Quantifying marine snow as a food choice for zooplankton using stable silicon isotope tracers

Aggregates of biogenic origin >0.5 mm, known as marine snow, represent a concentrated potential source of food for zooplankton. Little is known, however, about whether aggregates are commonly grazed by zooplankton in the field. While previous laboratory studies have shown that the euphausiid Euphausia pacifica, and the copepod, Calanus pacificus, common crustacean zooplankters, consume marine snow if it is the only food source available, it is not known if euphausiids will select marine snow in the presence of edible dispersed cells, as readily occurs in nature. To examine this question, we offered E. pacifica the diatom, Nitzschia angularis in aggregated and dispersed form as prey simultaneously. Aggregates and dispersed food contained identical cell types so that differing prey quality, taste or motility would not be a factor. A new method was developed to track food sources by labeling the frustules of aggregated cells with different naturally occurring, but rare, stable isotopes of silicon, Si and Si. Food selection was then estimated by measuring the isotopic composition of silica within fecal pellets produced by animals feeding on mixtures of the two labeled foods. Results indicate that E. pacifica consumed both aggregates and dispersed cells, even when more cells were made available in dispersed form than in aggregated form. This suggests that aggregates may indeed be a food source in the field, even when dispersed cells are relatively abundant. The method of labeling diatom cells with stable isotopes of silica may prove useful for future grazing experiments to distinguish identical cell types.