During some years Neocalanus plumchrus overwinters in Saanich Inlet while in other years they are absent from the fjord (Harrison et al., 1983). The cause of this variation is not known but the annual development of anoxic conditions followed by an intrusion of dense, oxygenated water, was suspected to influence their distribution and abundance. Vertical and horizontal haul samples collected from Saanich Inlet indicated the overwintering population in 1985 (September 1985 to March 1986) was sparse; in September 1986 the population of N. plumchrus was comparably low. In the spring and summer, N. plumchrus is introduced from the Strait of Georgia and Juan de Fuca Strait, and in some years may arise from reproduction within the inlet. So few adults were collected at the three stations in Saanich Inlet in January-March 1986 (<0.20 m⁻³) that their potential contribution to the spring population was considered negligible. Decline in the overwintering population in September 1985 and 1986 appears to be correlated with the occurrence of an extensive deep water renewal. The distribution of N. plumchrus during early stages of their deep water migration (June to August), is influenced by the low oxygen concentrations in the bottom of the inlet (i.e. 0.10-0.30 mL L⁻¹). During the renewal, copepods occurred above the oxygen minimum (75 m) possibly due to their displacement or their avoidance of the low oxygen zone. Subsequently, they were exposed to tidal transport out of the inlet and perhaps to increased predation. On two occasions in which N. plumchrus was present during the winter in Saanich Inlet (1969 & 1974), a high volume of dense water intruded, disrupting the copepod layer during the mixing of the two water masses. Even though a greater volume of water left the inlet some of the copepods could have remained in the water, below sill depth. Neocalanus plumchrus successfully overwinters and reproduces in Sechelt Inlet which is well-oxygenated but has a very shallow sill (15 m) that 'locks' the copepods into the inlet. The fifth copepodite stages also occupy deeper depths in Sechelt Inlet than in Saanich Inlet, even though the bottom depths are comparable. In low oxygen tolerance experiments many factors such as the period of captivity, and the region of origin (i.e. Saanich Inlet versus the Strait of Georgia) caused variable results. Although the minimum oxygen level tolerable during their migration could not be determined, N. plumchrus tolerated levels as low as 0.56 mL L⁻¹ (12% mortality). Sediment trap samples indicate that a massive die off could not account for the loss of N. plumchrus from Saanich Inlet. / Science, Faculty of / Zoology, Department of / Graduate

Although food quality is thought to play an important role in the survival of marine copepods, the extent of natural variability in food quality remains poorly characterized. Here I characterize the different scales at which food quality varies in copepods of the Strait of Georgia, British Columbia, Canada. Significant interannual variability occurs in the diet of Neocalanus plumchrus in the Strait of Georgia. Between 2001-06 the fatty acid profiles of N. plumchrus switched from omnivorous, oceanic signatures to herbivorous, diatom-dominated signatures. An index of food quality (DHA/EPA) is strongly correlated to the abundance of diapausing N. plumchrus, suggesting that the relative proportion of essential fatty acids provided by dinoflagellates and diatoms are related to the survival of this species. Combined fatty acid and stable isotope analysis indicated that the spring calanoid copepods of the Strait of Georgia occupy three trophic positions: Eucalanus bungii is herbivorous, Calanus marshallae and N. plumchrus are omnivorous, while Euchaeta elongata is carnivorous. Oceanic conspecifics of Strait of Georgia copepods experience a more omnivorous diet, as indicated by the presence of higher proportions of flagellate and carnivory markers, and lower proportions of diatom-based markers in their fatty acids. Despite spatial differences in the quality of their diets, the relative trophic positions of these copepods are constant as indicated by their stable isotope signatures. There is a correlation between the trophic information provided by stable isotopes and fatty acids. However, stable isotopes are not sensitive enough to capture the range of dietary variability observed in fatty acids, and fatty acids do not always provide reliable markers of carnivory and trophic position. Over the span of a season, copepods can utilize a wide range of dietary items including diatoms, flagellates, bacteria, detritus and microzooplankton. Copepods can switch from herbivory to carnivory in response to declining chlorophyll concentrations after the spring bloom, and are occasionally able to utilize detrital and bacterial sources. I conclude that the quality of copepod diets in the SoG varies on interannual, interspecific and seasonal scales. The implications of these findings are discussed in relation to ecosystem models of the area, and to copepod physiology.

Although food quality is thought to play an important role in the survival of marine copepods, the extent of natural variability in food quality remains poorly characterized. Here I characterize the different scales at which food quality varies in copepods of the Strait of Georgia, British Columbia, Canada. Significant interannual variability occurs in the diet of Neocalanus plumchrus in the Strait of Georgia. Between 2001-06 the fatty acid profiles of N. plumchrus switched from omnivorous, oceanic signatures to herbivorous, diatom-dominated signatures. An index of food quality (DHA/EPA) is strongly correlated to the abundance of diapausing N. plumchrus, suggesting that the relative proportion of essential fatty acids provided by dinoflagellates and diatoms are related to the survival of this species. Combined fatty acid and stable isotope analysis indicated that the spring calanoid copepods of the Strait of Georgia occupy three trophic positions: Eucalanus bungii is herbivorous, Calanus marshallae and N. plumchrus are omnivorous, while Euchaeta elongata is carnivorous. Oceanic conspecifics of Strait of Georgia copepods experience a more omnivorous diet, as indicated by the presence of higher proportions of flagellate and carnivory markers, and lower proportions of diatom-based markers in their fatty acids. Despite spatial differences in the quality of their diets, the relative trophic positions of these copepods are constant as indicated by their stable isotope signatures. There is a correlation between the trophic information provided by stable isotopes and fatty acids. However, stable isotopes are not sensitive enough to capture the range of dietary variability observed in fatty acids, and fatty acids do not always provide reliable markers of carnivory and trophic position. Over the span of a season, copepods can utilize a wide range of dietary items including diatoms, flagellates, bacteria, detritus and microzooplankton. Copepods can switch from herbivory to carnivory in response to declining chlorophyll concentrations after the spring bloom, and are occasionally able to utilize detrital and bacterial sources. I conclude that the quality of copepod diets in the SoG varies on interannual, interspecific and seasonal scales. The implications of these findings are discussed in relation to ecosystem models of the area, and to copepod physiology.