Quantification of dissolved oxygen dynamics in a semi-enclosed sea - A comparison of observational platforms

In this study, we present dissolved oxygen (DO) concentration data collected during an observational program conducted in the central Baltic Sea, six months after a Major Baltic Inflow (MBI) event from December 2014 has ended a long period of stagnation. The main working area was the Eastern Gotland Basin (EGB). The spatial distribution of DO was obtained by two ship based CTD-systems (classical CTD probe, towed undulating CTD probe) and by using a glider (Slocum), while the temporal variability of DO concentrations was revealed by a profiling mooring (GODESS - Gotland Deep Environmental Sampling Station). In this paper, we compare the performance of all platforms used and show that integrating modern oceanographic tools into an existing observation network can help to enhance the capability of the network to capture the spatiotemporal variability of DO in the Baltic Sea on both a large (seasonal; 10-500 km) and a small (diurnal; 1-10 km) scale. It was found that in comparison to a classical standard CTD system, a towed undulating CTD system (ScanFish) is better suited to resolve the meso- and submeso-scale distribution patterns of DO concentrations. In particular, in the sub-halocline layers where the variability of DO concentrations was consistently high (1-70 mu M). The highly dynamic post-inflow situation was also observed by the glider and the profiling mooring. For comparison purposes four different sub-halocline layers in a depth range from 80 m to 180 m depth were defined and the mean oxygen content of these layers was calculated for both platforms. It turned out, that the mean DO concentrations for the different layers, which were determined from glider and mooring measurements, differ only slightly from each other. Differences were greatest (up to 7 mu M) below the halocline (80-120m) and could be explained by the spatial and temporal variability of DO concentrations in this area and by the large response time of the gliders oxygen optode. However, the temporal development of inflow events could be determined and investigated in more detail by using autonomous platforms. The frequency and size of smaller inflows, for instance, that move along the halocline or in deeper layers thus could be evaluated.