Diversity in Fresh-water Phytoplankton

Abstract

Seasonal changes and mean values of diversity, calculated by the Shannon formula, are given for the phytoplankton communities of three freshwater bodies. Particularly in the more eutrophic bodies, diversity increased in summer but was low in winter. Diversity decreased with increased fertility of the water. The data confirm predictions made in the literature, but existing explanations of the phenomena are considered unnecessarily complex. An explanation based on the balance of cell numbers between overlapping populations of several species is given, supported by a simple mathematical model. The pigment diversity index and the nature of succession (periodicity) in freshwater phytoplankton are discussed. INTRODUCTION Diversity, though variously and precisely defined in several different formulae, is a concept to which it is difficult to give concrete meaning. Tendencies for numbers of organisms to become equally distributed among increasing numbers of species present are included in the recognition of progressively more diverse communities. However, different formulae, empirically or intuitively derived, may give different rankings to members of a series of communities. Similar discrepancies might be expected if standing crop or, better, individual annual production rather than numbers of individuals were used for calculation. Although Margalef (1968, p. 52) believes that diversity of a whole ecosystem is reflected accurately by the usually measured diversity of restricted taxonomic or trophic groups, this correspondence has yet to be tested. This paper takes one widely used diversity index, considers what it means practically, and examines two propositions: (1) diversity increases with time during the spring and summer development of phytoplankton populations in temperate waters (Margalef, 1958 ); (2) phytoplankton communities in infertile waters are more diverse than those in fertile waters (Margalef, 1964; 1968, p. 56). Analogies between seasonal development in phytoplankton communities and succession of vegetation types on land are also discussed. CHOICE OF DIVERSITY INDEX Phytoplankton communities contain cells of some species that from time to time divide and produce populations in the open water. Also present are many species which are not truly planktonic and do not reproduce in the open water, but are resuspended from algal comContribution number 236 from the W. K. Kellogg Biological Station.