Light absorption by phytoplankton and chromophoric dissolved organic matter in the drainage basin and estuary of the Neuse River, North Carolina (USA)

1. We examined the absorption of solar radiation by phytoplankton and chromophoric dissolved organic matter (CDOM) taking into account riparian shading in the rivers, reservoirs, swamps of the Neuse River Estuary and its drainage basin. 2. In the streams, CDOM typically absorbed 55 and 64% of photons in the spectral range of 400-700 nm (photosynthetically active radiation, PAR) and 500-600 nm, respectively. The large proportion of photons absorbed by CDOM indicates high potential for abiotic photochemial reactions in the 500-600 nm region. 3. Despite the high concentration of nutrients, phytoplankton contributed little (2%) to the total absorption of PAR in the streams. Small (<30 m wide) streams typically received only 7% of incident PAR that impinged onto the more exposed reservoirs and estuary. Riparian shading and the low contribution of phytoplankton to the total absorption resulted in conditions where phytoplankton absorbed nearly two orders of magnitude less PAR in the streams than in the estuary and reservoirs. 4. The results indicated that riparian shading and non-algal absorbing components can significantly restrict phytoplankton production in nutrient-rich streams with a high concentration of CDOM flowing throughout forested catchments.