Insight

Nutrients and algal blooms in the sea

Algae need nutrients to grow, but if too much nitrogen or phosphorus is introduced into the ocean, certain species can multiply en masse and cause problems.

Plants need nutrients to grow. This applies to trees, shrubs, and grasses on land, as well as to seagrass meadows and algal communities in the ocean. The main nutrients required are phosphorus and nitrogen, as well as micronutrients like iron and dissolved silicic acid in the water.

Large algae move under water — Algae forest under water (Picture: Dirk Schories)

How algae grow

Phosphorus and nitrogen enter the sea via rivers, are carried by the air, or are released during microbial recycling processes, such as in the sediment layer on the seabed. Nitrogen must then be converted into nitrate by cyanobacteria. Otherwise, algae can hardly make use of it.

Iron, on the other hand, is required by marine plants to form enzymes and proteins – especially those essential for photosynthesis. Important sources of iron include meltwater from Arctic and Antarctic glaciers, sediment-laden streams and rivers, desert dust clouds that release their sand over the ocean, and hydrothermal processes in the deep sea (such as black smokers), where iron-rich water emerges from the ocean floor.

If these nutrients are available in sufficient quantities, life in the ocean goes its usual way.

The over-fertilisation of the oceans

Filamentous green layers grow over seagrasses on the mudflats. They cover the seagrass meadow like a carpet and overgrow everything. — Green algae overgrow a seagrass meadow (Picture: Dirk Schories)

However, when nutrients are introduced in large quantities, massive algal blooms occur. Experts refer to this as eutrophication of the oceans. The problem is that when the algae die, the plant remains sink and are decomposed by microorganisms at mid-water depths or on the seabed. These microorganisms consume a lot of oxygen, which results in the formation of oxygen-poor zones in over-fertilized coastal waters, particularly in the summer, where few animals can survive.

The nutrient surplus in coastal waters largely comes from agriculture. When farmers apply too much manure or fertilizer to their fields, much of it is washed away by rainwater. Streams and rivers then carry the nutrient-rich load into the sea. Another relevant source of the nutrient surplus is wastewater, which also reaches the ocean via rivers⁽¹⁾.

Incidentally, seagrass meadows also have no future in over-fertilised waters. The algae blooms darken the water and deprive the grasses of the sunlight they need to survive. Seagrass meadows therefore thrive primarily in clear, nutrient-poor shallow water areas.

What can be done?

Foam caused by algae. — The over-fertilisation also stimulates the growth of algae, which are whipped into foam by the waves. This is seen in the picture, which was taken on the East Frisian island of Baltrum. (Picture: DAM)

Through targeted political measures, such as the Water Framework Directive and the Marine Strategy Framework Directive, the European Union aims to improve the water quality of European oceans and coastal waters. Key indicators for this include an adequate amount of oxygen, low concentrations of nutrients, and good biodiversity. The goal is to bring the marine environment into "good condition."

However, the results so far have not been convincing. For example, in the last review of the Marine Strategy Framework Directive in 2020, the goal of achieving good environmental status was significantly missed.

According to the report, 46 percent of European coastal waters do not have good ecological status. The Baltic Sea is the marine region with the highest proportion of coastal waters where nutrient conditions are a problem (58 percent)⁽²⁾.

There is a need for improvement: the levels of nitrogen and phosphorus are still too high. The German government is trying to meet the EU Commission's requirements at the national level with stricter regulations on fertilisers.

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(1) siehe auch Geomax 11, 2021: Die Reise des Stickstoffs. Warum wir immer wieder im Grünen baden. www.max-wissen.de/max-hefte/geomax-11-die-reise-des-stickstoffs/
(2) Bericht der Europäischen Kommission zur Umsetzung der Meeresstrategie-Rahmenrichtlinie (2020). eur-lex.europa.eu/legal-content/DE/TXT/PDF/