Interannual Variation in Photosynthetically Significant Optical Properties and Water Quality in a Coastal Blackwater River Plume Michael J. Durako & Piotr Kowalczuk & Michael A. Mallin & William J. Cooper & Jason J. Souza & David H. Wells Received: 6 August 2009 / Revised: 10 April 2010 / Accepted: 22 April 2010 / Published online: 18 May 2010 # Coastal and Estuarine Research Federation 2010 Abstract Surface water optical characteristics, nutrients, and planktonic chlorophyll a concentrations were analyzed in the Cape Fear River (CFR) plume over a 2-year period. CFR discharge during the dry year (109±105 m 3 s -1 ) was only 25% of the wet year discharge (429±337 m 3 s -1 ). Partitioning the contributions of phytoplankton pigments, non-pigmented particles, and colored dissolved organic matter (CDOM) to the absorption of photosynthetically active radiation (PAR) indicated that CDOM was the dominant contributor to PAR absorption. Particulate ab- sorption was relatively greater during the dry year. Pigment absorption was minor and varied little among stations or between years. Chlorophyll a concentrations were reduced at the most plume-influenced stations during the wet year, despite lower turbidity and higher nitrate concentrations. Ammonium and orthophosphate concentrations were not different between years. CDOM absorption [a CDOM (412)] ranged from 0.05 to 8.25 m -1 with highest values occurring near the CFR mouth. Our results suggest that for coastal ecosystems with significant blackwater river inputs, CDOM may exert a major limiting influence over near-shore primary production. Keywords South Atlantic bight . River plumes . Optical properties . Water quality . Colored dissolved organic matter . Chlorophyll Introduction The Cape Fear River (CFR) is one of the largest blackwater (high-colored dissolved organic matter, CDOM, levels) riverine systems on the eastern coast of the USA. The CFR watershed encompasses 23,310 km 2 , the largest river M. J. Durako (*) Department of Biology and Marine Biology and the Center for Marine Science, University of North Carolina Wilmington, 5600 Marvin Moss Ln, Wilmington, NC 28409, USA e-mail: durakom@uncw.edu P. Kowalczuk Polish Academy of Sciences, Institute of Oceanology, ul. Powstancow Warszawy 55, PL-81-712 Sopot, Poland e-mail: piotr@iopan.gda.pl P. Kowalczuk : M. A. Mallin : J. J. Souza : D. H. Wells Center for Marine Science, University of North Carolina Wilmington, 5600 Marvin Moss Ln, Wilmington, NC 28409, USA M. A. Mallin e-mail: mallinm@uncw.edu J. J. Souza e-mail: souzaj@uncw.edu D. H. Wells e-mail: wellsd@uncw.edu W. J. Cooper Department of Chemistry and Biochemistry and the Center for Marine Science, University of North Carolina Wilmington, 5600 Marvin Moss Ln, Wilmington, NC 28409, USA e-mail: wcooper@uci.edu Present Address: W. J. Cooper Urban Water Research Center and Department of Civil and Environmental Engineering, University of California, Irvine, CA 92697-2175, USA Estuaries and Coasts (2010) 33:1430–1441 DOI 10.1007/s12237-010-9302-5