Uncoupling indicators of water quality due to the invasive zooplankter, Bythotrephes longimanus

Abstract

Cultural eutrophication of lakes due to excess loading of nutrients is a global problem affecting the provisioning of freshwater ecosystem services. Lake water quality is often described by trophic state indicators such as water clarity or concentrations of nutrients, chlorophyll, and cyanobacteria toxins. While these indicators are widely assumed to covary, capturing the same basic aspects of water quality, they can be uncoupled by changes in lake food webs. Outbreak of the invasive predatory zooplankton, Bythotrephes longimanus, in Lakes Mendota and Monona, Wisconsin (U.S.A.) in 2009 led to a decline in the herbivore, Daphnia pulicaria, and in turn, water clarity. We use 20‐yr datasets for Lakes Mendota and Monona to evaluate interactions among nutrients, zooplankton, and phytoplankton that resulted in this trophic cascade. After invasion, diatoms increased in both lakes resulting in reduced water clarity, whereas cyanobacteria did not change in either lake. Multivariate time series analysis revealed that D. pulicaria grazing controlled edible diatoms in both lakes. Cyanobacteria biomass was positively associated with both D. pulicaria and summer nutrient concentrations. These results further clarify the limitations of top‐down control in eutrophic lakes: grazing by zooplankton can control edible phytoplankton taxa, but largely inedible cyanobacteria may be more limited by nutrient concentrations. Accordingly, despite declining clarity in Lake Mendota, the number of beach closures due to cyanobacteria blooms declined following Bythotrephes invasion. Our study reveals how these two indicators of water quality—water clarity and cyanobacteria blooms—can become uncoupled during a food web shift.

Climate and food web effects on the spring clear‐water phase in two north‐temperate eutrophic lakes

“…m −3 ). Walsh et al (2018) found that D. pulicaria biomass declined in Lakes Mendota and Monona after Bythotrephes invasion, but to a much greater extent in Lake Mendota. Because Lake Monona has been managed as a “panfish” or zooplanktivore lake while managing Lake Mendota as a “gamefish” or piscivore lake (i.e., strict regulations on harvest of piscivorous fishes) (Lathrop et al 2002; Walsh et al 2017), Bythotrephes may be controlled by planktivorous fish, such as bluegill, in Lake Monona.…”

“…In the summer of 2009, the invasive predatory zooplankton, spiny water flea (B. cederströmii) was detected in Lake Mendota and Lake Monona. In subsequent years, D. pulicaria biomass was notably lower, resulting in reduced water clarity especially in Lake Mendota (Walsh et al 2017(Walsh et al , 2018.…”

“…The monitoring of Lake Mendota and Lake Monona was conducted by the University of Wisconsin-Madison Center for Limnology and the Wisconsin Department of Natural Resources. Both lakes were sampled at the deepest location on a biweekly schedule beginning after iceout from spring through summer and sometimes monthly in fall, and at least once during winter when the lakes were icecovered (Lathrop et al 1996(Lathrop et al , 2002Walsh et al 2017Walsh et al , 2018. Although Secchi depth in some years was recorded more frequently than biweekly in Mendota, in this study we used biweekly Secchi depth data that corresponded to the sampling dates of zooplankton and nutrient monitoring data to match temporal resolution.…”

“…In this study, we defined the clear-water phase as the time period for which Secchi depth exceeded 3.5 m, a period following spring phytoplankton blooms and generally during April-June (and occasionally extending into July in a few years). While the threshold of Secchi depth for the definition of the clear-water phase has ranged from 2.5 to 7.0 m (Straile 2000;Winder and Schindler 2004b;Wagner and Benndorf 2007), previous studies on Lake Mendota used 3.0 m and 4.0 m as the threshold (Luecke et al 1990;Rudstam et al 1993;Lathrop et al 1999;Walsh et al 2017Walsh et al , 2018.…”

“…To test these hypotheses, we analyzed the long‐term limnological records of two north‐temperate eutrophic lakes; Lakes Mendota and Monona, Wisconsin, and quantified long‐term phenological patterns of the spring clear‐water phase. The lakes are adjacent to each other, separated by a narrow isthmus, but differ in physical characteristics (e.g., area, mean depth, water residence time, ice breakup date) and biotic characteristics (e.g., fish fauna, Bythotrephes density) (Magnuson et al 2006; Walsh et al 2018). Using a 38‐yr time series (1980–2017) of Secchi depth readings measured biweekly, we quantified the start date, maximum date (date of greatest water clarity), duration, and magnitude of the clear‐water phase in each year (Supporting Information Fig.…”