Very high secondary production at a lake outlet

SUMMARY. 1. Larvae and pupae of Simulium noelleri Fried, coated the concrete of parts of an artificial lake outlet in southern England. 2. In the first two (of three) summer generations, development was synchronous and this allowed the calculation of their secondary production by the instantaneous growth method. The production of the two summer generations was, respectively, 229.1 g C m−2 (7.4 g C m−2day−1) and 185.5 g C m−2 (8.8 g C m−2 day−1) The contribution of the third summer generation, and the overwintering generation, to annual production would be less than that of the first two summer generations. Nevertheless, annual production will have exceeded 500 g C m−2 at this site. 3. Larvae are suspension feeders and they captured the rich supply of particulate and dissolved organic material which passed over them after export from the lake. As food is brought to the larvae they only require space for attachment and can thus build up very high population densities (which exceeded 1 × 106 m−2 on some occasions during the summer). The high population densities result in a high biomass and hence in the high levels of production.