Seasonal changes of the biochemical composition of marine particulate matter with special reference to fatty acids and sterols

Seasonal changes in particulate size spectra, biochemical composition, fatty acid and sterol content were followed from winter to fall in a small north Atlantic coastal basin. Strong seasonality, related to both spring-bloom and summer biological production, was recorded for most chemical and biochemical descriptors. Size spectra were generally characterized by dominance of small particles in the size range 6.35–25.4 μm equivalent diameter, except in early summer when an additional component in the 32.0–64.0 μm size range became important. Changes in either C:N ratio or carbohydrate:protein ratio indicated physiological changes which could be related to nutrient limitation or senescence. The fatty acid and sterol composition of the lipid fraction displayed major seasonal changes which reflected: (1) the seasonal heterogeneity of the taxonomic composition of the particles; (2) the physiological changes within each group of organism; and (3) the various periods of low production or bloom decay. Spring bloom production of small to medium size particles (12.7–50.8 μm) was associated with C16 polyunsaturated acid, 20:5ω3, 24 methylene-cholesterol and desmosterol. Summer production of small particles (2.0–6.4 μm) was associated with C18 polyunsaturated acids, 22:6ω3, nor-24-cholesterol and isofucosterol, while the late winter-early spring period displayed close relationships between some saturates as well as monoenes (18:0, 18:1ω9, 20:1ω9, 22:1ω9) and cholesterol. Post-bloom decay associated another group of saturates and monoenes (14:0, 20:0, 22:0, 16:1ω9, 16:1tΔ3) and β-sitosterol. Overall particulate biochemical dynamics is discussed in terms of both phytoplankton metabolism and trophic influence for the zooplankton consumers.