Metabolic supply versus demand: direct and indirect effects of climate change in littoral Lake Tanganyika

Background/Question/Methods

Climate warming will increase the metabolism of aquatic poikilotherms. Whether this increase yields accelerated growth or represents a thermal stress depends upon each species thermal optimum. The capacity for an ecosystem to support increased consumer metabolic demand depends the effect of climate change on the productivity and nutrient content of autotrophs. The attached littoral algae (periphyton) in tropical Lake Tanganyika support a stunning diversity of algivorous cichlid fish. The lake is warming (0.1°/decade) and increased thermal stability may reduce nutrient loading to the littoral zone, which may, in turn, reduce the nutrient content of the periphyton resource.  We quantified periphyton carbon fixation (NPP) and nutrient content at 12 sites in Lake Tanganyika. We also measured densities, time budgets and total metabolic costs of the dominant algivorous cichlids (Tropheus and Petrochromis). We used a laboratory experiment to quantify the relative effects of warming and food quality on algivore metabolism and growth.

Results/Conclusions

The total metabolic demand of the algivorous fish assemblage was > 70% of average daily periphyton NPP (260 mg C/m²/d), suggesting near maximum consumption efficiencies at current temperatures.  However, the laboratory experiment demonstrated that Tropheus had a broad temperature tolerance: standard metabolism, ingestion rate, growth, and growth efficiency were not significantly different between individuals raised at 26° C, the current lake take temperature, and 29° C. Only the fish raised at 32° C failed to thrive. However, fish fed a lower quality algal diet grew 0.5 x the rate of fish on high quality algal food at all temperatures. Despite the tight consumer/resource coupling and high consumption efficiency in Lake Tanganyika’s littoral zone, the stress imposed by declines in food quality associated with warming may equal or exceed the stresses caused by increases in temperature.