Seasonal changes in the abundance and composition of plant pigments in particulate organic carbon in the lower Mississippi and Pearl Rivers |
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Authors: | Shuiwang Duan Thomas S Bianchi |
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Institution: | (1) Darling Marine Center, School of Marine Sciences, University of Maine, Walpole, ME 04573, USA;(2) Institute for Geophysics, University of Texas, J.J. Pickle Research Campus, Austin, TX 78758-4445, USA |
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Abstract: | Plant pigments in particulate organic carbon were examined in the lower Mississippi and Pearl Rivers (U.S.), along with physical
variables and nutrients to study seasonal changes in the abundance and composition of phytoplankton. Water samples were collected
monthly from September 2001 to August 2003 in the lower Mississippi River (MR; no samples were taken in February 2002) and
from August 2001 to July 2003 in the Pearl River (PR). High concentrations of total suspended solids (TSS), nutrients, and
chlorophylla (chla; dominated by diatoms) were observed in the lower MR. The smaller blackwater PR was characterized by lower nutrients and
chla, higher ultraviolet absorbance, and a phytoplankton biomass dominated by chlorophytes. Chla concentrations in the lower MR was high in summer low-flow periods and also during interims of winter and spring, and did
not couple with physical variables and nutrients, likely due to a combination of in situ production and inputs from reservoirs,
navigation locks and oxbow lakes in the upper MR and Missouri River. Chla concentrations in the PR was only high in summer low-flow periods and were controlled by temperature and concentrations of
chromophoric dissolved organic matter 9CDOM). The high, diatom-dominated phytoplankton biomass in the lower MR was likely
the result of decreasing TSS (increased damming in the watershed) and increasing nutrients (enhanced agricultural runoff)
over the past few decades. Lower phytoplankton biomass (dominated by chlorophytes) in the PR was likely linked with intense
shading by CDOM and lower availability of nutrient inputs. An increase in the relative importance of phytoplankton biomass
in large turbid rivers, such as the MR, could have significant effects on the age and lability of riverine organic matter
entering the ocean, the stoichiometric balance of nutrients delivered to coastal margins, and the sequestration of atmospheric
CO2 in these dynamic regions. |
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