Abstract
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.
Highlights
Sea level rise due to climate change is apparent along coastlines and is expected to continue at an accelerating pace in the decades to come (Nicholls and Cazenave, 2010; IPCC, 2019)
There was a total of 22 macroalgal genera with at least 24 species registered at the two monitoring stations in Gyldensteen Coastal lagoon during the first 5 years after flooding (Table 1), not all were present at the same time and location
The 5 year algal succession in Gyldensteen Coastal Lagoon had the following four major phases (Supplementary Figure S1): (1) A massive green algal bloom driven by excessive internal nutrient loading right after flooding (2014); (2) Reduced algal cover and increasing species diversity caused by gradually diminished DIN loading (2015–2016); (3) Development of a benthic cyanobacterial bloom stimulated by continued DIP loading when DIN limitation was strongest (2017); and (4) Higher growth and diversity of all algal groups driven by stimulated mineralization and nutrient loading in a warm and sunny year (2018)
Summary
Sea level rise due to climate change is apparent along coastlines and is expected to continue at an accelerating pace in the decades to come (Nicholls and Cazenave, 2010; IPCC, 2019). This poses a flooding threat to low lying coastal areas among which many are embanked and reclaimed agricultural land (Nicholls and Cazenave, 2010). Since the long-term ecological development after flooding agricultural land with seawater is almost unknown, there is a need for new research corroborating the development of best restoration practices and management of coastal areas threatened by sea level rise (Gerwing et al, 2020). The sequence of emerging benthic flora, including microalgal biofilms, macroalgae, and plants, is important for the long-term ecological quality of coastal lagoons restored by MR
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