Comparing Bongo net and N70 mesozooplankton catches: using a reconstruction of an original net to quantify historical plankton catch data

If Southern Ocean plankton communities are changing in response to climate, biases in various nets need to be evaluated to help understand regional and temporal differences between historical and contemporary sample collections. A comparison of the catching ability of a Bongo net (mesh aperture 200 mu m) and a reconstructed version of an N70 net (upper mesh aperture 445 mu m, lower mesh aperture 195 mu m) as used by the Discovery Investigations was therefore undertaken. Forty Bongo and forty N70 samples were obtained from 10 stations in the Southern Ocean during December 2009. Bongo net catch abundance was similar to 3 times greater than the N70 and similar to 4 times greater when only copepod instars < 0.5 mm body length were considered. The Bongo net captured more plankton at all chlorophyll a (Chl a) concentrations encountered, although the difference was less at high Chl a when the N70 appeared to filter all size classes of copepod more efficiently, as well as retaining a relatively greater proportion of non-copepod plankton. Application of these findings was made to a previous study in which N70 samples from 1926/1927 were compared to a series of Bongo net hauls made post-1995. By 'correcting' abundances for net bias among a common set of 45 taxa, N70 data were within the range of variability seen in the Bongo net samples. Making such inter-net comparisons allows us to use historical collections and better judge the nature and magnitude of change in these plankton communities.