Lacustrine Seston and Other Factors Influencing the Blackflies (Diptera: Simuliidae) Inhabiting Lake Outlets in Swedish Lapland

Abstract

The blackfly (Diptera: Simuliidae) assemblages of three localities in a river system in Swedish Lapland were examined. One locality was at a lake outlet, one further down in the same river and one in a nearby small forest stream. The three dominant species at the outlet were virtually absent from the other localities, where, conversely, most larvae belonged to species not present at the outlet. Total larval abundance was very much higher at the outlet, where larvae were present for a shorter period than elsewhere. Even within a few hundred metres from the outlet the relative growth rate of larvae was slower than at the outlet, and only a little further downstream the species prevailing at the outlet were replaced by other species. Large particles, such as phytoplankton and coarse detritus, occurred in similar numbers everywhere, and at all times, and therefore did not seem to control the abundance of blackfly larvae. After examining the role of temperature and substrate it is concluded that food, that is small particles possibly down to colloidal size, provide the resource maintaining the huge larval aggregations immediately below the lake outlet. These particles, produced on the lake bottom by decomposition processes during the winter, are washed out into the river at ice-melt. Changes in their availability and/or nutritive value limit their usefulness as food to the section of the river immediately downstream from the lake. Additional benefits may accrue to larvae growing up in these areas of extreme population density.