Effect of eutrophication on the distribution and ecophysiology of the mussel Mytilus trossulus (Bivalvia) in southern Baltic Sea (the Gulf of Gdansk)

The effect of eutrophication on benthos is usually reported as negative. In the case of the Gulf of Gdansk, eutrophication has increased the availability of food resources to filter feeders such as mussels, affecting their physiology and vertical distribution. Comparative studies of the mussel Mytilus trossulus from two depth zones (10 m and 40 m) over a seasonal cycle revealed ecophysiologic differences between deep- and shallow-water animals. For the same shell length, the shallow-water mussels were heavier (dry weight = 0.004L2.297) and showed a higher weight index (dry weight/volume), averaging 3.8 (plus-minus) 1.9 mg cm^-3, than the deep-water mussels (dry weight = 0.0002L1.726 and 2.5 (plus-minus) 1.1 mg cm^-3), primarily because of a nearly twofold greater carbohydrate store. In the shallow zone, females contained more carbohydrates (on average 5.8% dry weight) than males (3.8% dry weight) because females conserve energy for reproduction. Differences in physiologic variables, and subsequently physiologic performance of the mussels, were related to different nutritional conditions in the ambient water. The deep habitat had lower Chl a, averaging 1.1 mg L^-1, total particulate matter (TPM) 4.3 mg L^-1 and particulate organic matter (POM) 1.1% of TPM as compared with the shallow habitat, with average Chl a 2.5 mg L^-1, TPM 4.9 mg L^-1, and POM 1.5%. The eutrophication of the Gulf of Gdanʹsk has led to an increase in the food availability in the water column, which allowed efficient colonization of the deep zone by the mussels; however, the dominance of males over females (~3 : 1) suggests that the food sustains only spawning-related metabolic demands and is not sufficient for energy conservation in this zone. Starch gel electrophoresis of eight enzyme loci showed no statistical differences in the allele and genotype frequencies between the shallow- and deep-water mussels; thus, the differences in ecophysiologic traits between depths are due to acclimatization.