| Popis: |
In the summer of 1989 vertical profiles of chlorophyll a were taken in the North Atlantic. Stations were located along a transect following longitude 20-degrees-W, between 60 and 33-degrees-N. Maximum chlorophyll a levels were located near the surface in the north (2 mugl-1) but became gradually deeper towards the south (0.3 mug l-1 at a depth of 80-110m). Primary production varied only by a factor of 2.3 (226-533 mg C m-2 day-1), whereas the assimilation ratio showed far less variation (15.46 +/- 4.61 mg C mg Chl-1 day-1, n = 14). The northern part of the transect showed no thermal stratification, and nutrients were plentiful. Phytoplankton was distributed homogeneously, with a nearly constant signature of the cellular fluorescence characteristics as monitored by flow-cytometry. The dominant taxa were small prymnesiophytes (Emiliania huxleyi) and chroococcoid phycoerythrin-containing cyanobacteria. Cells were healthy, resulting in net growth rates (growth rate minus grazing) of up to 0.75 divisions per day. Of the total primary production, 68% was actually converted into new plant carbon. Midway along the transect (47-degrees-N) there was a sharp thermocline at a depth of 35 m. All nutrients (nitrogen, phosphate and silicate) were depleted in the upper water layer. Cyanobacteria dominated the phytoplankton community in numbers varying from 12,000 to 39,000 cells ml-1. Stabilization of the water column was apparently in progress, as could be derived from the gradual increase in mean cellular red fluorescence of the cyanobacteria with depth. Furthermore, net growth rates of the algae were relatively low, as was the proportion of primary production resulting in an actual increase of plant carbon (42%). At the southernmost station (33-degrees-N), the situation was typical of the oligotrophic subtropics: a sharp thermocline at 35 m, the 0.1% surface incident irradiance level at 150 m, and a nitracline at a depth of 100-120m. The most prominent (pico)plankters present were prochlorophytes, predominantly at the deep chlorophyll maximum (80-100 m, up to 95,000 cells ml-1). Cyanobacteria co-occurred but only in small numbers (maximum of 5500 ml-1), mainly in surface waters. The vertical (physical) stability of the water column was pronounced, and the mean cellular red fluorescence signal of cells found at the bottom of the euphotic zone was consequently higher than in surface waters. Most remarkable were the net growth rates of the various species suggesting a highly dynamic phytoplankton community. Extreme growth rates included positive as well as negative values, which were not always consistent with the greatest abundance of certain species. Despite these rapid changes, the actual increase in plant carbon was only 14% of the primary production. |
