Towards environmental assessment of river ecosystems by analyzing energy reserves of aquatic invertebrates

Environmental preferences of invertebrates in larger rivers are assumed to be displayed in their upstream-downstream distribution and in their abundance. Near the limits of a species' distribution, individuals must cope with rather hostile, energy consuming, environmental conditions. On the contrary, it is assumed that sites with high abundance are located more in the center of the distribution area and represent more optimal conditions. We hypothesized that at these sites also the fitness of the animals is better, which should be reflected by higher amounts of storage substrates. We measured the abundance and the energy storage (triglyceride and glycogen contents) of the mayfly Heptagenia flava (Ephemeroptera: Heptageniidae), the leech Glossiphonia complanata (Hirudinea: Glossiphoniidae), and the freshwater shrimp Dikerogammarus villosus (Crustacea: Gammaridae) along 560 km of the River Elbe during the summers of 2003 and 2004. H. flava and G. complanata reached highest densities upstream while D. villosus abundance increased continuously towards the river mouth. A model selection approach revealed that the factor sampling site was necessary to explain the data of energy storage for each species. In H. flava, triglycerides, the most efficient storage substrate, were observed to reach maximum levels at sites with high abundances. Although D. villosus and G. complanata followed a similar relation, both species displayed some exceptions from this pattern with highest triglyceride concentrations at their upstream distribution limit. It is speculated that storage substrates have the potential to become an important tool in environmental assessment because they yield long-term information about the fitness of a population, especially in merolimnic species, e.g. mayflies. As adult mayflies do not feed, the amount of accumulated energy reserves during the larval life is crucial for their reproductive success and therefore is a very good indicator for the assessment of a population's fitness. It is further assumed that the fitness of the other two species does not depend on the energetic status of a certain juvenile stage. Their storages can be refilled at any time, if enough food is available. Nevertheless, storage substrates as well as the total amount of available energy therein are appropriate criteria for the physiological fitness of both species. They result from long-term effects of the surrounding conditions, but may also reflect creeping changes in the environment, e.g. consequences of rising temperatures. Therefore, the use of those criteria as an assessment tool for the physiological fitness of invertebrate populations seems to be very promising. (C) 2008 Elsevier GmbH. All rights reserved.