The influence of different disturbance frequencies on the species richness, diversity and equitability of phytoplankton in shallow lakes

Abstract

The relationships between the species richness, diversity and equitability of phytoplankton is discussed in the context of Connell’s (1978, Science 199: 1304–1310) Intermediate Disturbance Hypothesis (IDH). The records of 759 vertical phytoplankton samples, which were obtained from four shallow central European lakes (Balaton, Neusiedlersee, and two small artificial ponds) at daily to weekly intervals were analysed.

• 1) The Shannon-Weaver function was used to measure diversity of the recorded species composition of the phytoplankton. It is shown on fictitious data that compositional diversity is sensitive to the number of coequilibrating species provided that the suspected interrelationship between diversity and ‘complexity’ is amenable to the application of this method.

• 2) The disturbance scale that was developed on the basis of the field records fits well to Reynolds’ (1988, Verh. int. Ver. Limnol. 23: 683–691) derivation: < 3 days qualifies as high frequency, approximately 3–8 days as intermediate frequency and > 8–9 days as low frequency of disturbance for phytoplankton.

• 3) Arithmetical means of the compositional diversity of phytoplankton under different frequencies of disturbance support the hypothesis that maximal diversity appears at intermediate frequencies.

• 4) There are different reasons for decrease in diversity at higher and lower frequencies. Inequitability diminishes diversity at low disturbance; while species number decreases at high frequencies.

• 5) The case of Neusiedlersee calls attention to the fact that it is difficult, if at all possible, to differentiate between the indices under continuous stress and high frequency of disturbance in lakes in temperate regions. Similar species number-equitability pattern are induced by both and it is also presumable that high frequency disturbance can itself effect a serious stress.

• 6) The striking effects that regular major periodic events (e.g. significant changes in the grazing pressure at the onset of the clear-water phase, autumnal cooling) in the plankton have on its species diversity are evident. Thus, the relative importance of intermediate frequency disturbances has its own seasonality: it is increasingly important in periods (partly in the spring, but mostly in the summer-autumn equilibrium phases), in which competition among phytoplankton species is increasing. This observation suggests a way by which the stochasticity-based IDH can be incorporated into rather more deterministic explanations (e.g. PEG-model; Sommer et al., 1986. Archiv für Hydrobiologie 106: 433–471) of plankton succession.

• 7) The most controversial issue and, therefore, the main difficulty, with IDH is that it not only maintains species richness in an ecosystem but it also supposes its presence. The lack of either early or late successional species in a given community can inactivate the mechanism. From the point of view of the diversity-species richness relationship, the persistence of disturbance at given frequencies is of greater importance than the temporal alterations themselves in the evolutionary ecology of the phytoplankton.

• 8) For characteristically unperturbed phytoplankton communities (no case was studied here), equilibrium concepts (niche diversification, etc.) should be more strongly applicable to their diversity and species richness.