Large scale seaweed cultivation in the sea will enhance water quality by feeding on nutrients and CO₂, therefore mitigating ocean acidity. This way seaweed will absorb important lost nutrients, washed out from the agricultural activities on land and transported to the ocean by our rivers. Especially the mining of phosphates is considered to reduce or even end in next decades, having a major effect on our crop yields on land. Seaweed can “capture” the lost phosphates if cultivation areas are placed in the right locations (e.g. near delta areas). Furthermore, offshore seaweed cultivation has a positive effect on fish populations, because the cultivation areas function as nurseries for smaller fish in restricted areas for ships and fishermen. AlgaeDemo will quantify these effects on the 1.4 ha scale.