Natural recovery of Fucus vesiculosus after reduction of the nutrient load to a coastal Baltic Sea area, Sweden

Nutrient enrichment from large outlets of sewage and runoff from land is a major problem in many coastal areas. The study area, situated in the northern Baltic Proper just north of Stockholm on the Swedish coast, has received high amounts of phosphorus and nitrogen for over a century. The main part came from untreated sewage. In this study, we documented occurrence of attached brown perennial macroalgae Fucus vesiculosus thalli for geographical and depth distribution and compared this to older surveys from the same area. We also used herbarium material for comparing the cover and species composition of epibiota on F. vesiculosus. Nutrient reduction in the studied area began in the 1960's. Today, phosphorus levels are similar to the 1930s. With reduced nutrient levels, Secchi depth improved from an average of 2.6 m in late 1960s to 4.5 m in the 2020 s. This increase in light penetration allowed for an increased depth distribution of F. vesiculosus. The nutrient reduction has also resulted in a natural recovery of F. vesiculosus in the area, and the innermost distribution is now back to where it was in the 1880s. The composition of epibiota on F. vesiculosus also showed changes, from a dense cover of filamentous algae to a high cover of filter-feeding invertebrates.