Seasonal Patterns in Production and Drift of Gammarus Pseudolimnaeus in Valley Creek, Minnesota

The stream amphipod, Gammarus pseudolimnaeus Bousfield, was sampled monthly for five consecutive years, 1968—1973, in Valley Creek, Minnesota, USA, and seasonal patterns in production rate, drift, standing stock, and growth were observed. Several modifications of the instantaneous growth rate method were attempted, and one, the Length Increment Alternative, employing size—frequency historgrams (both in numbers and biomass) to determine instantaneous growth rates, was selected as the most accurate to estimate production at approximately monthly intervals. Production, drift, and growth correlated well in a pronounced seasonal periodicity, with maxima in the warm months. A serious siltation occurred about midway in the study, reducing the population standing stock, production, and drift. During the 1st 2 yr prior to the siltation, when the stream was in normal condition, daily production (dry mass) reached summer maxima of about 0.12—0.14 g/m 2 , and maximum drift about 120—220 g dry mass°d — 1 °m — 3 °s — 1 , both falling to zero, or near, in winter. Daily instantaneous growth rate reached maxima of about 0.03 in all years, with zero growth in winter. Annual dry mass production by the Length Increment Alternative was 271, 238, 123, 57, and 91 kg/ha for the 5 yr, respectively. Calculations comparing daily drift to production indicated that production rates were sufficient to provide the observed drift without the necessity of compensatory upstream migration.