Seasonal variation in the relative utilization of carbon and nitrogen by the mussel Mytilus edulis: budgets, conversion efficiencies and maintenance requirements

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Abstract
Elemental balances for carbon and nitrogen in an open-shore population of the bivalve mollusc M. edulis underwent seasonal changes more characteristic of time-averaged than immediate optimization. Budgets emphasized the significance of shell and byssus towards both the carbon (8 and 44%, respectively) and nitrogen (8 and 21%, respectively) within total production, and indicated that between at least 20 and 67% of the nitrogen in faeces may be of metabolic origin. In addition to differing absorption rates, pronounced seasonal variations of net growth efficiency were effected for each element by changes in metabolic demand that not only reflected requirements per se, but also the ''gross efficiencies'' with which absorbed nutrients were used to offset net deficits. Changes of ''gross efficiencies'' were at least partially due to a variable subsidization of maintenance requirements from pre-stored reserves, rather than to possible variations of metabolic efficiency alone. Although net growth efficiencies for nitrogen consistently exceeded those for carbon, the associated seasonal patterns of utilization were different for each element, and are discussed in relation to physiological, storage and reproductive cycles documented elsewhere. Ratios between coincident maintenance requirements for the utilizable carbon and nitrogen in the alga Phaeodactylum tricornutum varied from 12:1 during summer to as much as 83:1 in winter. Further, relative to carbon, mussels regularly absorbed higher proportions of their maintenance requirements for nitrogen. These results, together with known fluctuations of nutrients in the natural environment, imply that there may have been transient limitation of organic processes by the available carbon, and emphasize the need for a multi-elemental approach in future studies of bivalve nutrition.