129-55 Spatial Variability in Drivers of Light Attenuation Across Massachusetts and New Hampshire Coastal Waters

Abstract:

Seagrass survival, productivity, and distribution depend critically on the quantity and spectral composition of ultraviolet (UV) and visible light reaching the seafloor. Because water-column constituents selectively absorb different wavelengths, understanding the drivers of vertical UV–visible light attenuation is essential for predicting seagrass habitat quality in coastal waters. Chromophoric dissolved organic matter (CDOM), non-algal particles, and phytoplankton are the primary contributors to light attenuation in nearshore waters, yet their relative importance and spatial variability across the coastal ecosystems of Massachusetts and New Hampshire remain poorly quantified. Here, we investigated the controls on the spectral diffuse attenuation coefficient of downwelling irradiance, K_d(λ), across the Plum Island Estuary, Great Bay, Essex Bay, and Waquoit Bay. We collected water samples and in situ K_d(λ) profiles between June 2019 and July 2026 and obtained laboratory measurements of CDOM (a_g(λ)), phytoplankton (a_ph(λ)), and non-algal particle (a_d(λ)) absorption from 250–700 nm. CDOM was the primary driver of K_d(λ) in the UV, blue, and green spectral regions, accounting for a median of 84.9%, 63.1%, and 45.2% of total absorption, respectively (R² = 0.88–0.99), whereas non-algal particles became the dominant predictor in the red (R² = 0.80). Despite this overall pattern, marked regional differences emerged. Absorption by CDOM varied by location due to river- and marsh-delivered terrigenous inputs and tidal flushing. This is reflected in the decreasing linear relationship between salinity and K_d(λ) at 340 and 443 nm (R² = 0.82–0.83). Waquoit Bay exhibited substantially greater non-algal particle contributions to UV–green attenuation compared to other regions, likely due to decomposed algal biomass and limited riverine CDOM inputs. Although phytoplankton generally played a limited role in light attenuation in the studied coastal sites, it exhibited a greater influence in offshore Great Bay, Plum Island Estuary, and Essex Bay stations. These findings demonstrate that although CDOM governs benthic UV–green light availability, regional differences in organic matter sources produce distinct underwater light environments likely to influence seagrass habitat suitability and productivity.

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