Alteration of lakeshores by urbanization and other human development has multiple effects on aquatic habitats, thereby affecting species interactions and energy flow through aquatic food webs. Stable isotope analyses are commonly used in food web studies to connect consumers to the location and identification of their food sources. There is currently some interest in quantifying the array of carbon sources supporting zooplankton production in lakes. Here we present carbon stable isotope data from a survey of 25 Pacific Northwest lakes and demonstrate that physical characteristics interact with lake productivity status to create the opportunity for metalimnetic feeding on isotopically light carbon sources by pelagic consumers.

Results/Conclusions

Stable isotope analyses show that zooplankton are depleted in ¹³C relative to particulate organic matter (POM) in lakes where potential for production in the metalimnion is high. Zooplankton are depleted in ¹³C relative to POM (by up to 4 ⁰/₀₀) as the depth to which light penetrates the water column deepens relative to the depth of the upper mixed layer in stratified lakes. Zooplankton are also depleted in ¹³C in lakes where the dissolved oxygen maxima depth is deep relative to the upper mixed layer. Both of these relationships are improved by adding lake trophic status as a predictor, as zooplankton become more enriched with increased chlorophyll a. That zooplankton are more enriched in ¹³C relative to POM on more urbanized lakes suggests that these trophic relationships regulated by physical characteristics of lakes are responsive to human influences.