Beach-cast seagrass wracks: greenhouse gas emissions and energy recovery

Seagrass meadows are among the most productive ecosystems in marine environments. Like many terrestrial higher plants, marine seagrasses lose their old leaves during annual or inter-annual senescence, and a significant proportion of these residues is transported in surface waters and washed up on shores by surf, tides and winds. These beach-cast seagrass wracks provide important ecosystem services, such as reducing wave impact, protecting beaches from erosion, providing habitat to birds and invertebrate species that colonize shorelines, and being a primary food resource for beach detritivores. However, seagrass residues accumulation on beaches, following meadows degradation, can negatively impact tourism. Therefore, wrack piles are frequently collected and disposed of in landfills or biomass waste facilities, and the adoption of these management practices implies substantial environmental and economic costs. On the other hand, wrack piles might be a significant source of greenhouse emissions (GHGs). Recent studies reported CO2 and CH4 emission rates and suggested possible mitigation options, such as energy conversion and biochar production through pyrolysis. Even though quantitative estimates of both seagrass coastal distribution and residues disposal to seashores are partially available, at least at regional level, the assessment of their contribution to global GHGs emissions is still lacking, due to a knowledge gap about the effects of peculiar beach ecosystems environmental conditions on seagrass decay rates. Moreover, many studies have proposed several reuse options of beached seagrass residues in order to reduce both economic costs of collection from the shoreline and disposal in landfills and to offer a more sustainable beaches management. seagrass biomass use for energy production is under consideration in several countries of the world, as it combines the continuous increase in energy demand, sustainable costs of applied technology and social acceptance. In this research, the seagrass wracks decomposition dynamics were investigated in both controlled conditions and experimental fields in North-East Italy, with focus on CO2 and CH4 emissions, as a function of temperature, salinity, water supply. Moreover, the problems and perspectives concerning the assessment of beach-cast wrack contribution to the global GHGs emissions were highlighted. Using obtained results, the research then focused on energy recovery of beached seagrass litter as biomass source for anaerobic digestion. It was determined the potential methane production, the average biogas yields using different relative concentrations of seagrass biomass and sewage sludge and salinity effect on anaerobic digestion. Moreover, through genetic analysis, salinity and temperature effect on the anaerobic bacterial community composition was highlighted and the most relevant microbial families for biogas production were determined.