Effects of detritivore density on detritus processing rates and nutrient feedbacks on benthic algae in shallow aquatic habitats

Background/Question/Methods

In aquatic foodwebs, detritus and algal pathways are often studied separately, in part because there is relatively little herbivory on vascular plants (most becomes detritus) compared to that of algae, much of which is consumed live. The goal of our study was to explore nutrient cross-links between the two pathways, and specifically to test the hypothesis that variation in detritivore densities, hence in mobilization of detrital nitrogen and phosphorus, results in variation in algal biomass. The experiments were based on 20 years of historical data that reveal dramatic fluctuations (< 1 to > 300 m²) in the densities of the dominant detritivore (aquatic larvae of a limnephilid caddisfly) in subalpine ponds at our study sites. In the first experiment, we manipulated caddisflies in littoral enclosures and quantified density effects on detritus breakdown, conversion of detrital biomass to secondary production, nutrient mobilization, and algal biomass. We repeated the experiment in closed microcosms using the same species and range of densities to isolate the signal between nutrient release and algae. We conducted a third experiment with detritus trays in open ponds to compare the relative importance of microbes (fine mesh prevented caddisfly access) and caddisflies (coarse-mesh) on decay rates.

Results/Conclusions

In the littoral cages, detritus decay rates increased three-fold, and the loss of nitrogen and phosphorus from detrital substrates doubled across the range of historically observed caddisfly densities. Water-column ammonium and soluble phosphorus concentrations also increased with caddisfly densities, but we did not observe an effect on algal biomass. Decay rates, nutrient release, and conversion of detritus to detritivore biomass all exhibited threshold or declining responses at the highest densities, which we attributed to intraspecific competition. In the microcosm experiment we observed similar density-dependence for all of the response variables. In addition, caddisfly density positively affected algal biomass, especially on surfaces from which caddisflies were excluded. We attributed the difference in algal responses between the experiments to the diffusion of nutrients out of the littoral cages.  Finally, in open ponds, breakdown rates in trays accessible to caddisflies were more than twice that in microbial controls. The latter suggests that the strengths of caddisfly effects on detritus breakdown observed in microcosms are ecologically relevant.  Our results provide evidence for a direct link between detritus breakdown and bottom-up control on benthic algae, and suggest that detritivores have the potential to regulate both pathways in aquatic foodwebs.