Experimental comparison of dissolved organic matter and nutrients leached from beach wrack by sea- and rainwater: A nutritional boost for the sandy beach subterranean estuary

Subterranean estuaries underlying high-energy beaches are efficient turnover sites for dissolved organic matter (DOM) and nutrients from marine and terrestrial waters. In addition, leaching of beach wrack during tidal inundation and precipitation can contribute to DOM and nutrient loads. However, the combined impact of diverse environmental settings on the release of DOM and nutrients from beach wrack has so far not been studied, although e.g., salinity oscillations and temporary exposure to sunlight are common in high-energy beach subterranean estuaries. Here, we present the results of an extensive beach wrack leaching experiment taking beach wrack type, age, sunlight exposure, and leaching matrix into consideration. A combination of UVA irradiation, advanced wrack age, and leaching by low-salinity artificial rainwater resulted in high dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) releases of mmoles- to moles per kg dry weight in the macroalga Fucus sp. Furthermore, jellyfish wrack released millimoles of TDN in artificial seawater incubations. Ultra-high-resolution analyses of DOM revealed a prevalence of molecular formulae resembling biochemical building blocks such as sugars, amino acids, and vitamins, indicating that the released DOM could be of substantial nutritional value for the heterotrophic microbial communities on and near beach wrack. An interesting finding was the high abundance of aromatic and humic-like DOM released from macroalgal beach wrack, which may impact typically used marine and terrestrial source- and sink proxies. As such, beach wrack DOM and nutrients could further complicate biogeochemical distribution patterns in the subterranean estuary.