Characterization of cDOM in the Elkhorn Slough estuary using EEM spectroscopy and its potential for macrophyte monitoring

Abstract

The Elkhorn Slough estuary is an upwelling-influenced system located in Monterey Bay, California. It is of great ecological importance, as it provides essential habitat to over 750 species, and is one of the largest estuaries in California. The sources and distribution of chromophoric dissolved organic matter (cDOM) in Elkhorn Slough are strongly influenced by geochemical processes linked to the transition between sea and continent, and to anthropogenic activity from adjacent lands. In 2016, water samples were collected from across the Elkhorn Slough estuary to assess the sources and spatial dynamics of cDOM and its components throughout the system. Using EEM spectroscopy, seven fluorophores (four humic-like and three protein-like), and three distinctive cDOM regions were identified through a combination of traditional peak analysis, Principal Components Analysis (PCA) and interpretation of different fluorescence indices. The majority of the sites in the lower slough, receiving water from Monterey Bay and the Old Salinas River watershed, presented a balanced mix of humic-like and protein-like materials. Sites in the proximity of eelgrass beds had distinctive cDOM signals with predominant protein-like components of autochthonous origin. Water from the upper slough, subject to high nutrient input and excessive macroalgal development, was mostly composed of humic-like matter linked to recent biological activity and macrophyte development. Distribution of cDOM along the main channel of the Elkhorn Slough estuary was influenced by hydrological processes, allochthonous input, and macrophyte distribution. The large contribution and high specificity of macrophyte-derived chromophores identified in this study suggest that EEM spectroscopy could be an effective tool to monitor macrophytes in locations where environmental conditions are unfavorable.