Do grazers respond to or control food quality? Cross-scale analysis of algivorous fish in littoral Lake Tanganyika

Abstract

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 (rETR) remarkably well (r = 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 (r = 0.44, P = 0.011) and decrease in algal biomass (r = 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 (r = 0.69, P = 0.00096), and algivore gut length increased with sedimentation (r = 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.

Blue Waters, Green Bottoms: Benthic Filamentous Algal Blooms Are an Emerging Threat to Clear Lakes Worldwide

“…For example, as was noted above, extreme weather events and watershed deforestation are increasing sediment loading to littoral habitats (Hambright et al 2004 ). In tropical Lake Tanganyika, sediments compromised the growth and feeding activity of algivorous fish, leading to a positive correlation between sediments and attached algal biomass (Munubi et al 2018 ). In addition to directly (Wagenhoff et al 2013 ) and indirectly (Munubi et al 2018 ) promoting filamentous algal growth, sediments fill interstitial spaces, reducing total habitat and promoting hypoxia in sediments.…”

“…In tropical Lake Tanganyika, sediments compromised the growth and feeding activity of algivorous fish, leading to a positive correlation between sediments and attached algal biomass (Munubi et al 2018 ). In addition to directly (Wagenhoff et al 2013 ) and indirectly (Munubi et al 2018 ) promoting filamentous algal growth, sediments fill interstitial spaces, reducing total habitat and promoting hypoxia in sediments. Filamentous algal proliferations themselves can create adverse environmental conditions for zoobenthos (Arroyo et al 2012 ).…”

Aquatic insects balance growth with future supply of algal food resources

“…Our experiment with T. gracilentus, in conjunction with a model fit to the experimental data, gave evidence that midges do not inherently overexploit their resources. Consumers often employ behaviors that mitigate the likelihood of overexploitation of their resources (Vuorinen et al 2021), such as enhancing resource growth rates through ecosystem engineering (Jones et al 1994) and abandoning low-resource habitat patches (Power 1984, Wiley and Warren 1992, Munubi et al 2018, both of which we observed by T. gracilentus. How consumers manage their resources in a dynamic environment may shape their somatic growth rates and subsequently their population dynamics.…”

Cryptic Constituents: The Paradox of High Flux–Low Concentration Components of Aquatic Ecosystems

“…In addition to depressing biomass, grazing increases nutrient turnover [27]. Because benthic consumers track primary producer availability, their grazing can result in uniformly low algal biomass even across strong contrasts in environmental productivity [28,29], particularly for the benthic algal groups that are preferentially grazed. However, compared to phytoplankton communities (which are dispersed throughout the euphotic zone), benthic biofilms are a spatially concentrated food.…”