Nutrient-Dissolved Oxygen Dynamics In Chesapeake Bay

Tuttle, Jon H.
Malone, Thomas C.
Jonas, Robert B.
Ducklow, Hugh W.
Cargo, David G.
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This study focused on the relationships of phytoplankton and microheterotrophs to the development and maintenance of anoxia in the mesohaline region of Chesapeake Bay. A series of 14 cruises were made from February to October, 1985, on which water quality, nutrient concentrations, phytoplankton production, and microheterotroph production and metabolism were assessed. Phytoplankton production from February to May generates a quantity of organic matter more than adequate to cause oxygen decline in mid-Bay deep waters. The development of the summer mazimum in phytoplankton productivity appears to depend upon the regeneration of nutrients from phytoplankton carbon produced in the spring which sinks into deep water and is remineralized by bacteria. These nutrients are recycled into the euphotic zone via vertical mixing and oscillations of the pycnocline. Bacterial biomass, production, and metabolism are tightly coupled to phytoplankton production during the spring and summer. Phytodetritus is the source of carbon fueling bacterial oxygen consumption in deep waters. A relationship for predicting oxygen consumption rates from bacterial abundance estimates has been developed. Water column oxygen consumption by microheterotrophs is a major contributor to the development of anoxia during the spring and early summer. Microbial sulfur cycling then becomes an important mechanism for maintaining anoxia. The potential for the establishment of anoxia is greatest in the northern regions of the mesohaline Chesapeake Bay. greater variation over shorter distances in an east to west direction. However, phytoplankton and bacteria parameters exhibit greater variatin over shorter distances in an east to west direction. Comparisons of biological measurements made in the summer of 1984, a high flow year in which anoxia was widespread, with measurements made in 1985, a low flow year in which anoxia was limited and of short duration, suggest that fundamental changes have occurred in the ecosystem of mid- Chesapeake Bay such that a significant portion of primary production is channeled into bacterial production. Thus, the severity of anoxic conditions depends upon climatic differences regulating freshwater flow, pycnocline tilting, and wind mixing rather than upon changes in biological parameters.
Phytoplankton , Micro-Heterotrophs , Summer Conditions 1985