Long-line mussel farming has been proposed as a mitigation tool in eutrophic coastal areas as nutrients are removed from the ecosystem upon harvest of the crops and transferred back to land. Further mussels filter the water and thereby increase water transparency and promote benthic plant growth. Intensive mussel farming may, however, negatively affect the nutrient cycling in the local environment through nutrient regeneration in the water column and through sedimentation of biodeposits resulting in organic enrichment of the underlying sediments leading to hypoxic conditions. The objective of this study was to explore the environmental interactions of a long-line mussel farm located in a eutrophic coastal area (Skive Fjord, Denmark) by studying the nutrient cycling in the water column and sediments and assessing their contribution to the nutrient dynamics and oxygen conditions in the fjord. The mussel lines contributed with nutrients, primarily ammonium, to the pool of nutrients in the water column and the contribution increased as the biomass of mussels in the farm increased. The sedimentation of biodeposits was only slightly higher at the farm (51-86 %) compared with a reference site, and the impact on the benthic environment was limited. During most of the production cycle the farm was a net sink of N due to uptake of N in the sediments, but after 1 year, the farm became a net source of N to Skive Fjord. Mussel excretion accounted for~82 % and sediments for~18 % of the N released from the farm. The study shows that mitigation of nutrients by long-line mussel farming will be most efficient, if mussels are harvested within the first year of the production cycle.

Abstract. Human activity has impacted many coastal fjords causing degeneration of the structure and function of the fish habitats. In Nørrefjord, Denmark, local fishermen complained of declining fish catches which could be attributed to eutrophication and extraction of sediments over several decades. This study aimed to establish blue mussel beds (Mytilus edulis) to increase structural complexity and increase the abundance of fish and epifauna in Nørrefjord. It was expected that the mussels would improve water transparency and increase the depth range and coverage of eelgrass (Zostera marina). New methods for mussel production and -bed construction were investigated in collaboration with local volunteer fishermen. The effect of the artificial mussel beds was most evident on a small scale. Video observations directly at the beds (Impact area) demonstrated increased biodiversity and a three times higher abundance of mesopredator fish compared to the Control area. Water clarity and eelgrass coverage were unchanged. Two methods for establishing mussel beds were tested. A total of 44 tons of blue mussels were produced and established in beds over an area of 121,000 m 2 . Production of blue mussels directly on hemp sacs hanging on long-lines was the most effective method. This new method is potentially a useful management tool to improve fish habitats.

How will coastal soils in areas newly flooded with seawater function as habitat for benthic marine organisms? This research question is highly relevant as global sea level rise and coastal realignment will cause flooding of soils and form new marine habitats. In this study, we tested experimentally the capacity of common marine polychaetes, Marenzelleria viridis, Nereis (Hediste) diversicolor and Scoloplos armiger to colonize and modify the biogeochemistry of the newly established Gyldensteen Coastal Lagoon, Denmark. All tested polychaetes survived relatively well (28–89%) and stimulated carbon dioxide release (TCO2) by 97–105% when transferred to newly flooded soils, suggesting that soil characteristics are modified rapidly by colonizing fauna. A field survey showed that the pioneering benthic community inside the lagoon was structurally different from the marine area outside the lagoon, and M. viridis and S. armiger were not among the early colonizers. These were instead N. diversicolor and Polydora cornuta with an abundance of 1603 and 540 ind m-2, respectively. Considering the species-specific effects of N. diversicolor on TCO2 release and its average abundance in the lagoon, we estimate that organic carbon degradation was increased by 219% in the first year of flooding. We therefore conclude that early colonizing polychaetes modify the soils and may play an important role in the ecological and successional developments, e.g. C cycling and biodiversity, in newly flooded coastal ecosystems. Newly flooded soils have thus a strong potential to develop into well-functioning marine ecosystems.

Managed realignment by deliberate flooding of coastal areas is an adaptation to sea level rise but may risk enriching the coastal zone with nutrients when seawater floods agricultural soil. This study focuses on the early development of macroalgae and their sources of nitrogen (N) in Gyldensteen Coastal Lagoon, Denmark. The lagoon was claimed for agricultural purposes in 1871 and reflooded by managed realignment 143 yr later (2014). Our hypotheses were: (1) that nutrients of agricultural origin from the newly flooded soil initiate opportunistic macroalgal blooms; and (2) that the isotopic composition of green algae rapidly reflects the origin of nutrient sources. We monitored macroalgal cover and conducted stable isotope (δ 15 N) analyses to assess the origin of N sources. Intense green macroalgal blooms occurred during the first summer after flooding and diminished in the 2 following years as a result of rapid water exchange. Low δ 15 N in macroalgae in the first year (mean ± SE, 4.2 ± 0.3 ‰) increased significantly in the next year (8.0 ± 0.1 ‰). A laboratory experiment tested the δ 15 N response of opportunistic green macroalgae (Ulva spp.) exposed to organic manure and synthetic inorganic fertilizers. Higher δ 15 N (11.1 ± 0.1 ‰) characterized manure-treated algae compared to fertilizer-treated algae (2.7 ± 0.2 ‰). Based on these field and laboratory results, we accept both hypotheses and conclude that the major N source supporting macroalgal growth in 2014 was derived from synthetic fertilizers; however, rapid tidal flushing during the following years resulted in nutrient limitation and lower macroalgal growth.

Managed realignment (MR) has been increasingly applied as an adaptation strategy to sea level rise in low-lying coastal areas, but the ecological consequences after flooding agricultural land with seawater are not well known. The restored Gyldensteen Coastal Lagoon represents one of the largest MR projects in Europe to date. The area served as agricultural land for about 150 years before being deliberately flooded with seawater in 2014. This study monitored for 5 years the succession of macroalgae and benthic cyanobacteria driven by changing internal nutrient (DIN = NH4+ + NO2– + NO3–, DON = dissolved organic nitrogen, and DIP = PO43–) loadings in the lagoon after flooding. A massive bloom of opportunistic green macroalgae (dominated by Cladophora spp.) occurred during the first year as response to a substantial loading of DIN and DIP from the newly flooded soils. The macroalgal cover was sparse the following years and the species richness increased with lower loading of particularly DIN. A cyanobacterial bloom controlled by declining DIN and steady DIP concentrations in the water dominated the lagoon and covered all solid surfaces 4 years after flooding. Highest macroalgal species richness with dominance of perennial Fucus vesiculosus and Agarophyton vermiculophylla was recorded 5 years after flooding following a temperature-induced stimulation of soil nitrogen transformation, leading to increased water column DON concentrations and DIN:DIP ratios. The lagoon remains therefore at an unstable tipping point where small and random changes in the DIN:DIP ratio control the balance between blooms of benthic cyanobacteria and high macroalgal species richness. Future MR projects involving agricultural land should prepare the soil to prevent algal blooms driven by sustained internal nutrient loading. Particularly P loading should be avoided to minimize the chances for recurrent blooms of benthic cyanobacteria.