Feeding rates and selectivity of adult Euphausia pacifica on natural particle assemblages in the coastal upwelling zone off Oregon, USA, 2010

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Filtration by adult Euphausia pacifica was measured before and during the upwelling season, using both “disappearance of chlorophyll” and “disappearance of cells” techniques. Results show that feeding rates and selectivity varied with food assemblages. Filtration rate (F) was best modeled by the Ivlev function: the average F on total Chl-a was 92 mL euphausiid−1 h−1, and 119 mL euphausiid−1 h−1 on microscopy cell counts. F averaged 36 for the <5 µm size fraction of Chl-a, 94 for the 5–20 µm fraction and 107 mL euphausiid−1 h−1 for the >20 µm fraction. The average F values were 155 and 163 mL euphausiid−1 h−1 for chain-diatoms and single diatoms, respectively, and115 and 137 mL euphausiid−1 h−1 for the <40 µm and >40 µm ciliates, respectively. Ingestion rates based on total Chl-a and size fractions, total cell counts and ciliates were significantly correlated using Hollings' models (P < 0.01). Maximum daily ration was 23% body C day−1 when a high food concentration (700 µC L−1) was available, but over the carbon range of 50–200 µg C L−1, daily ration averaged 4% body C day−1. Diatoms were consumed almost exclusively during blooms associated with summer upwelling events; larger types of ciliates and dinoflagellates were fed upon preferentially compared with their smaller counterparts.

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