Dissolved organic matter – variation and recommendations for monitoring

Most of the nutrients in the Baltic Sea originate from land. The rivers transport large amounts of carbon, nitrogen and phosphorus to the sea, and most of it is in organic forms. The bioavailability of DOC entering the Baltic Sea varies, but is generally quite low.

Future climate scenarios forecast an increase in river runoff, especially in the north part of the Baltic Sea. Monitoring of dissolved carbon, nitrogen and phosphorus will therefore become increasingly important. There are gaps in the monitoring programmes of today, both concerning spatial coverage and which variables are measured.

Spatial gradients

River water generally contains higher amounts of DOC compared to sea water. Consequently, the DOC concentrations are particularly high in estuaries. In coastal areas in general, the concentrations are high as a result of both the inflow of terrestrial carbon and the primary production benefitting from the inflow of nitrogen and phosphorus. A lot of the DOC seems to be bound in the coastal areas, and will never reach the open sea. In the open sea areas of the Baltic Sea, the DOC concentrations are quite similar, even if a larger part of it originates from land in the Gulf of Bothnia compared to more southern parts of the Baltic. The southern parts of the Baltic Sea receives water from the North Sea, which is very low in DOC. Therefore, there is also a gradient from east to west in this part of the Baltic.

Concentrations of DON and DOP are highly variable in the surface waters of the Baltic. DOM transported by the rivers supplies DON and DOP to the coastal areas of the Baltic Sea. In river estuaries the DON concentrations can be twice those in the open-sea surface water. However, 60 percent of DON and DOP can be bound in estuaries.

The concentration of CDOM decreases from the Bothnian Bay to the Skagerrak, as well as the concentration of humic substances. The concentrations also decrease from the coastal zone to the open sea. This reflects the impact of river inflow, and a higher contribution of aromatic compound in the northern regions.

Seasonal variations

The lowest concentrations of DOC are found during winter. The concentrations increase during spring, and reach their highest levels during summer and autumn. The increase is due to both inflow of terrestrial material and autochthonous production. The DOC concentrations are linked to both hydrodynamic conditions, such as river inflow and circulation of water masses, and biological metabolism.

The concentrations of organic nutrients are generally high in the Baltic Sea, and seasonal trends of DON and DOP differ between the basins. For example, DON concentrations showed no seasonal variations in the Baltic Proper, but accumulated during the growing season in the Gulf of Finland. DOP concentrations decreased during the growing season in the Baltic Proper. Further studies are required in order to identify reliable seasonal patterns. This is recommended, since organic nutrients may support autochthonous production for both bacteria and phytoplankton, and may thereby contribute directly to eutrophication of the Baltic Sea.

The humic substances also vary during the season. In the open Baltic Sea concentrations are low during summer, and increase from autumn to spring. The concentrations of humic substances are mainly linked to river inflow and circulation of water masses.

Future monitoring – recommendations

Today the spatial coverage of the DOM monitoring presents some gaps:

• Bothnian Bay: The two coastal sampling stations receive freshwater from the same river. The coast of the southern parts is not covered. A coastal station in the south part is recommended.
• Bothnian Sea: The two open sea sampling stations are close to each other, and the southern part is not sampled at all. Both a coastal and an open sea sampling station in the south part is recommended.
• Gotland, Bornholm and Arkona basins: the open sea sampling seems to be sufficient. Data are scarce from coastal areas. Since the rivers from the south carry nutrients from agricultural areas to the coastal areas, contributing to the eutrophication problems, a preliminary study of DOM in the southern coastal areas is required to make recommendations for the monitoring programme.
• Kattegatt and the Skagerrak: the lack of information makes it difficult to address the spatial coverage of the monitoring programme.

Optimizing the monitoring programme towards the survey of DOM would also include some changes in which variables are measured.

• DOC instead of total organic carbon.
• DON instead of total nitrogen.
• Entire DON pool instead of the different parts such as urea or amino acids.
• DOP instead of total phosphorus.
• Keep on measuring humic substances, but also include CDOM absorption.

Link to the report:

Brugel, S., Andersson, A. 2016. Monitoring of dissolved organic matter in marine Swedish waters - needs and methodology. Report to the Swedish Agency for marine and water management.

Editor: Kristina Viklund