Abstract:

Nearshore regions of lakes are important sources of fish, and can be strongly influenced by anthropogenic inputs
of nutrients as well as contaminants. This study characterizes food web structure, mercury concentrations,
and biomagnification of mercury in two embayments in northern Lake Victoria that differ in their
connectivity to the open lake, trophic status, and the influence of local anthropogenic pollution. Murchison
Bay is a semi-confined hypereutrophic bay in a densely populated region, while Napoleon Gulf is mesotrophic
and is well flushed with water from the open lake. Based on stable carbon and nitrogen isotope analysis, food
web structure was similar at both sites, with short food chains and conspecific fish occupying similar trophic
positions. However, there were strong differences in net phytoplankton d15N and d13C between sites; net
phytoplankton d13C was largely related to trophic status, while d15N values appeared to be influenced by inputs
of human waste and the prevalence of biological nitrogen fixation. Total mercury (THg) concentrations
in fish were consistently below 200 ng/g wet weight, and despite elevated THg concentrations in water in
Murchison Bay, THg concentrations in net phytoplankton and fish from both embayments did not differ,
highlighting that THg in water is not always a good predictor of concentrations in fish. We also observed
that biomagnification of mercury was occurring at a lower rate in Murchison Bay than in Napoleon Gulf, and
we propose that the hypereutrophic state of Murchison Bay may be acting to reduce potential Hg exposure for higher trophic level fish.