LATITUDINAL VARIATIONS IN MESOZOOPLANKTON GRAZING AND METABOLISM IN THE CENTRAL TROPICAL PACIFIC DURING THE US JGOFS EQPAC STUDY

Pigment ingestion rates by three size classes of mesozooplankton (200-500 mu m, 500-1000 mu m and 1000-2000 mu m) within the euphotic zone were measured during the Survey 1 (February/March) and Survey 2 (August/September) cruises of the 1992 United States Joint Global Ocean Flux Study (U.S. JGOFS) in the central equatorial Pacific (EqPac). Survey 1 was characterized by El Nino conditions while Survey 2 was characterized by typical climatological conditions, The small animals (200-500 mu m) contributed more than 50% (range: 34-80%) to the total mesozooplankton grazing reported here. Mesozooplankton grazing was higher within the equatorial region (5 degrees S-5 degrees N) than at higher latitudes (5 degrees S-12 degrees S, 5 degrees N-12 degrees N). The carbon-specific ingestion rates of both the large and small animals tended to be higher during Survey 1 than during Survey 2. In contrast to the carbon-specific ingestion rates, the mesozooplankton biomass during Survey 1 was lower than that during Survey 2. Thus, the higher ingestion rates during Survey 1 were offset by lower biomass and the mesozooplankton grazing was quite similar during Surveys 1 and 2. Mesozooplankton removal of chi a was higher within the high-phytoplankton-biomass equatorial region than at higher latitudes during Survey 1, but regional differences were not significant during Survey 2. Mesozooplankton community grazing was equivalent to an average daily removal of 3.0% (range: 0.5-7.7%) and 2.2% (range: 0.8-3.5%) of the total chi a standing stock within the euphotic zone during Surveys 1 and 2, respectively. However, mesozooplankton grazing was equivalent to a daily removal of 47% (range: 15-91%) and 36% (range: 23-66%) of the > 5 mu m chi a standing stock within the euphotic zone during Surveys 1 and 2, respectively. Assuming a carbon-to-chi a ratio of 58, we estimate that mesozooplankton grazing removed an average of 6% (range: 2-12%) of C-14 primary production during Survey 1, and 5% (range: 2-10%) during Survey 2. Removal rates were not significantly different between the equatorial region and the higher latitudes. Rates of respiration and ammonium excretion were estimated from empirical models based on animal weight and water temperature. Phytoplankton ingestion could not satisfy the estimated daily maintenance-carbon demands of mesozooplankton. The shortages were more pronounced in the large size fraction than in the small size fraction. The estimated ammonium excretion by the mesozooplankton could support 4-15% and 3-17% of C-14 primary production during Surveys 1 and 2, respectively.