Structure-dependent, protistan grazing and its implication for the formation, maintenance and decline of plankton patches

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Plankton distributions are frequently patchy and phytoplankton patches have long been suggested as important resources in otherwise nutritionally dilute environments. The present study confirms this hypothesis empirically in a coastal fjord with naturally forming phytoplankton patches and examines the implications for plankton distributions, abundance and rates of patch formation, maintenance and decline. Phytoplankton patches were identified using a CTD mounted fluorometer and sampled with a horizontally mounted 2 l Niskin bottle in 3 separate field seasons in summer 2007 and 2008 and spring 2009. We quantified chl a and macronutrient concentrations as well as phytoplankton growth and zooplankton (<200 μm) grazing rates. Average phytoplankton growth was equal inside and outside of patches (0.34 ± 0.07 d-1) and there was no indication of nutrient limitation to phytoplankton growth. Average grazing rate inside patches (0.25 ± 0.03 d-1) was significantly higher than outside of patches (0.09 ± 0.03 d -1). Grazing pressure was not simply a function of prey availability; there was no significant relationship between grazing rate and initial chl a concentration. Protistan grazing consumed on average 65% of primary production within and 26% outside of patches. Model predictions of population dynamics suggest that protistan grazing focused within patches and more rapid phytoplankton accumulation outside of patches eroded layer structures within hours to days. Formation of plankton patches due to phytoplankton growth was not supported by the data. Averaging rates, irrespective of phytoplankton distribution, greatly overestimated layer persistence and minimally underestimated primary production and its availability to higher trophic levels. These results emphasize empirically the importance of predator-prey interactions to the ubiquitous phenomenon of plankton patchiness and ultimately microbial food web dynamics. © Inter-Research 2010.

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Marine Ecology Progress Series