© 2004 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
A 9-year increasing trend in mesozooplankton biomass at the Hawaii Ocean Time-series Station ALOHA
a Department of Oceanography, University of Hawaii 1000 Pope Road, Honolulu, HI 96822, USA
b Integrative Oceanography Division, Scripps Institution of Oceanography La Jolla, CA 92093-0218, USA
*Correspondence to C. C. Sheridan: tel: +1 808 956 6050; fax: +1 808 956 9516. e-mail: sheridan{at}hawaii.edu; mlandry{at}ucsd.edu.
Mesozooplankton biomass in the North Pacific Subtropical Gyre (NPSG), as measured by the Hawaii Ocean Time-series program at Station ALOHA (22.45°N 158°W), increased significantly from 1994 to 2002. The changes occurred at a rate of 60 mg DW m–2 yr–1 for night-time collections and 45 mg DW m–2 yr–1 for daytime collections. Principal components analysis indicates that the 9-year trend was driven by an increase in small (0.5–2.0 mm) zooplankton that do not migrate on a diel cycle. This plankton class is known to increase during the summer at Station ALOHA when the water column is most stratified, and a strong summertime response is also apparent within the long-term trend from 1998 through 2002. Both long-term and seasonal changes in zooplankton biomass at Station ALOHA can be linked to an enhanced role of nitrogen fixation in ecosystem productivity. Climate forcing from El Niño–Southern Oscillation (ENSO) events may have influenced nitrogen fixation, general ecosystem productivity, and thus zooplankton biomass in the NPSG. However, it is difficult to evaluate the effect of climate cycles in this region without the benefit of a longer time-series at Station ALOHA. Because biomass trends in higher-level consumers like mesozooplankton can have cascading influences on lower levels, understanding the relative roles of bottom-up climate influences and top-down trophic processes will be important in resolving long-term trends in community composition and structure in the subtropical North Pacific Ocean.
Keywords: Central North Pacific, Hawaii Ocean Time-series, nitrogen fixation, subtropical gyre, zooplankton
accepted 23 March 2004.
1 tel: +1 858 534 4702; fax: +1 858 534 6500.
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