Summary
This report presents the main results of the Swedish national marine monitoring programme of the pelagic during 2017. The monitoring data of hydrography, nutrients and phytoplankton are analysed for the seas surrounding Sweden: Skagerrak, Kattegat, The Sound, Baltic Proper, Bothnian Sea and Bothnian Bay. The monitoring is carried out by SMHI (Swedish Meteorological and Hydrological Institute), SU (Stockholm University) and UMF (Umeå Marine Sciences Centre) and the monitoring programme is co-funded by SwAM (Swedish Agency for Marine and Water Management), SMHI, SU and UMF. Data is collected, analysed and reported with support from Swedish environmental monitoring and commissioned by SwaM.
The Baltic current along the Swedish west coast implies large variations in surface salinity and the unusually large outflow of brackish water from the Baltic Sea in 2017 was reflected as low surface salinity in Skagerrak and Kattegat in the beginning of the year. There were no major deep water inflows to the Baltic Sea during 2017 but a few inflows of minor magnitude. These minor inflows only temporarily improved the oxygen condition in the Bornholm Basin and in the southern part of the Eastern Gotland Basin.
The salinity below the halocline was above normal in the Gotland Basins and in the Northern Baltic Proper, and also in the surface layer in the Eastern Gotland Basin for almost the whole year.
In Skagerrak and Kattegat, surface concentrations of phosphate and dissolved inorganic nitrogen were normal while dissolved silica concentrations were elevated especially in spring. In the Baltic Sea, the concentration of silicate in the surface water was elevated in all basins. According to the estimated total content of silicate there has been an increase in silica content in the Baltic Sea since the early 1990’s. Surface concentrations of phosphate were above normal in the Gotland basins and the Northern Baltic Proper while inorganic nitrogen content was above normal in parts of the Arkona and Bornholm basins. During spring and summer, the inorganic nitrogen was consumed at greater depths than usual in the Baltic Proper. In particular concentrations of phosphate and dissolved silica were generally lower than normal in the bottom layer.
Instead of diatoms, the flagellate genus Pseudochattonella, which is potentially toxic to fish, bloomed in the Kattegat and Skagerrak areas in February – April. During autumn there was a prolonged diatom bloom though. In the Baltic Sea spring bloom occurred in April. The cyanobacteria bloom began in May already with Aphanizomenon flos-aquae. During June and July all three of the filamentous cyanobacteria, A. flos-aquae, Dolichospermum lemmermannii and the potentially harmful Nodularia spumigena were found in the phytoplankton samples in various amounts.
In the Bothnian Sea, the sea surface temperature during summer was lower than normal and the oxygen conditions in the bottom layer was not critical but still below normal levels.