Plankton Trophic Structure Within Lake Michigan as Revealed By Stable Carbon and Nitrogen Isotopes

Abstract

Zooplankton represent a critical component of aquatic food webs in that they transfer energy from primary producers to higher trophic positions. However, their small size makes the application of traditional trophic ecology techniques difficult. Fortunately, novel techniques have been developed that can be used to elucidate feeding information between zooplankton species. I used the analysis of stable carbon and nitrogen isotope ratios to estimate the trophic structure of Lake Michigan's zooplankton community. The major zooplankton species, three size classes of seston, and seston from specific water column depths were collected in 2011 and 2012 for stable isotope analysis. Trophic position was estimated for the major zooplankton species by relating their mean nitrogen stable isotope signatures to the assumed primary consumer Daphnia mendotae. Initial results suggested that the large calanoid copepod Limnocalanus macrurus occupies a trophic position of a tertiary consumer, nearly a full trophic position above all other copepods species. In 2013, subsequent sampling and analyses were done to determine if the high nitrogen stable isotope signature for L. macrurus was the result of trophic enrichment or some other mechanism. Zooplankton and seston samples were collected in 2013 from winter through summer. A seasonal trend in the stable nitrogen isotope ratios was observed in seston, with high values during the winter and low values during the summer. A dynamic model was developed and used to estimate the effects of this seasonal trend on the nitrogen stable isotope signature of L. macrurus and two other copepod species. The model results suggest that L. macrurus as well as two other copepods are reacting quickly to changes in the stable isotope signature of seston. Therefore, some other mechanism is likely causing the high stable nitrogen isotope values observed forL. macrurus.