World Seas are in peril: Hypoxia in Baltic Sea as an example

Perjantai 5.6.2020 klo 18:40 - Mikko Nikinmaa

Human activities have affected world seas in various ways. A three-volume book on environmental evaluation of world seas has just been published by Elsevier (World Seas: an Environmental Evaluation). In two volumes it evaluates the environmental statuses of different sea areas and in one volume the major environmental problems affecting marine environments. The book’s aim is both to evaluate the present statuworld_seas.gifs and to evaluate the future of the seas. It would be impossible to give a comprehensive review of the book’s contents, so I take one environmental problem, hypoxia, in one sea area, the Baltic Sea as an example.

There have always been virtually anoxic bottom areas in the Baltic Sea, but they have increased markedly because of human actions. For example, the area with low oxygen (less than 2 mg/l) has increased tenfold from preindustrial times. The main reason for hypoxic/anoxic sea bottoms is that for about 50 years paper and pulp mill industry’s wastewater was entered the sea uncleaned. The biological oxygen consumption of the waste is approximately the same as that of 100 million people’s waste. Since the waste of tens of millions of people also entered the sea without treatment, the sea bottom now contains so much organic material and nutrients that even without any further wastewater or fertilizer discharges, the Baltic Sea would remain eutrophicated and have large hypoxic areas for tens of years.

Hypoxia is caused by the consumption of oxygen by organisms or organic material, if it exceeds the diffusion of oxygen from atmosphere and its transport from oxygen-rich waters. In the Baltic Sea the water is stratified, and the high-temperature, low- salinity water which is oxygen-rich does not circulate with the dense oxygen-poor bottom water. The bottoms get oxygenated water only, when pulses of oceanic oxygen-rich waters displace the bottom areas. When this happens, the low-oxygen water, which has high nutrient content, will be transported to lower depths and to  Gulf of Finland and also in small amount to Gulf of Bothnia. Because large amounts of phosphorus and nitrogen are liberated, primary production increases, and generation of hypoxia occurs in new areas. The increase of temperature causes increased oxygen consumption of all poikilothermic animals. Thus, climate change increases the likelihood of hypoxic periods. Also, if the predictions of increased rainfall during winter remain accurate, the phopshorus and nitrogen discharges during natural river flow to the Baltic increase markedly.

Although eutrophication is usually linked only to increased plant, algal and cyanobacterial growth, the end result is always hypoxia. In shallow areas eutrophication leads to intermittent hypoxia.               During the day, when there is enough light for photosynthesis, green plants and algae liberate oxygen, and the water often becomes hyperoxic. At night plants, algae, bacteria and animals all only consume oygen, rendering water hypoxic. The oxygen consumption by animals always consumes oxygen, but probably the most important reason for hypoxia is the oxygen consumption of microbes, which eat up all the dead plants, algae, cyanobacteria and animals entering the bottom. There is a group of bacteria which forms bacterial mats to bottoms with very low oxygen.

If one considers communities, hypoxia generally decreases diversity and changes species composition. As an example, at group level, fish disappear first, and are replaced by jellyfish. Among macrofauna, polychaetes survive at lowest oxygen level. With lowering of oxygen level, microfauna and microbes replace macrofauna. Hypoxia reduces the growth of animals and as a consequence fish catches are reduced. Additionally, hypoxia-tolerant species are usually of reduced commercial value. Thus, the Baltic Sea fisheries are markedly suffering from the spreading of hypoxic areas.


Kommentoi kirjoitusta. Avainsanat: climate change, environmental pollution, eutrophication

Kaliningrad waste water treatment plant: ready at last

Sunnuntai 18.6.2017 klo 11:38

Kaliningrad area in the coast of Baltic Sea has been without waste water cleaning until about a week ago (June 2017). The waste of about a million people has been led to the Baltic Sea untreated. Among the present point sources of polluting, especially eutrophicating wastes, the area has recently become the most significant in the Baltic area. (After St. Petersburg waste water treating plants have started full function). Kaliningrad WWTP has been planned and built for the past 40 years, so after it was finally functioning, one can with good reason give a suck of relief.  

Kommentoi kirjoitusta. Avainsanat: Baltic Sea, eutrophication, water pollution

An Important Book on Baltic Sea

Tiistai 18.4.2017 klo 15:50 - Mikko Nikinmaa

Just recently Springer published Biological Oceanography of the Baltic Sea., edited by Pauline Snoeijs-Leijonmalm, Hendrik Schubert and Teresa Radziejewska. So if you are interested in what makes the Baltic to what it is, including what presently contaminates it, the book is worth reading. With 686 pages, and a list of prominent scientists as writers of its different chapters, it is the most authoritave work of the biology of the Baltic Sea published in many decades.

Kommentoi kirjoitusta. Avainsanat: oceanography, pollution, eutrophication