Quantifying the response of phytoplankton photosynthesis to ultraviolet radiation: Biological weighting functions versus in situ measurements in two Swiss lakes

The sensitivity of photosynthesis to ultraviolet radiation (UV) was assesse d for phytoplankton assemblages in two Swiss lakes, pre-alpine Lake Lucerne (Vierwaldstattersee) and alpine Lake Cadagno, using both in situ and labor atory incubations. Biological weighting functions for UV inhibition of phot osynthesis (BWFs) were determined in the laboratory using polychromatic exp osures in a Xe-lamp based incubator. Samples were concurrently incubated in situ under UV exposed and protected bottles (profiles 0-5 m), while additi onal spectral treatments were carried out at the 50% UV-B penetration depth : full spectrum, UV-A only (Mylar protected) and UV protected quartz tubes. Both particulate (>0.2 mum) and total organic carbon incorporation were me asured. Measured attenuation coefficients and incident UV spectral irradian ce data was used to evaluate a BWF/photosynthesis-irradiance model (BWF/P-I ) for in situ exposure conditions and compared with measurements. The BWFs showed sensitivity across the UV spectrum at similar, though somewhat lower , levels than an average BWF for marine assemblages. Relative photosynthesi s in situ (UV exposed/UV excluded) was about 40% at the surface and about 6 0% at the 50% UV-B penetration depth. Similar inhibition was predicted by t he BWF/P-I model. Generally, full spectrum (UV-B and UV-A) exposure had lit tle additional effect compared to UV-A only exposure. Reciprocal transfer o f samples between lakes showed enhancement of UV effects in L. Cadagno comp ared to incubation of the same sample in L. Lucerne, consistent with increa sed UV sensitivity due to the 5 degreesC cooler water temperature in L. Cad agno. Similarly, BWF prediction of in situ response in L. Cadagno was impro ved by increasing UV sensitivity according to a Q(10) of 2. Full profile ca lculations using the BWF/P-I model suggest stronger effects of UV on L. Luc erne compared to L. Cadagno phytoplankton due to greater sensitivity of the assemblage combined with higher overall transparency to UV relative to PAR in L. Lucerne. The BWF/P-I model was a good overall predictor of UV-depend ent photosynthetic performance in these lakes.