Scientific Report about marine measure for mitigating eutrophication

Abstract

The cultivation of sugar kelp has been suggested as a marine mitigation measure for the uptake and removal of nutrients from the marine environment. In 2022-2023,
sugar kelp was cultivated in the Limfjorden in a 12-hectare experimental facility to
document 1) biomass yields, 2) uptake of nitrogen, phosphorus and carbon and 3)
effects on the environment by large-scale cultivation. The results show that large-
scale cultivation of sugar kelp can remove up to 23 g N m-1 line year-1, corresponding to an annual nutrient removal potential of 23 kg N ha-1 and 0.8 kg P ha-1 in a standard cultivation system with 1000 m cultivation line ha-1. Yields and potentials for nitrogen removal in Danish waters vary and are highest in areas with high salinity and nutrient availability. The cost of nutrient removal varies accordingly, with the lowest cost of DKK 2805 kg N-1. Modelling scenarios indicate that a significant upscaling of sugar kelp cultivation is required to achieve any effect on the key environmental indicators for GES: “summer chlorophyll-a” and “light attenuation”. At high biomass densities, the environmental effects include 1) a negligible reduction of light to the seabed, 2) altered water flow, 3) reduced sedimentation rates, 4) limited periodic increases in pH and oxygen concentration under, in and near the seaweed cultivation site. Harvesting in April reduces the loss of carbon from the seaweed to the marine environment, and it is unlikely that seaweed cultivation can contribute to carbon sequestration in the inner Danish waters.