Environmental factors affecting the occurrence of different morphological forms of cyanoprokaryotes in the northern Baltic Sea

Abstract

Cyanoprokaryotes (blue-green algae, cyanobacteria) and environmental factors were studied in the Baltic Sea, off the SW coast of Finland, from April to December 1993. The aim was to outline the seasonal development of cyanoprokaryote populations and the occurrence of species and species groups in relation to prevailing hydrographic conditions and general features of the planktonic community. Altogether, 27 taxa of cyanoprokaryotes were encountered, representing the orders Chroococcales, Nostocales and Oscillatoriales. Different morphological forms of cyanoprokaryotes predominated during certain periods of the year: the spherical, gomphosphaerioid colonies of the genera Coelomoron, Coelosphaerium, Snowella and Woronichinia were rather abundant in early spring and the most abundant group in late autumn; minute-celled colonies with mucilaginous structures, e.g. the genera Aphanothece, Cyanonephron, Lemmermanniella and Merismopedia, were abundant during early summer; and filamentous, nitrogen-fixing species in late summer and early autumn. Each of the periods represents a distinct successional phase of the planktonic community with inherent environmental conditions, and it was obvious that the cyanoprokaryotic assemblage was modified by the general development of the planktonic community. According to canonical correspondence analysis (CCA), temperature and the weight ratio of inorganic nitrogen to inorganic phosphorus were the most important factors explaining variation in the cyanoprokaryote assemblage.