Clearance rates of < 6 µm fluorescently labeled algae (FLA) by estuanne protozoa. potential grazing impact of flagellates and ciliates

Abstract

Microzooplankton, numerically dominated by phagotrophic ciliates and flagellates, can have a significant grazing impact on phytoplankton, especially on ultraplanktonic (< 5 pm) algal cells, which are responsible for a large part of primary production in many pelagic systems. However, there are few analyses of actual in situ clearance rates of this size class of phytoplankton by natural protozoan assemblages. We estimated clearance rates of < 6 pm sized algal prey by heterotrophic flagellates and ciliates present in tidal creek water in a salt marsh estuary during the months of December 1989 to March 1990 via their rates of uptake of fluorescently labeled algae (FLA). Three types of FLA, ranging in size from 1.9 to 5.4 pm, were made from cultures of Nannochloris atomis, Chlorella capsulata, and Thalassiosira pseudonana. Both ciliates and flagellates ingested the FLA at rates greater than they ingested similarly sized fluorescent microspheres. Flagellates 5 to 10 pm in size predominantly ingested the 2 pm FLA, while oval cells 10 to 15 p in size, likely athecate dinoflagellates, were important consumers of 3.4 and 5.4 pm FLA. Estuarine ciliates, mostly choreotrichs 15 to 60 pm in size, ingested all types of FLA, but with lower clearance rates for the 2 pm FLA than for larger sized FLA. Clearance rates determined for flagellates ranged from 0.004 to 0.83 pl cell-' h-', and for ciliates from 0.24 to 8.3 p1 cell-' h-'. Estimated daily clearance rate by algivorous protozoa over the 4 mo period averaged 45 % of the water volume based on uptake of the 2 pm sized FLA, and 107 % of the water volume based on uptake of the larger sized FLA. The average grazing impact of flagellates was about 33 % that of ciliates for 2 pm sized prey, and 50 and 85 % that of ciliates for 5.4 and 3.4 pm sized prey respectively. Two experiments were carried out to determine the functional feeding response of algivorous flagellates and ciliates over an order of magnitude range in concentration of each FLA type. The 2 groups of protozoa had differing clearance-rate and FLA-ingestion-rate responses; clearance rates generally decreased with increase in FLA concentration. The results support the ideas that (1) total rnicrozooplankton herbivory is a combination of partial grazing pressures by individual species on different size classes of phytoplankton, and (2) the importance of algivory by < 20 pm sized phagotrophic flagellates should be considered in future studies.