Primary production, new production, and growth rate in the equatorial Pacific: Changes from mesotrophic to oligotrophic regime

Under an apparent monotony characterized by low phytoplankton biomass and production, the Pacific equatorial system may hide great latitudinal differences in plankton dynamics. On the basis of 13 experiments conducted along the 180degrees meridian (8degrees S-8degreesN) from upwelled to oligotrophic waters, primary production was strongly correlated to chlorophyll a (chl a), and the productivity index PI (chl a-normalized production rate) varied independently of macronutrient concentrations. Rates of total (C-14 uptake) and new (N-15-NO3 uptake) primary production were measured in situ at 3degreesS in nutrient-rich advected waters and at 0degrees where the upwelling velocity was expected to be maximal. Primary production was slightly higher at the equator, but productivity index profiles were identical. Despite similar NO3 concentrations, new production rates were 2.6 times higher at 0degrees than at 3degreesS, in agreement with much higher concentrations of biogenic particulate silica and silicic acid uptake rates (Si-32 method) at the equator. Furthermore, phytoplankton carbon concentrations from flow cytometric and microscopical analyses were used with pigment and production values to assess C:chl a ratios and instantaneous growth rates (mu). Growth rates in the water column were significantly higher, and C:chl a ratios lower at 0degrees than at 3degreesS, which is consistent with the more proximate position of the equatorial station to the source of new iron upwelling into the euphotic zone. For the transect as a whole, compensatory (inverse) changes of C:chl a and mu in response to varying growth conditions appear to maintain a high and relatively invariant PI throughout the equatorial region, from high-nutrient to oligotrophic waters.