A study of the vertical flow of carbon and nitrogen in controlled aquatic environments at the Knebel Bay, Denmark, with the use of the stable isotopes of nitrogen and carbon (N-15 y C-13)

Stable isotopes were used as tracers to characterize and quantify the downw ard flux of carbon and nitrogen. The experiments were conducted in Knebel b ay, Denmark (56 degrees 08' N, 10 degrees 11'E), in two controlled aquatic environments (mesocosm type). The addition of inorganic nutrients to the me socosms stimulated a bloom of the non-toxic flagellate Prorocentrum minimum . A similar pattern in the concentration of chlorophyll a (Chl-a), particul ate organic nitrogen (PON) and particulate organic carbon (POC) was observe d in both mesocosms. The elevated nutrient conditions resulted in low isoto pic discrimination, this was reflected in low delta C-13 values of the susp ended and sediment POC. The uncoupling between the peaks of PON, Chl-a and POC, as well as the rapid assimilation of the added nitrate (in less than t hree days) and a changing C/N ratio indicate that P. minimum has a great ab ility to assimilate inorganic nitrogen. The C/N ratio reached a minimum at the beginning of the experiment increasing later, once nitrate depletion wa s reached. The sedimentation of new nitrogen during this study was 10-11 % of the total originally added to the water column. No massive sedimentation of P. minimum was observed once the nitrate added to the mesocosms was uti lized.