Clearance rates of Oikopleura in cold coastal Newfoundland waters: a predictive model and its trophodynamic implications

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Abstract
Oikopleura vanhoeffeni inhabiting cold coastal Newfoundland waters are much larger than the extensively studied warm water species, O. dioica; nevertheless they possess fine-meshed feeding filters capable of retaining the nanoplankton that dominate the water column throughout much of the year. Total clearance rates were determined using radioactively labelled algae presented to individuals in feeding chambers in situ over a seasonal range of ambient water temperatures and food concentrations. Stepwise multiple regression analysis indicated that oikopleurid size explained 46% of the variance in clearance rate. An additional 10% of the variation was explained by the ambient biomass concentrations of chain-forming diatoms and of picoplankton (< 2 .mu.m in diameter), consistent with a filter-clogging interpretation. Water temperature was not a significant variable in the regression model and its coefficient was negative, indicating that clearing rates were highest at the near 0.degree. C temperatures characteristic of the spring bloom, a period when bacterial activity is suppressed. Water column clearing rates attributable to Oikopleura populations were comparable to those calculated for the copepod community based on published data. The large size and conspicuous pigmentation of O. vanhoeffeni should make them particularly susceptible to visually feeding planktivores and they may thus constitute an important link in the food chain supporting a large commercial fishery.