Shortening winter ice-cover duration in lakes highlights an urgent need for research focused on under-ice ecosystem dynamics and their contributions to whole-ecosystem processes. Low temperature, reduced light and consequent changes in autotrophic and heterotrophic resources alter the diet for long-lived consumers, with consequences on their metabolism in winter. We show in a survival experiment that the copepod Leptodiaptomus minutus in a boreal lake does not survive five months under the ice without food. We then report seasonal changes in phytoplankton, terrestrial and bacterial fatty acid (FA) biomarkers in seston and in four zooplankton species for an entire year. Phytoplankton FA were highly available in seston (2.6 µg L−1) throughout the first month under the ice. Copepods accumulated them in high quantities (44.8 µg mg dry weight−1), building lipid reserves that comprised up to 76% of body mass. Terrestrial and bacterial FA were accumulated only in low quantities (<2.5 µg mg dry weight−1). The results highlight the importance of algal FA reserve accumulation for winter survival as a key ecological process in the annual life cycle of the freshwater plankton community with likely consequences to the overall annual production of aquatic FA for higher trophic levels and ultimately for human consumption.

Accumulation of carotenoid pigments in copepods has often been described as a plastic adaptation providing photoprotection against ultraviolet radiation (UVR). However, reports of seasonal carotenoid maxima in winter, when UVR is low, challenge the proposed driving role of UVR. Therefore, we here evaluate the mechanistic connection between UVR and the seasonal pattern of copepod carotenoid pigmentation. We assessed the carotenoids, fatty acid content and reproduction of Leptodiaptomus minutus along with UVR exposure, water temperature, phytoplankton pigments, and fish predation in a boreal lake during 18 months covering two winter seasons. The predominant carotenoid astaxanthin occurred in free form as well as esterified with fatty acids. Mono-and diesters accounted for 62-93% of total astaxanthin and varied seasonally in close correlation with fatty acids. The seasonal variability in total astaxanthin content of the copepods was characterized by net accumulation in late fall of up to 0.034 lg (mg dry mass) 21 d 21, which led to the mid-winter maximum of 3.89 6 0.31 lg mg 21 . The two periods of net loss (20.018 lg mg 21 d 21 and 20.021 lg mg 21 d 21) coincided with peaks of egg production in spring and summer leading to minimum astaxanthin content (0.86 6 0.03 lg mg 21 ) in fall. This period was also characterized by the highest predation pressure by young-of-the-year fish. The results suggest that accumulation of astaxanthin in copepods is strongly related to lipid metabolism but not to UVR-photoprotection, and that seasonal changes of fatty acids and carotenoids are related to the reproduction cycle.The red pigmentation of many zooplankton has long puzzled biologists, and various hypotheses have been offered to explain the phenomenon via proximate and ultimate causes (e.g., Brehm 1938). The red coloration of copepods is due to carotenoids, a large family of lipid-soluble pigments that are synthesized only in primary producers but may be either accumulated by zooplankton or biologically converted to other carotenoids, notably astaxanthin, which is the primary carotenoid among crustaceans (Matsuno 2001;Andersson et al. 2003;Rhodes 2006). Astaxanthin is a powerful antioxidant (McNulty et al. 2007) occurring both in free form and esterified with fatty acids or associated with proteins (Cheesman et al. 1967;Matsuno 2001).In zooplankton, carotenoid accumulation is a highly variable trait that has been linked to photoprotection against ultraviolet radiation (UVR) in field studies comparing lakes with differential UVR exposure and in experimental studies (Hairston 1976;Moeller et al. 2005;Hylander et al. 2009;Rautio and Tartarotti 2010;Sommaruga 2010). The underlying mechanism ascribing astaxanthin photoprotection properties involves the quenching of singlet oxygen ( 1 O 2 ) produced during UVR exposure rather than direct absorption or reflectance of the hazardous wavelengths (Krinsky 1979;Kobayashi and Sakamoto 1999). UV-exposed copepods at low water temperatures have especially been suggested to profit from...

International audience1. Invasive species in aquatic systems are major drivers of changes in biodiversity. Amphipods are key species in freshwaters, with invasive amphipods either replacing or coexisting with native species and often damaging local biodiversity. However, the consequences of interactions among native and invasive amphipods for their habitat use and feeding ecology and ecosystem function are not yet well understood. 2. We examined a number of streams in Brittany and Northern Ireland, with native and invasive amphipods, to evaluate the consequences of species interactions for both habitat use and diet. Our field studies centred on testing two proposed models: a cohabitation model without competition between two native species (Gammarus pulex vs Echinogamm- arus berilloni), and a competition model between an invasive and a native species (Gammarus pulex vs Gammarus duebeni celticus). For these three species, alone and in combination, we assessed their habitat use and feeding patterns, the latter through gut contents and stable C and N isotope analyses of their tissues. 3. When existing as single-species populations, all three species used stream habitats broadly similarly, although G. pulex was more strongly associated with leaf litter and vegetation compared to pebble substrata than the other species. When G. pulex coexisted with either E. berilloni or G. d. celticus, the latter two changed to using all habitats equally, whereas the former retained its habitat preferences. 4. Similarly, all three species when alone had similar gut contents, with inorganic material predominating, followed by leaf and woody material and more rarely algae and invertebrates. When G. pulex coexisted with E. berilloni, the diet of the latter did not change; however, the frequency of inorganic matter, leaves and wood declined in the gut contents of G. pulex. When G. pulex coexisted with G. d. celticus, the pattern of gut contents did not change in either species. 5. When existing as single-species populations, G. pulex had a broader range of isotopic signatures, both for d13C and for d15N, than the two other species, indicating a more variable diet among individuals. When G. pulex coexisted with either E. berilloni or G. d. celticus, the latter two had similar ranges of d13C and d15N, whereas for G. pulex the range was much less for d13C and d15N, suggesting a less diverse diet. 6. Our results infer two different modes of coexistence between native and non-native amphipods. We have shown that the native species, which coexist stably, appear to show interference competition, leading to spatial habitat segregation, whereas competition for food and possible intraguild predation by G. pulex on G. d. celticus would explain why the distribution and density of the latter is affected by G. pulex. However, since all the species have a similar diet and feeding habit, we expect no great overall effect on ecosystem processes as a consequence of species interactions and displacements