Chesapeake Bay Seminar Series 2008

Regional, seasonal, and inter-annual variations of water quality, net O₂ production, and pelagic-benthic coupling were examined in relation to changes in nutrient loading and climatic forcing for the Patuxent River estuary.  Monthly rates of net biogeochemical production and physical transport of carbon, oxygen, and nutrients were calculated for six estuarine regions using a data-constrained salt- and water-balance model (box-model) and a time series of water quality and hydrologic data.  Assuming fixed metabolic stoichiometry for O₂, and carbon, we also derived estimates of particulate organic carbon (POC) sinking. Analyses of monthly mean rates for each estuarine volume revealed distinct regional and seasonal patterns in net O₂ production, including spring peaks in surface layer rates and generally higher rates in the middle estuary than other regions.  Rates of POC sinking, which also peaked during the spring bloom, were sufficient to support the majority of bottom layer nutrient regeneration and O₂ consumption at annual time-scales. Computed rates of net O₂ production and nutrient consumption in surface waters were generally enhanced by high river flow; however, muted effects of flow on bottom-layer respiration and nutrient regeneration suggest that much of the increased organic production in surface layers during high flow is transported to seaward regions. The box-modeling approach was also used to assess estuarine ecosystem responses to recent reductions in N and P loading from sewage treatment facilities under the variable climatic conditions of the last two decades. Reductions in nutrient concentrations, chlorophyll-a levels, and bottom layer O₂ consumption rates in the upper estuary since 1990 are closely tied to reductions in point-source nutrient loading rates (by 40-60%). The absence of clear trends in water quality and net O₂ production for the middle estuary appears to be attributable to persistently high diffuse-source nutrient loads, particularly during the high river flow conditions which characterized the mid- to late-1990s.  Despite significant declines in the seaward transport of N and P from the upper to the lower estuary, chlorophyll-a concentrations, net O₂ production, and turbidity have actually increased during this same time period.  These surprising trends coincide with a trend of increasing net inputs of DIN from the Chesapeake Bay to the Patuxent, related to larger nutrient concentration decreases in the Patuxent relative to Chesapeake Bay. In addition, a contemporaneous reduction in grazing pressure on phytoplankton appears to be related to a top-down trophic cascade that has allowed algal biomass and productivity to increase, with total N concentrations remaining relatively constant. These analyses of water quality trends and ecosystem processes illustrate (1) the value of long-term monitoring data, (2) the need for regional scale nutrient management that includes integrated estuarine systems, and (3) the potential water quality impacts of altered coastal food-webs.

Bacterioplankton are essential components of aquatic ecosystems, catalyzing critical biogeochemical reactions and serving as central members of planktonic food webs. Recent large-scale DNA sequencing projects show a staggeringly high genetic diversity in bacterioplankton communities, and yet several recent studies show predictable patterns in this diversity including spatial patterns related to environmental gradients and dispersal, and temporal patterns including seasonality and annual re-assembly. These biogeographic patterns reveal the tight connection between bacteria and their environment, and suggest that Metacommunity Theory and other ecological theories developed for macroscopic organisms may also apply to microscopic organisms. This talk will discuss these theories citing several examples of biogeographic patterns in bacterioplankton drawn from research on the Alaskan Tundra, a New England salt marsh estuary, and the anoxic zone of the Chesapeake Bay.

Existing individual-based models of clonal plants rely on simple rules of rhizome spacing, branching angle, and branching rate to predict colonization of space by asexually reproducing ramets. These models have provided unanticipated explanations for the emergence of nonlinear patch growth observed in real populations. However, no efforts have attempted to link these models\' growth or morphological parameters with environmental conditions, relying instead on constant or stochastic values. In the case of seagrasses, widespread decline has prompted managers to ask for predictive tools that might provide better understanding of the landscape-level response of these clonal plants to water quality. The Virtual Eelgrass Meadow (VEM) was created to explore how the simple rules of a modular, clonal, architecture might be linked with environmental variables such as light, temperature, and nutrients to simulate eelgrass patch dynamics. Insights from VEM simulations suggest that changes to growth rates and resource allocation in response to the physical environment provide a mechanistic explanation for the morphological plasticity of ramets, which ultimately affects patch expansion rates.

The recent significant decline in Arctic sea ice has captured public interest in the challenges of mitigating and responding to climate changes that seem to be unambiguously underway at high latitudes. Despite the obvious physical differences between ice-covered seas and open water, predicting ecosystem response and biological changes that are likely to result is much more difficult. For example, it is thought that declining sea ice coverage will increase light penetration and increase primary production on arctic continental shelves, which might be globally significant because the continental shelves in the Arctic are the world’s largest in extent. However, in comparisons between chlorophyll biomass in the Bering Sea for years with light versus heavy ice coverage, open water conditions do not lead to significantly higher water column chlorophyll biomass possibly because high winds significantly mix phytoplankton in open water and sea ice provides an attachment structure to hold sea ice algae close to the sea surface. The timing of seasonal sea ice retreat is also hypothesized to play a role in structuring the food web with better development of zooplankton populations in some circumstances. The northern Bering and Chukchi continental shelves currently have short, efficient food-chains that deposit organic material synthesized during the brief, but intense production period directly to the shallow sea floor without much exploitation by zooplankton. Specialized apex predators such as walrus, gray whales, bearded seals and diving sea ducks exploit the rich benthos as a food resource, but there is also evidence that fish are becoming more important in structuring the food web and zooplankton may also become more important in intercepting primary production too. The North Pacific Fisheries Management Council has also begun to assess how fisheries should be managed and what the potential conflicts will be for the existing benthic-based food-web as commercial exploitation of Bering Sea fisheries is poised to expand northward. I will present data on what is known about how the Bering and Chukchi Sea food webs and biological systems are changing in response to regime shifts and seasonal sea ice retreat. Our intent is to use these data as a starting point for developing a predictive capability to understand how the arctic biological system will respond to the stresses of climate change.