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 dis solved organic matter (CDOM) is remnant of diluted inputs from the terrestr ial biosphere, is the product of in situ biological processes, or is a deri vative of both sources. The development of a persistent phytoplankton thin layer (2-4 m thick) in East Sound, Washington, provided an opportunity to s tudy 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 observ ed 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 d evelopment of the thin layer, deviations from this a(g)(412)-salinity relat ionship were observed. To isolate only the changes in a(g)(412) resulting f rom in situ processes, the initial state, represented by the a(g)(412)-sali nity relationship, was subtracted from the measured a(g)(412). The remainin g "in situ change" or residual a(g)(412) fraction was linearly correlated w ith oxygen supersaturation and a(p)(440). Above the phytoplankton layer cen tered at 3-6 m the a residual was negative, suggesting removal by photodegr adation. A positive a(g) residual within the thin layer is the first direct evidence for rapid in situ CDOM production associated with phytoplankton p rimary production. Although the in situ fraction was 10% or less of the tot al CDOM absorption in East Sound during this period, in the open ocean this fraction may comprise nearly all of the absorption by dissolved materials.