Case Study 1: Eutrophication (Stansfield et al. 1986)
This scenario includes the loss of submerged plant communities with eutrophication and the use of organochlorine pesticides (Stansfield et al. 1989). The research of Brian Moss and associates has shown that some of the shallow lakes of the Norfolk Broads have switched from submerged plant dominance to phytoplankton dominance during the 1950-60s. It is proposed that the likely mechanism was a poisoning of the community of Cladocera which graze on the algae and are associated with the plants. This allowed phytoplankton to take advantage of the nutrient loadings and increase their biomass.
Sediment cores were taken from Hoveton Great Broad, in which the switch to phytoplankton has occurred, and from two Broads (Upton and Marham South Broads) in which submerged plants are still dominant. The chydorid remains were grouped after Whiteside (1970) into clear water (plant-associated - C), non-specific (largely mud-associated - N) and turbid water (T) groups.
In Hoveton Great Broad, a switch from clear water associated chydorids was found to coincide with the loss of aquatic plants in the 1950's. No Daphnia remains were found. In the other two cores, clear water chydorids, mostly plant associated, were found throughout the cores.
DDT derivatives were particularly associated with the end of the phase of submerged plant dominance and the beginning of that of phytoplankton dominance.