Importance of algae in the diet of subarctic populations of Gammarus lacustris and Pontoporeia affinis

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Abstract
Algae represented up to 13.5% by volume of the gut contents of G. lacustris collected between June 1975 and May 1976 from a subarctic stream. While most species were consumed in proportion to their abundance in the environment, there was considerable selection against adnate and filamentous forms. The chloroplasts of most algae were readily broken down in the gut and the maximum length of an ingested cell was 120 .mu.m. All animals fed continuously during the summer but most ceased feeding in winter. Algae represented only 0.1-1.5% of the gut contents of P. affinis collected from 3 subarctic lakes. Most algae were ingested, except for colonial Cyanophyta and algae attached to sand grains. As with G. lacustris, few cells contained intact chloroplasts. The maximum length of an ingested organism was 100 .mu.m. Feeding occurred continuously throughout the summer and appeared to cease between Oct. and March.