Comparisons of plankton at 59 degrees N and 37 degrees N during two Lagrangian drift experiments in the North Atlantic in June and July 1996

Data are summarised for two Lagrangian experiments in the North Atlantic in early summer 1996. At 59 degreesN 20 degreesW, plankton dynamics was studi ed in an SF, tracer release experiment within a mesoscale eddy over a 9-day period. At 37 degreesN 20 degreesW, a second experiment followed a driftin g buoy for 7 days. The data obtained in these two experiments have been ave raged for 3 depth strata; the euphotic zone, the surface mixed layer (SML), and the seasonal thermocline immediately beneath the surface mixed layer. At 59 degreesN, the euphotic zone was only marginally deeper than the SML, but at 37 degreesN the SML was ca 30 m and the euphotic depth was ca 110 m. At 37 degreesN, nutrient concentrations in the SML were low but significan t new production occurred in the thermocline because of light penetration i nto the nutricline. The particulate organic carbon (POC) concentration of t he SML at 59 degreesN was 13-15 mu mol C kg(-1), but at 37 degreesN POC con centrations were 4 mu mol C kg(-1). These POC measurements include biota an d detritus. As a way of investigating latitudinal differences in the plankt on communities, estimates have been made of the carbon and nitrogen content of phytoplankton, bacterioplankton, microzooplankton and mesozooplankton. At both 59 degreesN and 37 degreesN, phytoplankton was the largest componen t, accounting for ca 50% of the planktonic biomass in the SML. At 59 degree sN, microzooplankton was 16% of the planktonic carbon, but at 37 degreesN t his reduced to 8% of the total. Mesozooplankton was a relatively constant p roportion (ca 20%) of the planktonic carbon in the SML at both 59 degreesN and 37 degreesN. Bacterioplankton was 14% of the biomass at 59 degreesN, in creasing to 24% in the microbial loop-dominated system at 37 degreesN. Mean carbon fixation rate in the oligotrophic southern station was 24% of that at the north, with more carbon fixation below the SML at 37 degreesN than a t 59 degreesN. Respiration rates showed little variation with latitude, and the rates at 37 degreesN were 80% of those at 59 degreesN. Nitrate and amm onium uptake rates were very low in the oligotrophic conditions in the SML at 37 degreesN, but nitrate uptake in the euphotic zone was comparable to t hat at 59 degreesN. Ammonium uptake by phytoplankton was also significantly greater at 37 degreesN, in both the euphotic zone and thermocline, but upt ake in the SML was only 20% of that in the SML at 59 degreesN. (C) 2001 Els evier Science Ltd. All rights reserved.