Ecology of desmids in Flemish softwater ponds

Chapter 1: Desmids in softwater ponds in the Kempen region in Belgium: current status and comparison with historical records A new species list for desmids (Desmidiales, Chlorophyta) in the Kempen region in Belgium was compiled using data from 89 ponds and an additional survey of over 300 ponds. This list was compared with historical data from before 1940. A comparison between the species that have been lost since 1940 and the new species found, revealed tendencies of eutrophication and a loss of sensitive species. To preserve the sensitive species that are still present, regions with high diversity should be protected and regions with declined water quality should be restored. Chapter 2: Substrate specificity of periphytic desmids in shallow softwater ponds in Belgium At six sites in five mesotrophic softwater ponds in the wetland Zwart Water (Belgium), periphyton samples were collected on different substrates ranging from macrophytes to mosses and sandy sediment. Significant differences between substrates were observed at five out of six sites studied. The differences between the substrates, however, could not be related to known effects of these substrates on their chemical environment (e.g. excretion of H+ ions, CO2 or allelopathic substances) nor to their morphology. Therefore, differences between substrates were probably related to differences in local environmental conditions associated with these substrates. Differences in desmid community composition between substrates within a pond were always smaller than differences with samples from other ponds. Chapter 3: Desmid based inference models for environmental variables The relationship between periphytic desmid communities and 13 environmental variables from 88 lakes in the Flanders region in Belgium was explored. Using canonical correspondence analysis, variables were selected that could be used in weighted averaging inference models. We constructed models with good predictive power for pH (bootstrap-r2= 0.76 and RMSEP/range= 0.13) and tP (bootstrap-r2=0.61 and RMSEP/range= 0.16). These results indicate that periphytic desmids have good potential to be used as bio-indicators for acidification and eutrophication. We compared the constructed models to models based on planktonic desmids or on reduced datasets. Models based on planktonic desmids had the same predictive power of those based on periphytic desmids. The good predictive power was also preserved after transformation to presence/absence data or to a dataset in which only the presence of the 5 most abundant species in each sample was used. The use of a dataset in which abundances of species were combined at the genus level, lead to a clearly reduced predictive power. We conclude that desmids communities can be used for biomonitoring, especially for pH and tP, but that further research is necessary to improve the model. Chapter 4: Unimodal response of desmid diversity to productivity indicators in shallow softwater ponds The relationship between species richness and environmental variables has been the subject of many studies. Especially the often observed unimodal response to productivity has led to different theories regarding the underlying mechanism leading to this pattern. These theories are all based on models of competition within the community and do not take into consideration the role of regional processes. Regional processes are, however, known to potentially have an impact on local species richness. In this study we use lakes as models for metacommunities, with their planktonic and periphytic desmid communities as interacting local populations. In this way, we can evaluate the importance of these interactions for the determination of local species richness. Both planktonic and periphytic desmid communities showed a very similar unimodal response to variables related to productivity (tP and pH). We also found indications for strong interactions between these two communities, and the resulting similarity between the communities showed a U-shaped response over pH. The observed patterns could partly be explained by the resource heterogeneity or the keystone predator hypothesis. It is, however, difficult to test the predictions of these theories due to a limited knowledge of the ecological characteristics of the species. Moreover, some of the observations did not fit the expected pattern. An alternative hypothesis was formulated, stating that the unimodal response of planktonic and periphytic desmid species richness to indicators of productivity was caused by the coexistence of two differentiated communities at intermediate productivity levels. Strong interactions between these communities resulted in the enrichment of each community with species from the other community. We conclude that these dynamics between planktonic and periphytic communities have the potential to significantly influence the relationship of species richness with productivity, especially for microbial communities. Chapter 5: Relative importance of local and spatial factors in structuring communities of planktonic and periphytic microorganisms Spatial processes as dispersal, colonization and extinction have become generally recognized to have an important influence on community composition. For microscopic species, the importance of spatial variables is more debated, as they are often considered to be ubiquitous and only influenced by local conditions. In this study we investigate the influence of both local and spatial variables on the composition of periphytic and planktonic desmid communities in four pond systems. The pond systems differ regarding the degree of connectivity and regarding regional water quality. We found that local variables were generally most important in structuring desmid communities, but that an additional influence of spatial variables could sometimes be found. In the pond system with the highest connectivity, mass effects between neighboring ponds were observed for planktonic desmid communities only. In the pond system with the lowest connectivity, the results indicate that both periphytic and planktonic desmid communities were dispersal limited. In the systems with intermediate connectivity, no influence of spatial variables was detected. We therefore conclude that the importance of spatial processes is dependent of the characteristics of the pond system (connectivity and water quality) and of the characteristics of the community studied (planktonic or periphytic).Introduction and outline Chapter 1 Desmids in softwater ponds in the Kempen region in Belgium: current status and comparison with historical records Chapter 2 Substrate specificity of periphytic desmids in shallow softwater ponds in Belgium Chapter 3 Desmid based inference models for environmental variables Chapter 4 Habitat diversity and source sink dynamics determine the unimodal response of desmid diversity to productivity indicators in shallow softwater ponds Chapter 5 Relative importance of local and spatial factors in structuring communities of planktonic and periphytic microorganisms General discussion Summary Samenvattingstatus: publishe