Colored dissolved organic matter in shallow estuaries: relationships betweencarbon sources and light attenuation

[摘要] Light availability is of primary importance to the ecological function ofshallow estuaries. For example, benthic primary production by submergedaquatic vegetation is contingent upon light penetration to the seabed. Amajor component that attenuates light in estuaries is colored dissolvedorganic matter (CDOM). CDOM is often measured via a proxy, fluorescingdissolved organic matter (fDOM), due to the ease of in situ fDOM sensormeasurements. Fluorescence must be converted to CDOM absorbance for use inlight attenuation calculations. However, this CDOM–fDOM relationship variesamong and within estuaries. We quantified the variability in thisrelationship within three estuaries along the mid-Atlantic margin of theeastern United States: West Falmouth Harbor (MA), Barnegat Bay (NJ), andChincoteague Bay (MD/VA). Land use surrounding these estuaries ranges fromurban to developed, with varying sources of nutrients and organic matter.Measurements of fDOM (excitation and emission wavelengths of 365 nm(±5 nm) and 460 nm (±40 nm), respectively) and CDOM absorbance were takenalong a terrestrial-to-marine gradient in all three estuaries. The ratio ofthe absorption coefficient at 340 nm (m⁻¹) to fDOM (QSU) was higher inWest Falmouth Harbor (1.22) than in Barnegat Bay (0.22) and Chincoteague Bay(0.17). The CDOM : fDOM absorption ratio was variable between sites withinWest Falmouth Harbor and Barnegat Bay, but consistent between sites withinChincoteague Bay. Stable carbon isotope analysis for constraining the sourceof dissolved organic matter (DOM) in West Falmouth Harbor and Barnegat Bayyielded δ¹³C values ranging from −19.7 to−26.1 ‰ and −20.8 to−26.7 ‰, respectively. Concentration and stable carbonisotope mixing models of DOC (dissolved organic carbon) indicate acontribution of ¹³C-enriched DOC in the estuaries. The most likelysource of ¹³C-enriched DOC for the systems we investigated isSpartina cordgrass. Comparison of DOC source to CDOM : fDOM absorption ratios at eachsite demonstrates the relationship between source and optical properties.Samples with ¹³C-enriched carbon isotope values, indicating a greatercontribution from marsh organic material, had higher CDOM : fDOM absorptionratios than samples with greater contribution from terrestrial organicmaterial. Applying a uniform CDOM : fDOM absorption ratio and spectral slopewithin a given estuary yields errors in modeled light attenuation rangingfrom 11 to 33 % depending on estuary. The application of a uniform absorptionratio across all estuaries doubles this error. This study demonstrates thatlight attenuation coefficients for CDOM based on continuous fDOM records arehighly dependent on the source of DOM present in the estuary. Thus, lightattenuation models for estuaries would be improved by quantification of CDOMabsorption and DOM source identification.

[学科分类] 地球化学与岩石