Document: BEA-3-74-3

Pelagic food web responses to temporal heterogeneity

BEISNER, B.E.*

University of British Columbia, Vancouver, B.C. V6T 1Z4 ¹

Abstract:
The pelagic environment within which plankton live varies in space and time. Upwelling events that arise at various frequencies from storm and wind events have been associated with nutrient pulsing. This phenomenon has been well studied in laboratory chemostats but the response of phytoplankton in the presence of the entire plankton food web in the field has yet to be determined. A series of mesocosm (5000L) field experiments investigated the response of phytoplankton community structure in the presence and absence of higher trophic levels, in systems of low and high productivity, subject to intermittent upwelling at high (3-5 days), intermediate (15 days) and low (21-30 days) frequencies. Natural phytoplankton communities responded to temporal heterogeneity in upwelling only under enriched conditions; Daphnia modified the response of the phytoplankton community, while a complete zooplankton community eradicated the response of phytoplankton. There was also some evidence to suggest that the frequency with which intermittent vertical mixing occurs had consequences for higher trophic levels. Finally, it was apparent that the effect of upwelling goes beyond simple nutrient pulsing, especially in oligotrophic systems. Changes to organismal encounter rates as a result of turbulent mixing and altered light regimes likely play equally important roles.

Keywords: phytoplankton, community structure, temporal heterogeneity, food webs, nutrients

This abstract is being presented at: 10:45 AM in session:
Oral Session #54: Lake Ecology.