3Friedrich-Alexander Universität, Staudtstr. 5, D-91058 Erlangen, Germany

The effects of solar radiation (PAR, UV-A, UV-B) on community productivity and structure were assessed during primary succession of primary producers and consumers on artificial substrate in a rocky shore of the Saronikos Gulf near A. Korinthos, Greece. The communities were exposed to PAR, PAR+UV-A, and PAR+UV-A+UV-B at 0.5, 1.0 and 1.5 meters of depth. Pennate diatoms were the main taxonomic component of the developing periphytic communities during the study period. Exposure to solar UV-B initially reduced the biomass and altered the structure of the diatom assemblages. The highest biomass of diatom assemblages was observed under PAR (49.2 g/m²). This value was significantly higher than the biomass of assemblages growing under PAR+UV-A+UV-B, but not significantly different compared to the biomass of assemblages exposed to PAR+UV-A. These differences, however, did not persist at later stages. The most abundant invertebrate groups present were Polychaetes and Crustaceans. Solar UV-B did not have significant effects on invertebrate biomass. Analysis of the invertebrate assemblage structure revealed time-course differences but no clear trends among the different treatments. Sample separation primarily reflected differences in species number and number of individuals.

Key words: algae, grazers, invertebrates, solar UV radiation, artificial substrate, Greece, rocky shore, mesocosm.

The rapid decline in stratospheric ozone concentrations results in increased levels of UV radiation, especially UV-B (280-315 nm) incident on the Earth's surface (Madronich et al. 1995, Kerr & McElroy 1993). Peak global losses of ozone are expected around the year 1998-99, coinciding with peak stratospheric chlorine and bromine abundances (World Metereological Organization 1995). The maximum ozone loss relative to the late 1960s will likely be about 6-7% at northern mid latitudes in winter/spring.

Short-term UV screening experiments confirm that UV radiation can reduce the growth rates of benthic diatom communities in shallow freshwater during summer at mid-northern latitudes (Bothwell et al. 1994). Prolonged exposure to UV radiation increased diatom biomass and community diversity. This effect was attributed to the greater UV-B susceptibility of grazers (larval chironomids) than their food, the algae.

Literature on the effects of UV radiation on periphyton is rather scarce (Schreiber & Pennock 1995, Santas et al. 1996, Santas et al. 1997a). Direct UV-B effects on diatom assemblages grown on ceramic tiles in a natural marine habitat in Saronicos Gulf, Greece included shifts in species composition and temporary inhibition of biomass production (Santas et al. 1996).

The sensitivity and tolerance of marine invertebrates to UV-B radiation may vary considerably (Hunter et al. 1982). Cleavage reduction has been observed in sea urchin eggs as a result of exposure to ultraviolet radiation (El Sayed 1988). On the other hand, the egg capsules of the gastropod Nucella emarginata provide embryos with substantial protection from UVR. These capsules allow less than 5% of the incident UV-B at 300 nm and less than 55% of incident UV-A at 360 nm to enter the capsule chamber (Rawlings 1996). The sensitivity of corals to UV-B may depend on the depth at which they grow (Siebeck & Böhm, 1987). A correlation between depth and concentration of UV-absorbing pigments has been found in different organisms (Maragos 1972, Jokiel & York 1982).

While our earlier attempts aimed at assessing solar UVR effects on diatom assemblages (Santas et al. 1996, Santas et al. 1997a), in the present study, the role of solar ultraviolet radiation was investigated in the grazers (invertebrates) associated with the diatom assemblages during the process of primary ecological succession in benthic communities of Greece.

The experiment was conducted at a distance of 50 m from an east-facing rocky shore of Saronikos Gulf, near Korinthos, Greece (37⁰ 58' N, 23⁰ 0' E). Using UV-transmitting Plexiglas and plastic foil filters, 9 treatment combinations (3 UV regimes x 3 depths; Table 1) were performed in duplicate. The filters used were UV-transmitting Plexiglas (Plexiglas GS 2458) and plastic cut-off filters, (295 Ultraphan; PR Montagefolie 320 nm Art. Nr. 10155099; and Ultraphan UV Opak; thickness: 0.3mm). Each experimental unit consisted of eight 10x10cm ceramic tiles placed on a polypropylene screen fixed onto a 3/4", 45 x 90 cm PVC frame. The 18 units were suspended in random pairs from nine 2 x 1 m rafts at 0.5, 1.0 and 1.5 m. The rafts were constructed from 2.5" PVC pipe filled with polyurethane for waterproofing and tied together in tiers of three. Each raft tier was anchored at one end only to allow free swinging of the apparatus with the current. The filters were cleaned regularly to prevent alteration of the transmittance properties due to biofouling. Light trasmittance was periodically checked for replacement of defective filters.

Attenuation of the PAR, UV-A and UV-B bands in the water column was measured with an Optronic 752 double monochromator spectroradiometer using a sensor in 4p geometry. Irradiance levels at 1.5 m were ca. 35% lower than immediately below the surface for PAR; 20% for UV-A; and 6% for UV-B.

Surface irradiance measurements were obtained using an ELDONET dosimeter equipped with three sharp band sensors (Gröbel, Ettlingen) for PAR, UV-A and UV-B calibrated against the 752 Optronic spectroradiometer (Santas et al. 1997b). The signals from the sensors were amplified, digitized and stored in a dedicated computer (Visual Basic program ‘Windose 2000’ written by M. Lebert, Univ. of Erlangen, Germany). Daily surface irradiance averages were 10,027 kJ/m2 (PAR), 1,110 kJ/m2 (UV-A), and 28 kJ/m2 (UV-B).