The Seasonal Dynamics of The Chesapeake Bay Ecosystem

Abstract

The full suite of carbon exchanges among the 36 most important components of the Chesapeake Bay mesohaline ecosystem is estimated to examine the seasonal trends in energy flow and the trophic dynamics of the ecosystem. The networks provide information on the rates of energy transfer between the trophic components in a system wherein autochthonous production is dominated by phytoplankton production. A key seasonal feature of the system is that the summer grazing of primary producers by zooplankton is greatly reduced due to top—down control of zooplankton by ctenophores and sea nettles. Some of the ungrazed phytoplankton is left to fuel the activities of the pelagic microbial community, and the remainder falls to the bottom where it augments the deposit—feeding assemblage of polychaetes, amphipods, and blue crabs. There is a dominant seasonal cycle in the activities of all subcommunities, which is greatest in the summer and least in the cold season. However, the overall topology of the ecosystem does not appear to change substantially from season to season. Matrix operations can be employed to assess the various direct and indirect pathways by which each trophic group obtains energy. Often, indirect linkages reveal interesting differences. For example, although the bluefish and striped bass are both piscivorous predators, 63% of bluefish intake depends indirectly on benthic organisms, whereas striped bass depends mainly on planktonic organisms. Nearly all higher trophic species exhibit significant indirect dependencies upon the upper components of the microbial loop, especially during summer. The complicated trophic network can be mapped into an eight—level trophic chain in the sense of Lindeman. Such analysis reveals that detritivory is about 10 times greater than herbivorous grazing in the Chesapeake system and that 70% of detritus results from internal recycle. Annual efficiencies of trophic levels decrease as one ascends the chain. Major seasonal shifts in trophic efficiencies at higher levels appear to be modulated by how effectively microscopic zooplankton (mostly ciliates) are cropped by their predators. Average trophic efficiency is 9.6%. Despite the existence of eight trophic levels, the average level at which each species feeds always remains below 5. One "pest" species (the coelenterate Chrysaora quinquecirrha) feeds rather high on the trophic pyramid and may exert a heretofore unappreciated level of control on the planktonic food chain. The number of cycles present in the network is surprisingly few, despite the fact that a relatively large and seemingly constant amount (23.2%) of total system activity is devoted to recycling. This combination of factors possibly indicates a stressed ecosystem. A study of the rate—limiting links in the seasonal networks of recycling of material within the plankton reconfirms the shift of predator control from crustaceous zooplankton in springtime to the sea nettle (Chrysaora quinquecirrha) during summer months. The collection of cycles present in the system is disjoint; there is no overlap between the cycles among the planktonic community and the circulations among the deposit feeders and nekton. The filter—feeding benthos and fish do not participate in any cycling, but serve rather as bridges to shift carbon and energy from the planktonic community into the benthic—nektonic subsystems. Neither do most of the members of the microbial loop engage in any recycle of carbon, functioning instead as a dissipative shunt of energy out of the system.