Community patch‐dynamics governs direct and indirect nutrient recycling by aggregated animals across spatial scales

Animals can have pervasive effects on ecosystems as they modify their
biogeochemical and physical environments. In particular, when animals
occur in high densities these effects can result in dramatic changes in
the physical environment and biogeochemical hotspots or hot moments. While
most research to date has focused on the direct role of animals in
biogeochemical cycles, few have examined how animals indirectly influence
biogeochemical cycles across scales. Freshwater mussels occur as spatially
heterogeneous, dense and species-rich aggregations in many river
ecosystems worldwide. Here we examined how mussel communities (1) directly
influence the flux of particulate and dissolved nutrients and (2)
indirectly effect the flux of N2 production, via denitrification, across a
gradient of mussel biomass and differences in community composition at the
patch- (0.25 m2) and stream reach-scales (60-80 m). We combined
measurements of ammonia (N) and soluble reactive phosphorous (P) excretion
and C, N, and P biodeposition rates for ten species with biomass and
distribution estimates for seven mixed-species aggregations to quantify
direct mussel contributions to biogeochemical cycling and the spatial
heterogeneity of their impact. Additionally, we sampled sediments at a
fine spatial scale to determine how mussel biomass and richness influence
potential denitrification (indirect flux) rates at the patch- and
reach-scales. We predicted that increasing mussel biomass would lead to
greater direct and indirect fluxes of nutrients, manifesting in
heterogeneous nutrient redistribution within and among stream reaches. We
also predicted that variation in community composition would result in
differential nutrient excretion and egestion stoichiometries. Our results
indicate that mussel aggregations directly influence soluble and
particulate nutrient fluxes with community composition, particularly
phylogenetic tribe composition, controlling the stoichiometry. Mussel
aggregations also indirectly influenced nutrient fluxes as greater mussel
biomass and species richness resulted in higher denitrification rates as
mediated by their interactions with the sediments and enhancement of
nutrient availability. Our results underscore the importance of patchy
communities in acting as biogeochemical control points.