Imbalance of fatty acids in the base of the Baltic Sea food web -: a mesocosm study

A reproductive disturbance in Baltic Sea Atlantic salmon (Salmo salar), the M74 syndrome, has been reported since early 1970s and has occasionally caused up to 90% mortality for newborn fry. Previous research has revealed that the M74 syndrome may be due to reduced levels of the vitamin thiamin, the carotenoid astaxanthin, and elevated ratios of omega 3/omega 6 fatty acids in salmon eggs. Using mesocosm experiments, we compared the quantity (mu g center dot L-1) and quality (mg center dot g(-1) C) of fatty acids in microalgae and copepods in the southern Baltic Sea where the M74 syndrome is common with those in a habitat in the Norwegian Sea where the syndrome has not been observed. Daily additions were made of the nutrients N and P or N, P, and Si, copepods were added after 6-7 days, and nutrient additions were stopped after 9-10 days. Flagellates dominated completely in the Baltic Sea, whereas higher phytoplankton diversity was found in the Norwegian Sea. We found elevated omega 3/omega 6 ratios in phytoplankton and abnormally high docosahexaenoic acid/arachidonic acid ratios (22:6 omega 3/20:4 omega 6) in copepods in the Baltic Sea mesocosms compared with those in the Norwegian Sea. Our results suggest that imbalance in fatty acid composition may prevail in the basic food web of the Baltic Sea.