The microphytobenthos and its role in aquatic food webs

Microphytobenthos represents an important component in freshwater and marine habitats as it contributes significantly to the primary production in shallow-water ecosystems and benthic microalgae biomass supports higher trophic levels. This study had the main aim of elucidating which microphytobenthic community structures and key organisms were important in situ and then, based on this, to differentiate key grazer-microalgae interactions. Because of the man-hour problems associated with differentiating the variability of microphytobenthos populations spatially in situ a new multi-algal fluorometer was devised and tested. This probe enabled a rapid evaluation of microphytobenthic communities, instantaneous monitoring of total chlorophyll concentrations and differentiation of major taxonomic groups on an non-retrospect approach. The information on community structures gleaned from the in situ investigations was then used as a fundament to carry out three types of grazer-microphytobenthos experiments: (1) Investigations on the grazer efficiency and active selectivity of the freshwater snail Potamopyrgus antipodarum, (2) Experiments on the functional role of consumer presence and nutrient supply on microphytobenthic assemblages in macrophyte ecosystems and (3) Labelling experiments on selectivity patterns and competitive interactions between coexisting benthic grazers.Chapter 1 General introduction 5 Chapter 2 Seasonality and diversity patterns of freshwater and marine microphytobenthic communities 12 2.1 Introduction 13 2.2 Material & Methods 14 2.3 Results 18 2.4 Discussion 28 Chapter 3 ‘Spectral fingerprinting’ for specific algal groups on sediments in situ: a new sensor 37 3.1 Introduction 38 3.2 Material & Methods 39 3.3 Results 44 3.4 Discussion 50 Chapter 4 Microcosm experiments on grazing efficiency and selectivity of the freshwater hydrobiid snail Potamopyrgus antipodarum preying upon microphytobenthic assemblages. 53 4.1 Introduction 54 4.2 Material & Methods 55 4.3 Results 57 4.4 Discussion 61 Chapter 5 Factors influencing microphytobenthos community structures beneath macrophyte beds- a marine and freshwater mesocosm study. 66 5.1 Introduction 67 5.2 Material & Methods 68 5.3 Results 72 5.4 Discussion 83 Chapter 6 Selectivity and competitive interactions between two benthic invertebrate grazers (Asellus aquaticus and Potamopyrgus antipodarum)- an experimental study using 13C- and 15N-labelled diatoms. 101 6.1 Introduction 102 6.2 Material & Methods 103 6.3 Results 105 6.4 Discussion 111 Chapter 7 General Discussion 117 Chapter 8 Summary 125 Chapter 9 Reference list 12