Jul 8, 1984 - ABSTRACT. Data on the distributions in summer of phytoplankton and inorganic nutrients in the Gulf of. Maine and across Georges Bank are ...
Journal of Marine Research, 42, 1051-1073,
1984
The significance of subsurface chlorophyll, nitrite and ammonium maxima in relation to nitrogen for phytoplankton growth in stratified waters of the Gulf of Maine by P. M. Holligan,' W. M. Balch2 and C. M. Yentsch3 ABSTRACT Data on the distributions in summer of phytoplankton and inorganic nutrients in the Gulf of Maine and across Georges Bank are presented. The chlorophyll maximum represents a phytoplankton biomass maximum and occurs at a depth where both light and nitrate availability allow net growth of the population. The dominant species were generally flagellates and included the toxic dinoflagellate, Gonyaulax tamarensis var. excavata, at some stations. The ammonium and nitrite profiles suggest that nitrification is occurring at the base of the pycnocline below the chlorophyll maximum, and this may be an important source of nitrate during the summer months. The highest levels of nitrite and ammonium were found over the slopes of Georges Bank.
1. Introduction Temperate coastal waters that are stratified during the summer months are generally characterized by well defined subsurface chlorophyll maxima associated with a relatively shallow (10-40 m) main pycnocline and nitracline (e.g. Cullen and Eppley, 1981, for the Southern Californian Bight; Pingree et al., 1978, for the northwest European Shelf). The mean light levels in these layers is as high as 10% of the surface irradiance so that phytoplankton growth rates are likely to reach at least 1 division day-I under optimal conditions of illumination and nutrient supply. In areas where the water depth is 10% surface irradiance) and a "nitrate dependent"
1068
Journal of Marine Research
[42,4
production term (which varies from 0 at nitrate 1.0 J.lM). From a general consideration of ecological and experimental data on the effects of light and nitrate on phytoplankton growth, these ranges appeared to be the most appropriate, although clearly they do not take into account daily variations in ambient light or the effects of other nitrogen sources on nitrate utilization. Potential rates of new production were considered to reach minimal or zero values in surface layers at nitrate levels
