The greatest problem faced by the Baltic Sea is eutrophication, caused by excess nutrients. Even though we have been able to halve the nutrient load of the Baltic Sea in the past few decades, the symptoms of eutrophication, such as toxic blue algae blooms and anoxic seabeds, continue to afflict the Baltic Sea.
The eutrophication of the Baltic Sea is caused by excessive nutrients, which feed the growth of algae and water plants in the Sea, thereby increasing marine productivity. For decades already, the Sea has received nutrients, nitrogen and phosphorus in particular, in excess.
Nutrients end up in the sea with e.g. urban and industrial wastewaters, and rainwater from forests and fields. Some of the nitrogen discharges from transportation are also carried airborne to the Baltic Sea. With advancing climate change, increasing rainfall and mild winters threaten to further increase the nutrient load that enters the Sea from land.
In the Baltic Sea countries, wastewaters are these days treated quite well for nitrogen and phosphorus, even though there is still room for improvement in the efficiency of nitrogen removal in particular. Agriculture plays a large role, as more than half of the nitrogen and phosphorus in the nutrient load originates in agriculture. According to new research results, the load from forestry to the waterways is more significant and more enduring than we knew. Forests on drained peatland, in particular, are a source of nutrients, solids, and dissolved carbon runoff to waterways.
Nutrients and agriculture
Nutrients like nitrogen and phosphorus are needed in crop farming. Once they end up in the sea, however, a surplus of nutrients is harmful. Moreover, when nutrients are retained by the soil of farmed fields, the nutrient runoff from the field will increase for a long period of time.
Animal manure contains large quantities of nutrients that can be utilized as fertilizer for crops. Improving the efficiency of agriculture has, however, led to crop and animal farming centralization in different areas, so that in one area there is a shortage of nutrients, whereas in another area more nutrient-rich manure is generated than is needed by the fields nearby.
It is expensive to transport manure as such from one area to another, which is why in those areas where animal farming is predominant – such as the western coast of Finland and Southwest Finland – there are many instances of too much manure being spread on fields, leading to phosphorus, in particular, accruing in the farm soil. There is even a need to have more fields for the purpose of spreading manure. Agriculture in Southwest Finland, which generates a major load for the Archipelago Sea, is in fact Finland’s only blemish on the hot spot list for discharges maintained by the Baltic Marine Environment Protection Commission HELCOM.
The impact of climate change threatens to further increase nutrient discharges from agriculture, and long-term solutions that reduce runoff are sorely needed. Accelerating the utilization of fast-acting and proven measures, such as gypsum spreading, is also important.
Impact of eutrophication on the Baltic Sea
The Baltic Sea is small and shallow, and consequently especially vulnerable to eutrophication. In an almost closed-up inland sea, water is exchanged very slowly through the narrow Danish straits, and nutrients and environmental toxins stay in and affect the Sea for a long time.
The water in the Baltic Sea is permanently stratified by salinity, making it slow to mix. Eutrophication and the stratification of the water maintain anoxia in the deep basins of the seabed. In an environment that is low on oxygen, nutrients retained in the seabed dissolve to the water again, and accelerate eutrophication. This phenomenon is also known as the vicious circle of the Baltic Sea’s internal load.
Eutrophication threatens the biodiversity of marine nature. The key species of the Baltic Sea, bladderwrack and eelgrass, suffer from cloudy waters and thread algae, which suffocate perennials. The habitats formed by the key species are important growth areas and habitats for numerous other Baltic Sea species. Bottom-dwelling organisms, who suffer from anoxic seabeds, are also an important source of nutrition for many other species.
Eutrophication has been a boon also for the common reed, which grows by the shores of the Baltic Sea. As thickets of reeds take over the shores, we lose many open, mosaic-like coastline habitats that are important for various species.
Blue-green algae, e.g. cyanobacteria, are amongst species of the Baltic Sea, but their volume has increased in the past few decades as a result of eutrophication. Unlike other algae, blue-green algae can take all the nitrogen they need directly from the atmosphere. This is why their growth speed is determined, in particular, by the volume of phosphorus in the water. Some of the blue-green algae are poisonous, and can be harmful especially to pets and small children.
Like anyone on holiday, blue-green algae love warmth, which is why the most abundant blooms appear on the Baltic Sea right in the middle of the best summer holiday season. When the weather is warm and calm, the algae pile up on the surface of the water, forming large blooms, and if the winds are right, they drift to the beaches where people swim. Even if there are lots of nutrients in the water, masses of blue-green algae blooms do not appear in cool and windy weather.
As the climate heats up, seawater, too, will become warmer, boosting blue-green algae growth in the sea. Forecasts indicate that as a consequence of climate change, increasing rainfall and mild winters will also increase nutrient loads from land to the Baltic Sea, leaving an even greater abundance of phosphorus in the Sea for the use of blue-green algae.
Anoxic seabeds and saline pulses
A vast area in the bottom of the Baltic Sea has no oxygen and no life. Incidental anoxia has tested the depths of the Baltic Sea even before the load created by man had an impact. The eutrophication of the Sea has, however, led to the extension of these anoxic areas, and to a vicious internal nutrient load cycle.
Because of the stratification of the water, the basins of the Baltic Sea receive new, oxygen-rich ocean water through the Danish straits only with a strong current of saline water, i.e. a saline pulse. The oxygen-rich water brought by a saline pulse prevents nutrients from being released from the seabed. However, a saline pulse flows to the Baltic Sea only approximately once a decade, when the weather conditions are right.
In the past few decades, saline pulses have become rarer, and at the same time, the area of seabed that is anoxic has grown. The latest saline pulse that entered the Baltic Sea was not enough to bring oxygen to all the depths of the Sea. It also pushed old, low-on-oxygen and nutrient-rich water out of its way towards the surface layers of the Sea, where the algae can easily reach it.
The saline pulses are not enough to make eutrophication take a turn for the better.