Seasonal changes In the size spectrum and biochemical composition of the suspended particulate matter were recorded for 2 yr in Logy Bay, southeast Newfoundland. Canada. The seston load was lower than in many shalloiv marine bays, owing partly to a relatively small terrigenous input and little silt, and the quantity of the parhculate organic matter primarily depended on phytoplankton productivity. The ratio chlorophyll a:phaeopigments was high during the phytoplankton bloom, due to the living nature of the organic seston at this time, compared with the rest of the year, when lower values were associated with the highly degraded suspended material. The size-frequency distribution of the suspended particulate matter reflected the succession of the phytoplankton community throughout the year. The spnng peaks were composed mainly of single diatoms and chain-forming diatoms in the size range 20 to 60 pm (equivalent spherical diameter). Variations in carbohydrate, lipid and protein in the seston were primarily associated with the periods of high and low standing stocks of phytoplankton observed in this environment. The seasonal trends were very similar for each biochemical component. and values were highly correlated with those for other properties of the seston, such as organic matter, chlorophyll a and particulate organic carbon and nitrogen. The nutritional quality of the seston was expressed by a food index, calculated as the ratio of food material (protein + carbohydrate + lipid) to total seston. This index followed the cycle of the phytoplankton bloom, with maximum values during spring and summer, and much lower values during winter. The results suggest that the gross analysis of the suspended particulate matter or the measurement of a single chemical variable cannot describe fully the nutritive value of the seston to a suspension-feeder. An analysis of the size spectrum and biochemical composition of the material is required for this purpose.

A population of Modiolus modjolus (horse mussel) inhabiting a sub-arctic environment in Logy Bay, Newfoundland, Canada, was studied for 2 yr. The main objective was to gain insight into the relationship between environmental factors (temperature and components of the seston) and the physiological response of this species. The highest values for energy acquisition (ingestion and absorption rates) by M. rnodiolus coincided with the spring phytoplankton bloom occurring during April-May in Logy Bay, whereas energy expenditure (oxygen uptake and ammonia excretion rate) was greatest during the summer (July and August). The result was a clear seasonal fluctuation in the 2 physiological integrations, scope for growth (SFG) and net growth efficiency (K,), for which lower values were associated with a high metabolic rate, high temperature and low quality of the food supply. Conversely, higher values of SFG and K2 were associated with a low metabolic rate, low temperature and a n energy-rich food supply provided by the phytoplankton bloom. M. modiolus appears to be a species which is able to compensate physiologically for the poor food conditions occurring in Logy Bay during a large part of the year. It does this by reducing feeding activity during fall and winter, when there is a poor food supply. Conversely, it increases clearance rate during spring and early summer, when the seston is mainly composed of phytoplankton. Furthermore, M modiolus has a high absorption efficiency, which may be an adaptation to the low seston concentration in Logy Bay Such physiological compensations minimise the periods dunng which SFG is negative, and enable the horse mussel to survive in a n environment characterised by an intermittent and often inadequate food supply.