DISTRIBUTION OF EPIPHYTIC DIATOM SPECIES COMPOSITION, DIVERSITY AND BIOMASS ON DIFFERENT MACROALGAL HOSTS ALONG SEASONAL AND SALINITY GRADIENTS IN THE BALTIC SEA

Abstract

In spring 1990 and 1991, some weeks after ice-break, macroalgae and their epiphytes were sampled from 8 areas along the stable salinity gradient of the Baltic Sea from the southern Baltic Sea proper (8‰) to the northern Bothnian Sea (5‰). The macroalgal hosts included the brown alga Pilayella littoralis (L.) Kjellm., the red alga Ceramium gobii Wærn, and the green alga Cladophora glomerata (L.) Kütz. In addition, seasonal succession was studied from monthly samples throughout one growing season (March-November 1991) at one of the sampling sites (5‰ salinity). Host preference was highest in spring when the diatoms were newly colonizing the rapidly growing macroalgae, and higher in the Baltic Sea proper (higher salinity) than in the Bothnian Sea (lower salinity). Biomass was generally highest on Pilayella and lowest on Cladophora, whereas community diversity was generally highest on Ceramiutn and lowest on Pilayella. The differences in epiphytic diatom community composition and biomass between the three macroalgal hosts could be partly explained by morphology and life cycle of the host; however, physiological aspects may also be involved. With decreasing salinity the total number of diatom cells increased and community diversity decreased on all three hosts, but diatom species richness was not affected. Biomass increased with decreasing salinity on Cladophora but not on Pilayella or Ceramium. Between a salinity of 5 and 6‰ there was a marked transition from an epiphytic flora dominated by diatoms with marine affinity to one dominated by diatoms with freshwater affinity.