Do grazers respond to or control food quality? Cross-scale analysis of algivorous fish in littoral Lake Tanganyika
Food quality determines the growth rate of primary consumers and ecosystem trophic efficiencies, but it is not clear whether variation in primary consumer densities control, or is controlled by, variation in food quality. We quantified variation in the density and condition of an abundant algae-eating cichlid, Tropheus brichardi, with respect to the quality and productivity of algal biofilms within and across rocky coastal sites in Lake Tanganyika, East Africa. Adjacent land use and sediment deposition in the littoral zone varied widely among sites. Tropheus brichardi maximized both caloric and phosphorus intake at the local scale by aggregating in shallow habitats: algivore density decreased with depth, tracking attached algae productivity (rETRMAX) remarkably well (r2 = 0.84, P = 0.00033). In contrast, algivore density was unrelated to among-site variation in algal productivity. Rather, there was significant increase in algal quality (r2 = 0.44, P = 0.011) and decrease in algal biomass (r2 = 0.53, P = 0.0068) with T. brichardi density across sites, consistent with strong top-down control of primary producers. The amount of inorganic sediment on rock surfaces was the strongest predictor of among-site variation in algivore density (r2 = 0.69, P = 0.00096), and algivore gut length increased with sedimentation (r2 = 0.36, P = 0.034). These patterns indicate extrinsic and top-down forcing of algal food quality and quantity across coastal landscapes, combined with adaptive habitat selection by fish at the local scale. Factors that degrade food quality by decreasing algal nutrient content or diluting the resource with indigestible material are likely to depress grazer densities, potentially dampening top-down control in high-light, low-nutrient aquatic ecosystems.
KeywordsAlgivore Microphytobenthos Periphyton Food quality Productivity Tropheus Littoral Sediment C:N:P
We thank Dr. Rashid Tamatamah, Dr. Ismael Kimirei, and the Tanzanian Fisheries Research Institute for facilitating this research. We gratefully acknowledge the field help of George Kazumbe, Len Kenyon, Ryan Satchell, Erica Hile, Sam Drerup, Ellen Hamann and Leslie Kim. Funds were provided by the US National Science Foundation (DEB 0842253 to YV and DEB 1030242 to PBM) and Wright State University’s Environmental Sciences Ph.D. Program.
Author contribution statement
RM conceived of and designed the study, collected and analyzed data, and wrote the manuscript. YV advised on study design, collected and analyzed productivity data and wrote the manuscript. PBM advised on study design, collected community fish data, and provided editorial contributions to the manuscript
Compliance with ethical standards
Statement of human and animal rights
All applicable institutional and/or national guidelines for the care and use of animals were followed.
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