This paper discusses the similarities between biofilms and periphyton mats in aquaculture systems. The structure of both biofilms and periphyton mats is briefly reviewed, the process of ammonia removal is explained and quantified, solid trapping and detachment from films and mats are described and the possible impact on the oxygen budget in culture systems is examined. In addition, the performance of a recirculating aquaculture system (RAS) using a sedimentation tank and trickling biofilter is compared to the functioning of a periphyton-based RAS.
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.
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