MEPS 318:187-201 (2006)  -  doi:10.3354/meps318187

Zooplankton response to storm runoff in a tropical estuary: bottom-up and top-down controls

R. S. Hoover1, 3,*,**, D. Hoover1, M. Miller1, M. R. Landry2, E. H. DeCarlo1, F. T. Mackenzie1

1Department of Oceanography, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA
2Integrative Oceanography Division, Scripps Institution of Oceanography, La Jolla, California 92093-0227, USA
3Present address: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, California 95039, USA
* Email: **Former name: R. D. Scheinberg

ABSTRACT: Zooplankton successional patterns and response times were characterized in a tropical estuary following a major storm-runoff event to evaluate the effects of a nutrient perturbation on community composition and dynamics. Intensive water-column monitoring in southern Kaneohe Bay, Hawaii, showed that dissolved macronutrients—NO3 + NO2, SRP (soluble reactive phosphorus) and Si(OH)4—increased significantly immediately following the initial runoff event. Bottom-up effects were evident in both phytoplankton and zooplankton communities. An initial phytoplankton bloom was dominated by small cells and lasted only a few days, while post-bloom pigment concentrations showed a more gradual increase in total chlorophyll a and a shift to a diatom-dominated community. The initial bloom had an unexpectedly large influence on zooplankton growth and reproduction on extremely short time scales. Appendicularians exhibited the most dramatic response, with biomass increasing 6-fold in 1 d, and abundances reaching values only rarely observed in these waters. Response covaried with organism size, with larger components of the community, especially calanoid copepods and gelatinous zooplankton, increasing as new resources became available. Post-bloom changes in zooplankton and phytoplankton community structure also suggest significant top-down controls on phytoplankton and zooplankton community biomass and structure, with increased predation on appendicularians and copepods resulting in partial release of grazing pressure on small and large cells, respectively. Nutrient-rich runoff can have significant and surprisingly rapid impacts on zooplankton population dynamics in tropical coastal waters via direct, pulsed, food influences on the growth and reproduction of omnivorous organisms and the indirect stimulation of secondary consumers.

KEY WORDS:Zooplankton · Phytoplankton · Succession · Response times · Nutrients · Runoff · Bottom-up · Top-down

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