Seasonal fouling by diatoms on artificial substrata at different depths near Piran (Gulf of Trieste, Northern Adriatic)

Abstract

       Seasonal fouling by diatoms was studied in the heavily polluted and eutrofied area near Piran in the Gulf of Trieste. Concrete plates (50 x 50 cm) were placed at l m, 3 m and 7 m depths, with the fouling observed monthly for one year, from March to October. Two plates were used at each level: one was scratched clean monthly to get an insight into the seasonality of fouling, while from the other only representative samples were taken in order to follow the fouling succession. In the eulittoral two quadrats of the same dimension were scratched clean on a vertical concrete wall. Diatoms proved to be the main fouling component sublittorally, while in the eulittoral green algae determined the physiognomy of the experi mental surfaces during spring. The present contribution deals only with the diatoms. Peaks of diatom colonization were found in April and August in the eulittoral, and sublittorally in July. Regarding the depth distribution, maxima in the number of recorded species were found at 3 m in spring, and at 7 m in autumn. The fouling populations were heterogenous, including epilithic, epipsammic and epipelic species with different affinities (marine, brackish and even freshwater). Colonial forms belonging to the genera Berkeleya, Navicula and Licmophora were outstanding and covered most of the experimental surfaces. Achnanthes species were among the primary colonizers, while Nitzschia species joined the fouling communities in autumn, along with several epipelic species. Seasonal recolonization on the monthly denuded plates was usual for species found sublittorally, either the whole year around, or only in autumn. Species found during spring did not recolonize monthly, and the same was true of the eulittoral ones.