SEASONAL-CHANGES IN PHYTOPLANKTON COMMUNITY STRUCTURE IN RELATION TO PHYSICOCHEMICAL FACTORS IN LOCH-LOMOND, SCOTLAND

An intensive seasonal sampling programme at two contrasting sites in L och Lomond, the largest lake in Britain, showed the 1992-93 status of the loch to be mesotrophic in the shallow south basin and oligotrophic in the deeper north basin. There is evidence for an increase in both phytoplankton production and reactive phosphorus concentrations in the loch during the two decades up to 1993. Canonical Correspondence Anal ysis (CCA), plus classification (by Two-Way Indicator Species Analysis : TWINSPAN) of the dataset of time-position samples from the loch reve aled that seasonal factors (particularly the thermal regime of the loc h, and associated variables) remain more important than non-seasonal e nvironmental variables in predicting changes in phytoplankton communit y composition. As in the 1970s, the community remains dominated by a d iatom-desmid assemblage, but cyanobacteria blooms, albeit at fairly lo w abundance, are becoming a frequent phenomenon in the south basin of the loch.