According to observations made by R/S Aranda, the oxygen-depleted deep water layer with hydrogen sulphide has grown in extent and thickness. The major part of the water layer at more than 100 m depth is oxygen-depleted and contains large quantities of hydrogen sulphides. At some sites, hydrogen sulphide was detected already at 80 m.
The salinity of deep water in the Baltic Sea has continued to increase after August 2008, particularly in the Sea of Åland and in the Bothnian Sea. In the Bothnian Bay, too, the salinity of deep water has continued to increase and the oxygen concentration has decreased. However, there is no reason to speak about oxygen depletion in the Bothnian Bay. The deep water plankton communities are doing well and the overall situation is much better than in the Baltic proper.
In the Bothnian Sea and in the Bothnian Bay, the nutrient concentrations have continued to increase, possibly because of the inflow of of deep water from the Baltic proper to the Gulf of Bothnia over the threshold in the Sea of Åland.
In the surface layer of the sea, nutrient concentrations were normally low in the Baltic proper, in the Gulf of Finland and in the Bothnian Bay. In August, nutrients are mostly bound to the phytoplankton biomass.
At the research sites of Aranda, bluegreen algae were at this stage only observed in small quantities in the Gulf of Finland and in the northern parts of the Baltic proper. They were mixed with the water mass. The potentially noxious dinoflagellates Heterocapsa triquetra ja Dinophysis acuminata were observed in small quantities.
The fishhook waterflea Cercopagis pengoi was found in large quantities in the Gulf of Finland and in the Baltic proper. Another rapidly spreading alien water flea species, Evadne anonyxia, was found in the same sea areas. Water fleas in general have become more numerous. Taken as a whole, the composition and overall quantities of zooplankton were as usual.
Best conditions in ten years in the Gulf of Finland
The warm winters of the last few years and the limited extension of the ice cover have made possible the mixing of the water masses in the Gulf of Finland. Oxygen-rich water has reached even the deep water areas along the coast. The improved oxygen situation can be seen as better conditions in the bottom sediments, particularly in the outer archipelago. The numbers of benthic animals at coastal observation sites have increased from the previous years.
In the western open sea parts of the Gulf of Finland the oxygen situation of the sea floor has not essentially improved from last August. In the inner archipelago of the Gulf of Finland the oxygen situation is poor in areas where the water exchange is slow.
The research vessel Muikku has since 1999 monitored the situation of the sea floor in the Gulf of Finland. 30 permanent observation sites have been monitored every year, and the oxygen situation was good at 20 sites. Last time the overall oxygen situation was as good as now in 1999. Only three years ago, more than 80 percent of the sites were without oxygen.
The improved oxygen situation of the sea floor can be seen as improved conditions for the zoobenthic communities. Of the 30 examined sites, only two were completely without benthic fauna, whereas most sites were without benthic animals in 2006 when the situation was worst.
In the southern and central parts of the Archipelago Sea, the condition of the sea floor was even this year better than in the Gulf of Finland. Only two of the monitored sites were without oxygen. At most sites there were lots of benthic animals and the communities were diverse.
Due to the improved oxygen situation at the sea floor, thousands of tons of dissolved phosphorus have again been bound into the bottom sediment. Thereby the phosphorus concentration in the Gulf of Finland has considerably decreased. This favourable change can be seen as decreased amounts of algae during the summer, especially in the eastern Gulf of Finland.
Mr Juha Flinkman, leader of Aranda's tour, SYKE, phone. 040 750 3911, firstname.lastname@ymparisto.fi
Mr Seppo Knuuttila, leader of Muikku's tour, SYKE, phone 040 760 9232, firstname.lastname@ymparisto.fi
Mr Riku Lumiaro, information officer, SYKE, phone 040 509 8654, firstname.lastname@ymparisto.fi
white = no observation
yellow = oxidised
black = anoxic
