On-board Diary: Areas in the Baltic Sea which suffer from eutrophication
Author: Christina Abel
Date: June 2, 2011
We arrived in the Gulf of Riga Tuesday afternoon. The Estonian’s Muhu archipelago separates the Gulf of Riga from the rest of the Baltic Sea, and several large rivers provide fresh water to the gulf. The gulf is quite shallow with 67 m as maximum depth. The salinity is lower here than in the Baltic Proper. Because of these conditions the Gulf of Riga is in risk of eutrophication and algae bloom in the spring and summer times. The first dive we had here was in a MPA close to a small island, called Ruhne Island. Here the divers had a dive with bad visibility, due to algae. This situation was the same with all our others scuba dives, as well as with some of the ROV dives in this region. We observed only few species in the gulf, including the familiar isopod, viviparous blenny and different kind of clams.
Thereafter we sailed to the south of Gotland Deep to do a couple of ROVs. Earlier we had made one ROV recording in the Gotland Deep, but this time we wanted check if the situation was the same as in the northern part the deep. We found a similar situation with an anoxic bottom without life. Before reaching the bottom we filmed beautiful jellyfish in the water column, with meters long tentacles. At around 100 meters depth we saw a clear horizontal line, which is the thermocline and/or the halocline. Our CTD data will later clarify the physical phenomenon for us.
As we now can see the end of the expedition, with only few days of fieldwork left, we moved quickly on to the next place in Swedish water where we will do underwater filming.