1. The prevalence of mycosporine-like amino acids (MAAs) -a group of potential ultraviolet (UV)-photoprotective compounds -was surveyed across 11 species of freshwater copepods from 20 lakes of varying ultraviolet radiation (UVR) transparency in North America, New Zealand and Argentina. Co-occurring cladocerans were also analysed (seven species from 12 lakes). Many of the calanoid copepod populations were red with carotenoid pigmentation, allowing comparison of MAA and carotenoid accumulation as photoprotective strategies. 2. In two Pennsylvania (U.S.A.) lakes, MAA and carotenoid contents were followed during the early spring to mid-summer period of lake warming. A pronounced seasonal pattern of higher carotenoid/low MAA content in spring, shifting to low carotenoid/higher MAA content in summer, was observed in calanoids from the more UV transparent lake. 3. All copepod samples contained MAAs. Visibly red calanoids, especially southern Hemisphere Boeckella, often had moderate to high concentrations (2.5-11 lg MAA mg )1 dw), but low concentrations (0.04-1 lg MAA mg )1 dw) in some N. American red calanoids show that high carotenoid pigmentation (e.g. 5-10 lg carotenoid mg )1 dw) does not necessarily imply high MAA content. 4. No cladoceran sample had more than trace amounts of MAAs (<0.05 lg mg )1 dw). Therefore, MAA accumulation does not seem to be a photoprotective strategy utilized by Daphnia (five species from nine lakes) or other cladocerans. 5. Seven identifiable MAAs were widely distributed among both calanoids and cyclopoids. Shinorine was ubiquitous and was usually the most abundant MAA in N. American samples. In contrast, porphyra-334 was the predominant MAA in the southern Hemisphere Boeckella. 6. Copepods from higher UVR lakes tended to have a higher MAA content, but this relationship was statistically weak overall and taxon-specific when found.

The objective of this study was to expand the spatial scale of previous experiments on the effects of ultraviolet radiation (UVR) on diel vertical migration (DVM) by freshwater zooplankton. We conducted an in situ mesocosm experiment in highly UVR transparent Lake Giles, Pennsylvania, in which we imposed two treatments: ambient UVR and UVR-shielded. Mesocosms (3440 L, 0.74 m diameter, 8 m deep) were large enough to include a spatial refuge from UVR and permit relatively large-scale DVM. Daphnia catawba adopted a significantly deeper distribution during the day in the ambient UVR treatment compared to the UVR-shielded treatment, but effects of UVR were absent at night. In contrast, DVM by Leptodiaptomus minutus was unaffected by the UVR treatment. In both treatments, Leptodiaptomus minutus were most abundant at the bottom of the mesocosms during the day and exhibited a more uniform distribution across depths at night. These results suggest that UVR, along with temperature, algal resources, and predators, may affect zooplankton DVM in aquatic ecosystems.

1. Anthropogenic stressors such as climate change, ozone depletion and acidification may act in concert to alter ultraviolet (UV) light and temperature regimes in freshwater ecosystems. These physical and chemical changes will inevitably affect zooplankton community dynamics, but little is known about their relative effects on different species in natural communities. During spring, species that migrate to surface waters to take advantage of warmer temperatures may be especially vulnerable as UV levels can be high. 2. The objective of this study was to investigate the in situ effects of UV and temperature on a natural assemblage of planktonic rotifers and crustaceans during the spring. We performed in situ exposure experiments in two lakes with different surface temperatures. 3. Exposure to UV had a significant effect on the abundance and/or reproduction of four rotifers: Gastropus spp., Kellicottia bostonensis, Kellicottia longispina, Keratella spp.; two cladocerans: Holopedium gibberum, Daphnia catawba, and one copepod: Leptodiaptomus minutus. Incubation under cooler temperatures had a negative effect on K. longispina and H. gibberum. Temperature and UV had a significant interactive effect on abundance and/or reproduction of L. minutus and Ploesoma truncatum. Our results indicate that changes in underwater UV and temperature can significantly influence the composition of the zooplankton community and ultimately food web dynamics.

1. The d 13 C and d 15 N signatures of zooplankton vary with dissolved organic carbon (DOC), but inconsistent and limited taxonomic resolution of previous studies have masked differences that may exist among orders, genera or species and are attributable to dietary and ⁄ or habitat differences. Here we investigate differences among the isotopic signatures of five zooplankton taxa (Daphnia, Holopedium, large Calanoida, small Calanoida and Cyclopoida) in Precambrian shield lakes with a sixfold range of DOC concentration. 2. d 13 C signatures of Daphnia, small calanoids and large calanoids became more depleted with increasing lake DOC, whereas Holopedium and cyclopoid d 13 C became enriched with increasing DOC concentration. 3. The variability of d 13 C and d 15 N isotopic signatures among zooplankton groups was reduced in high-DOC, compared to low-DOC lakes, especially for d 13 C. Differences in d 13 C and POM-corrected d 15 N accounted for up to 33.7% and 19.5% of the variance, respectively, among lakes of varying DOC concentration. 4. The narrow range of signatures found in higher DOC lakes suggests that different taxa have similar food sources and ⁄ or habitats. In contrast, the wide range of signatures in low-DOC lakes suggests that different taxa are exploiting different food sources and ⁄ or habitats. Together with the variable trends in zooplankton isotopic signatures along our DOC gradient, these results suggest that food web dynamics within the zooplankton community of temperate lakes will change as climate and lake DOC concentrations change.