Long-term variations in primary production in a eutrophic sub-estuary. II. Interannual variations and modeling

Abstract

A 19 yr time series of annual primary production in the eutrophic Rhode River subestuary of the Chesapeake Bay in Maryland (USA) was analyzed in relation to climatological and ecological factors. The objectives of this work were to understand factors controlling interannual variations in primary production in a eutrophic estuary and develop a model for tracking future variations in production, in the absence of direct measurements of photosynthetic carbon uptake. Annual production (P-A) averaged 328 (range 152 to 612) g C m(-2) yr(-1). Interannual variability was statistically significant, but there was no significant linear trend or significant non-random variations over the available 19 yr. Climatological indices based on North Atlantic Oscillation or flow of the Susquehanna River, the principal N source to the upper Chesapeake Bay, were not significant predictors of P-A. A classification of years based on magnitude of the spring dinoflagellate bloom and timing of nitrate depletion was a significant predictor of P-A. Phytoplankton biomass, B, and the light saturated photosynthetic rate normalized to chlorophyll, P-max(B), were of similar magnitude in their influence on the variance in P-A. The high degree of variability in P-max(B) weakened efforts to model both daily and annual production from measurements of chlorophyll and light attenuation. Between 4 and 15 yr of measurements of chlorophyll and light attenuation would be needed to detect a change in trophic status of the sub-estuary, depending on the level of reduction achieved in P-A. Average daily production would have to be reduced below 1052 mg Cm-2 d(-1) to achieve mesotrophic status.