Effect of silt, water and periphyton quality on survival and growth of the mayfly Heptagenia sulphurea

The herbivorous mayfly Heptagenia sulphurea is characteristic of rivers with stony bottoms. Records from the 20th century showed that this species had disappeared from the Common Meuse in the Netherlands, probably due to river regulation or changes in water quality. A field survey in 2003 showed that H. sulphurea was present in the Geul tributary, approximately 300 m upstream of its confluence with the Common Meuse. H. sulphurea has not recolonized the Common Meuse despite improvements in water quality over the last decades. Concentration of suspended sediments in the River Meuse, however, is still high, much higher than in the beginning of the 20th century. The presence of a silt layer may limit the return of H. sulphurea in this river by reducing availability and quality of its food. The prime objective of this study was to investigate the impact of silt on survival and growth of H. sulphurea in a laboratory experiment. Furthermore, the impact of water and periphyton quality from the Common Meuse on survival and growth of this mayfly was also investigated. Results showed that neither water quality nor cultured periphyton from the Common Meuse reduced survival and growth of H. sulphurea. The presence of a silt layer resulted in a significantly lower growth rate of the mayfly larvae. It is concluded that the silt layer reduces the accessibility of the food. Covering of food is possibly one of the main factors limiting the recolonization of H. sulphurea and probably other benthic grazers in the Common Meuse

A method for measuring effects of bioturbation and consolidation on erosion resistance of aquatic sediments

Sediment erosion by water movement affects turbidity and thus benthic communities in numerous aquatic systems. This aspect has been widely studied in coastal habitats and estuaries, but less studied in freshwater systems such as shallow lakes. Here we present a simple device to study the effects of biota on the resistance of aquatic sediments to erosion by water movement. Circular 75 l tanks are used to incubate selected sediments with microbial communities. The tank size allows addition of organisms such as small fish, invertebrates and aquatic plants. Effects are studied by gradually increasing water movement by means of a rotor and continuously monitoring the increase of suspended solids. The water speed needed for resuspension is a measure of sediment erosion resistance. With this device we found a linear increment of erosion resistance over time if sediments are left undisturbed, and a significant reduction of erosion resistance with small holes punched randomly in the top layer of sediment (to mimic the effect of benthivorous fish foraging behaviour). Fish biotic perturbation was indicated by a reduction of sediment resistance associated with benthivorous fish presence. However, three spined stickleback did not create the same effect. Measurements with this simple device are reasonably precise and suggest that the set-up can be used to study effects of numerous factors that may affect sediment erosion resistance. For example, sediment consolidation associated with light, temperature, and microbial and plant colonization, along with sediment perturbation related to big invertebrates and fish behaviour

Variation in stonefly (Nemoura cinerea Retzius) growth and development in response to hydraulic and substrate conditions

Spatial heterogeneity among microhabitat patches in aquatic ecosystems creates refuges (e.g., substrate interstices) that protect organisms against a variety of environmental constraints. Aquatic insects have evolved the ability to alter their life-history traits in response to environmental variability. Our objective was to determine whether differences in hydraulic and substrate conditions cause phenotypic plasticity in nymphs of the stonefly Nemoura cinerea Retzius. We experimentally manipulated near-bed current velocities (3 and 9 cm/s) and substrate types (medium sand and coarse gravel) in indoor artificial channels. Changes in the head capsule width, mesothoracic wing pad length, total body length, body mass, and behavior were studied over a 4-wk period. Morphometric and behavioral differences were primarily associated with the mechanism of refugium use (although confounded by substrate size), which reduced energy expenditure. Growth (body size and mass) was reduced in sand (absence of refugia) relative to growth in gravel, regardless of current velocity, and nymphs made almost no investment in body length (variation in allometry). The effect of current velocity on behavior was less marked in sand than in gravel because of a possible confounding effect of direct exposure of nymphs to flow on sand (i.e., no available refugia). In sand, individuals were located mainly on the food source (erect leaf discs) at current velocities of 3 cm/s, but feeding was depressed. Therefore, nymphal development was slow on sand. In gravel, nymphs followed different developmental trajectories depending on current velocity because hydraulic stress seemed to trigger accelerated development. Our study shows that physical factors associated with habitat structure can result in adaptive phenotypic plasticity among aquatic insect populations, and that a lack of shelter might have a strong impact on stonefly fitness

Habitat-mediated cannibalism and microhabitat restriction in the stream invertebrate Gammarus pulex

In cannibalistic species, small individuals often shift habitats to minimize risk of predation by larger conspecifics. The availability of diverse size-structured habitats may mediate the incidence of cannibalism by larger individuals on smaller individuals and increase fitness of smaller individuals. We tested these hypotheses in a series of laboratory studies with Gammarus pulex, a freshwater amphipod inhabiting substrates with varying interstitial pore space sizes. In the absence of larger, potentially cannibalistic individuals, small Gammarus actively used all pore space sizes offered. They used only substrates containing food and preferred food items that provided cover to food items that did not. In the presence of larger G. pulex, small individuals almost exclusively used smaller pore spaces from which larger individuals were excluded. Small individual survival was significantly lower in the presence of larger Gammarus than in controls without larger individuals regardless of substrate size, but availability of mixed pore sizes significantly increased survival. Food consumption and growth per individual were not affected by the presence of larger individuals or substrate composition. Our results suggest that the distribution and availability of complex and high-quality habitats may affect the occurrence and significance of cannibalism in size-structured populations

Haften weer terug in de Grensmaas?

Haften (Ephemeroptera) vertoonden in het verleden een grote diversiteit, maar die is in de loop van de tijd sterk gereduceerd in de Grensmaas. In het kader van afstudeerwerk is een aantal factoren van vermindering van de macrofauna onderzoch