The Intermediate Disturbance Hypothesis and its applicability to planktonic communities: Comments on the views of Padisak and Wilson

The relevance of Connell's Intermediate Disturbance Hypothesis (IDH) to explanations of diversity and co-existence among plant species generally and in the phytoplankton in particular has been debated recently. Compared to terrestrial vegetation, planktonic communities experience distorted time and space scales, Generation times are in the order of days, not years to decades, Advective fluid transport raises the critical patch size to the order of kilometers, Within these scales, species survival and growth, community assembly and successional development in the phytoplankton conform to all the standards (compositional, strategic, thermodynamic) of conventional community organisation. These processes are known to move toward competitively excluded outcomes, Equally, they are liable to be interrupted by externally imposed disturbances, which reset the succession or alter its potential outcome, These findings are not only illustrative of intermediate disturbance but are instructive in the nature of diversity- disturbance relationships generally, IDH has considerable potential to explaining persistent species co-existence.     

Stress and disturbance in the phytoplankton community of a shallow,hypertrophic lake

The validity of Connell's intermediate disturbance hypothesis in phytoplankton communities was tested on data from a hypertrophic, shallow lake, Hjarbæk Fjord, Denmark.The present data from Hjarbæk Fjord demonstrate the difficulties in distinguishing stress from disturbance in a phytoplankton community, and show that great changes in the phytoplankton community can take place within few days.A collapse of blue-green algae in late June 1986 caused remineralization of nutrients and resulted in a rapid increase of fast-growing small chlorococcal green algae and phytoplankton species diversity, without any external disturbances acting on the lake. External disturbances in the form of wind action and brackish water intrusion occurred several days after the onset of these events. Carbon depletion and pH 11.0 were severe stress factors on the phytoplankton community. They were induced by calm, warm weather, but eventually acted as a kind of disturbance to the normally well circulated lake.     

Influence of experimental soil disturbances on the diversity of plants in agricultural grasslands

Aims and Methods Disturbance is supposed to play an important role for biodiversity and ecosystem stability as described by the intermediate disturbance hypothesis (IDH), which predicts highest species richness at intermediate levels of disturbances. In this study, we tested the effects of artificial soil disturbances on diversity of annual and perennial vascular plants and bryophytes in a field experiment in 86 agricultural grasslands differing in land use in two regions of Germany. On each grassland, we implemented four treatments: three treatments differing in application time of soil disturbances and one control. One year after experimental disturbance, we recorded vegetation and measured biomass productivity and bare ground. We analysed the disturbance response taking effects of region and land-use-accompanied disturbance regimes into account.Important findings Region and land-use type strongly determined plant species richness. Experimental disturbances had small positive effects on the species richness of annuals, but none on perennials or bryophytes. Bare ground was positively related to species richness of bryophytes. However, exceeding the creation of 12% bare ground further disturbance had a detrimental effect on bryophyte species richness, which corresponds to the IDH. As biomass productivity was unaffected by disturbance our results indicate that the disturbance effect on species richness of annuals was not due to decreased overall productivity, but rather due to short-term lowered inter- and intraspecific competition at the newly created microsites. Generally, our results highlight the importance of soil disturbances for species richness of annual plants and bryophytes in agricultural grasslands. However, most grasslands were disturbed naturally or by land-use practices and our additional experimental soil disturbances only had a small short-term effect. Overall, total plant diversity in grasslands seemed to be more limited by the availability of propagules rather than by suitable microsites for germination. Thus, nature conservation efforts to increase grassland diversity should focus on overcoming propagule limitation, for instance by additional sowing of seeds, while the creation of additional open patches by disturbance might only be appropriate where natural disturbances are scarce.     

