences in filtration efficiency and seasonality that result in maximum top-down grazer control of oysters at a time when the phytoplankton is already subject to heavy grazing. These results suggest that oyster restoration must be achieved at levels as much as 25-fold present biomass to have a meaningful effect on phytoplankton biomass and as much as 50-fold to achieve effects similar to a 50% nutrient load reduction. The unintended effect of oyster restoration at these levels on other consumers represents a trade-off to the desired effect of reversing eutrophication.

Long-Term (1939 to 2008) Spatial Patterns in Juvenile Eastern Oyster (Crassostrea virginica, Gmelin 1791) Abundance in the Maryland Portion of Chesapeake Bay

ABSTRACT Stocks of eastern oysters (Crassostrea virginica, Gmelin 1791) have undergone dramatic declines in the Chesapeake Bay since the mid 1800s. As a result, substantial efforts have been made to try and reverse this decline to provide support for a commercially and socially important fishery and, more recently, to restore the oysters important ecological role. Since 1939, juvenile oyster abundance has been measured at sentinel oyster bars in the northern portion of Chesapeake Bay, Maryland. We conducted a cluster analysis on these data and detected 4 distinct spatial patterns. These patterns were related to juvenile oyster abundance (i.e., bars that experienced high overall juvenile abundance grouped together) and salinity. Of the sentinel bars sampled since the mid 1980s, our analysis identified 13 bars that were characterized by high juvenile oyster abundance and low variation among years, which makes them prime candidate bars for protection to aid in restoration. A comparison with bars already protected within oyster sanctuaries revealed some overlap (4 total bars in common); however, the 13 bars we identified were found over broader geographical and salinity ranges. Juvenile oyster abundance on this group of 13 prime bars was intercorrelated significantly, suggesting that interannual variability in juvenile oyster abundance affects each region similarly. Significant correlations between the juvenile oyster abundance time series and the Palmer hydrological drought index suggest that variations in wet/dry cycles are the cause of this interannual variability. Our analysis also indicates that the entire oyster population of the northern Chesapeake Bay may respond in similar manner to climate change effects.