Dissolved and Particulate Amino Acids in the Lower Mississippi and Pearl Rivers (USA)

Seasonal changes (monthly samples) in abundance and composition of dissolved and particulate amino acids were observed at one station in the lower Mississippi and Pearl Rivers (MS, USA) from September 2001 to August 2003. Spatial variability was also observed during a 4 day transmit from river-mile 225 to river mouth (Head of Passes, LA) in the Mississippi River, and a two-day downstream sampling from Jackson (MS) to Stennis Space Center (MS). Temporal data in the lower Mississippi River showed significantly lower concentrations of dissolved combined amino acids (DCAA, 0.45-1.4 μ M) and dissolved amino acids in high molecular weight fraction (HMW DAA, 0.13-0.27 μ M) than in the Pearl River (DCAA, 0.91-2.8 μ M; HMW DAA, 0.25-0.95 μ M). DCAA and HMW DAA in both rivers were generally higher during high-flow periods. DFAA was significantly lower than DCAA in both rivers (0.05-0.08 μ M), and displayed minimal seasonal variability. Total particulate amino acids (PAA) in both rivers were in the same range (0.7-1.4 μ M). A C- normalized yield of PAA (PAA-C/POC) was negatively correlated with suspended particulate matter and positively with chl-a in both rivers. No significant difference in PAA composition was observed in the two rivers. However, PAA in both rivers was relatively enriched in arginine, alanine, methionine and leucine, and depleted in aspartic acid, serine, and non-protein amino acids, compared to DCAA. While DCAA spatial variability in the lower Mississippi River was minimal, decreases in PAA (from 1.06 to 0.43 μ M) were consistent with particulate organic carbon (POC) and particulate nitrogen (PN). Frequent variations in the PAA-C/POC ratio were inversely correlated with suspended particulate matter and PAA (R = -0.7, n = 48), suggesting short- scale sedimentation and resuspension events. A gradual increase in % non-protein AA along with a loss of phytoplankton biomass along the river, suggested was indicative of bacterial utilization of labile phytodetritus. The abundance and composition of DAA and PAA in these river systems provides important information on nitrogen and carbon cycling in very different rivers entering the coastal ocean within the same coastal plain.