Measured and inferred gross energy content in diapausing Calanus spp. in a Scotian shelf basin

Calanus finmarchicus stage-C5 and C. hyperboreus stage-C4 diapausing in Scotian Shelf basins are high-quality food sources because they are abundant and high in energy content. When combined, these two variables are informative for quantifying energy density, carrying capacity and energy transfer in marine ecosystems. However, measuring energy content directly can be difficult and expensive. Here, we present energy (caloric) content estimates for two co-located diapausing copepod species; C. finmarchicus C5 and C. hyperboreus C4 collected during late-summer when their energy content is at or near the annual maximum. We then develop several practical energy content calibrations, each designed to estimate energy content using simple measures of Calanus spp. body size or weight. Weight-specific energy content (energy quality, kJ g(1)) did not differ between C. finmarchicus and C. hyperboreus; i.e. body size explained the majority of inter-species variation in individual energy content (J ind.(1)). Energy content estimated directly using calorimetry did not differ from energy inferred from oil sac volume (OSV), demonstrating that energy content can be inferred when direct energy estimates are unavailable. We also assess the effect of formalin preservation on energy content to determine the utility of using archived samples when addressing energy-related questions. There was no difference between directly measured energy content in frozen- and formalin-preserved specimens. However, formalin preservation appears to alter oil sac shape in C. hyperboreus, thereby altering energy content estimates inferred from OSV for the species. Formalin-preserved samples can be used for direct energy content estimation if individuals with intact oil sacs are selected.