ASSESSING THE IMPACT OF SOFT-WATER CALCIUM DECLINE ON THE LIFE-HISTORIES STRATEGIES OF VARIOUS DAPHNIA SPECIES
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Soft-water calcium decline is of growing concern for Ca-rich aquatic organisms such as Daphnia, a cladoceran zooplankton. Survival and reproduction thresholds across daphniid species have not been well studied in Ca-limiting conditions. In hopes to establish Ca-thresholds, the life-history of ten iso-female Daphnia clones, composed of Daphnia pulex/pulicaria, D. catawba and D. ambigua, were tracked over 21-day trials exposed to five Ca-treatment groups (1.0, 1.5, 2.0, 2.5 and 5.0 mg of Ca/L). D. catawba and D. pulex/pulicaria survivorship was significantly affected by the Ca-gradient, while the survivorship of the smaller-bodied D. ambigua was not. Increased survivorship across the Ca-gradient was most notable in D. catawba, which is surprising as it is often viewed as a species tolerant to low-Ca conditions. However, it is believed that species-specific Ca responses are driven by large variations in survivorship, attributed to differences both within and across clone lines. At low-Ca (< 2.5 mg of Ca/L) all species exhibited premature mortality (<10 days), most likely due to high-Ca demands of juveniles. In general, survivorship did not increase with increasing Ca-level, as the greatest survivorship within all species was seen at moderate Ca-levels (2.5 mg of Ca/L). It is suggested that acclimation to moderate Ca-levels (2.5 mg of Ca/L) is from mother clone lines and batch cultures being raised in the same Ca-concentration. Acclimation to 2.5 mg of Ca/L conditions is further supported by evidence that this Ca treatment also experienced the greatest reproductive output as well as fewer delays in reproduction. Nevertheless, inherent concerns over establishing conservative Ca-thresholds in the laboratory emerge; this in combination with the difficulties that arise here in culturing Daphnia, may highlight the need for in situ studies to identify Ca-thresholds across daphniids.