Temporal and spatial variability of phytoplankton and chlorophyll a: lessons from the south coast of Norway and the Skagerrak

Abstract

Regular monitoring of phytoplankton has been carried out on the Norwegian Skagerrak coast three times a week since 1989, and samples from the upper 3 m of the water column have been analysed for chlorophyll a (Chl a) concentrations and dominant species. Selected potentially toxic or harmful species of phytoplankton have also been quantified. Chl a appears to fluctuate considerably on a short time scale (2–3 days), and simple analyses suggest that Chl a should be measured at least twice a week to obtain reliable annual estimates. By contrast, at the inshore monitoring station Chl a is significantly correlated with transect measurements across most of the Skagerrak, indicating a high spatial homogeneity. Hence, measurements at the inshore stations are likely to reflect concentrations over a large area and meteorological conditions are suggested to be the most likely driving force controlling the variability. The traditional view of a marked spring and autumn bloom in temperate stratified waters does not seem to be a predominant feature of the production cycle in the Skagerrak. In fact, Chl a concentrations > 4μg l⁻¹ rarely last for more than a week. In addition to short-term variability, there is also high inter-annual variability in the production cycle. This picture is reinforced by large variations in species composition of the phytoplankton community; the abundance of all species analysed varies substantially from one year to the next.