The effects of species gain and species loss on nutrient storage and cycling in freshwater ecosystems: Summarizing patterns across ecosystems

Background/Question/Methods

In both terrestrial and aquatic ecosystems, organisms directly affect nutrient storage and cycling by sequestering nutrients through growth and remineralizing nutrients via excretion and egestion. Although many studies have documented the effects of organisms on nutrient dynamics, there has not been a synthesis examining the effects of species losses and additions across systems. Anthropogenic activities often result in the loss of biodiversity while enhancing the ability of exotic species to invade and persist in novel habitats and this may have significant effects on nutrient dynamics in freshwater ecosystems that may render habitats functionally impaired. To understand the potential effects of the addition or loss of species on nutrient dynamics, we need to examine the consequences of changes in nutrient cycling and storage. This study summarizes the influence of organisms on nitrogen (N) and phosphorus (P) storage and cycling in freshwater ecosystems to promote comparison of the effects across systems. To accomplish this, we compiled the work of three ongoing studies in a variety of freshwater ecosystems:  two studies of native species’ that are threatened with extirpation (mussels in Oklahoma and tadpoles in Panama) and one species of highly invasive fish (in Mexico), on nutrient storage and remineralization by freshwater organisms.

 

Results/Conclusions

Collectively, our results indicate that the roles of animals in nutrient dynamics are both context and biomass/density dependent. Areal excretion rates for native mussels at nine sites ranged from 94-439 μmol N/m2/hr and 8.5-29.2 μmol P/m2/hr. Storage by mussels was 3.6-97.5 g N/m2 and 1.0-32.5 g P/m2. Variation was due to differences in densities and species composition of mussel communities. Similarly, nutrient cycling and storage by native tadpoles in Panama was primarily density-dependent. Areal excretion of pre-amphibian decline sites was 5.9-47.6 μmol/m2/hr NH4-N and 1.3-22.5 μmol PO4-P/m2/hr and nutrient storage was approximately 0.014 g N/m2 and 0.003 g P/m2. In the Chacamax River, armored catfish (Loricariidae) are highly invasive and occur at a higher biomass than native fishes. Daytime excretion rates of loricariids were more than 25 times the amount of N (191 µmol N/m2/hr1 vs. 7.5 µmol N/m2/hr) and P (4.5 µmol P/m2/hr vs. 0.18 µmol P/m2/hr) than excreted by the native fish assemblage. Further, loricariids stored 20g N/m2 and 10g P/m2. This highlights the importance of cross-ecosystem comparisons to describe the functional roles of aquatic organisms to allow scientists and managers to predict the effects of species loss and addition on ecosystem function across systems.