Biological-Physical Coupling in the Chesapeake Bay Plume
A major focus in understanding plankton dynamics has been the relative roles of physical and biological controls in structuring the spatial and temporal patterns of biomass and production. Bounded or semi-bounded ocean features such as eddies, rings and buoyant plumes are attractive areas to study these controls on plankton dynamics because the distance hydrography and persistence of these structures allow the investigator to study populations/communities over time and to quantify growth rates, mortality rates, physical inputs and losses. Coastal plumes are characterized by a distinct salinity structure and high biological production relative to ambient shelf waters. Low salinity water discharged from Chesapeake Bay usually makes a broad, anticyclonic turn as it flows onto the continental shelf, subsequently narrowing into a coastal jet as it flows south along the coast (Chao and Boicourt 1986). The spatial and temporal scales of the Chesapeake Bay plume are effected by the discharge rate of water from Chesapeake Bay as well as wind speed and direction. Under downwelling (Northerly) winds, the plume forms a narrow coastal jet which can propagate along the coast at speeds reaching 7 km d-1 which can result in low salinity Chesapeake Bay water traveling over 100 km S of the Bay mouth (Boicourt et al. 1987). In contrast, upwelling (Southerly) winds oppose the anticlyclonic turn of the plume, and - because of Ekman circulation - result in a seaward spreading and rapid thinning and dilution of plume waters. Thus the coastal plume of Chesapeake Bay can have a horizontal scale of 10 to 100 km, a vertical scale of 5 to 20 m, and a time scale of 1 to 20 days (Boicourt et al. 1987). These temporal scales overlap with the scales of growth and patch structure of plankton (bacteria, phytoplankton, protozoa and copepods). Therefore the physics of the plume can influence the growth and trophic interactions among plankton populations of the plume and their mixing rates with surrounding shelf waters.
Drifter experiments conducted in the Chesapeake Bay plume showed that, in general, there was a shift from an autotrophic to a heterotrophic plankton community with time in the plume waters. Chlorophyll decreased (lost to both sinking and grazing) as bacteria, flagellates and copepods increased in the plume waters (Boicourt et al. 1987; Roman et al. 1988; Malone and Ducklow 1990; McManus and Fuhrman 1990; Glibert et al. 1991). Heterotrophic processes such as bacterial production, ammonium regeneration and copepod grazing usually increased over time as the plume traversed the continental shelf (Boicourt et al. 1987; Roman et al. 1988; Malone and Ducklow 1990; Glibert et al. 1991). The convergence areas associated with frontal processes of the plume usually had high concentrations of copepods.
A convenient biological tracer of plume waters are the larvae of benthic invertebrates which spawn in estuaries. Although many invertebrate larvae have adopted strategies such as vertical migration which help retain them in estuaries with twolayer flow patterns, a number of estuarine invertebrate species have pelagic larvae which are transported to shelf waters where they develop and then return to the estuary. Because these latter invertebrate species have a fairly unambiguous source, they can be used along with more conservative tracers such as salt to trace the flow pattern of plume water on the shelf. However, unlike the salinity of plume water which rapidly (hours to days) increases because of mixing with shelf waters, patches of crab larvae often actively aggregate in surface waters, thus their distribution can provide a signature of past flow events.
As part of the MECCAS (Microbial Exchanges and Coupling in Coastal Atlantic Systems) Project, crab larvae were collected in the shelf waters off Chesapeake Bay in April, June and August (Roman and Boicourt 1999). We conducted both hydrographic (temperature, salinity, nutrients) and biological (chlorophyll, copepods) mapping surveys in conjunction with Eulerian and Lagrangian time studies of the vertical distribution of crab larvae in the Chesapeake Bay plume. These abundance estimates are used with both current meter records and drifter trajectories to infer mechanisms of larval crab dispersion to the shelf waters and recruitment back into Chesapeake Bay. The highest numbers of crab larvae were usually associated with the Chesapeake Bay plume, suggesting that the dominant source of crab larvae to shelf waters was Chesapeake Bay. However, patches of crab larvae also were found in the higher salinity shelf waters, as possible remnants of previous plume discharge events. The distribution of crab larvae in the shelf waters changed on 12 day timescales as a consequence of both variations in the discharge rate of the Chesapeake Bay plume and local winddriven currents. Downwellingfavorable winds (NW) intensified the coastal jet and confined the plume and crab larvae along the coast. For example, in April when NW winds predominated, crab zoeae were transported southward along the coast at speeds that at times exceeded 168 km d-1 during a downwelling wind event. In contrast, during June and August upwellingfavorable winds (S,SW) opposed the anticyclonic turn of the plume and, via Ekman circulation, forced the plume and crab larvae to spread seaward. Plume velocities during these conditions generally were less than 48 km d-1. The recruitment of crab larvae to Chesapeake Bay is facilitated in late summer by the dominance of S winds which can reverse the southward flow of shelf waters. Periodic downwelling favorable winds can result in the flow of surface waters and crab larvae towards the entrance of Chesapeake Bay. In addition, approximately 27% of the larval crabs spend at least part of the day in bottom waters which have a residual drift towards the Bay mouth. Thus there appears to be a variety of physical transport mechanisms which can enhance the recruitment of crab larvae into Chesapeake Bay.References
Boicourt, W.C., S. -Y. Chao, H. W. Ducklow, P.M. Glibert, T.C. Malone, M.R. Roman, L.P. Sanford, J.A. Fuhrman, C. Garside and R.W. Garvine. 1987. Physics and microbial ecology of a buoyant estuarine plume on the continental shelf. EOS 86: 666-668.
Chao, S.-Y. and W.C. Boicourt. 1986. Onset of estuarine plumes. J. Phys. Oceanogr. 16: 2137-2149.
Glibert, P.M., C. Garside, J.A. Fuhrman and M.R. Roman. 1991. Time-dependent coupling of inorganic and organic nitrogen uptake and regeneration in the plume of the Chesapeake Bay estuary and its regulation by large heterotrophs. Limnol. Oceanogr. 24: 683-696.
Malone, T.C. and H. W. Ducklow. 1990. Microbial biomass in the coastal plume of Chesapeake Bay: Phytoplankton-bacterioplankton relationships. Limnol. Oceanogr. 35: 296-312.
McManus, G.B. and J.A. Fuhrman. 1990. Mesoscale and seasonal variability of heterotrophic nanoflagellate abundance in an estuarine outflow plume. Mar. Ecol. Progr. Ser. 61: 207-213.
Roman, M.R., K A. Ashton and A.L. Gauzens. 1988. Day/night differences in the grazing impact of marine copepods. Hydrobiol. 167/168: 21-30.
Roman, M.R. and W.C. Boicourt. 1999. Dispersion and recruitment of crab larvae in the Chesapeake Bay plume: Physical and biological controls. Estuaries: In Press.
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