COS 37-5 - CANCELLED - Multiple bioindicators in a size-structured framework differentiate food web pathways of zooplankton – a tale of two time series

Abstract: Marine food webs are strongly size-structured and size-based ecology is a useful approach to study an entire community at once in a systematic and replicable way. Fatty acid analysis is commonly used to identify diet information of individuals and species, but is rarely applied to size structured communities. It is a powerful complement to more commonly applied stable isotope analyses, and can differentiate among processes that are confounded by isotopes. We applied fatty acid analysis to size-structured marine zooplankton communities for the first time to characterize how prey resources and trophic pathways vary with size. Using two separate datasets, we investigated resource use over different temporal scales and body sizes.

To track relationships between prey and consumers, we first analyzed a nine-month time series of seven size classes of plankton, from > 0.7 μm particulate organic matter through zooplankton > 2000 μm. Size classes of zooplankton were generally distinguishable by their dietary fatty acids, while stable isotopes revealed more seasonal variability. Zooplankton fatty acids were correlated with those of their prey sources (particulate organic matter and smaller zooplankton) and identified trophic pathways that support the observed differences in their fatty acid composition. With the second data set, we examined interannual differences in consumers using a four-year time series of five size classes of zooplankton, from > 250 μm through > 4000 μm. Across all four years, the dietary fatty acids of zooplankton were again differentiated by size class and season, but with clear interannual variability. In some years, fatty acid composition was differentiated by size class to a high degree (2017; R² = 0.38), while in others it was weaker (2018; R² = 0.21). In both data sets, the DHA:EPA ratio was the fatty acid trophic marker most consistently correlated with trophic position estimated from δ¹⁵N. Multiple bioindicators allowed for the separation of different processes e.g., when fatty acid trophic markers indicated that trophic position was likely underestimated due to the consumption of protists with low fractionation factors. Applying tools like fatty acid analysis within a size-structured framework allows for greater inference into trophic pathways than offered by isotopes alone, while maintaining the ability to relate food web characteristics across regions, time periods, and to ecosystem models.