Cruise report from M/V Aura week 45-46, 2017

Type: Report
Author: Örjan Bäck


Initial problems with a winch caused the expedition to be delay already from the start, tough weather during the expedition, primarily in the Baltic, resulted in further delays. Because of these delays we had to skip three stations (P2, BY20 and ref M1V1) and we could only sample a few things at our second stop at Anholt E. Technical problems with the metre wheel on one of the winches made it impossible to sample zooplankton and jellyfish in the Skagerrak and the Kattegat, however the metre wheel could be repaired during the passage through the Sound and the sampling could continue like normal after that.

Oxygen concentrations close to zero were observed in large parts of the deep water of the Baltic Proper. Anoxic conditions, when hydrogen sulphide forms, were found in the Western Gotland Basin from 80 metres depth, in the northern part at BY31 Landsortsdjupet from 90 metres. Hydrogen sulphide was also found in the Eastern Gotland Basin but only values above the limit of quantification at BY15 Gotlandsdjupet closest to the bottom. Areas with acute hypoxia (dissolved O2<2ml/l) where found in the Bornholm Basin and in the Eastern and Western Gotland Basin at around 60-70 metres, also in the Bay of Hanö from 70 metres.

Nutrient concentrations in the surface were at normal levels for the season. Higher concentrations were found beneath the halocline which is normal, however the phosphate levels were below normal beneath the halocline in the Western Gotland Basin and in the northernmost part of the Eastern Gotland Basin. Silicate concentrations in the entire Baltic Proper continued to be higher than normal above the halocline with values around 14-15 μmol/l.

The surface water temperature was normal for the season in the entire survey area and varied between 8-12 °C, coldest in the Baltic Proper and warmest in the Skagerrak. The salinity values were somewhat above normal both above and beneath the halocline in the Eastern Gotland Basin, also in the northernmost part of Western Gotland Basin the values were somewhat above normal beneath the halocline. In the Arkona Basin, Bay of Hanö and the Bornholm Basin the salinity was somewhat lower than normal above the halocline, values were normal beneath the halocline except for in the Bornholm Basin where the salinity beneath 70 metres was slightly higher than normal.