Comparison of biomass and size-spectra derived from OPC data and net samples. Application to the assessment of mesoplankton distribution in the NW and N Iberian shelf

摘要

Data from vertical net hauls and from a submersible optical plankton counter (OPC) werecompared in terms of biomass and slope of the normalized biomassesize spectra (NBeSS),a proxy for the size structure of the community. The relationship between the estimates ofbiomass in the overlapping range sampled by both methods (0.2e2 mm equivalentspherical diameter (ESD)) was linear and not significantly different to 1 (BOPC : BNET ¼1:1G0:1, r2 ¼ 0:84). However, the relationship varied depending on the size fraction considered;the ratio BOPC:BNET was 0:10G0:04 (r2 ¼ 0:30), 0:8G0:2 (r2¼ 0:66), and2:4G0:5 (r2¼ 0:64) for the size fractions 0.2e0.5, 0.5e1, and 1e2 mm ESD, respectively.The discrepancies between methods were presumably due to the combined effect ofthe limitations of the instrument in the lower detection limit and the errors in the volume ofwater sampled by the net for the smallest size fraction, and to net avoidance enhanced byclogging for the largest size fraction. The agreement between methods improved when theNBeSS of the different data sets were compared. The slope (b ¼ 1:1) and the intercept(a ¼ 14:6) of the NBeSS integrated across stations were not significantly different(Student’s t-test) for the linear model fitted to net samples, OPC data, or pooled data fromboth methods. Station by station, the slopes of the NBeSS from the net (bNET) and the OPC(bOPC) were not significantly different in 61% of the stations. This percentage increased to78% when the comparison was limited to the mesoplankton size range. As an example ofthe applicability of the OPC, we showed the distribution of mesoplankton biomass and sizestructure along the NW and N Iberian Shelf during the winterespring transition of 2002 andits relationship with the hydrographic scenario.