Net community production in high nutrient-low chlorophyll waters of the tropical and antarctic oceans - grazing vs iron hypothesis

An analysis of tropical upwelling systems shows a great range in the rate of increase of the standing stock expressed in terms of the water column chlorophyll content. Grazing appears to be the main factor responsible for lowering the rate of increase, especially in moderate and strong High Nutrient-Low Chlorophyll (HNLC) waters. A simple model comparing Net Community Production (NCP) rates with observed chlorophyll increase rates, leads us to conclude that phytoplankton population must present high specific growth rates on a daily rhythm in order to overcome grazing. Maximum specific growth rates (V(max)) calculated for daily total production (TP) are, in the two main upwelling areas, 137 d-1 off NW Africa and 1.04 d-1 off Peru. For extreme HNLC conditions in the Costa Rica Dome, V(max) remains high, 0.98 d-1. The Atlantic equatorial upwelling in the Gulf of Guinea is characterized by moderate HNLC conditions. From an analysis of a nutrient-temperature diagram and heat fluxes, we deduce a nitrate-based new production (NP) rate of 0.368 g C m-2 d-1, corresponding to af-ratio of 0.3. The Peruvian coastal upwelling, during the strongest upwelling period in austral winter, shows HNLC conditions extending far offshore and reaching the Galapagos Islands. NCP could not be determined in these waters. Antarctic nutrient consumption diagrams are also presented. An overall consumption evaluation along a 1,550 km North-South section leads to atomic assimilation ratios of DELTA-Si/DELTA-N/DELTA-P = 55.9/12.3/1. From the end of winter to end of summer, the average nitrate consumption is 0.38 Mol M-2 . For this five month period, the calculated NP is 0.2 g C M-2 d-1. From the silicon-nitrogen ratios during nutrient consumption and those in the particulate material, we deduce af-ratio slightly lower than 0.5. This f value confirms earlier data derived from N-15 uptake studies. The model application leads to a V(max) (TP) value of 0.54 d-1, which does not seem excessively low in the low temperature regime of the Southern Ocean. We discuss an Antarctic scenario starting with "grazing" limitation and finishing with iron limitation. Despite evidence in favour of the grazing hypothesis which allows us to classify regions geographically into "high and low" speed areas of phytoplankton development, a global chart of dust inputs into the ocean strongly supports the iron limitation hypothesis. This observation constitutes a challenge to the "grazing only" hypothesis and militates in favour of more detailed future studies.