More river water decreases the coastal sea production

[2017-02-01] When the discharge of river water into the sea increases, for example as an effect of climate change, the ratio between phytoplankton and bacteria production will change. Phytoplankton growth is disfavoured by both the organic carbon in the river water and the changes in depth of the upper layer of water. This is shown in a new study performed within the research programme EcoChange.

Climate change will lead to increased precipitation in northern Europe. As a result of this, larger amounts of river water will be transported to the sea. The salinity will decrease, and the depth of the mixed, low-saline upper layer of water will increase.

The river water entering the sea contains both nutrients and dissolved organic carbon originating from land. Bacteria are favoured by the dissolved carbon, since they are able to use it as a source of carbon. Phytoplankton, on the other hand, are disfavoured by the increased organic carbon, since this carbon makes the water darker.

Largest effect in the north

The conditions in the Baltic Sea differs in many ways between the northern and the southern parts. In the north, the growth of bacteria is dependent on the landbased carbon, while bacteria in the southern parts of the Baltic to a larger extent make use of dissolved carbon originating from phytoplankton. Data from long-term monitoring shows a clear relationship between high river discharge and decreased phytoplankton/bacteria ratio. The results are comparable to studies in lakes, where even moderate changes in concentrations of dissolved carbon have been shown to generate major shifts in the phytoplankton/bacteria ratio. However, it was not expected that the changes would be so radical in such large water volumes as the coastal seas.

Mesocosm facility
Ulf Båmstedt working in the mesocosm facility of Umeå Marine Sciences Centre. Photo: Andrea Gillgren.

A study of different scenarios
So, salinity, stratification, concentrations of dissolved carbon and nutrients will change in costal waters as an effect of the increased discharge of river water to the sea. The question is how the combined effect of this will be on the coastal ecosystem. To study this, experiments were performed in a mesocosm facility, where realistic conditions could be tested. By manipulating both the stratification of the water column and the concentrations of dissolved organic carbon, the researchers could get a picture of what happens to the ratio between phytoplankton and bacteria with different future scenarios. A careful control of the conditions was required, so that the levels of impact on the ecosystem would be as realistic as possible.

Deeper upper layer – less phytoplankton

There are several layers in sea water, caused by differences in density. The upper layer has lower salinity, and is well mixed by wind and other forces. Phytoplankton are primarily found in the upper layer, while bacteria can be found in the whole water column. The mixing distributes the phytoplankton in the entire upper layer.

When the amount of river water increases in the coastal sea, the concentrations of dissolved organic carbon will increase as well. This will deteriorate light conditions in the water, which will disfavour the light-dependent phytoplankton. The inflow of freshwater will also deepen the upper layer of water. Thereby the light-dependent phytoplankton will appear deeper down, where light conditions are deficient. The combined effect of a deeper upper layer and an increase in dissolved organic carbon is a decrease in the phytoplankton production. Bacteria are not dependent on light, and are able to make use of the riverine carbon.

Explains low production in the Botnian Bay

The results from this study can also explain the generally low phytoplankton production in the Bothnian Bay in the northernmost part of the Baltic. Up until now, this low production has been explained by low concentrations of phosphorus. This study proves that a combination of a high inflow of freshwater and an increased concentration of dissolved organic carbon is enough to explain the low phytoplankton production in the Bothnian Bay compared to other parts of the Baltic.

Rivalry for nutrients

One might expect that the increase of the amount of nutrients would favour phytoplankton over bacteria. However, this does not seem to be the case. Despite an increase in the amount of nutrients, the phytoplankton are disfavoured of the increased river inflow. Instead, the bacteria seem to compete with the phytoplankton for nutrients, and will win the struggle of growing the fastest.

Inflicted detour

In a food web based on bacteria, the carbon will be forced through a detour with additional steps before it reaches the higher levels of the food web, such as fish. Energy is lost in every step, and therefore the detour will lead to a lower total production. The seemingly trifling change in the ratio between phytoplankton and bacteria in the very base of the food web could in the end cause large changes in how much fish and shellfish the ecosystem produces.

Article in Swedish

Båmstedt, U., Wikner, J. 2016. Mixing depth and allochthonous dissolved organic carbon: controlling factors of coastal trophic balance. Mar Ecol Prog Ser 561:17-2.

The research programme EcoChange

The mesocosm facility used in the study

Editor: Kristina Viklund

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