Sorption and desorption dynamics of bulk dissolved organic matter and amino acids in the Mississippi River plume a microcosm study

摘要

Particulate and dissolved carbon, nitrogen and amino acids were analysed in a microcosm experiment where particulate material from the Mississippi River was mixed with surface waters of different salinities (2.0, 20.4 and 28.9) collected from the Mississippi River plume. There was evidence for the partitioning of amino acids in different salinity treatments during a 24-h period. Dissolved amino acids in the productive mid-salinity region of the plume had higher binding efficiencies than did amino acids from non-productive waters, indicative of the relative importance of phytoplankton sources of reactive amino acids. Basic amino acids were adsorbed more readily to particles, whereas acidic amino acids remained in the dissolved pool, indicating that electrostatic mechanisms affected amino-acid partitioning. Neutral amino-acid enrichment onto the particles was proportional to their respective hydrophobicity. Because riverine particles enter the higher salinity-shelf waters of the Mississippi River plume, changing ionic strength is a key in controlling amino-acid sorption and desorption kinetics. We propose that the short-term partitioning of amino acids between dissolved and particulate amino-acid pools across salinity gradients in the plume were affected by (1) electrostatic characteristics of suspended riverine sediment surfaces, (2) the availability of 'freshly' produced amino acids from phytoplankton and (3) the functional groups of amino acids.