Anomalous rise in algal production linked to lakewater calcium decline through food web interactions

Abstract

Increased algal blooms are a threat to aquatic ecosystems worldwide, although the combined effects of multiple stressors make it difficult to determine the underlying causes. We explore whether changes in trophic interactions in response to declining calcium (Ca) concentrations, a water quality issue only recently recognized in Europe and North America, can be linked with unexplained bloom production. Using a palaeolimnological approach analysing the remains of Cladocera (herbivorous grazers) and visual reflectance spectroscopically inferred chlorophyll a from the sediments of a Nova Scotia (Canada) lake, we show that a keystone grazer, Daphnia, declined in the early 1990s and was replaced by a less effective grazer, Bosmina, while inferred chlorophyll a levels tripled at constant total phosphorus (TP) concentrations. The decline in Daphnia cannot be attributed to changes in pH, thermal stratification or predation, but instead is linked to declining lakewater [Ca]. The consistency in the timing of changes in Daphnia and inferred chlorophyll a suggests top-down control on algal production, providing, to our knowledge, the first evidence of a link between lakewater [Ca] decline and elevated algal production mediated through the effects of [Ca] decline on Daphnia.[Ca] decline has severe implications for whole-lake food webs, and presents yet another mechanism for potential increases in algal blooms.

Long-term changes in invertebrate size structure and composition in a boreal headwater lake with a known minnow introduction

“…In contrast to the palaeolimnological-based findings of Korosi et al (2012), which demonstrate that Daphnia may control primary production through grazing, we note a large decline in VRS Chl-a levels, while Daphnia abundances remain exceptionally stable and low (Figs. 4 and 5).…”

“…Chaoborus), as well as its common prey (i.e. Bosmina), we build upon other paleolimnological studies of historic shifts in zooplankton size structure from eastern North America (Leavitt et al, 1989;Korosi et al, 2010Korosi et al, , 2012. Whereas most paleolimnological studies recognize the importance of top-down controls of predation on zooplankton (i.e.…”

Changes since the onset of acid deposition among calcium-sensitive cladoceran taxa within softwater lakes of Ontario, Canada

“…changes among algal grazers due to Ca limitation facilitating increased algal production; Korosi et al 2012), and the potential cascading impacts for terrestrial systems (St. Louis and Breebaart 1991;Watmough and Aherne 2008). The ability to predict the consequences of reduced aqueous Ca for aquatic ecosystems is of considerable interest for scientists, managers and policy makers; however, forecasting the future response of Ca-sensitive communities from direct monitoring records of Ca declines and concurrent ecological changes remains difficult.…”

Establishing past environmental conditions and tracking long-term environmental change in the Canadian Maritime provinces using lake sediments

“…Declines in Daphnia relative abundance related to Ca decline since pre-industrial times have been extensively documented for softwater Precambrian Shield lakes in south-central Ontario (DeSellas et al 2011;Jeziorski et al 2012). For lakes examined in southwest Nova Scotia, however, Daphnia were rare in both modern and pre-industrial sediment intervals (Korosi and Smol 2012a), and only two of over 50 lakes examined in southwest Nova Scotia showed reductions in Daphnia relative abundances consistent with Ca decline (Jeziorski et al 2008;Korosi et al 2012;Smol 2012a,2012b). Little Wiles Lake, located near Bridgewater, Nova Scotia, showed a striking decline in Daphnia in the 1970s to near extirpation, with no recovery (Jeziorski et al 2008) (Fig.…”

“…Lakes that have undergone declines in DI pH > 0.2 pH units (see Table 1 for actual pH changes for acidified lakes) since pre-industrial times are represented by red boxes, and lakes that have not undergone DI pH declines> 0.2 pH units are represented by black circles. White triangles represent lakes that showed evidence of a negative impact of lakewater calcium decline on zooplankton (Jeziorski et al 2008;Korosi et al 2012). …”