SEASONAL SUCCESSION OF PHYTOPLANKTON IN A LARGE SHALLOW LAKE (BALATON, HUNGARY) - A DYNAMIC APPROACH TO ECOLOGICAL MEMORY, ITS POSSIBLE ROLE AND MECHANISMS

1. The phytoplankton of the large shallow Lake Balaton was studied quantitatively on 211 consecutive days between April and November 1980. In total 417 taxa were found during the period of sampling. The number of planktonic species was c. 300. Compared with the few dominants, most of the species were extremely rare. 2. Relative frequencies (RF) within the total number of enumerated specimens (c. 100000) were used to analyse the quantities of both frequent and rare species on a common quantitative base. Approximately two-thirds of the 300 planktonic species were evenly distributed in time, the others exhibited temporal peaks. Based on population dynamics, 100 species (RF > 2000(-1), i.e. a relative frequency greater than 500 out of 100000 specimens) are considered to characterize the community responses to changes in both major and minor resources set by the given pattern of environmental background. Approximately 15 of these 100 species might have had an allochthonous origin. 3. Approximately half of the 200 rare species are velocity specialists, able to grow rapidly in spatially and temporally randomly distributed, nutrient-rich micro-environments; the other 100 are termed the 'ecological memory' of the phytoplankton community. Memory is considered here to be the capacity of past states or experiences to influence present or future responses of the community. 4. The memory of the community comprises all the potential recruit-species which are not completely excluded because of spatial and temporal disturbances (heterogeneity). These species may out-compete the regular dominants of the most probable seasonal successional pattern if the competitive arena changes. 5. Several cases (years with exceptional meteorological conditions, limnocorral experiments) in the history of planktonic events in Lake Balaton and in other lakes support the hypothesis.