Internal Nutrient Loading Controls Macroalgal and Cyanobacterial Succession in a Coastal Lagoon Restored by Managed Realignment of Agricultural Land

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.

Prioritising ecosystem opportunities and threats of floating solar photovoltaics

<p>Floating solar photovoltaic installations are an emerging form of solar energy deployed on varying types of water bodies globally. Deployments have proliferated in recent years, particularly in land-scarce areas, as the drive to decarbonise the energy-mix intensifies. However, the potential ecosystem opportunities and trade-offs of floating solar photovoltaic installations remain unclear, often acting as a barrier to deployment. Exploiting floating solar photovoltaic knowledge systems, we synthesise evidence and insight from scientists and industry stakeholders, through a systematic review, international survey and workshop, to evaluate potential opportunities and threats to ecosystems. We found that reduced evaporation is the greatest perceived opportunity of floating solar, while detrimental chemical impacts, such as anoxia and internal nutrient loading, are perceived as the greatest threat. Using this knowledge, we assessed the overarching sustainability of floating solar, using the United Nations Sustainable Development Goals (SDGs) as a framework. We identified that floating solar photovoltaic installations may impact on eight of the seventeen SDGs. Given the need to rapidly develop understanding, in light of the anticipated growth rates, we prioritise the knowledge gaps and improvements critical to ensuring floating solar photovoltaic installations minimise ecosystem threats and maximise opportunities, safeguarding overall sustainability.</p>

Instantaneous Effects of Sediment Resuspension on Inorganic and Organic Benthic Nutrient Fluxes at a Shallow Water Coastal Site in the Gulf of Finland, Baltic Sea

Climate change is leading to harsher resuspension events in shallow coastal environments influencing benthic nutrient fluxes. However, we lack information on the quantitative connection between these fluxes and the physical forces. Two identical experiments that were carried out both in May and August provided novel knowledge on the instantaneous effects of resuspension with known intensity on the benthic dissolved inorganic (phosphate: DIP, ammonium: NH4+, nitrite+nitrate: NOx, silicate, DSi) and organic nutrient (phosphorus: DOP, nitrogen: DON, carbon: DOC) fluxes in the shallow soft bottoms of the archipelago of Gulf of Finland (GoF), Baltic Sea. Resuspension treatments, as 2 times the critical shear stress, induced effluxes of one to two orders of magnitude higher than the diffusive fluxes from the studied oxic bottoms. The presence of oxygen resulted in newly formed iron oxyhydroxides and the subsequent precipitation/adsorption of the redox-dependent nutrients (DIP, DSi, organic nutrients) affecting their fluxes. Resuspension-induced NH4+ and NOx fluxes were associated with the organic content of sediments showing the highest values at the organic rich sites. NH4+ showed the strongest responses to resuspension treatments in August, but NOx at the time of high oxygen concentrations in near-bottom water in May. Foreseen increases in the frequency and intensity of resuspension events due to climate change will most likely enhance the internal nutrient loading of the studied coastal areas. The fluxes presented here, connected to known current velocities, can be utilized in modeling work and to assess and predict the internal nutrient loading following climate change.

Organic N and P in eutrophic fjord sediments – rates of mineralization and consequences for internal nutrient loading

Nutrient release from the sediments in shallow eutrophic estuaries may counteract reductions of the external nutrient load and prevent or prolong ecosystem recovery. The magnitude and temporal dynamics of this potential source, termed internal nutrient loading, is poorly under\\-stood. We quantified the internal nutrient loading driven by microbial mineralization of accumulated organic N (ON) and P (OP) in sediments from a shallow eutrophic estuary (Odense Fjord, Denmark). Sediments were collected from eight stations within the system and nutrient production and effluxes were measured over a period of ~ 2 years. Dissolved inorganic nitrogen (DIN) effluxes were high initially but quickly faded to low and stable levels after 50–200 days, whereas PO43− effluxes were highly variable in the different sediments. Mineralization patterns suggested that internal N loading would quickly (< 200 days) fade to insignificant levels, whereas internal PO43− loading could be sustained for extended time (years). When results from all stations were combined, internal N loading and P loading from the fjord bottom was up to 121 × 103 kg N yr−1 (20 kg N ha−1 yr−1) and 22 × 103 kg P yr−1 (3.6 kg P ha−1 yr−1) corresponding to 6 (N) and 36% (P) of the external nutrient loading to the system. We conclude that the internal N loading resulting from degradation of accumulated ON is low in shallow eutrophic estuaries, whereas microbial mineralization of accumulated OP is a potential source of P. Overall it appears that, in N-limited eutrophic systems, internal nutrient resulting from mineralization of ON and OP in sediments is of minor importance.

Organic N and P in eutrophic fjord sediments – rates of mineralization and consequences for internal nutrient loading

Nutrient release from the sediments in shallow eutrophic estuaries may counteract reductions of the external nutrient load and prevent or prolong ecosystem recovery. The magnitude and temporal dynamics of this potential source, termed internal nutrient loading, is poorly understood. We quantified the internal nutrient loading driven by microbial mineralization of accumulated organic N (ON) and P (OP) in sediments from a shallow eutrophic estuary (Odense Fjord, Denmark). Sediments were collected from 8 stations within the system and nutrient production and effluxes were measured over a period of ~ 2 years. DIN effluxes were high initially but quickly faded to low and stable levels after 50–200 d, whereas PO43− effluxes were highly variable in the different sediments. Mineralization patterns suggested that internal N-loading would quickly (< 200 days) fade to insignificant levels whereas PO43−release could be sustained for extended time (years). When results from all stations were combined, internal N-loading and P-loading from the fjord bottom was up to 121 x 103 kg N yr−1 (20 kg N ha−1 yr−1) and 22 x 103 kg P yr−1 (3.6 kg P ha−1 yr−1) corresponding to 6% (N) and 36% (P) of the external nutrient loading to the system. We conclude that the internal N-loading resulting from degradation of accumulated ON is low in shallow eutrophic estuaries, whereas microbial mineralization of accumulated OP is a potential source of P. Overall it appears that in N-limited eutrophic systems, internal nutrient resulting from mineralization of ON and OP in sediments is of minor importance.

The Effects of Estuary Dredging on Removing Nitrogen and Phosphorus in Dianchi Lake, China

Sediment dredging is currently the most commonly selected option for getting rid of contaminated sediments. In this study, the effects of estuary dredging on removing nitrogen and phosphorus were investigated by comparing the different vertical nutrient content between the dredged and un-dredged areas in Dianchi Lake. The results showed that the contents of total nitrogen and total phosphorus in the un-dredged areas were relatively higher than that in dredged areas. Besides, the contents of bio-available nitrogen and phosphorus represented the similar results, only a few dredged spots showed a higher potential releasing capacity. Therefore, with the reduction of internal nutrient loading, it indicated that sediment dredging might be an effective and reliable way to improve such eutrophic lakes.