Novel Methods in Cladoceran Size Structure Analysis Reveal ~60 Years of Trophic Change in a Sediment Core From a Great Lakes Area of Concern (Bay of Quinte, Lake Ontario)

ABSTRACT Nutrient loading, aquatic invasive species, and climate warming have altered food web dynamics in the Great Lakes. Long‐term data are required to characterise the response of the lower food web to multiple environmental drivers and develop trophic indicators for future research and management efforts. We measured a suite of size metrics in cladoceran subfossils from a dated sediment core from the Bay of Quinte, Lake Ontario, which has a long history of environmental degradation. We collected bosminid carapace and mucro lengths and calculated the percentage of Bosmina longirostris cornuta ‐type subfossils. Further, we developed novel methods by separating subfossil measurements by size class (as a proxy for juvenile and reproductive instars) and by measuring Chydorus sphaericus rostrum length as an indicator of invertebrate predation pressure. Cladoceran size structure was significantly related to observed water clarity regime. Different size classes showed significant trends over time and between observed clear‐water and turbid conditions. Small‐class B. longirostris mucro length and large‐class Eubosmina coregoni carapace length were significantly correlated with clear‐water conditions, while small ‐ class E. coregoni carapace length and the percentage of cornuta‐ type B. longirostris were significantly correlated with turbid conditions. Chydorus sphaericus rostrum length did not show significant change in the sediment core but was correlated with B. longirostris mucro length and clear‐water conditions. The most notable change in size structure followed dreissenid invasion and the turbid to clear‐water transition, which appears to have decreased fish planktivory pressure on larger zooplankton and consequently increased invertebrate predation pressure on smaller cladocerans. It is likely that dreissenids enhanced benthic production and the availability of benthic food sources, causing planktivorous fish to decline relative to benthivorous fish. The development of novel methods for cladoceran size structure analysis will assist future research on trophic change in complex aquatic ecosystems. Separating measurements by size class decreased variability and revealed trends that would have been obscured by an aggregated metric. To our knowledge, this study is the first use of C. sphaericus rostrum length as a trophic indicator.