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

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 compositionof the phytoplankton. It is shown on fictitious data that compositional diversity is sensitive to the numberof 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, approximatel3–8 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 ofdisturbance support the hypothesis that maximal diversity appears at intermediate frequencies.
4. There are different reasons for decrease in diversity at higher and lower frequencies. Inequitabilitydiminishes 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 differentiatebetween the indices under continuous stress and high frequency of disturbance in lakes in temperateregions. Similar species number-equitability pattern are induced by both and it is also presumablethat 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 pressureat the onset of the clear-water phase, autumnal cooling) in the plankton have on its species diversity areevident. 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 equilibriumphases), in which competition among phytoplankton species is increasing. This observation suggestsa way by which the stochasticity-based IDH can be incorporated into rather more deterministicexplanations (e.g. PEG-model; Sommer et al., 1986. Archiv für Hydrobiologie 106: 433–471) of planktonsuccession.
7. The most controversial issue and, therefore, the main difficulty, with IDH is that it not onlymaintains species richness in an ecosystem but it also supposes its presence. The lack of either earlyor late successional species in a given community can inactivate the mechanism. From the point of viewof the diversity-species richness relationship, the persistence of disturbance at given frequencies is ofgreater importance than the temporal alterations themselves in the evolutionary ecology of the phytoplankton.
8. For characteristically unperturbed phytoplankton communities (no case was studied here), equilibriumconcepts (niche diversification, etc.) should be more strongly applicable to their diversity andspecies richness.

     

Phytoplankton succession and diversity in a warm monomictic,relatively shallow lake: Lake Volvi,Macedonia, Greece

In Lake Volvi, phytoplankton dominance was examined in relation to the main phases of the fluctuating physical state of the water column and nutrient levels. Four stages of algal succession were identified. The successional pattern was, in general, similar from year to year. External disturbances such as increased wind mixing and high floods had the effect of setting the succession back to an earlier stage.Nanoplanktic diatoms and flagellates dominated during the first stages of succession. The succession was running from r-selected species in early stages to K-strategists in summer and towards a mixed community in the terminal stage.Diversity was expressed by the indices of Gleason and Shannon (based on physical units and biomass). Interpretation of species diversity took into account the differential variations of its two components, the number of species and the evenness of their distribution. Diversity was more affected by evenness than by number of species.Diversity increased in late stages of succession corresponding to the complexity of the community (high number of species, high evenness). Physical disturbances influenced positively the diversity. The maximal diversity appeared in the transition periods between the compositional changes generated by disturbances and the true successional development. Disturbances may generate a more uniform distribution of diversity throughout the whole water column.The role of physical disturbances in increasing the phytoplankton diversity may be demonstrated from comparisons of diversity — evenness — species number of the same stages under different meteorological conditions.     

Does the intermediate disturbance hypothesis comply with thermodynamics?

A unique data set from Keszthely Bay, Lake Balaton has been applied to develop a dynamic structural model able to describe the observed changes in phytoplankton biomass and diversity. We tested whether the model reacts according to the Intermediate Disturbance Hypothesis and according to the hypothesis that ecosystem reactions attempt to maximize the thermodynamic function exergy under prevailing conditions. If the answer to these tests are confirmatory, it can be considered a support for IDH and for the use of the exergy maximization principle as a general principle to explain ecosystem reactions.     

The effect of colonization and competition processes on the relation between disturbance and diversity in plant communities

Many theoretical and field studies have emphasized the impact of disturbance in the dynamics and diversity of sessile organism communities. This view is best reflected by the Intermediate Disturbance Hypothesis (IDH), which states that a maximum of diversity is found in ecosystems or communities experiencing intermediate disturbance regimes or at an intermediate stage of development since the last major disturbance event. Although theoretical models based on competitive interactions tend to validate this hypothesis, a recent meta-analysis of field experiments revealed that the mono-modal relationship between disturbance and diversity might not be a general pattern. In this article, we investigate the relationship between disturbance and diversity through the study of patch models, combining two types of competitive interactions: with or without competitive hierarchy, with two mechanisms influencing colonization: negative frequency dependence in colonization rates and immigration. These combinations led to various disturbance-diversity patterns. In the model without competitive hierarchy (founder effect model), a decreasing relationship appeared to be the rule as mentioned in previous studies. In the model with competitive hierarchy, the IDH pattern was obtained for low frequency dependence and low immigration. Nevertheless, high negative frequency dependence in colonization rates led to a decreasing relationship between disturbance and diversity. In contrast, high immigration led to an increasing relationship. The coexistence window (the range of disturbance intensity allowing coexistence) was the widest for intermediate immigration rates. For random species assemblages, patterns with multiple peaks were also possible. These results highlight the fact that the mono-modal IDH pattern should not be considered a rule. Competition and colonization mechanisms have a profound impact on the relationship between disturbance and diversity.     

