Stable isotope patterns in food webs of Minnesota lakes reflect productivity gradients

Background/Question/Methods

Stable isotopes ¹³C and ¹⁵N are often used in food-web studies of lakes to assess energy sources and trophic position, respectively. However, δ¹³C and δ¹⁵N can also be influenced by biogeochemical processes in lakes, but these processes are poorly understood. We sampled δ¹³C and δ¹⁵N in mussels, cisco (Coregonus artedi), and in seston and zooplankton in both the epilimnion and hypolimnion in 22 Minnesota (USA) lakes. The lakes were dispersed across a 70,000 km² area of Minnesota that captured a large gradient in lake productivity with total phosphorus values ranging from 2 to 51 μg L^-1. We also measured temperature-oxygen profiles, total phosphorus, and photosynthetic compensation depth in each lake to assess factors influencing isotope patterns. All isotope samples were baseline corrected using mussels in each lake (sample - mussel) to control for landscape-level differences in isotope values

Results/Conclusions

Results showed δ¹³C in cisco was positively related to hypolimnetic zooplankton δ¹³C, and zooplankton δ¹³C was positively related to hypolimnetic seston δ¹³C. Hypolimnetic seston δ¹³C, in turn, was negatively related to the ratio of compensation depth: mixed layer depth, such that seston was most depleted in lakes where light penetrated deepest into the hypolimnion. This was likely due to hypolimnetic phytoplankton using respired CO₂ depleted in δ¹³C in clear-water lakes. Results also showed a positive relationship between cisco δ¹⁵N and hypolimnetic zooplankton δ¹⁵N, and that hypolimnetic zooplankton δ¹⁵N were positively related to hypolimnetic seston δ¹⁵N. Hypolimnetic seston δ¹⁵N, in turn, showed a negative relationship with total phosphorus, likely due to differences in microbial degradation of seston and denitrification rates between low and high phosphorus lakes. Overall our results indicate differences in the abiotic environment and microbial processes can generate patterns in isotopic differences between epilimnetic organisms (mussels) and hypolimnetic organisms that change along lake productivity gradients. These patterns should be accounted for in isotope studies of whole-lake food webs.