We made an assessment of the hydrography in the Bothnian Sea based on data collected by the Argo floats during the first 6 years of operation in the Bothnian Sea (2012)(2013)(2014)(2015)(2016)(2017). We evaluated the added value of Argo data related to the pre-existing monitoring data. The optimal usage and profiling frequency of Argo floats was also evaluated and the horizontal and vertical coverage of the profiles were assessed. For now we lose 4 m of data from the surface due to sensor design and some meters from the bottom because of the low resolution of available bathymetry data that is used to avoid bottom collisions. Mean monthly temperature and salinity close to surface and below halocline from the float data were within the boundaries given in literature, although some variation was lost due to scarcity of winter profiles. The temporal coverage of the Argo data is much better than that of ship monitoring, but some spatial variability is lost since the floats are confined in the over 100 m deep area of the Bothnian Sea. The possibility to adjust the float profiling frequency according to weather forecasts was successfully demonstrated and found a feasible way to get measurements from storms and other short term phenomena unreachable with research vessels. First 6 years of operation have shown that Argo floats can be successfully operated in the challenging conditions of the Bothnian Sea and they are shown to be an excellent addition to the monitoring network there. With multiple floats spread in the basin we can increase our general knowledge of the hydrographic conditions and occasionally get interesting data related to intrusions and mixing during high wind events and other synoptic scale events.

Argo floats have been used in the environmental monitoring of the very shallow Bothnian Sea, a sub-basin of the Baltic Sea, for 5 years as part of the Finnish Euro-Argo programme. The Bothnian Sea is so far considered to be an environmentally healthy part of the Baltic Sea because the deep waters of the basin are well-ventilated by inflowing oxygen-rich saltier and heavier surface layer waters of the Baltic Sea proper. Thus the deep water flow is of interest in the Bothnian Sea. In this study, we used Argo float data from six different long-term missions, from 111 to 512 days, to analyze the deep-water flow in the Bothnian Sea where no continuous monitoring of currents exist. We estimated mainly the flow below the expected halocline from the paths of the floats. We analyzed the movements statistically and estimated the error caused by the surface drift of the floats during their stay at the surface by using 3D hydrodynamic model results as reference data. Our results show a northward flowing resultant current in the deep trench of the Bothnian Sea. There seemed to be very little exchange between coastal zone and open-sea waters in deeper layers. The drifting speed of the floats in the deep layers of Bothnian Sea generally was around 2 cm/s but instantaneous speeds of up to 30 cm/s in the middle-layer (50 dbars) were observed. In the Bothnian Sea deep, the deep trench on the Finnish side of the Bothnian Sea, the vast majority of the observations showed deep currents from south to north, with the same average speed of around 2 cm/s but the instantaneous maximum was smaller at 13 cm/s. Our study indicates that the routine Argo float observations can be used to get information on the deep currents in the basin in addition to hydrographic observations.

Argo floats have been successfully used for more than 10 years in the world's ocean. The Finnish Meteorological Institute began to develop practices to use Argo floats in the shallow brackish water Baltic Sea in 2011. Since 2012, Argo floats have been in continuous use in the Baltic Sea and are now a part of the Euro-Argo European Research Infrastructure Consortium (ERIC). The floats are kept in the different basins of the Baltic Sea, usually for a year and then recovered and replaced with a new float. The observation cycle is usually a week in monitoring mode, but we have also used shorter intervals up to one day. With proper piloting practices, Argo floats are of great value in monitoring and for research of shallow marginal seas, as they give regular and frequent data around the year, regardless of weather conditions. Operating the floats has matured to a level where we can state that Argo monitoring of the Baltic Sea is an operative reality.

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Publication II: "Argo floats as a novel part of the monitoring the hydrography of the Bothnian Sea" NH wrote the first draft with significant help from LT and held the main responsibility for the analysis of the observation data and production of the illustrations. PR, S-MS, LT, and PA contributed to the illustrations. Background research was mainly done by NH, PR, and PA. TP was responsible for the technical support and acquisition of data. All the authors contributed to the evaluation of the data, wrote sections for this manuscript, and revised and approved the sent version. Publication III: "Estimating currents from Argo trajectories in the Bothnian Sea, Baltic Sea" All the authors contributed to the design of this study. PR wrote the first draft and largely did the analyses for the observations and model results. S-MS and NH contributed to analyses and illustrations. PR, vii Publication V: "Ensemble forecasting of harmful algal blooms in the Baltic Sea" PR had the main responsibility for outlining the article and conducting the analyses. PR was also responsible for the writing and handling the publication process. AW implemented the forecasting system and validated the model. AN analysed a case study and drew the figures. TS instructed this