1. Earlier spring warming as predicted for climate change will alter combinations of water temperature and photoperiod that act as emergence cues for zooplankton resting stages. As a result, water temperature cue thresholds will be experienced at shorter photoperiods, a variable independent of weather variations. Also, light intensity, another potentially important cue for zooplankton emergence, could decrease in many lakes if symptoms of climate change resemble those of eutrophication. 2. We designed a laboratory experiment to test the effects of three factors, temperature (6, 9 and 12°C), photoperiod (13L : 11D and 16L : 8D) and light intensity (20 and 35 lE m )2 s )1 ) on hatchling abundance and timing of hatching of daphniids (Daphnia ambigua) and rotifers (Keratella spp. and Synchaeta pectinata) from resting eggs. Further, we investigated the implications of potential changes in hatching dynamics, following variations in hatching cues, on zooplankton spring population development using predator-prey simulation models. 3. For hatchling abundance and timing of hatching, photoperiod had a significant effect for D. ambigua but not rotifers. Daphnia ambigua hatchling abundance decreased by 50% when incubated at conditions mimicking early spring (12°C + 13-h photoperiod) compared to a later spring (12°C + 16-h photoperiod). Light intensity has a significant effect only for S. pectinata, producing greater hatchling abundance at lower light intensity. 4. Simulation models suggest that in contrast to a later spring, an early warming produces a shift in spring zooplankton community composition, from daphniid to rotifer dominance. These patterns are primarily driven by differential zooplankton emergence response with variations in temperature-photoperiod cues. 5. Overall, our laboratory experiments and simulation models suggest that lakes with strong dependence on the 'resting egg-bank', characteristic of many shallow northtemperate lakes or in years with low winter survivorship of adult zooplankton, may be most susceptible to climate change. Further, fewer large grazers such as daphniids with an earlier spring may result in less control of cyanobacterial blooms in eutrophic lakes.
After decades of use of polybrominated diphenyl ethers (PBDEs) as flame retardants, a large reservoir of these toxins has accumulated in ecosystems worldwide. The present study used an innovative approach to examine whether the fully brominated PBDE decabromodiphenyl ether (decaBDE) degrades to more toxic congeners in aquatic environments. The authors incubated intact sediment microcosms with high-purity [(13)C]decaBDE in a remote boreal lake to assess its debromination under ambient conditions. Although the addition of [(13)C]decaBDE increased total PBDE concentrations in sediment more than 10-fold, the relative amount of [(13)C]decaBDE in sediment did not change significantly over a 1-mo incubation. However, observation of small quantities of lower-brominated [(13)C]BDEs lent support to the hypothesis that decaBDE is slowly debrominated. The authors observed a significant increase in octaBDEs and nonaBDEs in profundal, but not littoral, sediment over 30 d. A second experiment in which sediment was incubated under different light and oxygen regimes yielded a surprising result-oxygen significantly stimulated the formation of octaBDEs and nonaBDEs. The authors also conducted a large-scale in situ enclosure experiment in which they followed the fate of experimentally added decaBDE in sediment over 26 mo, but that study yielded little evidence of decaBDE debromination. Overall, the authors suggest that the debromination of decaBDE occurs very slowly, if at all, in natural sediment of boreal lakes, in contrast to the rapid degradation kinetics reported by most laboratory-based studies, which are usually conducted by dissolving decaBDE in organic solvents. The findings reinforce the need for field studies on contaminant fate to inform environmental policy decisions.
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