Vertical migration of phytoplankton in coastal waters with different UVR transparency

Chlorophyll a maxima of mainly motile dinoflagellates were observed in situ at all days and at both study sites (open marine, brackish waters), independent on prevailing weather conditions or cDOM concentrations. Phytoplankton migration was triggered solely by irradiance in the 400- to 700-nm wavelength range (PAR) at the particular water depth, irrespective of PAR/UVR ratios and surface UVR (290 to 400 nm), after an illumination period of about 40 min. Interestingly, the PAR tolerance levels of the phytoplankton, which have been lower in cDOM-rich waters, matched their light acclimation values determined by parallel PAM measurements.The response of the phytoplankton to PAR is not a sufficient protection strategy versus increasing UVR levels, which might have wide ecological implications beyond the level of primary producers to impact important ecosystem functions such as the delicate trophic interactions.Over the last decades, there have been increasing reports on the depletion of the stratospheric ozone layer (reviewed in [1]). At first, the thinning ozone layer and ozone holes have been mainly associated with polar regions [2], yet this has been observed in the northern hemisphere too [3-5]. Although the transmission of ultraviolet radiation (UVR) is highly variable due to e.g. solar variability or meteorological conditions, climate change is expected to enhance UVR, consisting of UVA (320 to 400 nm) and UVB (290 to 320 nm), in the long run [6,7]. These reports initiated numerous investigations on possible effects of UV radiation on terrestric and aquatic (marine and freshwater) organisms. In the pelagic zone, an elevated sensitivity of phytoplankton and zooplankton versus UVR could be observed with effects on growth, production, cell biochemistry, ontogeny, genome and mortality (reviewed in [8-10]).Beyond the UVR effects on single organisms, the ecosystem functions of the aquatic habitats may be threatened considering for instance the delicate balance of trophic