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Temporal variations in the abundance and composition of intact polar lipids (IPLs) in North Sea coastal marine water were assessed over a one-year seasonal cycle, and compared with environmental parameters and the microbial community composition. Sulfoquinovosyldiacylglycerol (SQDG) was the most abundant IPL class, followed by phosphatidylcholine (PC), phosphatidylglycerol (PG) and diacylglyceryl-(N,N,N)-trimethylhomoserine (DGTS) in roughly equal concentrations, and smaller amounts of phosphatidylethanolamine (PE). Although the total concentrations of these IPL classes varied substantially throughout the year, the composition of the IPL pool remained remarkably constant. Statistical analysis yielded negative correlations between IPL concentrations and dissolved inorganic nutrient concentrations, but possible phosphorous limitation during the spring bloom did not result in changes in the overall planktonic IPL composition. Significant correlations between SQDG, PC, PG and DGTS concentrations and chlorophyll-a concentrations and algal abundances indicated that eukaryotic primary producers were the predominant source of IPLs at this site. However, whilst IPL concentrations in the water were closely tied to total algal abundances, the rapid succession of different algal groups blooming throughout the year did not result in major shifts in IPL composition. This shows that the most commonly occurring IPLs have limited chemotaxonomic potential, and highlights the need to use targeted assays of more specific biomarker IPLs. [ABSTRACT FROM AUTHOR]