Interannual variability in the relationship between in situ primary productivity and somatic crustacean productivity in a temperate fjord

ABSTRACT: Simultaneous and precise measurements of primary and secondary productivity are required when examining energy transfer from phytoplankton to zooplankton. We examined the relationship between primary and crustacean productivity over 2 yr in Saanich Inlet, British Columbia, Canada, to determine how temporal variations in primary productivity influence crustacean productivity and trophic transfer efficiency (TTE). Primary productivity was measured via the 13C tracer method, while the crustacean moulting enzyme chitobiase was used to estimate community-level somatic crustacean productivity. Peak primary productivity occurred much earlier in 2010 (late April; 9.17 g C m-2 d-1) than in 2011 (mid-June; 5.01 g C m-2 d-1) due to a higher abundance of diatoms. Fatty acid analyses revealed that one of the dominant copepods, Calanus marshallae, was feeding on a higher proportion of diatoms than dinoflagellates (lowest DHA:EPA ratios) in May 2010 and June 2011. Crustacean productivity ranged between 0.01 and 0.65 g C m-2 d-1 over both sampling years. Average TTE was 14% in 2010 and 8% in 2011, indicating that the earlier peak in primary productivity in 2010 resulted in more efficient energy transfer from phytoplankton to crustacean zooplankton compared to 2011. Results from this study highlight the need for incorporating routine field estimates of crustacean productivity into oceanographic studies with the same resolution as primary productivity measurements. Together, these estimates are critical in terms of investigating the impact of a potential increase in the occurrence of mismatches between lower and higher trophic levels in predicted future warming scenarios.