Seasonal changes in the biomass, distribution, and patchiness of zooplankton and fish in four lakes in the Sierra Nevada, California

Abstract

Abstract Zooplankton are important components of lentic ecosystems, affecting phytoplankton, water clarity, and nutrient cycling, as well as transferring primary production to upper trophic levels. Many of these processes are temporally and spatially heterogeneous, but are difficult to observe at fine scales with traditional sampling methods. High‐resolution sampling has been especially rare in remote and high‐altitude lakes. We measured the vertical distribution of zooplankton and fish in four lakes in the Sierra Nevada Mountains of California, U.S.A. (Independence Lake, Lake Tahoe, Cherry Lake, and Lake Eleanor) using a dual‐frequency echosounder, and estimated lake‐wide biomass in all lakes except Tahoe. For zooplankton, we also quantified trends and patchiness in their horizontal distribution. In two of the lakes, Cherry and Eleanor, surveys were repeated four times at seasonal intervals between autumn 2013 and autumn 2014. Zooplankton were most abundant in these lakes in the spring and summer of 2014, with peak wet‐weight biomasses estimated at 31,000 kg in Lake Eleanor in April and 68,000 kg in Cherry Lake in June. The biomass and vertical distribution of fish also varied, increasing and moving shallower in the water column in June in both Cherry Lake and Lake Eleanor. Zooplankton density was not horizontally homogeneous, displaying gradients at the lake basin scale (5–6 km), and nested patchiness at a range of smaller scales (0–2 km). This small‐scale spatial variability may be generated biologically, not physically. While it is well‐known that the distribution of zooplankton is often patchy, this aspect of their ecology has not been quantified in most lakes, especially in remote montane locations. These results illustrate how acoustic sampling can rapidly and simultaneously measure the biomass and spatial distribution of multiple trophic levels in small lakes. This capability provides unique opportunities to study the processes that generate and maintain gradients and patchiness in these components of the ecosystem.