A review of the effects ofBythotrephes longimanusand calcium decline on zooplankton communities — can interactive effects be predicted?

Abstract

Anthropogenic stressors including acid deposition, invasive species, and calcium (Ca) decline have produced widespread damage to Canadian Shield lakes, especially to their zooplankton communities. Here, we review current knowledge on the individual effects on zooplankton by the non-indigenous predator Bythotrephes longimanus and Ca decline; we identify knowledge gaps in this literature and examine the likely interactive impacts of Bythotrephes invasions and Ca decline on zooplankton. The negative impacts of Bythotrephes longimanus on zooplankton communities are well known, whereas current understanding of the effects of declining Ca on zooplankton is restricted to Daphnia spp.; hence, there is a large knowledge gap on how declining Ca may affect zooplankton communities in general. The co-occurring impacts of Bythotrephes and declining Ca have rarely been studied at the species level, and we expect daphniids, particularly Daphnia retrocurva and Daphnia pulicaria, to be the most sensitive to both stressors. We also expect a synergistic negative interaction on cladocerans in lakes with both stressors, leaving a community dominated by Holopedium glacialis and (or) copepods. Our predictions form testable hypotheses but since species and ecosystem response to multiple stressors are difficult to predict, we may actually see ecological surprises in Canadian Shield lakes as Bythotrephes continues to spread and Ca levels continue to fall.

The impact of calcium decline on population growth rates of crustacean zooplankton in Canadian Shield lakes

“…We provide the first field evidence of a positive effect of Ca on three cyclopoid copepod species, A. robustus, M. edax, and T. extensus. Although copepods have low Ca content (0.05-0.4% dry weight (DW); Azan et al 2015 and references therein) that has been used to infer tolerance to low [Ca], we detected reduced growth rates for all three copepod species at low [Ca] under low food conditions. While per capita growth rates for A. robustus and T. extensus decreased, as the [Ca] declined, their mean growth rates were positive in each Ca treatment, possibly due a low food threshold concentration compared to Daphnia.…”

“…Laboratory studies using Daphnia pulex have proposed 1.5 mg Ca/L as an important biological threshold for reproduction (Ashforth and Yan ) and the development of anti‐predator defenses (Riessen et al ), but this threshold is likely dependent on temperature and food availability. Similar studies using Ca‐rich Daphnia (e.g., Daphnia galeata , Daphnia magna , and Daphnia pulex ) have documented increased mortality rates, as well as impaired survival, growth and reproduction in Ca poor waters under a range of Ca from 0.5 mg/L to 5 mg/L (Azan et al and references therein). In addition, declining aqueous Ca is expected to influence the distribution of Ca‐rich Daphnia , as higher occurrences have been detected in lakes with high ambient [Ca] (Tessier and Horwitz ; Hessen et al ; Wærvågen et al ; Cairns ).…”

“…Our ability to predict the effect of Ca decline on the broader zooplankton community, however, remains limited as laboratory and field studies have mainly focused on Ca‐rich Daphnia (Alstad et al ; Hessen and Rukke ; Hessen et al ; Rukke ; Ashforth and Yan ; Jiang et al ; Prater et al ), and body Ca content is not always a reliable predictor for species response to low Ca (Tan and Wang ; Azan et al ). Although field studies have demonstrated increases in the relative abundance of bosminids and Holopedium glacialis as Ca declines (DeSellas et al ; Jeziorski et al ), manipulative experiments to assess causal associations have not been conducted.…”

Decoupled trophic responses to long‐term recovery from acidification and associated browning in lakes

“…More recent studies have shown that Ca body content is not always a reliable predictor of species sensitivity to low Ca. (Azan & Arnott, ; Azan, Arnott, & Yan, ; Tan & Wang, ). Thus, the strong relationship between declining Ca and declining copepod biomass observed here suggests that copepods may be more sensitive to low Ca than previously thought, though more research is needed.…”

Calcium decline reduces population growth rates of zooplankton in field mesocosms

“…In general, aqueous Ca concentrations below 0.5 mg Ca/L are lethal to Daphnia in laboratory studies (reviewed in Cairns and Yan 2009). Field thresholds, based on species occurrence data, range from 0.5 to 1.69 mg Ca/L for most daphniids (Azan et al 2015). Reduced body size, delayed maturity (Cairns and Yan 2009), and possible increased energy expenditure (Hessen and Rukke 2000a) are expected to result in reduced population growth rates for daphniids experiencing low Ca, while birth rates and population growth rates of small cladocerans and copepods are hypothesized to be unaffected by low Ca (Azan et al 2015).…”