Disturbance, diversity and phytoplankton production in a reservoir affected by inter-basin water transfers

The effect of water transfers between two reservoirs on the phytoplankton community of the receiving reservoir was investigated over a 9-year period. Canonical correspondence analysis was used to demonstrate the significance of water transfers as an anthropogenic disturbance to the phytoplankton community and its diversity. A mass balance associated with a Granger causality test was applied to discriminate between the cell transport effect from the upstream reservoir and internal processes within the receiving reservoir, and to quantify the net phytoplankton growth in the receiving reservoir. Low and high disturbance regimes were identified and diversity was maximised during low disturbance conditions. The decrease of diversity during high disturbance conditions was explained by decreasing retention time, increasing silica loads and by the transport of specific phytoplankton genera, i.e. diatoms, from the upstream reservoir. Disturbance regimes significantly affected the relationship between phytoplankton production and diversity. Low disturbance regimes were described by phytoplankton dynamics likely influenced by complementarity effects, while high disturbance regimes were characterised by a phytoplankton community dominated by highly productive species and increased productivity, thus indicating an advantage of selection behaviour over complementarity effects. The phytoplankton diversity, expressed as evenness, was identified as a key variable explaining the relationship disturbance-diversity-phytoplankton production.     

Intermediate disturbance in the ecology of phytoplankton and the maintenance of species diversity: a synthesis

This paper concludes a collection of contributions presented at the 8th Workshop of the International Association of Phytoplankton Taxonomy and Ecology. It derives a consensus as to the virtues and strengths of J. H. Connell's Intermediate Disturbance Hypothesis (IDH), its applicability to phytoplankton ecology and its theoretical and practical weaknesses. The view is expressed that the IDH is too useful a concept to reject and that, as a word model, it provides a powerful link between diversity and disturbance. The more robust investigations that are necessary to consolidate the tenancy of IDH need to concentrate upon the separation and quantification of the stimulus- and response-components of disturbance.     

Exposure of non-target vertebrates to second-generation rodenticides in Britain, with particular reference to the polecat

The Intermediate Disturbance Hypothesis [IDH] and the Gradual Climate Change Hypothesis [GCC] offer intuitively appealing, verbal non-equilibrium explanations to species coexistence in competitive communities, but so far they lack a solid theoretical background and a proper experimental methodology. To make them testable and comparable on a solid methodological basis, they should be formulated as well-defined non- equilibrium community dynamical models. We suggest that this is possible, if explicit assumptions on the spatiotemporal structure of the environment and the pattern-generating mechanisms of the species assemblage in question are given. In the framework of a non-spatial population dynamical model we show that disturbance and climate change effects can be safely distinguished, and the "intermediate" level of external effects leading to maximum community diversity can be quantified. Based on the information statistical analysis of field data and simulation results, we explain why it is necessary to consider simultaneously the spatiotemporal patterns of the vegetation, the abiotic environment and the disturbances in order to predict the consequences of external effects regarding community diversity.     

Seasonal succession of phytoplankton and its diversity in two highly eutrophic lakes with different conditions of stratification

Seasonal succession of phytoplankton biomass, its diversity and its photosynthetic activity in two highly eutrophic lakes have been compared. In order to test the intermediate disturbance hypothesis, the lakes have been chosen with almost the same level of trophy but different conditions of stratification, through two ice-free periods of open water with different weather conditions.High phytoplankton diversity throughout the period of investigation was characteristic for the shallower Lake Lobardzu. The number of species here was usually more than 30 and the Shannon diversity changed from 1.2 to 4.2. Owing to the frequent external disturbances, periods characterized by autogenic succession with establishing dominance and declining diversity alternated with periods of biomass reduction and rises of diversity and photosynthetic activity. In the warmer summer of 1983, with more intense warming of bottom layers and predominance of blue-greens, phytoplankton biomass was higher and diversity lower than in the cold summer of 1982.In stratified Lake Rudusku, phytoplankton diversity and number of species were usually much lower. During the long summer stratification up to three-four dominant species of blue-greens and dinoflagellates become established and competitive exclusion leading to low diversity advanced. Some changes in biomass and diversity, were caused by zooplankton activity.     

