Macroalgae

Macroalgae, commonly known as seaweed, contain chlorophyll and perform photosynthesis but lack roots or vascular tissue to transport nutrients. Instead, they absorb nutrients directly from the water through their outer surfaces. Unlike flowering plants, algae do not produce seeds but reproduce either asexually, for example through fragmentation, or sexually through the fusion of "gamete cells".

There are many different species of red, brown, and green algae, which thrive under varying degrees of wave exposure and light levels.

Macroalgae forests provide the foundation for a rich diversity of marine life. In the past, extensive growths of macroalgae were found in deeper waters along coasts and in fjords. This vegetation contributes to a stable aquatic environment, partly by binding nutrients in the plants during the growing season, which makes the nutrients unavailable to planktonic algae. Additionally, macroalgae beds serve as nursery grounds and habitats for many groups of fauna, including numerous fish species.

Macroalgae produce oxygen, which supports the transformation of ammonium into nitrate and subsequently into free nitrogen, reducing the availability of nitrogen in the water. At the same time, they diminish phosphorus exchange between the sediment and water column, thereby limiting the release of phosphorus from the sediment.

The decline in macroalgae coverage has therefore had significant negative impacts on the marine ecosystems.

Common to perennial macroalgae is the need to attach to solid surfaces, such as crevice-forming or scattered stone reefs, and growing at depths where light is available. Most macroalgae grow on hard rocky substrates and cannot anchor themselves to soft sandy sediments. Therefore, hard rocky bottoms are essential for the presence of macroalgae. These algae withstand currents and wave forces through strong attachment to the substrate and a flexible stalk with high tensile strength.

Seaweed forests are common in Denmark, especially in open marine areas with good water flow. However, due to nutrient pollution and murky water, the distribution of macroalgae has been reduced in Danish waters. Furthermore, stone fishing, which removes the solid substrates that macroalgae depend on for growth, has also reduced their occurrence.

Reestablishing hard substrates, such as single stones on the seabed or actual stone reefs, provide anchoring grounds for seaweed forests, resulting in increased biomass and greater local biodiversity, while also having a coastal protection effect.

With the significantly reduced natural state in many coastal areas, including the near-total absence of larger fish, there is a need for active marine conservation alongside the implementation of measures to reduce or immobilize nutrients if tangible effects are to be achieved within a shorter timeframe.

By reestablishing stone reefs, macroalgae will have the opportunity to regenerate seaweed forests, improving conditions for marine fauna and increasing biodiversity.