Biomass Size Spectra: Linking Spectra Across Trophic Levels as Indicators of Estuarine State

Sukgeun Jung 1, David G. Kimmel 2, Jason Adolf 2, Edward D. Houde 1, Michael R. Roman 2, and Lawrence W. Harding, Jr. 2,3

1 University of Maryland Center for Environmental Science, Chesapeake Biological Laboratory (UMCES-CBL), Solomons, Maryland.
2 University of Maryland Center for Environmental Science, Horn Point Laboratory (UMCES-HPL), Cambridge, Maryland
3 Maryland Sea Grant, University of Maryland, College Park, Maryland

Biomass size-spectra potentially may serve as integrative indicators of estuarine status. In the Atlantic Coast Environmental Indicators Consortium (ACE INC), spectra are being constructed for the pelagic community of Chesapeake Bay and include organisms from phytoplankton to pelagic fish. Variability in spectra on inter-annual, seasonal, and regional scales is being evaluated with respect to hydrological and anthropogenic factors. To date, synthesis of existing data (1995-2000) has produced spectra for mesozooplankton to fish in the size range 0.0038 to 4,096 kcal, based on zooplankton data from an Optical Plankton Counter and fish data from a midwater trawl. Slopes of normalized spectra for each of the trophic levels varied inter-annually, seasonally, and regionally. Annual mean slopes of fish spectra were positively correlated with freshwater input, the major factor that modified fish community structure. Slopes of zooplankton spectra also were responsive to variability in freshwater discharge, the major controller of nutrient and organic matter input into the estuary. Preliminary analysis indicated that the 'backbone' slope (a measure of trophic efficiency) of combined zooplankton-fish spectra was -1.4, a value significantly different from a theoretical -1.0 and steeper than the slope for each trophic level (-1.2 for zooplankton and -1.0 for pelagic fishes). Annually, the 'backbone' slope was steepest and the regression intercept (a measure of relative productivity) lowest in 1996, the wettest year. Regionally, the 'backbone' was steepest in the middle Bay and the intercept highest in the lower Bay. Defining spatial and temporal trends in statistical properties of biomass size spectra can characterize variability in community structure and provide an integrative indicator for defining ecosystem state.

Keywords: biomass size spectra, indicator, Chesapeake bay, estuary and ecosystem