Seasonal differences in concentrations of particulate lipids, proteins and chitin in the North Sea

Abstract

The concentrations of particulate lipids, proteins and chitin were determined in the North Sea in January 1992, October 1992 and March 1993, with additional measurements of total suspended matter concentrations and organic matter contents in the suspended matter (POC) by ashing and wet-oxidation. During each of the three cruises, the proteins were by far the most important of the three components measured, followed by the lipids. The concentrations of chitin were always very low. The concentrations of these three components were highest in March and lowest in January; there was also a variation in the distribution over the North Sea between the three cruises. In January 1992, the suspended matter contained more organic matter in the northern North Sea than in the southern North Sea, but the higher concentrations of total suspended matter in the southern North Sea resulted in an inverse distribution for the concentrations of organic matter per volume of water. In March 1993, highest concentrations of the three organic compounds were observed in the Skagerrak and along the eastern side of the North Sea, decreasing towards the west. The suspended matter was richest in organic compounds in the Skagerrak and in the Norwegian Channel. In October 1992, the situation was very similar to that observed in January 1992, but the concentrations of organic compounds in the suspended matter and per volume of water were higher. The ratio of proteins to lipids was similar to this ratio in phytoplankton. The composition of the particulate organic matter became very close to that of phytoplankton if we assume that we missed a large amount of carbohydrates; the small differences persisting were probably due to the contributions of zooplankton, bacteria and terrestrial sources. The total contents of particulate lipids + proteins + chitin were low, but when estimated amounts of carbohydrates were added, the total particulate organic-matter concentrations found were very similar to the total particulate organic carbon concentrations found by wet-oxidation. During the period of sampling, no large concentrations of phytoplankton occured.