Seasonal patterns of mesozooplankton abundance and biomass at Station ALOHA

The zooplankton community of the central North Pacific has been sampled at Stn. ALOHA since 1994 as part of routine data collections for the Hawaii Ocean Time-series (HOT) Program. Typically, three day and three night tows are collected in the upper 150 m on each cruise with a l-m, 200-mum mesh net. Size-fractioned carbon and dry weight biomass have been determined for each sample, and one day and one night sample per cruise have been enumerated microscopically through 1996. The accumulated data show statistically significant seasonal signals, with peak biomass and abundance during the summer months for the total community and for smaller size fractions (0.2-0.5, 0.5-1, 1-2 mm). Interannual variability is seen as significantly higher nighttime biomass during summer 1996, which is also related to the enhancement of small size fractions. Comparisons of the present composition of the community to the results of classic studies at the CLIMAX site show remarkable similarities in the species sampled and their abundance ranges. Nonetheless, biomass estimates, corrected for relative capture efficiencies of the different net systems, suggest that there may have been an increase of about a factor of two in zooplankton standing stocks over the past two decades. This hypothesis raises issues about the comparability of the two sites, but the increase, if real, would be consistent with the observed decadal-scale increase in phytoplankton chlorophyll a. The small harpacticoid copepod, Macrosetella gracilis, demonstrates a strong summer maximum consistent with the increased abundance of nitrogen-fixing Trichodesmium spp. during mid-summer months, as described in recent studies at Stn. ALOHA. Accordingly, nitrogen fixation could provide the source of new nutrients to support higher productivity, larger phytoplankton, and enhanced zooplankton standing stocks in the summer, when the upper water column is most stratified and isolated from nutrient influxes from below. (C) 2001 Elsevier Science Ltd. All rights reserved.