COS 259-5 - Nutrient abatement shifts the complexity and pathways of energy flow in the Bay of Quinte food web

Background/Question/Methods

Despite frequent implementation of remedial nutrient management, holistic pictures of whole-food web response to changing nutrient levels are rare owing to a historical paucity of comprehensive, highly resolved, long-term data in ecology. The Bay of Quinte, a large embayment in eastern Lake Ontario, historically experienced high nutrient loading from agricultural and wastewater inputs, with total phosphorus (TP) levels exceeding 70 ug/L. Nutrient abatement measures were implemented in 1978, resulting in a progressive decline in TP to 30-40 ug/L by the mid-1980s.

Results/Conclusions

Here, harnessing >40 years of data collected by multiple agencies, we show that phosphorus abatement shifted the overall structure and flow of energy through the Upper Bay of Quinte food web. (1) We demonstrate that the food web shifted from a simple to a more diverse community with nutrient abatement. Community biomass was dominated by a few detritivorous species at the highest phosphorus levels and had greater functional group diversity as eutrophication lessened. (2) Using stable δ¹⁵N isotope data we document a shift from shorter to longer food chain length with nutrient abatement, with evidence that the shift is due to declining omnivory among consumers at intermediate trophic levels. (3) Finally, we show that the food web was dominated by the benthic energy channel at highest levels of TP and shifted to a mixed dominance of pelagic and benthic energy channels with phosphorus abatement. Overall these data attest to a short, simple, detritus-dominated food web at the highest TP levels, contrasted with a longer, more complex, mixed-pathway food web with declining TP.