Oxygen and nutrients

Plants on land and in the ocean require nutrients to grow. However, when nutrient levels - particularly nitrogen and phosphorus - become too high, it leads to eutrophication. The term eutrophication is also used in aquatic environments where high nutrient concentrations cause excessive growth of planktonic algae.

Nutrients in the ocean primarily originate from land, entering via streams and rivers. Shallow coastal waters are therefore typically more nutrient-rich and naturally eutrophic compared to deeper offshore areas. The naturally higher nutrient levels along the coasts support greater production, biomass, and species diversity than in open ocean areas. However, excessively high nutrient concentrations can ultimately degrade living conditions significantly, leading to sharp declines in biodiversity.

Increased nutrient concentrations give planktonic algae a competitive advantage because they grow quickly and have short generation times. The algae make the water murky and can reach such high densities that sunlight can no longer penetrate to the seabed. As a result, eelgrass and macroalgae, which grow on the seabed, do not receive enough light and can only survive in very shallow waters. Additionally, high nutrient levels can lead to mass occurrences of free-floating, fast-growing seaweed species such as sea lettuce, which can form large, continuous "carpets" in shallow waters, outcompeting perennial eelgrass beds and macroalgae.

For many years, Danish marine and fjord areas have been undergoing eutrofication due to decades of excessive nitrogen and phosphorus inputs from cities, industries, and agricultural land. As a result, most coastal waters are nutrient-rich with unclear water. This has led to a significant reduction in the depth extent of eelgrass across all Danish fjords and coastal areas. In the past, eelgrass could grow at depths of 5–6 meters, but today, its depth limit has been reduced to approximately 2–3 meters in most areas. Overall, there is significantly less eelgrass in Danish waters than earlier. Reduced light penetration has also limited the growth of macroalgae at greater depths.

When sunlight cannot penetrate deep enough and eelgrass and macroalgae are restricted to shallow waters, the vegetation no longer plays its stabilizing role, which is crucial for maintaining biological balance and providing nursery areas for fish and water birds. Furthermore, eelgrass and seaweed beds act as wave breakers, meaning their reduced coverage increases sediment resuspension and decreases coastal protection.

Nitrogen is the primary limiting nutrient for plankton algae growth in the marine environment. This means that reducing nitrogen inputs from land to the sea is crucial. However, in certain fjords, phosphorus can regulate algae growth during specific periods of the year, making it important to also reduce phosphorus inputs in these areas. 

Since the 1990s, there has been a significant decrease in the input of both nitrogen and phosphorus into the marine environment. However, progress has stalled in recent years, and considerable efforts are still needed to reduce nutrient inputs to a level that allows for a satisfactory environmental state in Danish fjords and coastal waters.

As long as light reaches the seafloor, the conditions enable growth of eelgrass and seaweed, which can sustain oxygen production even in the deeper parts of fjords and bays. However, when these plants are outcompeted due to shading, most of the oxygen production shifts to the water column, where it is generated by phytoplankton and free-floating macroalgae such as sea lettuce. When the planktonic algae and sea lettuce die, they contribute with organic material to the seafloor. The breakdown of this organic matter can lead to oxygen depletion.

Oxygen depletion occurs naturally, but only to a very limited extent and typically in deeper waters with slow water exchange. It is eutrophication that causes oxygen depletion beyond natural levels. In Danish fjords and enclosed marine areas, bottom waters often suffer from poor oxygen conditions during summer and autumn due to the oxygen-consuming decomposition of organic matter.

Over the past hundred years, oxygen depletion has increased in frequency, extent, duration, and severity as a result of eutrophication and climate change.

In the worst cases, oxygen depletion can lead to sulfide eruptions. These occur when sulfur bacteria decompose dead algae, releasing hydrogen sulfide, which rises through the seabed and up to the water surface. The toxic hydrogen sulfide drags along black sludge from the seafloor, killing all marine life in its path.

Oxygen depletion affects both eelgrass and macroalgae. Eelgrass is particularly vulnerable to low oxygen levels, as its basal growing point - which produces leaves and rhizomes - can be destroyed after a relatively short period of oxygen deficiency. Macroalgae, on the other hand, have a greater ability to recover from short-term anoxic conditions, meaning that temporary oxygen depletion typically only has a transitory negative effect on already established populations.

Oxygen depletion in Halkær Bredning in 2022.

It is essential to significantly reduce nutrient inputs into our marine areas. The primary sources of excess nutrients are wastewater and, most notably, agricultural production. Reducing nitrogen discharges into coastal waters must primarily be achieved by lowering the amount of nitrogen leaching from farmland.

Active nature management can serve as an important supplement in achieving improved environmental and ecological conditions. Marine protection efforts can create a better basis for the successful restoration of habitats or key species within the protected areas. Therefore, integrating marine protection with ecological restoration efforts is often crucial.