STABILITY OF OLIGOTROPHIC AND EUTROPHIC PLANKTONIC COMMUNITIES AFTER DISTURBANCE BY FISH

The objective of our study was to investigate the relationship between resource availability and stability (resistance and resilience) in fr eshwater zooplankton communities and to assess their recovery after pr edation impact. We used 12 in-lake enclosures (3.8 m diameter x 3.5 m deep) treated with 6 combinations (replication n = 2) involving 2 nutr ient concentrations and 3 fish densities. The experiment lasted 14 wee ks and zooplankton and water chemistry were monitored throughout. The protocol involved 3 treatment-specific time periods: (a) the before fi sh treatment period comprising the first 2 weeks, (b) the during fish treatment period comprising the next 5 weeks, and (c) the after fish t reatment period which lasted 6 weeks following fish removal. In terms of biomass and numbers, we found that the zooplankton in the fish trea tments varied most and that the eutrophic biomasses were higher. In te rms of stability, the zooplankton in the oligotrophic enclosures were more resistant to disturbance by fish and were more resilient after th e fish were removed. Conversely, zooplankton communities in the eutrop hic enclosures responded more strongly to fish predation and were less likely to return to the pre-disturbance community structure. Zooplank ton at the top of the food web (top carnivores) were the most suscepti ble to planktivory, but recovered as quickly as the other zooplankton groups. Studies of trophic structure showed that the omnivorous, inter mediate species group, had the highest species interaction levels and contributed most to community recovery in all of the treatments. Resou rce availability and disturbance magnitude are likely to determine pos t-disturbance biomass production. The stability of low productive syst ems seems to be maintained through strong food web links, while more p roductive systems are more loosely structured.