Trait response in communities to environmental change: effect of interspecific competition and trait covariance structure

The response of ecological communities to environmental disturbances depends not just on the number of species they contain but also on the functional diversity of the constituent species; greater variation in the tolerance of species to different environmental disturbances is generally thought to confer greater resistance to the community. Here, I investigate how the functional diversity of communities changes with environmental disturbances. Specifically, I assume that there is variation in traits among species that confer tolerance or sensitivity to environmental disturbances. When a disturbance occurs, variation in species tolerances causes changes in the relative abundances of species, which in turn changes the average tolerance of the community. For example, if tolerance to an environmental disturbance is conferred by large body size, then the environmental disturbance should be expected to increase the average body size of individuals in the community. Despite this expectation, ecological interactions among species can affect the average community response. For example, if larger species are also strong competitors with each other, then this might reduce the increase in average body size in the community, because interspecific competition limits the grow in population density of large bodied species. Similarly, when disturbances affect multiple traits, the covariance in the distribution of trait values among species may restrict the response of any one trait; if two traits provide tolerance to the same disturbance but negatively covary among species, then the response of one trait will limit the response of the other trait at the community level. Using a Lotka–Volterra model for competitive communities, I derive general formulae that generate explicit predictions about the changes in average trait values in a community subject to environmental disturbances. These formulae demonstrate that competition can impede the change in average community trait values. However, the impediment is not considerable in comparison to the predominant factors of trait variances and species selection effects when species with the most similar trait values also experience the greatest interspecific competition. Similarly, negative covariances among different traits that confer resistance to the same environmental disturbance will impede their responses. I illustrate these results using phytoplankton data from a whole-lake experiment in which manipulation to the zooplankton community created a disturbance to the phytoplankton that changed the selective consumption of large vs. small phytoplankton.     

Disturbances by human trampling alter the performance,sexual reproduction and genetic diversity in a clonal woodland herb

Disturbances play a major role in structuring the community composition in a variety of ecosystems. The potential impact of disturbances on sexual reproduction and genetic diversity of plants has so far received little attention. We examined the effects of disturbance by human trampling on the performance, sexual reproduction and clonal diversity in the woodland herb Anemone nemorosa in six sites disturbed by intensive recreational activities and in six undisturbed sites in suburban forests in Switzerland. Disturbance by human trampling decreased shoot height of A. nemorosa and altered the size-dependent flowering probability function. Furthermore, in disturbed sites an increased percentage of aborted seeds was found, resulting in a reduced sexual reproductive potential of A. nemorosa populations. Both clonal and genetic diversity of A. nemorosa were reduced in disturbed sites. Our study shows for the first time that disturbance by human trampling can decrease the genetic diversity in a clonal woodland plant. This suggests that genetic aspects of woodland plants should be considered in restoring degenerated forest sites.     

Phytoplankton communities and antecedent conditions: high resolution sampling in Esthwaite Water

1. The succession of a phytoplankton community was investigated through an intensive period of sampling and related to physical, chemical and biological conditions sampled at an equal, or higher, temporal resolution. 2. Phytoplankton samples were taken on a weekly basis from June to September 2004 and analysed for diversity, species composition, and contribution of different functional groups to total biomass. Physical and chemical data were collected on the sampling days, and physical environmental factors were also logged continuously throughout the period by automatic measuring stations. This continuous logging allowed community structure to be compared with physical data averaged over periods from a day to a week before each sampling date. 3. The Schmidt stability of the lake, a measure of the strength of stratification calculated from thermal data, showed a negative correlation with phytoplankton species diversity. This is consistent with the hypothesis that mixing was preventing exclusion by species that would otherwise dominate in stratified conditions. 4. At a functional level, stress tolerant (S‐type) species dominated during the stratified summer conditions, with small, colonising species (C types) and ruderal, disturbance tolerant species (R types) contributing little to the overall biomass. Of the stress tolerant species, the faster growing (S_C) phytoplankters were significantly favoured by more stable, stratified conditions and higher solar radiation. Increased abundance of this group resulted in decreased species diversity. Correlations were generally strongest when using the 6‐ to 7‐day averaged physical data, stressing the importance of continuous measurements of these drivers in phytoplankton studies.     

