Bimonthly quantitative day and night samples from one monitoring station located in the St. Lawrence Estuary (in silt–clay, at a depth of 119 m), with a constant yearly temperature (1–4 °C), reveal that densities of the detrivorous and necrophagous lysianassid amphipod Hippomedon propinquus were similar (0.4–9.7 individuals/100 m3 in 1970, 1971, and 1973) to densities at another station in Chaleur Bay with comparable sediments, temperature, and depth (4.2–7.5 individuals/100 m3 in 1969 and 1971). However, its rank among the gammaridean community (10th to 15th in the bay, 3rd to 5th in the estuary) and a faster growth in the estuary suggest better "success" in the latter. Vertical migrations are mainly diurnal and of higher amplitude in the bay than in the estuary, where these are mostly nocturnal, except in June and July. Life expectancy is over 2 years with a maximum of 13 molts, 4 to 5 of which occur in the adult stage. Growth is slower, life expectancy is shorter, but rank is higher (third) at another Chaleur Bay station where H. propinquus also displays increased swimming activity. In both ecosystems, reproduction is mostly a continuous, year-round process, but juvenile recruitment is more important in spring and summer, conforming to our prediction that life cycle is more independent of primary production and seasonal sestonic fallout when species occupy higher positions in the food web. Average size of ovigerous females decreases while fecundity increases in summer. Females produce larger eggs in Chaleur Bay. Poorer and less predictable primary production in the St. Lawrence Estuary gives opportunistic species like H. propinquus an advantage over more strictly detrivorous crustacean species.
Temperature Affects the Time Required to Discern the Relationship between Primary Production and Export Production in the Ocean