On the Absorption of Light in the Orinoco River Plume

The spectral absorption coefficients of particles (phytoplankton and detritus) and colored dissolved organic matter (CDOM) were measured in the Orinoco River plume in the Gulf of Paria (GOP) and southeastern Caribbean (SEC) Sea in the wet (June, October) and dry (February, March) seasons of 1998, 1999, and 2000. During both seasons and over the three years, there was substantial patchiness in water color off the Orinoco Delta, and CDOM accounted for 61-98% of the total light absorption coefficient at 440 nm in the GOP and SEC at most stations (over(x, -) ≈ 89 ± 7.6 %; N = 44). Detritus dominated absorption due to particles (over(x, -) ≈ 70 ± 20 %; N = 27) near the Orinoco Delta, while phytoplankton dominated particle absorption in the northern GOP and in the SEC (over(x, -) ≈ 65 ± 17 %; N = 42). Absorption coefficients of CDOM, phytoplankton, and detritus at 440 nm ranged 0.23-2.59, 0.016-0.55, and 0.002-0.545 m- 1 during the dry seasons, and 0.38-3.21, 0.013-0.113, and 0.003-0.754 m- 1 during the wet seasons, respectively. The ratio of CDOM to phytoplankton absorption in the Orinoco plume [aCDOM (440) / aph (440)] ranged from 3.3 to 139 (over(x, -) ≈ 30 ± 24; N = 44), which are extremely high compared to other coastal regions. These values are explained by the unique characteristics of the Orinoco River (e.g. low phytoplankton biomass and high discharge of sediments and CDOM which inhibit phytoplankton growth). Our results demonstrate that it is fundamentally difficult to estimate phytoplankton absorption or chlorophyll-a concentrations from space using wavelengths in the blue portion of the light spectrum in coastal and estuarine waters affected by large rivers like the Orinoco. New chlorophyll algorithm development efforts need to focus on the red wavelengths (e.g. fluorescence peak) where CDOM influence is minimal