Crustacean zooplankton sedimentary remains from calcium-poor lakes: complex responses to threshold concentrations

Abstract

Declines in lakewater calcium (Ca) concentrations are occurring in many softwater lakes of the Canadian Shield, and likely elsewhere, and there is growing interest regarding the potential impacts that reduced Ca availability may have on aquatic ecosystems. Here, we test the hypotheses that the Ca limitations of Daphnia pulex (reduced growth/survival when Ca \1.5 mg L -1 ) identified in laboratory analyses and paleolimnological case studies can be observed among cladoceran assemblages from surface sediments along a narrow Ca gradient (1-3 mg L -1 ), and also whether the impacts of low Ca concentrations can be distinguished from those of low lakewater pH. Significant differences among daphniid communities were observed across the 1.5 mg L -1 Ca threshold; however, these differences did not extend to the broader cladoceran community and considerable variability in Ca tolerance was observed within the D. pulex species complex. Of the measured environmental variables, only pH and lake depth explained a significant amount of variation among the entire cladoceran community, although when enlarging the dataset to extend the Ca and pH ranges, Ca replaced pH as a significant explanatory variable. Our data adds support to our previous observations that, within the cladoceran community, daphniids are the most responsive to Ca decline, however responses differ within daphniid species complexes and Ca thresholds under natural conditions may be higher for some species than their laboratory-identified values.

Relative importance of environmental, geographic, and spatial variables on zooplankton metacommunities

“…The dominance of pH suggests that zooplankton differ in their environmental tolerance ranges for pH, a finding noted elsewhere (Holt et al 2003), which has implications for community composition under the threat of lake acidification. Further, the impact of pH may also include the impact of other variables, as pH is often correlated with other water quality metrics such as calcium (Ca), dissolved organic carbon (DOC), and total organic phosphorus (Jeziorski et al 2012). The importance of DOC and pH to zooplankton communities is supported by previous analyses in boreal shield lakes influenced by acidification (Derry et al 2009).…”

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

“…), due to the large amounts of Ca lost during each moult when the carapace is shed and subsequently regenerated (Alstad et al 1999). The dependence of cladocerans upon aqueous Ca availability has been demonstrated in both field surveys and examinations of present-day sedimentary remains that have identified lakewater Ca concentration as a significant explanatory environmental variable for cladoceran communities (Waervågen et al 2002;DeSellas et al 2008;Jeziorski et al 2012). The importance of aqueous Ca is likely due to speciesspecific differences in Ca requirements and Ca content (i.e.…”

Using morphological characters of subfossil daphniid postabdominal claws to improve taxonomic resolution within species complexes

“…In summer 2007, sediment cores were collected from the deepest basins of Young, Dunbar, and Oudaze lakes using a Glew (1989) gravity corer (Jeziorski et al, 2011). A Glew (1988) vertical extruder was used to extract the top 0.25 cm of sediment from each core, which in this region typically represents the last 0-3 years of sediment accumulation (e.g., Mills et al, 2009).…”

“…D. catawba is an acidtolerant taxon that can thrive in low-Ca lakes (Cairns 2010), and is one of the most common daphniid taxa of the naturally acidic, low-Ca lakes of the Canadian Shield (Hebert & Finston, 1997). D. catawba has been observed to replace D. pulicaria in Plastic Lake (Muskoka, Ontario) when Ca concentrations declined (N. Yan, York University, personal communication), and Jeziorski et al (2011) identified coarse species resolution and differential species tolerances to low [Ca] to be detrimental to paleolimnological investigations on the response of daphniids to aqueous [Ca] decline. Furthermore, in areas such as Nova Scotia (Canada), where no long-term zooplankton monitoring datasets exist, our current understanding of how keystone daphniid communities have changed over time in response to acidification-related stressors relies solely on paleolimnological data (Korosi & Smol, 2011).…”

“…In addition, in eastern Canada, D. ambigua (D. longispina complex) and D. catawba (D. pulex complex) are relatively acid-tolerant (Keller & Pitblado, 1984), and are also tolerant of low calcium (Ca) concentrations (Cairns 2010), well below the 1.5-mg/l fitness threshold identified for D. pulex (Ashforth & Yan, 2008). Therefore, by grouping species together into complexes, we reduce the effectiveness of cladoceran subfossils as paleolimnological indicators of environmental stressors such as lakewater acidification and aqueous Ca decline (e.g., Jeziorski et al, 2011). Hebert & Finston (1997) showed that the number of stout spines on the middle comb of the postabdominal claw was a useful feature for separating D. pulex from D. catawba; D. catawba typically has 3-4 stout spines, and D. pulex ordinarily has 5?.…”