Physical and chemical variability of the Baltic Sea: a joint experiment in the Gotland Basin |
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Authors: | A. Aitsam H.P. Hansen J. Elken M. Kahru J. Laanemets M. Pajuste J. Pavelson L. Talpsepp |
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Affiliation: | 1. Department of the Baltic Sea, Institute of Thermophysics and Electrophysics, Academy of Sciences of the Estonian SSR, Paldiski St. 1, Tallinn 200031, USSR;2. Department of Marine Chemistry, Institut für Meereskunde, Düsternbrooker Weg 20, D-2300 Kiel 1, FRG |
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Abstract: | 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. |
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