A Decade of Change in the Skidaway River Estuary. III. Plankton |
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| Authors: | Peter G Verity and David G Borkman |
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| Institution: | (1) Skidaway Institute of Oceanography, 10 Ocean Science Circle, Savannah, GA 31411, USA;(2) Graduate School of Oceanography, University of Rhode Island, Narragansett, 02882, RI, USA |
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| Abstract: | The Skidaway River estuary, GA (USA), a tidally dominated subtropical system surrounded by extensive Spartina salt marshes, is experiencing steady increases in nutrients, chlorophyll, and particulate matter and decline in dissolved
oxygen, associated with cultural eutrophication. A long-term study is documenting changes in these parameters: previous papers
Verity (Estuaries 25:944–960, 2002a, Estuaries 25:961–975, b) reported on hydrography, nutrients, chlorophyll, and particulate matter during 1986–1996; plankton community responses are
reported here. Phytoplankton, bacteria, heterotrophic nanoplankton and dinoflagellates, ciliates, and copepods exhibited strong
seasonal cycles in abundance driven by temperature and resource availability, typically with summer maxima and winter minima.
However, cultural eutrophication coincided with altered planktonic food webs as autotrophic and heterotrophic communities
responded to increasing concentrations and changing ratios of inorganic and organic nutrients, potential prey, and predators.
Small (<8 μm) photosynthetic nanoplankton increased in absolute concentration and also relative to larger cells. In contrast,
diatoms did not show consistent increases in abundance, despite significant long-term increases in ambient silicate concentrations.
Mean annual bacteria concentrations approximately doubled, and eukaryotic organisms in the microbial food web (heterotrophic
and mixotrophic flagellates, dinoflagellates, ciliates, and metazoan zooplankton) also increased. All plankton groups except
copepods showed trends of increasing annual amplitudes between seasonal high and low values, with higher peak concentrations
each year. These observations suggest that the eutrophication signal was gradually becoming uncoupled from regulatory mechanisms.
Theory and evidence from other more impacted waters suggest that, if these patterns continue, changes in the structure and
function of higher trophic levels will ensue. |
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