Coastal bacteria thrive on river water
Large amounts of dissolved carbon discharged in river waters enable bacteria to thrive in the northernmost part of the Baltic. In the coastal zone bacteria dominate the base of the food web, instead of the normally prevalent phytoplankton. Increases in the dissolved carbon discharges can lead to a less efficient food web, and climate changes are expected to further promote bacteria production.
Bacteria are so small that you just barely can see them even if you use a microscope. Even so there might be millions of bacteria in a single milliliter of water. Their role in the marine ecosystem is not to cause diseases, as we usually might expect from bacteria, but rather to decompose and recycle organic matter.
There is a lot of organic matter in sea water, especially in estuaries and inland waters. During the spring flood you can even see the darker colouring of the river water as it discharges into the sea, due to the terrestrial organic matter it carries. The transported organic matter contains dissolved compounds and particles that absorb light giving the brownish color to the water. In early spring the amount of dissolved carbon from the rivers increases in coastal areas, and the bacteria thrive. They either transform the organic matter into biomass, which can then transfer through the food web to higher trophic levels, or else they use the carbon for their own metabolism and respiration.
Bacteria-based food web
To study the aspects of food web efficiency and climate change, a series of measurements were performed in the northernmost part of the Gulf of Bothnia by a group of Ecochange researchers. The studied area is placed in an estuary receiving river discharge from an unregulated river, the Råne river. The large majority of the dissolved carbon in the sea water is of terrestrial origin, and the bacterial production is threefold higher than the phytoplankton production. In other words, the food web in the studied area is heavily reliant on bacterial production.
Rivers can influence coastal production
The river-borne carbon mostly consists of large molecules dissolved in the water, and it is generally not easy for bacteria to digest these large molecules without particular specialization. Still, in this study the bacteria seemed able to live on the river-borne material, and in certain cases were clearly limited by the amount of suitable dissolved carbon. The concentrations of other vital nutrients varied in their importance across the season and the sampled area. In spring and early summer the carbon effect was clear, while the nutrients became more important and limiting as the summer progressed. In addition the bacteria in the waters with closer proximity to the coast were more limited by carbon than bacteria further out to sea, which to a higher extent were limited by nutrients such as phosphorus and nitrogen.
More bacteria - less fish
The efficiency of a food web indicates how much energy is required for the carbon to transfer from the lowest trophic levels to higher levels in the food web, such as fish. A food web based on bacteria will contain additional steps compared to a phytoplankton based food web as these tiny microbes are only edible for smaller members of the food web. Thus numerous additional intermediate organisms shape the food web. Since energy is lost at each step the bacterial-based food web will therefore be less efficient. The relationship between bacterial production and phytoplankton production will have a great impact on the food web efficiency. If the proportions change, so that the bacteria production increases in importance, the productivity at higher trophic levels is expected to decrease, as well as the total productivity of the coastal ecosystem. In simple terms: more bacteria - less fish.
Changes in the future
The expected future alterations to the marine ecosystem due to climate change have been widely reported. Subarctic areas are often pointed out as areas of particular interest, since changes are anticipated to be some of the largest in these areas. In this region both precipitation and river discharge are expected to increase. The increased river discharge will lead to a higher inflow of river-borne dissolved organic matter to the coastal zone, which in turn will promote bacterial production. Simultaneously, the phytoplankton production will decrease as a result of both competition with bacteria for nutrients and the deteriorating light conditions. A glance into the future shows us a shift towards more bacterial-based marine food webs, and thereby an altered subarctic Baltic Sea ecosystem.
Figueroa, D., Rowe, O.F., Paczkowska, J., Legrand, C., Andersson, A. 2016. Allochthonous Carbon—a Major Driver of Bacterioplankton Production in the Subarctic Northern Baltic Sea. Microbial Ecology, DOI 10.1007/s00248 015 0714 4.