Dissolved organic matter composition differs across water types in Mid-Atlantic and Great Lakes coastal regions

Dissolved organic matter (DOM) in coastal surface waters influences local water quality and is an important component of biogeochemical cycling in coastal systems. Many studies have highlighted transformations of DOM along longitudinal river to estuary transects; however, processes that alter DOM composition along lower reaches of rivers and nearshore estuarine waters is poorly understood. The high productivity of coastal environments and their limited representation in Earth System Models further highlights a need for better understanding of DOM transformations along coastal reaches. We leverage a spatially distributed community sampling effort that spans coastal ecosystems across two regions to identify broad spatial drivers of surface water DOM composition and identify transferable trends between regions and surface water types. Samples were collected by community members from 47 locations within the mid-Atlantic and Great Lakes coastal regions. The samples used in this study focused on surface waters from small tidal streams and rivers and nearshore estuarine/lacestuarine environments. The DOM was characterized by excitation-emission fluorescence and Fourier transform ion cyclotron resonance mass spectrometry. We observed that optically active DOM did not display systematic regional trends but instead was primarily distinguishable across water types; optically active DOM was notably lower in nearshore estuarine/lacestuarine environments compared to tidal streams and rivers. At the molecular scale, DOM also did not display systematic regional differences. However, heteroatom containing DOM (e.g., nitrogen, sulfur) distinguished water sources. We further observed strong linkages between DOM and surface water quality parameters, such as pH, that indicate potential for transferability of DOM processing across coastal domains. Collectively, our results highlight a broad similarity and transformation of terrestrial signatures that may be conserved in coastal surface waters across regional scales. Such results have important implications for making scalable predictions of coastal biogeochemical processes and their responses to future perturbations.