Cladoceran remains as evidence of change in trophic state in three Swiss lakes

Species composition and density of cladoceran populations changed in lakes Zuerich, St. Mortiz and Baldegg as human populations increased in these watersheds. Lake Zuerich sediments became annually laminated in the 1890's as a result of increased organic input as the size of the cities surrounding the lake grew. At the same time, the Bosmina species changed from a oligotrophic form (longispina ) to a eutrophic form (longirostris ). An increase in Daphnia spp. populations also occurred at this time in the lake's history. Bosmina longispina reappeared in the lake in 1965 as the lake's trophic status changed from eutrophic to mesotrophic due to effective sewage treatment facilities. Annual laminations appear in the Lake St. Moritz sediments about 1910. Shortly thereafter, a shift from B. longispina to B. longirostris occurred. This change in trophic status is associated with increased tourism in the area. Lake Baldegg sediments also show annual laminations beginning in 1885 and a similar shift in the Bosmina species.