Eutrophication, Hypoxia and Trophic Transfer Efficiency in Chesapeake Bay

Abstract

Coastal eutrophication is a global problem that has contributed to loss of estuarine habitats and potentially decreased fisheries production. Hypoxia is often observed in eutrophic estuaries where it can be an important cause of habitat loss. This study utilized a suite of empirical analyses to examine key linkages relating coastal eutrophication to hypoxia, trophic structure, and trophic transfer efficiency in Chesapeake Bay (CB), USA. A salt- and water-balance model, or "box" model, was developed to quantify large-scale physical transport for CB, an input to many subsequent analyses. Historical ( 1950-1999) dissolved oxygen (DO) data for CB showed that moderate hypoxia (DO<2.0 mg1^-1) increased ~3-fold, modulated by spring river flow. Severe hypoxia (DO<0.7 mg1^1) occurred only in high flow years during 1950-1967, but was present annually since 1968. Analysis using tree-structured regression showed that hypoxia was the most important factor determining patterns of macrobenthic biomass in Chesapeake Bay. Carbon budgets showed that, where habitat quality was poor, macrobenthic biomass was much less than could be supported by the organic carbon supply. In these cases, even dramatic reductions in carbon supply would not be expected to limit benthic production and by extension, trophic transfers to upper trophic levels via the benthos. The effect of eutrophication and hypoxia on trophic structure and trophic transfer efficiency were examined by estimating trophic flow networks for three regions of CB during summer. In addition, a series of "rules" were described and used to infer the trophic flow network for a "restored" middle CB from historical data, comparative ecological relationships and mass balance constraints. Excessive carbon now through bacteria was the most pronounced symptom of eutrophication in the modern mid Bay. The microbial food web transferred organic matter to trophic levels comparable to large piscivorous predators, maintaining average trophic transfer efficiency, even as the fraction of primary production transferred to top predators decreased. In the restored Bay, increased macrobenthic production shifted metabolic activity away from the microbial food web, increasing the potential trophic transfer to fish by 7-fold, even as total primary production decreased to 63% of the current average.