Physical and Chemical Variability of the Baltic Sea - a Joint Experiment in the Gotland Basin

Aitsam, A, Hansen HP, Elken J, Kahru M, Laanemets J, Pajuste M, Pavelson J, Talpsepp L.  1984.  Physical and Chemical Variability of the Baltic Sea - a Joint Experiment in the Gotland Basin. Continental Shelf Research. 3:291-310.


An area of 25 × 30 nmi with a subarea of 20 × 20 nmi in the Baltic Sea Gotland Basin was surveyed for about one month by vertical CTD and fluorometric chlorophyll a casts, and by towed profiles with a chemical profiler (nutrients, CTD, O2, pH) and an undulating CTD. Autonomous current meter data were available for the same period.Five CTD surveys with a 5-mile spacing showed the existence of synoptic eddies and several other perturbations. A first mode eddy (with all the isopycnals lifted within the halocline) had reduced salinities in the upper layer and chlorophyll concentrations were reduced by 2 to 3-fold. A second mode eddy (with the isopycnals lifted in the top of the halocline and lowered in the bottom layers) with a diameter of 20 km was under prolonged observation. Anticyclonic currents at 96-m depth were up to 25 cm s−1. Along the section across the eddy at 80-m depth, the mean nitrate concentration dropped from about 7 to 8 to 5 μmol dm−3 while other chemical parameters lacked intense variations. An increased activity of the intrusive fine-structure was detected in the eddy center. The Baltic eddies are essentially non-linear, and transport water in their ‘nuclei’. An internal front, separating waters with different T,S composition, was discovered in the upper layers.The observations are discussed with respect to discovering long-term trends while monitoring the Baltic environment.