From Sink to Source: Dynamic of Greenhouse Gases Emissions from Beach Wrack Accumulations in a Temperate Coastal Bay

Coastal ecosystems such as salt marshes, seagrass meadows, and kelp forests add to climate regulation as carbon sinks. However, coastal ecosystems may also act as carbon sources as beach wrack accumulations may release greenhouse gases (GHG) during decomposition. Little is known about the magnitude of GHG emissions of beach wrack accumulations under natural conditions, hampering accurate blue carbon accountings. In this study, the spatio-temporal variability of GHG emissions from beach wrack accumulations on a temperate sandy beach was assessed to quantify CO, CH and NO release and to identify the environmental determinants. Beach wrack accumulations, dominated by Zostera marina and opportunistic brown macroalgae, presented variable spatio-temporal dynamics. Annual beach wrack GHG emissions achieved up to 77,915 mg m d (expressed in CO equivalents) and varied largely throughout the study period due to interactive effects of temperature, wave exposure, beach wrack biomass moisture, abundance and species composition. Our findings showed that new and drifting wrack methane emissions reached up to 100 mg m d, representing up to 57% of annual CO equivalents emissions and occurring throughout the year. Nitrous oxide emissions were highly variable among the substrates investigated and comprised a minor extent (i.e., up to 4%) of annual CO equivalents emissions. Together, wrack CH and NO emissions provided extra 13.69 g CO m per year to the atmosphere. Special attention was also paid to potential correlations between beach wrack composition and GHG release to get robust estimates of annual budgets. Positive relationships between CO and NO emissions and opportunistic macroalgae abundance highlighted that eutrophication-driven excessive wrack accumulations may increase GHG emissions. Our study supports that whilst beach wrack depositions are a natural and essential part of coastal ecosystems, they may provide an extra source of GHG to the atmosphere, potentially counteracting the role of vegetated coastal ecosystems as carbon sinks.