Invasive invertebrate predator, Bythotrephes longimanus, reverses trophic cascade in a north‐temperate lake

Abstract

Trophic cascades can determine the structure of aquatic food webs, a role often used to manage water quality in lakes. However, trophic cascades are subject to multiple interacting drivers of environmental change. For example, invasive species can dramatically alter food webs, though we know less about invasive species effects on managed trophic cascades. Here, we investigate the changing food web dynamics of Lake Mendota (Wisconsin, U.S.A.) over a 40‐yr time period. Piscivore stocking (biomanipulation) beginning in 1988 coupled with a 1987 die‐off of cisco (the dominant zooplanktivore) led to decreased zooplanktivory that cascaded to increases in the large‐bodied herbivorous zooplankton Daphnia pulicaria and greater water clarity. In 2009, the invasive spiny water flea Bythotrephes longimanus was first detected in Lake Mendota and soon reached record densities. As a result of this invasive invertebrate predator, zooplanktivory increased to 47% of the earlier cisco period. Concomitant with the spiny water flea population explosion were declines in D. pulicaria and water clarity as both exhibited a strong negative relationship to zooplanktivory across the 40‐yr time series. We argue that the trophic cascade of 1987–1988 created an environment of high food resources and low fish planktivory that promoted the proliferation of Bythotrephes as it functionally replaced cisco in Lake Mendota's food web. Our results highlight how a manipulated, eutrophic lake was vulnerable to adverse impacts by invasive species, calling attention to the importance of understanding interactions among drivers of environmental change.

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

“…We used the model to estimate average annual Bythotrephes density. Considering that Bythotrephes reaches maximum abundances in autumn in the study lakes and persists at high densities until the lake freezes (Walsh et al 2017), one could imagine that the Bythotrephes density of the previous year ( Bythotrephes density t −1 ) affects the spring Daphnia populations and clear‐water phase metrics of that year by influencing Daphnia overwintering success. Therefore, we used Bythotrephes density t −1 in the subsequent statistical analyses.…”

Detecting species at low densities: a new theoretical framework and an empirical test on an invasive zooplankton

“…, Walsh et al. ). Our general theoretical relationship between density and the effort needed to reliably detect a species at low densities also highlights the benefits of targeting search efforts to areas and times of higher expected densities within regions selected for monitoring and reinforces Royle and Nichols' () point that abundance is a key factor in determining detectability.…”

“…For example, Bythotrephes dispersal is strongly driven by propagule pressure, namely boater movement (MacIsaac et al 2000, Weisz andYan 2010). Further, Bythotrephes is more likely to occur in large, deep, and nutrient-poor lakes (Branstrator et al 2006, Wang andJackson 2011) but is more abundant in productive lakes (Brown et al 2012) as well as lakes with higher prey availability (Young et al 2011, Walsh et al 2017. Our general theoretical relationship between density and the effort needed to reliably detect a species at low densities also highlights the benefits of targeting search efforts to areas and times of higher expected densities within regions selected for monitoring and reinforces Royle and Nichols' (2003) point that abundance is a key factor in determining detectability.…”

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

“…In this study, we show that two indicators of lake water quality are influenced by different ecological interactions in Lakes Mendota and Monona. An enormous increase in predation pressure on D. pulicaria (Walsh et al ) produced cascading change in the phytoplankton communities of Lake Mendota and Lake Monona toward a higher biomass of small diatoms in spring, in turn dampening and shortening the length of the clear water phases of each lake (here defined as Secchi depth > 4 m; Fig. C,D).…”

“…The invasive predatory zooplankton, spiny water flea ( Bythotrephes longimanus ) was detected in Lake Mendota and downstream Lake Monona in 2009. Predation by Bythotrephes has since reversed much of the gains in D. pulicaria biomass and water clarity that resulted from the 1987 fish die‐off and subsequent lake biomanipulation (Walsh et al ), incurring large economic damages to Lake Mendota (Walsh et al 2016 a ). Yet, seasonally the largest decline in clarity was observed in the spring, likely because of lower D. pulicaria biomass.…”