Responses of phytoplankton diversity to physical disturbance under manual operation in a large reservoir, China

In aquatic ecosystems, physical disturbances have been suggested to be one of the main factors influencing phytoplankton structure and diversity. To elucidate whether large-scale artificial operation of a hydroelectric reservoir has potential impacts on phytoplankton diversity, the impact on phytoplankton biodiversity of physical disturbances under artificial operation from May 2007 to April 2008 in tributaries of the Three Gorges Reservoir (TGR), China, was analysed. Two disturbance parameters, i.e. the absolute incremental rates of discharge (R _d,i ) and precipitation (R _p,i ), were created in this study for evaluating physical disturbance intensities during low and high water level periods of the TGR. Results showed that river discharge seemed to be the main factor controlling the phytoplankton diversity in low water level periods (≤151 m), and that precipitation was a potential promoter of the physical disturbance. During the 156-m impoundment process, the species diversity clearly decreased due to the high dilution effect on the phytoplankton communities. At high water level periods (>151 m), the low levels of disturbance eventually allowed the phytoplankton community to approach competitive exclusion in late February 2008. Sharply declining diversity values appeared when the Dinophyta blooms occurred in late March and late April 2008 (Peridinium and Ceratium, respectively).     

Disturbance–diversity models: what do they really predict and how are they tested?

The intermediate disturbance hypothesis (IDH) and the dynamic equilibrium model (DEM) are influential theories in ecology. The IDH predicts large species numbers at intermediate levels of disturbance and the DEM predicts that the effect of disturbance depends on the level of productivity. However, various indices of diversity are considered more commonly than the predicted number of species in tests of the hypotheses. This issue reaches beyond the scientific community as the predictions of the IDH and the DEM are used in the management of national parks and reserves. In order to compare responses with disturbance among measures of biodiversity, we used two different approaches of mathematical modelling and conducted an extensive meta-analysis. Two-thirds of the surveyed studies present different results for different diversity measures. Accordingly, the meta-analysis showed a narrow range of negative quadratic regression components for richness, but not evenness. Also, the two models support the IDH and the DEM, respectively, when biodiversity is measured as species richness, but predict evenness to increase with increasing disturbance, for all levels of productivity. Consequently, studies that use compound indices of diversity should present logical arguments, a priori, to why a specific index of diversity should peak in response to disturbance.     

Evaluating the role of fire disturbance in structuring small reptile communities in temperate forests

Fire is an integral disturbance shaping forest community dynamics over large scales. However, understanding the relationship between fire induced habitat disturbance and biodiversity remain equivocal. Ecological theories including the intermediate disturbance hypothesis (IDH) and the habitat accommodation model (HAM) offer predictive frameworks that could explain faunal responses to fire disturbances. We used an 80 year post-fire chronosequence to investigate small reptile community responses to fires in temperate forests across 74 sites. First, we evaluated if changes in species richness, abundance and evenness post-fire followed trends of prior predictions, including the IDH. Second, using competing models of fine scale habitat elements we evaluated the specific ways which fire influenced small reptiles. Third, we evaluated support for the HAM by examining compositional changes of reptile community post-fire. Relative abundance was positively correlated to age post-fire while richness and evenness showed no associations. The abundance trend was as expected based on the prior prediction of sustained population increase post-disturbance, but the trend for richness contradicted the prediction of highest diversity at intermediate levels of disturbance (according to IDH). Abundance changes were driven mainly by changes in overstorey, ground layer, and shelter, while richness and evenness did not associate with any vegetation parameter. Community composition was not strongly correlated to age since fire, thus support for the HAM was weak. Overall, in this ecosystem, frequent fire disturbances can be detrimental to small reptiles. Future studies utilizing approaches based on species traits could enhance our understanding of biodiversity patterns post-disturbance.     

