Food web structure and mercury transfer in two contrasting Ugandan highland crater lakes (East Africa)

Volcanic crater lakes scattered throughout western Uganda are important local sources of water and fish. Two representative but contrasting crater lakes near the Kibale National Park were sampled in 2000; the hyper-eutrophic Lake Saka, which is highly affected by agricultural practices, and the mesotrophic Lake Nkuruba that is still surrounded by intact forest. The food web structures in these two lakes were assessed using stable nitrogen (dN) and carbon (dC) isotope analyses, and the mercury (THg) transfer patterns were quantified. The dN results indicate that food webs in both lakes are abbreviated, with only one to two trophic levels from primary consumers. The Lake Saka biota had distinctively enriched dC values compared with those in Lake Nkuruba, which may be due to C-limited phytoplankton blooms in this lake. In Lake Nkuruba, two introduced tilapiine species and the introduced guppy Poecilia reticulata fed predominantly upon invertebrates and decomposed terrestrial plant material. In Lake Saka, the introduced Nile perch Lates niloticus appeared to occupy the top trophic position, but stable isotope values of the endemic haplochromine cichlids exclude those as Nile perch prey items. THg was found to biomagnify through the food web, reaching highest concentrations in P. reticulata in Nkuruba, which tended to be higher than for L. niloticus in Saka, suggesting increased bioavailability of THg in Nkuruba. Maximum THg concentrations in fish never approached WHO recommended guidelines (200 ng g) designed to protect at-risk groups.