A Field Manipulation of Trophic Interactions in Rock-Pool Plankton

Three rock-pools (ca. 10 Daphnia 1-1, ca. 100 Daphnia 1and > 100 Daphnia 1-1) were each bisected with a plastic curtain. One side of each pool was left intact, while planktivorous fish were introduced into the other side. The succession of the zooplankton and phytoplankton communities in the rock-pools was followed, using quantitative sampling, for one summer. In the experiment the fish eliminated large (> 0.5 mm) herbivorous cladocers from the zooplankton within 20-40 d of introduction. An increase in the numbers of small zooplankton (rotifers, cladocers) then occurred. In the control sides large Daphnia thrived well, though with fluctuating densities, throughout the summer. The response of phytoplankton to the elimination of large herbivores varied between the rock-pools. In terms of species diversity and evenness no obvious differences were observed between control and experiment in the rock-pool with low Daphnia density. In the medium-density rock-pool both species diversity and evenness were lower in the experimental part, while in the high-density rock-pool the opposite was observed. In these two pools elimination of Daphnia resulted in an increase in total numbers of cells 11 of 2-3 orders of magnitude. The difference between control and experimental parts was due to the increase in small cells (< 64 Bm diameter). The elimination of large herbivores resulted in lowered densities (low-density) or elimination (medium and high-density) of filamentous algae from the phytoplankton.