A multifaceted ecosystem response to perturbation along nutrient and depth gradients

Background/Question/Methods

Perturbations are often employed to study ecosystem dynamics, and responses to perturbations are typically quantified and equated somehow to stability. Whole ecosystem responses are obviously the result of interactions between components, making generalizations about ecosystem structure and stability difficult. Therefore, characterizing responses to perturbations from multiple perspectives and linking component-level responses to ecosystem-level responses should improve both our understanding of the links between ecosystem structure and function as well as our ability to predict responses more generally. We perturbed large in situ aquatic mesocosms that had experienced different nutrient subsidies (control – no addition, medium and high concentrations or inorganic N and P at a fixed molar ratio relative – relative to the observed range for similar bodies of water) for 8 weeks by shading them (at 60% ambient PAR) for one week. We measured dissolved oxygen (DO), chlorophyll a and biomass in three size classes (greater than 100microns, between 35 and 100 microns, between 20 and 35 microns) prior to and following the perturbation in both the epilimnion and in the hyoplimnion. We performed individual counts for preserved samples of each of the three size classes prior to and following perturbation as well.

Results/Conclusions

The most pronounced and immediate effect of the perturbation was a decrease in DO in both the epilimnion and hypolimnion. The high nutrient replicate exhibited the highest pre-perturbation DO and the largest post-perturbation decline in both the epilimnion and hypolimnion. Average DO and the pre-post change in DO were ordered along the nutrient addition gradient in the eiplimnion, but not in the hypolimnion. The epilimnion and hypolimnion were slowest to return to pre-perturbation DO concentrations in the control replicate, and the high nutrient replicate typically exhibited the shortest return time. Pronounced increases in both chlorophyll a and biomass in the 35 and 100 micron size classes occurred in the high nutrient replicate at both depths, but were delayed by a week relative to the DO response. This significantly different increase in the high nutrient replicate was associated with a unique shift in species composition in the 35 and 100 micron size classes. Our comprehensive characterization of ecosystem-level and community-level responses to a perturbation indicates that ecosystem components bearing the brunt of a perturbation will exhibit the most immediate response in terms of a flux, but the response will later cascade to alter biomass and species composition in functionally different taxonomic groups.