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Marine Ecology Progress Series

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MEPS 226:27-33 (2002)  -  doi:10.3354/meps226027

Extending the analysis of the dilution method to obtain the phytoplankton concentration at which microzooplankton grazing becomes saturated

Anna M. Redden1,*, Brian G. Sanderson2, David Rissik3

1School of Applied Sciences, University of Newcastle, PO Box 127 Ourimbah, NSW 2250, Australia
2Environmental Modelling Solutions, 38 Dora St, NSW 2250, Australia
3Estuaries Branch, Ecosystems Directorate, Dept Land and Water Conservation, GPO Box 39, Sydney, NSW 2001, Australia

ABSTRACT: The application of the dilution method to productive coastal waters has been limited, in part by difficulties in interpreting the nonlinear grazing dynamics. A split-function model of microzooplankton grazing is used to adapt the dilution method for analysis of mesotrophic and eutrophic systems. Phytoplankton growth rate µ and microzooplankton grazing rate g are obtained using the conventional analysis with dilutions sufficient to ensure that grazing rate is proportional to phytoplankton concentration. When phytoplankton concentration is sufficiently great, dilution experiments indicate that grazing becomes independent of phytoplankton concentration (i.e. grazing is saturated). A straightforward calculation yields the phytoplankton concentration Ps that saturates grazing. The saturated grazing is Gs = gPs. Grazing balances primary production when the phytoplankton concentration is Pb = gPs/µ. In productive waters, determination of Ps is important because microzooplankton grazing becomes an increasingly irrelevant constraint on exponential phytoplankton growth as the phytoplankton concentration further exceeds Pb. Data were analyzed from a dilution experiment conducted in a mesotrophic coastal lagoon in New South Wales, Australia. Values of µ, g, Ps, and Gs were 0.53 d-1, 0.65 d-1, 3.8 µgchl l-1, and 2.5 µgchl l-1 d-1, respectively. In this particular experiment the in situ phytoplankton concentration exceeded Pb by a factor of 1.7.

KEY WORDS: Microzooplankton grazing · Dilution method · Nonlinear models · Zooplankton · Phytoplankton growth · Saturated grazing

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