Separating in situ and terrigenous sources of absorption by dissolved materials in coastal waters

It is presently unclear to what extent coastal and oceanic chromophoric dissolved organic matter (CDOM) is remnant of diluted inputs from the terrestrial biosphere, is the product of in situ biological processes, or is a derivative of both sources. The development of a persistent phytoplankton thin layer (2-4 m thick) in East Sound, Washington, provided an opportunity to study the link between CDOM formation and in situ phytoplankton production. Spectral CDOM absorption a(g)(lambda), spectral particulate absorption a(p)(lambda), and hydrographic parameters were simultaneously recorded in high-resolution vertical profiles. Significant fine-scale variability was observed in a(g)(412). The majority of this fine structure covaried with salinity, consistent with mixing between an oceanic end member water type (low CDOM, high salinity) and a riverine one thigh CDOM, low salinity). During the development of the thin layer, deviations from this a(g)(412)-salinity relationship were observed. To isolate only the changes in a(g)(412) resulting from in situ processes, the initial state, represented by the a(g)(412)-salinity relationship, was subtracted from the measured a(g)(412). The remaining in situ change or residual a(g)(412) fraction was linearly correlated with oxygen supersaturation and a(p)(440). Above the phytoplankton layer centered at 3-6 m the a residual was negative, suggesting removal by photodegradation. A positive a(g) residual within the thin layer is the first direct evidence for rapid in situ CDOM production associated with phytoplankton primary production. Although the in situ fraction was 10% or less of the total CDOM absorption in East Sound during this period, in the open ocean this fraction may comprise nearly all of the absorption by dissolved materials.