Lake macroinvertebrates and the altitudinal environmental gradient in the Pyrenees

The distribution of different macroinvertebrate groups inhabiting the littoral zone of 82 mountain lakes in the Pyrenees was investigated in relation to the altitudinal environmental gradient. For each lake, altitude, longitude and latitude, together with 28 environmental variables, relating to chemical and physical characteristics and to lake general productivity, were considered. Using Principal Component Analysis (PCA) we showed that the altitudinal environmental gradient (i.e. altitude and altitude-related variables) represented the largest gradient of environmental variability. We found that incidence was related to altitude in about 50% of macroinvertebrate groups, most relationships being inverse, and also that the number of macroinvertebrate groups found per lake was better described by a second-order polynomial function than by simple linear regression. However, this relationship was linear for a subset of high-altitude lakes above 2,500 m a.s.l., suggesting an ecological threshold around this altitude. Redundancy Analyses (RDAs) showed the importance of environmental factors varying with altitude for the distribution of macroinvertebrate groups. Organic matter, salmonid presence, fine substrate dominance, macrophyte coverage, temperature and altitude by itself were, in this order, the most relevant factors. Partial RDAs showed that different combinations of these variables contributed to the explanation of the distribution of each group. However, the variable that uniquely explained most variability differed from group to group. We conclude that the altitudinal gradient is a multi-faceted ecological factor, which impinges on each group by means of some specific environmental variable(s) that are particularly relevant for the life history of that group.