Importance of Long-Term Intensive Monitoring Programs for Understanding Multiple Drivers Influencing Lake Ontario Zooplankton Communities

Drivers of lower food web composition and productivity in Lake Ontario have undergone extensive changes in the last 40 years, including nutrient abatement, fluctuations in planktivores (Alewife), and invasion by dreissenid mussels and predatory cladocerans. Temporally intensive long-term index stations are critical for understanding these drivers and interpreting the results of periodic lake-wide spatially intensive surveys such as Cooperative Science and Monitoring Initiative (CSMI). We compare epilimnetic physical-chemical parameters and zooplankton metrics at a Kingston Basin biomonitoring site (Station 81) over three time stanzas (1981-1986, 1987-1995 and 2007-2017). In the most recent stanza, mean May-October temperature increased by 2.5 degrees C, and despite static total phosphorus levels, chlorophyll has significantly decreased and Secchi depth has increased. Between Stanzas 2 and 3, epilimnetic density, biomass and production of crustacean zooplankton have declined by 88%, 79% and 67%, respectively. Bosminids, Daphnia retrocurva, Diacyclops and juvenile cyclopoids are most impacted, whereas larger taxa (calanoids, Daphnia galeata, Holopedium and predatory cladocerans) have remained stable or increased. While some taxa have increased in size over time, zooplankton egg ratios have remained stable. Dreissenid veligers are now numerically dominant and have replaced some of the lost crustacean production. Redundancy Analysis showed environmental drivers (Secchi and temperature) significantly influenced zooplankton during the 1981-1995 period but not in the recent stanza. Alewife were not a significant driver despite substantial declines since the 1990s. Resource competition by Dreissena for the strongly reduced phytoplankton productivity, combined with predation by invasive cladocerans Cercopagis and Bythotrephes have also likely influenced Kingston Basin zooplankton. Crown Copyright (c) 2022 Published by Elsevier B.V. on behalf of International Association for Great Lakes Research. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).