(COASTES = COmplexity And STressors in Estuarine Systems)
Academy of Natural Sciences, Estuarine Research Center
Dr. Denise Breitburg
10545 Mackall Rd.
St. Leonard, MD 20685
breit@acnatsci.org
To understand the effects of numerous stressors on coastal ecosystems, the complexity of such systems must be recognized and incorporated into experimental and conceptual frameworks. We are currently addressing this complexity through a 6-yr multi-investigator program- COASTES - by studying a subestuary of the Chesapeake Bay at multiple levels of organization. Our approach includes 1) mesocosm and field-enclosure tests of the effects of stressors (nutrients, inorganic toxics, dissolved oxygen) on ecological processes within the Patuxent River estuary, 2) an examination of the relationships between land-use patterns, geology and watershed loadings of stressors, 3) models of ecological effects of stressors from the individual through ecosystem level, including spatially explicit fisheries models, and 4) an economic evaluation of management practices. Estuarine experiments specifically test the importance of trophic complexity and functional redundancy to the way the estuary responds to stress.
1) The first is our emphasis on the role of ecological complexity in the response of an estuarine system to anthropogenic stress, and in determining the scientific information needed to make sound management decisions. We are considering the importance of trophic complexity and variability as well as spatial heterogeneity. In addition, we address complexity in a broader sense by linking system response to landscape-level processes, management issues and sociological concerns.
2) Second, our emphasis is on multiple stressors. We will consider nutrients, contaminants and low oxygen separately, but our emphasis is on asking how individuals, populations and entire systems respond when they are challenged with several stressors at the same time.
3) Third, the project has at its core an integrated team of watershed experts, ecologists, economists and managers who designed an integrated approach to the issue. By examining land use, ecology, fisheries, management issues, and economics in a coordinated effort, we insure that we will be able to integrate information generated to improve our scientific understanding and management of the estuary.
A fisheries modeling component of COASTES developed with funding from MD Sea Grant
Low dissolved oxygen concentrations commonly occur in stratified waters where density differences within the water column enable microbial respiration of excess primary production to reduce dissolved oxygen in bottom waters. Because surface waters can remain at oxygen concentrations near saturation even when bottom layers are anoxic or have extremely low dissolved oxygen concentrations, dissolved oxygen itself imparts an important additional component to the spatial structure of the water column. The three-dimensional oxygen structure of the water column may be especially important in predicting how subpycnocline oxygen depletion will affect trophic interactions. Low dissolved oxygen can directly affect nearly every aspect of predator-prey interactions because both behaviors and processes limited by physiological processes can be influenced.
We have developed spatially-explicit individual-based predation models to predict how low dissolved oxygen in a vertically stratified water column will affect survival of estuarine fish larvae subjected to predation by scyphomedusae and fish in Chesapeake Bay. This issue is especially important to management of Chesapeake Bay and other eutrophic estuaries because the most important negative effect of excess nutrient loads on fish populations may be summer oxygen depletion in bottom waters. The use of a spatially explicit individual-based model allows for the integration of large-scale distributional effects and small-scale behavioral changes along with physiological effects of dissolved oxygen in a stratified water column.
The spatially-explicit individual-based models indicate that bottom dissolved oxygen can strongly affect predation mortality of fish larvae because of its effect on the vertical distributions of larvae and their predators, the ability of predators to capture larvae, and the growth rate of larvae. Thus, there is the potential for eutrophication to strongly affect larval fish recruitment even in the absence of direct effects of low oxygen on mortality of larvae, or effects of nutrient enrichment on the abundance of larval prey or predators. For any particular water column depth X predator combination, bottom dissolved oxygen altered larval survival during a 30-d simulated larval duration from between 60% to more than two orders of magnitude. Not surprisingly, the importance of the dissolved oxygen effect increased with increasing water depth as a larger proportion of the water column became subject to oxygen depletion.
The bottom dissolved oxygen concentration leading to the highest survival of fish larvae varied with predator characteristics and water column depth, making predictions about the relationship between eutrophication and predation mortality complex. Nevertheless, several important, and sometimes counterintuitive results emerge from these simulations. First, a pristine water column (i.e. all water layers at or above dissolved oxygen concentrations that had no effect on larval growth or the distribution of capture rates of predators) rarely yielded the highest larval survival. Second, lowest larval survival always occurred with either an anoxic/severely hypoxic bottom layer (dissolved oxygen concentrations of 0.00-0.99 mg/L) or when oxygen concentrations in the bottom layers were high. An anoxic/severely hypoxic bottom layer often led to extremely low larval survival even in the absence of any direct mortality caused by exposure to extremely low oxygen concentrations. Instead, high predation mortality resulted primarily from larvae and their predators crowding into the surface and pycnocline layers and thus greatly increasing encounter rates. Finally, the physiological (e.g. growth) and behavioral responses to dissolved oxygen of each individual species involved in the predator-prey interaction were important. Effects of dissolved oxygen on growth, distribution and prey capture rates all contributed to the overall effects of dissolved oxygen on predation mortality seen in these simulations. However, the relative importance of each of those factors, as well as the magnitude, direction and additivity of their effects varied among predators and with the oxygen structure of the water column.
Overall, these models indicate that dissolved oxygen imparts a substantial degree of spatial structure to the water column, and that this spatial structure can alter the qualitative as well as quantitative result of predator prey interactions. The low survival of fish larvae in some simulations of high bottom dissolved oxygen indicates that caution needs to be taken in assuming that improving water quality will automatically increase larval survival.
Estuarine Research Center
10545 Mackall Rd
St. Leonard, MD 20685
sellner@acnatsci.org
This program routinely measures chlorophyll distributions vertically and horizontally along the axis of the river as well as phytoplankton spp. composition, biomass and productivity at 3 stations in tidal-fresh, oligohaline and mesohaline portions of the river/estuary. Microzooplankton spp. composition and biomass for the >44 um fraction are also determined at these stations. Data from August, 1984 through present with monthly sampling in fall and winter and twice per month in spring and summer.
Primary research interest is determining fate of recurring phytoplankton blooms in the river/estuary, pri ncipally dinoflagellates. We have documented spatial and temporal heterogeneity of these blooms as well as role in primary production of the system, in the sedimentation of organic matter and as a source of carbon for the domiant micro- and mesozooplankton. This is an on- going effort with future work focusing on the role of these blooms in DOM cycling, perhaps the principal fate of particulate production in the blooms.
As an active member of the NOAA-COP COASTES program, my primary role has been to assess the importance of nutrient and metal additions to zooplankton within the overall design of increasing trophic complexity. To date, nutrient effects have been demonstrable in the copepods as well as significant top- down impacts on these planktonic grazers. However, there has been no detectable organism-nutrient interaction. This work is in its second year and is now focusing on metal-nutrient effects on zooplankton as trophic complexity increases.