Comparisons of Calanus finmarchicus fifth copepodite abundance estimates from nets and an optical plankton counter

The response of an optical plankton counter (OPC) to concentrations of Calanus finmarchicus fifth copepodites (C5) ranging from 2 to 1621 copepods m(-3) was examined during the summers of 1999-2001 over the continental shelf of the northwest Atlantic Ocean. Net tows from either a bongo net or a multiple opening/closing net and environmental sensing system (MOCNESS) were collocated with vertical OPC casts to provide comparable data. OPC-derived particle abundance in the 1.5-2.0 mm equivalent circular diameter range was strongly correlated with net-derived abundance of C. finmarchicus C5 (r(2) = 0.655 and 0.726 for comparisons in two independent datasets). Particle abundance in this size range increased with increases in the descent speed of the vertically profiled OPC, which indicated avoidance of the small sampling aperture by C. finmarchicus C5. A regression model was developed to relate OPC particle abundance in the 1.5-2.0 mm size range to the abundance of C. finmarchicus C5 and the descent speed of the OPC. The data fitted the model well (r(2) = 0.684) and the inverted model was used as a calibration equation to predict C. finmarchicus C5 abundances from OPC measurements in an independent comparison to net abundances. In that case, the calibration equation underestimated net abundance by an average factor of 2. However, anomalously low OPC particle abundances for some casts suggest that spatial heterogeneity (patchiness) can confound such comparisons.