Seawater infiltration transforms porewater dissolved organic matter composition

Abstract:

Hydrologic flow facilitates the transport and transformation of organic materials from terrestrial to aquatic ecosystems, but less is known about how seawater influences terrestrial ecosystem biogeochemical cycling and function. The two-way exchange of water along coastal interfaces exposes them to a diversity of geochemical constituents and biological communities. Exposure of coastal ecosystems to seawater is predicted to increase due to sea level rise and increased frequency and magnitude of extreme events. We examine how persistent, episodic exposure of a first-order coastal watershed in the temperate Pacific Northwest to estuarine forcings modulates dissolved organic matter composition, a key fuel for food webs in both terrestrial and aquatic ecosystems. Tidal exchange in the study watershed was recently restored, which exposed the lower floodplain to periodic seawater inundation. Modeled hydrological flow, built upon high-resolution in-situ sensor data, suggests that the majority of water in the subsurface of the floodplain now originates from seawater infiltration. Using a variety of bulk- and high-resolution analytical tools, we evaluated how porewater and surface water organic matter composition varied across the lateral terrestrial-aquatic continuum from the river to the upland forest hillslope. We observed a consistent link between tidal amplitude and organic matter quality both within the river, floodwaters, and in floodplain soil porewaters. Furthermore, porewaters exhibited distinct molecular signatures laterally from river to hillslope, reflective of spatio-temporal variations in organic matter cycling across the floodplain. These observations suggest that the estuarine forcings play a dominant role in controlling terrestrial processes and the complex hydro-biogeochemical feedbacks occurring within this coastal watershed.