Connecting the ditches: a spatial perspective on biodiversity in Dutch polder landscapes

The ditch networks in polder landscapes are a prime example of aquatic ecosystems where biodiversity and ecology are under threat because of intense agricultural land use. These connected water systems constitute a large part of the fresh water surface of the Netherlands. Due to their connected nature, conservation and management of biodiversity may benefit from a more regional perspective. Within this thesis I studied the biodiversity and environmental quality of 21 polder landscapes in a region in the west of the Netherlands. My work (Chapter 2, Chapter 3) confirms previous work showing that high dissimilarity in community composition between ditch reaches across different species groups exists. Furthermore, I show that different polder landscapes harbour different communities (Chapter 2) and these are complementary to eachother. I found land use and nutrient loading to be important predictors of biodiversity across levels of spatial scale (local to regional) and across species groups (Chapter 2 and 3). Eutrophication of aquatic systems can lead to changing nutrient incorporation in stoichiometrically flexible primary producers. In chapter 4 I illustrate that aquatic plants are partly flexible in their nutrient incorporation. I also showed that different plants may exhibit a different tissue elemental content under similar environmental nutrient conditions. Consumers tend to be less flexible, leading to elemental mismatches between producers and their consumers that impacts consumer community structure and functioning. In chapter 5 I study how elemental mismatches impact consumer community composition using a phytoplankton-zooplankton community as a model system. Here I showed that consumer communities cope with mismatches in phytoplankton food quality through both shifts in species composition and intra-specific elemental plasticity. Also, producer nutrient enrichment led to an enrichment of sedimenting detrital matter, leading to an enriched sediment. Sediment enrichment is an important part of the natural succession of vegetation in ditch systems. Through management (i.e. dredging, mowing) succession in ditches is reset, allowing for the redevelopment of successional stages and their associated communities. In chapter 6 I show that allowing for many successional stages to exist throughout the landscape is beneficial for overall diversity, as successional stages were found to be partly complementary to one another. Excessive enrichment however, may lead to the local ditch being dominance by free floating plants. In chapter 7 I used a spatially explicit ecological simulation model to examine the effects of land-use configurations on functional vegetation group dominance in a polder landscape. Results indicated that the location of intensively and extensively used fields in the landscape can strongly impact what vegetation groups proliferate and where. Together, these chapters highlights the importance of maintaining successional stage diversity and its heterogeneity in nutrient status, while avoiding state shifts to free-floating plant dominated conditions which are also linked to nutrient supply. All in all, this thesis contributes to maximisation of biodiversity in Dutch ditch systems across scales and functional groups through understanding of drivers of community composition and diversity and development of methodological approaches to analyse potential management scenarios