Zooplankton Grazing during Dinoflagel‐late Blooms in a Cape Cod Embayment, with Observations of Predation upon Tin‐tinnids by Copepods

Abstract

Abstract. The patterns and rates of feeding by the copepod Acartia hudsonica and larvae of the polychaete Polydora sp. were investigated during the spring in an estuarine embayment. These dominant macrozooplankters fed upon the natural particulate assemblages (predominantly dino‐flagellates) spiked with the toxic dinoflagellate Gonyaulax tamarensis. G. tamarensis was ingested by both zooplankters, as was Heterocapsa triquetra, the most abundant alternative food item. Ingestion rates generally increased as dinoflagellate concentrations increased, resulting in a relatively constant (and low) filtration rate for each grazer. Dinophysis acuminata, another dominant dinoflagellate, was essentially ungrazed. Thus, the zooplankters did not ingest one dinoflagellate and consumed others in proportion to their availability but at low rates. Based on these low rates and the small number of A. hudsonica observed during the spring, we infer minima! grazing impact on a 1980 G. tamarensis bloom. In contrast, the impact of polychaete larvae may have been substantial, since their extreme numerical abundance more than compensated for low filtration rates.In one instance, the tintinnid Eutintinnus pectinis was accidentally included in a grazing experiment. Our ingestion data demonstrate that A. hudsonica, when presented with a combination of tintinnids and several species of phytoplankton (G. tamarensis and D. acuminata) ingested the tintinnids at high rates, in proportion to their high abundance. Since the nanoflagellate Chroomonas amphioxea was found within the loricae of many of the surviving tintinnids, this also provided a qualitative demonstration of the often‐hypothesized nanoplankton to tintinnid to copepod link in a marine food chain.