The time course of phytoplankton biomass and related limnological factors in shallow and deep lakes: a multivariate approach

Weekly studies of phytoplankton biomass and environmental variables were made over one year in a shallow stratifying, hypertrophic El Porcal lake near Madrid (Spain). Data were collected on abiotic factors, primary production, biomass and phytoplankton losses and subjected to reduction by means of several principal component analyses. Furthermore, weekly data on the same variables were gathered from published studies on Überlinger See, an embayment of the much deeper, mesotrophic Lake Constance (Central Europe), and treated in the same way. The two first principal components of PCAs on biological variables explained more than 60% of overall variance in both lakes. They could be ascribed to phytoplankton production + biomass and photosynthetic physiology + phytoplankton losses, respectively. The ordination of the biological trajectories in the data space of the two first principal components revealed six stable states of phytoplankton biomass in the shallow lake and seven in the deep lake. The breakpoints between stable states could be due to environmental, abiotic variables in some cases but biological interactions were suspected to be the cause of the other breakpoints. The abiotic effects on phytoplankton biomass took longer to occur in the deep lake. Also, short-term dynamics (one-three weeks) were demonstrated for both phytoplankton communities.A preliminary comparison between phytoplankton biomass dynamics in stratifying, shallow and deep lakes suggests that differences may be attributed partly to differences in depth.     

Issues affecting the measurement of disturbance response patterns in herbaceous vegetation – A test of the intermediate disturbance hypothesis

The 'intermediate disturbance hypothesis' (IDH) predicts maximum species diversity at intermediate levels of disturbance. Disturbance levels are measured by intensity or frequency of disturbance or by the time since disturbance. The IDH has been tested over a wide range of studies and communities with results either accepting or rejecting the hypothesis. The objectives of this study were to determine how observed disturbance response patterns for the same herbaceous plant community are influenced by modelling techniques, particularly in relation to the expression of disturbance gradients, choice of species diversity indices, and time of assessment since disturbance. Response patterns were examined using a multi-factorial disturbance experiment involving grazing (absent and present); fire (absent and present); soil cultivation (none, 5 and 20 cm); and amendment (none, fertiliser, and fertiliser plus clover seeds). Generalized linear models with a b-spline function were used to define response patterns for five different disturbance gradients over a 24-month period using three indices of diversity. Seven basic disturbance response models were recognized ranging from the classical IDH bell-shaped pattern through increasing or decreasing trends, to a no-change model with increasing levels of disturbance. Only 6.7% of all models were consistent with the IDH. The no-change model was found in nearly half the cases investigated and increased in occurrence with time since disturbance. The choice of the disturbance intensity gradient, the species diversity index used and the time of assessment after disturbance significantly influenced the frequency of occurrence of the disturbance response models observed. Consequently, the responses of vegetation to disturbance gradients show many patterns depending on how they are defined and modelled rather than the simple bell-shaped curve as predicted by the IDH, but the ecological mechanisms supporting some of these patterns need to be further investigated.     

Disturbance events affecting phytoplankton biomass,composition and species diversity in a shallow,eutrophic, temperate lake

The seasonal changes in phytoplankton biomass and species diversity in a shallow, eutrophic Danish lake are described and related to different disturbance events acting on the phytoplankton community.Both the spring diatom maximum and the summer bloom of the filamentous blue-green alga, Aphanizomenon flos-aquae (L.) Ralfs, coincided with low values of phytoplankton species diversity and equitability. Diatom collapse was mainly due to internal modifications as nutrient depletion (Si, P) caused by rapid growth of phytoplankton, and increased grazing activity from zooplankton. A large population of Daphnia longispina O.F. Müller in June effectively removed smaller algal competitors, thus favouring the development of a huge summer bloom (140 mm³ l^–1) of Aphanizomenon flos-aquae. Heavy rainfall and storms in late July increased the loss of Apahnizomenon by out-flow and disturbed the stratification of the lake. These events caused a marked decline in phytoplankton biomass but had no effect on species diversity. A second storm period in late August circulated the lake completely and was followed by a rapid increase in phytoplankton diversity, and a change in the phytoplankton community structure from dominance of large, slow-growing K-selected species (Aphanizomenon) to small, fast-growing r-selected species (cryptomonads).