The measurement of mesozooplankton biomass in the ocean requires the use of analytical procedures that destroy the samples. Alternatively, the development of methods to estimate biomass from optical systems and appropriate conversion factors could be a compromise between the accuracy of analytical methods and the need to preserve the samples for further taxonomic studies. The conversion of the body area recorded by an optical counter or a camera, by converting the digitized area of an organism into individual biomass, was suggested as a suitable method to estimate total biomass. In this study, crustacean mesozooplankton from subtropical waters were analyzed, and individual dry weight and body area were compared. The obtained relationships agreed with other measurements of biomass obtained from a previous study in Antarctic waters. Gelatinous mesozooplankton from subtropical and Antarctic waters were also sampled and processed for body area and biomass. As expected, differences between crustacean and gelatinous plankton were highly significant. Transparent gelatinous organisms have a lower dry weight per unit area. Therefore, to estimate biomass from digitized images, pattern recognition discerning, at least, between crustaceans and gelatinous forms is required.

During the austral summer, zooplankton excretion along the western Antarctic Peninsula was studied in a contrasting hydrographic regime including coastal and oceanic waters. In coastal waters, ammonium supply by mesozooplankton indicated a low contribution to fuel primary production. In oceanic waters, however, Antarctic krill Euphausia superba contributed a significant percentage to the nitrogen requirements of primary producers. Thus, the ontogenetic migration of adult krill during austral summer should be a key factor regulating the regenerated ammonium for primary production. A significant coupling of ammonium concentration in the water column and in situ krill biomass supported the significant role of krill excretion in the epipelagic realm. Results from short-term experiments with E. superba indicated that ammonium excretion rates were much higher than previously found. Because the use of experimental metabolic rates that are close to field rates would be more appropriate, we suggest to re-assess the ammonium supplied by the epipelagic marine biota. Moreover, the outcomes of experimental krill excretion rates, in situ measurements of ammonium and a review of data on primary production suggest that Antarctic krill sustain a high proportion of the daily phytoplankton production.

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