Abundance and composition of dissolved and particulate organic matter in the lower Mississippi and Pearl rivers
Description
Chemical Biomarkers, along with bulk carbon and nitrogen measurements were examined in the lower Mississippi (MR) and Pearl Rivers (PR) (USA) to study seasonal changes in the abundance and composition of organic matter and riverine phytoplankton. Water samples were collected monthly from September 2001 to August 2003. Surveys of spatial variability (225 km downstream in the MR and from Jackson to Stennis Space Center in the PR) in total DOC, DON and amino acids were also conducted in both rivers in June 2003 Higher phytoplankton abundance (dominated by diatoms) was observed in the lower MR than the PR (by chlorophytes), likely the result of decreasing TSS (increased damming in the watershed) and increasing nutrients (enhanced agricultural runoff) over the past few decades. Phytoplankton abundance in the lower MR was high not only in summer low-discharge periods, as observed in the PR, but also during winter and spring, indicating the inputs from reservoirs in primary tributaries Seasonal variations in the abundance and composition of DOM in the PR were highly correlated with water discharge, indicating a coupling between local carbon inputs (soil and wetlands) and regional precipitation events in the PR. Conversely, seasonal variability of bulk composition and chemical biomarkers of DOM in the lower MR was controlled by spatial variability of an integrative signal from watershed inputs and in-situ production from upriver sources. Spatially, very little change in total DOM in the downstream survey of the lower MR, compared to large decreases in the PR, likely suggested that OM has been subject to long-term in-situ processing An increase in the relative importance of phytoplankton biomass in large turbid rivers could have significant effects on the sources, lability of riverine organic matter, and the stoichiometric balance of nutrients delivered to coastal margins. Longer residence time of DOM and POM in large river systems from the drainage basin to the coast is an important difference in the overall processing of organic matter small versus large systems. If we are to better understand the controls of organic matter delivery to the coastal zone from both small and large rivers, sampling strategies need to be adjusted to account for the different scales of hydrologic response time and in-situ processing associated with different residence times