Six decades of data indicate reduced zooplankton density with warming conditions in Iliamna Lake, Alaska, USA

Abstract Zooplankton, a key component in lake ecosystems, are affected by planktivore grazing pressure, phytoplankton, and climate‐driven factors such as water temperature and, in lakes at mid‐high latitudes, the duration of ice coverage. Efforts to assess the relative effects of climate and planktivores on zooplankton are complicated in many large lakes by anthropogenic changes in shoreline development, water quality, non‐native species, and other factors. In contrast, Iliamna Lake, Alaska, is a large (2622 km 2 ), undeveloped lake that has experienced warmer summer air and surface water temperatures over the past six decades. These changes have been associated with earlier spring breakup of ice cover and varying abundance of planktivorous sockeye salmon, Oncorhynchus nerka , threespine stickleback, Gasterosteus aculeatus , and ninespine stickleback, Pungitius pungitius . Here, we first describe lake‐wide patterns of temperature, ice coverage, planktivore and zooplankton density and then examine data spanning six decades (1963–2022) to assess the relative roles of climate (peak summer surface water temperature and date of ice‐out) and planktivore abundance on the density and composition of zooplankton in Iliamna Lake. Lake‐wide sampling indicated that the relative and absolute densities of these fishes and their zooplankton prey varied greatly among months and regions of the lake. The long‐term data at the east end of the lake revealed that water temperature had the dominant influence (negative, lower peak density in warmer years) on the four major taxa of zooplankton. The negative temperature effect was strongest for cyclopoid copepods (55% of all zooplankton after removing nauplii and rotifers), intermediate in calanoid copepods (13%) and Bosmina (24%), and weakest in Daphnia (5%). Increased fish density had only a slight negative effect on total zooplankton density. These results, contrasting with those in other lakes in the region and elsewhere, reveal the varying ways in which climate and planktivorous fishes can affect the zooplankton community in lakes, as well as their spatial heterogeneity and temporal changes linked to climate processes.