Sciknow Publications Ltd. ABR 2014, 2(1):1-12 Aquatic Biology Research DOI: 10.12966/abr.02.01.2014 ©Attribution 3.0 Unported (CC BY 3.0) Springtime Crustacean Zooplankton Abundance under Cold Water Conditions Is Determined Mainly by Temperature Regardless of Fish Predation Minna Rahkola-Sorsa 1 , Ari Voutilainen 1, 2,* , Olli Urpanen 3 , Hannu Huuskonen 1 , Timo J. Marjomäki 3 , Markku Viljanen 1 , and Juha Karjalainen 3 1 Department of Biology, University of Eastern Finland, Joensuu, Finland 2 Department of Nursing Science, University of Eastern Finland, Kuopio, Finland 3 Department of Biological and Environmental Science, University of Jyväskylä, Jyväskylä, Finland *Corresponding author (Email: ari.voutilainen@uef.fi) Abstract - The seasonal succession of plankton communities is initiated as temperature starts to increase in early spring. Here, we examined the abundance of crustacean zooplankton in six large Finnish boreal lakes a few days after the melting of the ice for a period of ten years. We related zooplankton abundance to relevant abiotic and biotic factors, including temperature, wind and predation by fish and assessed the annual variability in zooplankton abundance at three spatial scales (lake, part of a lake, and habitat) and one temporal scale (year). Zooplankton abundance was positively associated with water temperature regardless of fish predation pressure and the spatial scale examined. This finding is significant as mean water temperature in the lakes concerned was very low (5.7 °C), although the observed temperature range was actually quite wide (2-13 °C). Predation by newly-hatched coregonids affected zooplankton density only in one lake. The effect of wind in one lake was significant in the littoral but not in the pelagic habitat. Temperature may be the ultimate factor determining zooplankton dynamics, particularly under cold water conditions. Keywords – Boreal Lakes, Crustacean Zooplankton, Fish Predation, Spatiotemporal Variation, Water Temperature, Zooplankton Abundance 1. Introduction Inter-annual variation in temperature affects zooplankton both directly and indirectly. Direct effects are mediated via the physiology of the zooplankton, including metabolic and reproductive processes (Herzig 1983, Maier 1989). Along with photoperiod, temperature controls the hatching of cladoceran and copepod resting eggs so that the hatching success is higher in warmer water (Vandekerkhove et al. 2005, Hansen et al. 2010). The development time of copepod nauplii and copepodid stages, on the other hand, is inversely related to temperature, although considerable differences may be found in the rate of development between species at lower temperatures (Maier 1989). Indirect effects include increased grazing rates at higher temperatures (Mourelatos & Lacroix 1990), for instance. Thus temperature is critical for the survival, growth, reproduction (Allan & Goulden 1980, Gillooly et al. 2002), and population dynamics (Gerten & Adrian 2000, Winder et al. 2009) of zooplankton, particularly in boreal and temperate lakes in spring (Gaedke et al. 1998, Pinel-Alloul et al. 1999, Romare et al. 2005). However, the full range of temperature effects and their implications for the distribution of zooplankton abundance have not been studied in boreal lakes in very early spring, when the average water temperature is very low (<6 °C), but variations in temperature at different spatial scales are large and rapid. The seasonal succession of plankton communities is initiated as temperature increases in spring. It has been proposed that conditions during the winter determine the inoculums of particular species in spring and affect their succession patterns (Adrian et al. 1999, Hülsmann et al. 2012). In the temperate zone, from where most of the published literature originates, lakes do not have ice-cover every winter, and ice-free winters have a strong effect on plankton dynamics (Adrian et al. 1999, Hülsmann et al. 2012). In the boreal zone, lakes are always ice-covered for several months each year and water temperature is very low, typically between 0 and 4 °C. The zooplankton in boreal lakes goes through a seasonal succession (Rahkola-Sorsa 2008) during a much