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Abstract
The vertical migration of phytoplankton was investigated in natural waters using in situ fluorescence profiling, chlorophyll a concentrations and life counts at two study sites differing in coloured dissolved organic matter (cDOM) concentrations. The data from the corresponding water depths (50-cm intervals down to 10 m) and times (hourly, before dawn to sunset, several days) were related to the highly resolved (2 nm) underwater ultraviolet radiation (UVR)/photosynthetic active radiation (PAR) transparency (290 to 700 nm). 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.
Highlights
The vertical migration of phytoplankton was investigated in natural waters using in situ fluorescence profiling, chlorophyll a concentrations and life counts at two study sites differing in coloured dissolved organic matter concentrations
Deviation of the measured points from the ideal calibration curve showed no correlation to sampling depth or time which could indicate an influence of NPQ or phytoplankton composition
The reports on a continuous decrease in ozone and a subsequent shift in the ultraviolet radiation (UVR)/photosynthetic active radiation (PAR) ratio are of concern since UVR effects have been shown to be detrimental to phytoplankton with possible implications on higher trophic levels of the pelagic food chain
Summary
The vertical migration of phytoplankton was investigated in natural waters using in situ fluorescence profiling, chlorophyll a concentrations and life counts at two study sites differing in coloured dissolved organic matter (cDOM) concentrations. 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. The UV radiation can directly or indirectly influence the phytoplankton-zooplankton interactions with severe cumulative effects up to higher trophic levels in the pelagic foodweb
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