The deep chlorophyll maxlmum (DCM) IS a widespread feature In the subtropical gyre of the North Paclflc In mid-August 1985 chlorophyll concentration increased In the DCM, then dechned over time at 95 to 110 m depth near 28"N 155"W During the higher chlorophyll perlod the DCM lay w~t h i n the top of the nitracline Later, as the chlorophyll concentratlon decllned and the DCM deepened, following the 24 9 slgma-t surface, the maxlmum was above the nitracllne The DCM was also a maxlmum In abundance of autotrophlc flagellates Particulate nltrogen was retained over time in the euphotic zone relative to particulate carbon and chlorophyll Phytoplankton growth rate at 90 to 110 m depth averaged 0 11 d-' and phytoplankton carbon was about 10 pg I-', about 50 % of the total paruculate carbon INTRODUCTIONThis is a report of observations on the deep chlorophyll maximum (DCM) region mid-August to early September, 1985, at 28"N 155"W DCM's are thought to result from in situ growth of phytoplankton at low Light levels near the base of the euphotic zone where growth is dependent both upon regenerated nutrients and on nitrate diffusing upward across the nutricline (Fasham et al. 1985). The DCM in the North Pacific is continuous across the ocean basin at depth above the nutricline (Venrick et al. 1972, Venrick 1979. The feature has been s t u d e d in considerable detail at the CLIMAX (S10 1974) station, 28"N 155" W, where it is found at about 100 m depth. Studies in the 1970's suggested it was neither a phytoplankton biomass (Beers et al. 1975(Beers et al. , 1982) nor a primary production maximum. Apparently the elevated chlorophyll reflected a photoadaptive response of the phytoplankton, altering the chlorophyll/biomass ratio (Eppley et al. 1973
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