Macroinvertebrate assemblages in acidified mountain lake inflows differs from lake outflows: the influence of lakes

This synoptic study of inflows and outflows of eight atmospherically acidified glacial lakes in the Bohemian Forest, the Czech Republic and Germany, investigated how the presence of lakes on headwater streams changes the water chemistry and concurrently macroinvertebrate assemblages. The extent of changes in water chemistry was assessed in relation to lake morphometry (max. depth, lake volume and area, water residence time (WRT)). The inflows exhibited a wide pH gradient, from 3.9–6.3, while pH of outflows ranged from 4.6–5.9. Lakes generally increased the temperature, reduced the acidity of outflows and were net sinks for dissolved nutrients but sources of their particulate forms and N-NH 4 + . However, we only found significant relationships between N-NH 4 + and WRT; other relationships between differences in physico-chemical parameters and lake morphometry were nonsignificant. Comparisons of macroinvertebrate groups indicate that the numbers of Plecoptera, Trichoptera, and Diptera taxa were significantly higher in inflows than in outflows. Plecoptera dominated in inflows whereas the abundance of Ephemeroptera was significantly higher in outflows. The higher abundance of Diptera in outflows was due to the family Simuliidae, probably benefiting from the supply of food (seston) from lakes. We found a significant positive relationship between pH and the number of macroinvertebrate taxa in inflows, but not in outflows. This lack of significancy was most probably caused by the low number of sites (7), the narrow pH range, and several factors that influenced outflows: higher temperature, altered water chemistry, the input of lake taxa, and higher content of food.