In situ thymidine uptake by particulate matter and zooplankton grazing rates on 3H-thymidine-labelled particulate matter were measured in the euphotic zone in various Warm Core Gulf Stream Rings (WCR), the North Atlantic Slope Water, Northern Sargasso Sea and at four stations
in the Ría de Arosa estuary in Spain. Thymidine [3H] uptake by particulate matter was greatest in the estuary and lowest in the Sargasso Sea and Slope Water. The percent of total 3H uptake (>0.2 μm) which was >3.0 μm ranged from 13% in
the Slope Water to 40% in the Ría de Arosa. Correlation between 3H uptake by the >3.0 μm and 0.2–3.0 μm fractions was highest in the Sargasso Sea and Slope Water in summer and lowest in the estuary and Slope Water in spring and fall. Average
weight-specific ingestion of 3H-labelled particulate matter by 64–333 μm zooplankton was at least 25% greater than the >333 μm zooplankton size fraction. Weight-specific ingestion of 3H-labelled particulate matter by zooplankton multiplied
by their standing stock (i.e., total ingestion by zooplankton) was highest in Warm Core Rings > Slope Water > Sargasso Sea. Zooplankton grazing rates (ml mg C–1 h–1) calculated by assuming that only >3.0 μm thymidine-labelled particulate
matter was “available” food were similar to published laboratory grazing results. However, based on their thymidine [3H] uptake, zooplankton removed only 0.02 to 0.38% of the labelled >3.0 μm particles and 0.01 to 0.08% of the labelled particles >0.2
μm during the hour incubation. These results are consistent with other published studies which suggest that grazing by >64 μm zooplankton has little effect on populations of free-living and attached bacteria in the sea.
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