Estimates of oceanic mesozooplankton production: a comparison using the Bermuda and Hawaii time-series data

Mesozooplankton growth rates were estimated for the Hawaiian (HOT) and Bermuda (BATS) ocean time-series stations using the empirical model of Hirst and Lampitt (Marine Biology 132 (1998) 247), which predicts copepod growth rate from temperature and body size. Using this approach we derived seasonal and annual estimates of mesozooplankton production as well as rates of mesozooplankton ingestion and egestion using assumed growth and assimilation efficiencies for the period 1994–1997. Annual mesozooplankton production estimates at HOT (average 0.79molCm−2yr−1) were higher than production estimates at BATS (average 0.33molCm−2yr−1) due to both higher mesozooplankton biomass and higher estimated mesozooplankton individual growth rates. Annual primary production at the two sites was similar (average 14.92molCm−2yr−1 at HOT and 13.43molCm−2yr−1 at BATS). Thus, mesozooplankton production was a greater fraction of primary production at HOT (0.05) as compared to BATS (0.02). Mesozooplankton potentially contributed more to the gravitational flux of carbon at HOT, where the ratio of the average annual estimate of mesozooplankton fecal pellet carbon production/annual estimate of carbon flux at the base of the euphotic zone was 1.03 compared to the same ratio of 0.39 at BATS. Mortality estimates were similar to estimates of mesozooplankton production when compared over the entire study period. The higher mesozooplankton biomass and derived rate parameters at HOT compared to BATS may be due to the more episodic nature of nutrient inputs at BATS, which could result in mis-matches between increases in phytoplankton production and the grazing/production response by mesozooplankton. In addition, there is evidence to suggest that there are periodic blooms of gelatinous macrozooplankton (salps) at BATS that may not be captured sufficiently by the monthly sampling program. Thus the gelatinous zooplankton would add to the overall grazing impact on the phytoplankton at BATS as well as the contribution of zooplankton to the gravitational flux of biogenic material via fecal pellet production.