New production in the North Atlantic derived from seasonal patterns of surface chlorophyll

Abstract

Annual new production resulting from winter nitrate has been estimated for the North Atlantic using data from the Coastal Zone Color Scanner (CZCS) between 1979 and 1983. Twelve monthly mean surface chlorophyll images, based on 5-year averages of CZCS data, were used to identify three zones with distinct seasonal patterns. A mid-latitude zone, with an area of 7 × 10 6 km 2, exhibited a spring bloom followed by oligotrophic conditions at the surface throughout the summer. The decline in surface chlorophyll following the spring bloom was assumed to indicate that winter nitrate in the mixed layer was exhausted and that a nitracline had formed at depth. Based on observations by Strass and Woods ( Deep-Sea Research, 38, 35–56, 1991) in the North Atlantic, we estimate that the nitracline deepened at a rate of 10 m per month, starting at the base of the mixed layer. By determining the timing of the onset of oligotrophic conditions from the satellite data, we can estimate the volume of nitrate-depleted water lying above the nitracline in late summer. This was combined with an estimate of the nitrate concentration at the start of the growing season to derive new production. The model for the initial nitrate concentration is based on an empirical relationship between winter nitrate from Glover and Brewer ( Deep-Sea Research, 35, 1525–1546, 1988) and the maximum chlorophyll at the end of the spring bloom. The resulting new production was 24 g C m −2 y −1 (4.2 g N m −2 y −1) in this mid-latitude transitional zone. Applying the same method to subtropical zone of approximately equal area yields an estimate of new production of 18 g C m −2 y −1 (3.1 g N m −2 y −1); the area-weighted average for both zones was 21 g C m −2 y −1 (3.7 g N m −2 y −1). In the subtropical zone, the maximum chlorophyll occurred in the winter and the minimum in late summer, suggesting that production was nutrient-limited throughout the year. The third smaller zone (2.5 × 10 6 km 2) was located in subpolar regions to the north, where surface chlorophyll was minimum in winter and maximum in late summer. Assuming that all winter nitrate had been assimilated by the end of the summer within the upper 40 m of the water column in this zone, new production is estimated to be 43 g C m −2 y −1 (7.6 g N m −2 y −1). Our results underestimate new production because they are based solely on winter nitrate assimilated by phytoplankton between winter and late summer. Nevertheless, the values are comparable in magnitude to previous estimates of total productivity in oligotrophic oceanic regions ( Koblentz-Mishke et al., in Scientific exploration of the South Pacific, W.S. Wooster, editor, pp 183–193, 1970).