UVB radiation and depth interaction during primary succession of marine diatom assemblages of Greece

Exposure to UVB radiation caused shifts in the species composition of diatom assemblages developing on ceramic tiles in a natural marine habitat near Korinthos, Greece. These differences in community structure were more pronounced during the first month of community development. The diatom species Mastogloia crucicula and Nitzschia constricta were sensitive to both UVA and UVB radiation, while Amphora delicatissima, Amphora veneta, Opephora olsenii, Synedra baccilaris, and Synedra robusta were excluded by exposure to UVB but not to UVA. Amphoru robusta, Cocconeis jluminensis, Mastogloia erythraea, and Mastogloia ovalis were absent from 0.5 m while Nitzschia bilobata was present only in 1.0 m. Mastogloia badjikiana, Mastogloia labuensis, Nitzschia lanceolata, and Synedra laevigata were present during different stages of succession. The fact that differences in community structure do not persist at later successional stages suggests that periphytic communities of the upper euphotic zone possess adjustment mechanisms to the stress posed by increased solar ultraviolet radiation. The increase in solar UVB irradiation caused by the rapid decline in stratospheric ozone concentrations has been confirmed by satellite measurements (Molina and Molina 1992). The deterioration of this phenomenon, as predicted by mathematical models (Madronich et al. 1995), has brought about scientific and public concern about the expected global-scale adverse effects in the future. The impact of solar UV radiation on aquatic ecosystems and their primary productivity has been the subject of intensive investigations in recent years (Donkor et al. 1993; Hader 1993; Hessen et al. 1995; Hader et al. 1995; Williamson 1995). Because marine habitats exceed in size the terrestrial habitats almost by a factor of two, severe UV effects in the marine environment are expected to have a significant global-scale climatic impact (Caldwell et al. 1995; Liu et al. 1995). Several studies focus on UV effects on phytoplankton productivity (Hader and Schafer 1994; Kim and Watanabe 1994; Gerber and Hider 1995a), while others focus on changes in motility, photoorientation, and velocity caused by UV (Gerber and Hader 1992; Gerber and Hader 1995b; Schafer et al. 1993; Tirlapur et al. 1993; Nielsen et al. 1995). In contrast to the plethora of phytoplankton studies, literature on the effects of UV radiation on periphyton, i.e. algae found attached to many types of substrate, is rather scarce (Santas 1989; Schreiber and Pennock 1995; Santas et al. 1996). Periphytic communities are ecologically and economically important due to their high productivity and their diversified flora and fauna. The present study investigates the combined role of solar UV radiation and depth during the process of ecological succession and establishment of Mediterranean diatom assemblages. The experiment was conducted at a distance of 50 m from an east-facing rocky shore of Saronikos Gulf, near Korinthos, Greece (37”58’N, 23”O’E). Nine treatment combinations (3 UV regimes X 3 depths) were performed as shown in Table 1. The filters used were UV-transmitting plexiglass (Plexiglas GS 2458) with plastic foil cutoff filters (295 Ultraphan; PR Montagefolie 320 nm Art. Nr. 10155 099; and 395 Ultraphan UV Opak; thickness, 0.3 mm). Each experimental unit consisted of eight 10 X lo-cm ceramic tiles placed on a polypropylene screen fixed onto a 0.75-inch, 45 X 90-cm PVC frame. Two replicates for each of the nine treatment combinations were suspended from PVC rafts anchored at one end only to allow free swinging of the apparatus with the current. Attenuation of the photosynthetically active radiation (PAR: 400-700 nm), UVA (320-395 nm), and UVB (290-320 nm) bands in the water column was measured with an Optronic 752 double monochromator spectroradiometer. Average daily doses were obtained using three sharp band sensors (Grobel, Ettlingen) for PAR, UVA, and UVB calibrated against the Optronic spectroradiometer. The signals from the sensors were amplified, digitized, and stored in a dedicated computer (Visual Basic program Windose written by Michael Lebert, Univ. of Erlangen, Germany).