Material cycling mediated by overlapping aggregations of mussel and fish consumer creates spatiotemporally variable control points in stream ecosystem function

Background/Question/Methods

Consumers provide simultaneous top down and bottom up controls on stream ecosystem function. Seasonal hydrology may influence the degree of interaction between and biogeochemical effects of, dense aggregations of consumers. This may lead to variable activation of mussel beds as ‘control points’ or hotspots in the flux and composition of materials linked to ecosystem functions. To test the effects of seasonal overlap in the biological processes of sessile and mobile consumers on ecosystem function, we conducted an eight-week mesocosm experiment manipulating the presence of a grazing cyprinid within unionid mussel beds. In flow-through stream mesocosm experiments, we measured nutrient excretion rates and biomass of consumers to estimate consumer driven material fluxes and composition (e.g. stoichiometry). We further measured gross primary production and ecosystem respiration as ultimate functional responses to consumer material cycling.

Results/Conclusions

Our preliminary results suggest that, availability and stoichiometry of ammonium, and soluble reactive phosphorus were correlated to mussel biomass and fish presence. In mussel only treatments, ammonium and SRP concentrations were negatively related (R²=0.23, p<0.05) and the ratio of N to P was positively related to the production to respiration ratio (P/R) (R²=0.58, p<0.05). However, in treatments with both mussels and fish, the relationship between nutrients was positive (R²=0.54, p<0.05), and ratio was unrelated to P/R. This suggests that fish and mussel feeding and material recycling traits may interact to produce different control points in ecosystem function.

Across mesocosms, in treatments without fish, the P/R ratio was three times more variable upstream of the mussel bed and roughly equal at the most downstream extent compared to treatments with fish. Longitudinally within mesoscosms, variance in P/R was two times greater in mesocosms without fish than those with fish. We attribute these differences to the combined effects of both groups’ feeding and excretion traits. Thus, overlap of consumer groups, particularly ones with different traits, may be important for stabilizing function within ecosystems. This suggests that consumer communities with diverse traits may produce control points whose rates of function are more predictable.