Species sorting along a subsidy gradient affects community stability

Background/Question/Methods

The movement of resources between terrestrial and aquatic ecosystems may have strong effects on resident communities, especially microbial communities. Inputs can alter the available resource pool, and depending on quantity and quality may impose new selective pressures on residents. In addition because these inputs are donor controlled, many have hypothesized that depending on concentration and quality, inputs may stabilize or destabilize local communities by dampening internal dynamics or decoupling local food webs respectively. Inputs of terrestrially derived dissolved organic carbon (tDOC) into aquatic ecosystems represent a large influx of low quality resource; thus, has the potential to alter both community diversity through selective pressures and resilience to perturbations through changes in stability. To address both changes in diversity and resilience we experimentally manipulated inputs of tDOC in experimental ponds by adding a terrestrial carbon analog (SuperHume) to create a tDOC loading gradient (0 g m^-2 – 212 g m^-2). We determined how this terrestrial input altered selective pressures and resilience by measuring how community diversity changed along the gradient and how responsive the altered communities were to a pulse perturbation of inorganic phosphorous.

Results/Conclusions

As a result of our experimental tDOC loading gradient, we determined that the aquatic microbial community changed through decreasing species richness and evenness, and this corresponded to major changes in community composition indicating strong species sorting pressures. In addition, we determined that due to these changes, the microbial community became less responsive to our pulse perturbation indicating increasing resilience. Together, these results suggest that tDOC loading has strong selective pressures on the aquatic microbial community and these pressures alter community composition by selecting for oligotrophic organisms that specialize on the low quality inputs. These changes in composition lead to a stable community which was not able to efficiently respond to the nutrient pulse perturbation.