Phosphorus and the Baltic Sea: A brief review

Dana Zimmer,
Leibniz ScienceCampus for Phosphorus Research Rostock, Leibniz Institute for Baltic Sea Research Warnemuende (IOW),

Ulrich Bathmann,
Leibniz ScienceCampus for Phosphorus Research Rostock,

Leibniz Institute for Baltic Sea Research Warnemuende (IOW),

The Baltic Sea is a telling example of a semi-enclosed and stratified water body that receives high fresh water input from a large, densely populated catchment area, has a limited water exchange (with the North Sea) and therefore is very prone to the negative impacts of high input of nutrients such as phosphorus (P). In the last decades, the nutrient input from major point sources such as waste water treatment plants were systematically reduced so that today the majority of nutrients that newly enter the Baltic Sea are nearly background loads of rivers or originate from diffuse sources like agricultural land. Therefore, despite the notable reduction of nutrient contamination, the observed changes towards a better ecological status of the Baltic Sea are almost indiscernible. So how can the “Good Ecological Status” be reached, as it is determined by the “Marine Strategy Framework Directive” (MSFD) of the EU?

Hydrological models clearly demonstrate that further reduction of point sources will not result in a significant improvement of water quality with regard to P concentrations. Furthermore, a decrease of the natural riverine background P load is hardly achievable. Consequently, a further substantial decrease in yearly P loads of rivers draining into the Baltic Sea can be achieved by combined measures tackling many of the following diffuse upstream problems: Restoration and re-naturalisation of upstream lakes and channelled water courses, implementation of more sufficient P and N fertilisation practices with lower nutrient loss to the environment, measures to reduce soil erosion by wind and water, installation of devices against overflow of manure tanks and other farm waters (e.g. silos), controlled drainage, riparian strips as nutrient filters, constructed wetlands (including harvest e.g. reed), implementation of specific P reduction measures in small and measures against overflow in all waste water treatment plants, to name a few.

But even if such complex sets of external measures -that is outside the impacted system- would be stringently implemented, the effects in reduction might not be totally satisfying alone. Studies from semi-enclosed lagoon systems – the so-called “Bodden” along the Baltic coast – showed that even a strong reduction of the yearly riverine P inputs did not lead to a switch-back to the former macrophyte dominated ecosystem status with notably clearer waters. Several reasons might be responsible for the difficulties to reverse the effects of eutrophication: Atmospheric P deposition to water bodies might have been increased, depending on land use of the region and distance from the coast. Furthermore, sediments act as internal P sources, either under anaerobic conditions or when they are re-suspended by wind and waves. As a consequence, dense blooms of primary producers (e.g. algae), which mainly result from the high P availability early in the year and are also formed by N-fixing cyanobacteria (“blue-green algae”) that can store excess of P in their cells to support growth during lower P-input, keep on occurring very often throughout the year and cause a permanently increased turbidity. Thus, phytoplankton keeps being dominant in turbid waters. This hinders the growth of the light-dependent benthic macrophytes in the Bodden, which initiates all kinds of self-amplifying processes: Macrophytes normally decrease the strength of the currents and thus reduce re-suspension of P-rich sediments. They also accumulate nutrients in their biomass and thereby reduce the nutrient availability for phytoplankton. So, the lack of macrophytes caused by an excess of planktonic algae results in a positive feedback by stabilizing the phytoplankton dominance.

These studies from the Bodden ecosystems highlight, why it is not enough to solely reduce the nutrient input from external sources and how important it is, to re-establish a persistent submerged macrobenthic vegetation within the Baltic Sea to reach the target values for a good environmental status as specified by the MSFD. Therefore, a combination of external measures with internal restoration -that means measures inside the system- is proposed as the best option to increase water transparency for growth of macrophytes and decrease anaerobic zones, especially in the deeper basins, where they are very persistent. However, internal measures such as the planting of submerged macrophytes to decrease current velocity and act as a nutrient sink or to establish mussel farms for local nutrient entrapment and, thus, to increase the water clarity, might have side effects and have to be evaluated comprehensively before implementation.

A system approach is needed to evaluate all possible eutrophication mitigation measures. The investigation of trade-offs for any measure should include many aspects: Environmental legislation protocols, the monitoring of real potential for nutrient removal or for binding capacities in the short- and long-term, the effect and costs of nutrient reduction measures at the source locations on land, the effect and costs of nutrient removal measures at sea or in lagoons (e.g. dredging), the environmental and ecological impact of a measure relative to the costs, damage, biodiversity and long-term effects, just to name a few. In conclusion, a targeted, scientifically based combination of external and internal measures is needed to re-establish a good ecological status of the Baltic Sea.

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