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 Oc ean Time-series (HOT) Program. Typically, three day and three night tows ar e 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 eac h sample, and one day and one night sample per cruise have been enumerated microscopically through 1996. The accumulated data show statistically signi ficant 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 nightt ime biomass during summer 1996, which is also related to the enhancement of small size fractions. Comparisons of the present composition of the commun ity to the results of classic studies at the CLIMAX site show remarkable si milarities in the species sampled and their abundance ranges. Nonetheless, biomass estimates, corrected for relative capture efficiencies of the diffe rent net systems, suggest that there may have been an increase of about a f actor 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 inc rease in phytoplankton chlorophyll a. The small harpacticoid copepod, Macro setella 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, nitroge n fixation could provide the source of new nutrients to support higher prod uctivity, larger phytoplankton, and enhanced zooplankton standing stocks in the summer, when the upper water column is most stratified and isolated fr om nutrient influxes from below. (C) 2001 Elsevier Science Ltd. All rights reserved.