THE CHANGING ECOSYSTEM OF A SHALLOW, BRACKISH LAKE, HICKLING BROAD, NORFOLK, UK .1. TROPHIC RELATIONSHIPS WITH SPECIAL REFERENCE TO THE ROLE OF NEOMYSIS-INTEGER

1. Hickling Broad, a shallow, brackish lake in England, changed from dominance by submerged aquatic plants to dominance by phytoplankton in the 1970s. These changes were ascribed to the effects of guanotrophication by black-headed gulls, and increased salinity which together resulted in fish kills caused by a toxic alga, Prymnesium parvum. A mysid, Neomysis integer, was believed to be important in switching the system to plankton dominance through its presumed selective feeding on Cladocera and increasing population size as fish predation decreased. 2. Studies on laboratory predation rates of the mysid on Cladocera and on the population dynamics, predation rates and diet of the mysid and the populations of the major zooplankter, Eurytemora affinis in the Broad in the 1980s, have shown that the former explanations were incomplete. The mysid could have markedly reduced the cladoceran community, for it has potentially high predation rates, but Cladocera were probably lost through salinization. Neomysis feeds efficiently on Eurytemora affinis, but the latter reproduces rapidly and its populations are unlikely to be controlled by mysid predation. Nauplii and small copepodites are selectively taken. Alternative and probably major food sources for the mysid are periphyton and benthic algae and detritus. 3. Roach and bream readily take the mysid but the present fish stock in the Broad is very low and can now exert minimal pressure on the mysid population. An invertebrate predator on the mysid, Palaemonetes varians, is also unlikely to have major effects. The former model of the operation of the Broad's ecosystem needs reconsideration in view of the findings.