Pore waters as a contributor to deep-water amino acids and to deep-water dissolved organic matter concentration and composition in estuarine and marine waters

Abstract

Amino acids (AA), including the bacterial D-enantiomers (D-AA), and bacterial muramic acid were quantified in bulk particulate and dissolved organic matter (POM and DOM) from the St. Lawrence system (Canada). These tools were used to reveal the origin of POM and DOM and their mechanisms of formation and transformation across the sediment-water interface. Results show that pore waters were much more enriched in DOM and AA (6 to 25 times) than deep waters. Pore waters are thus a source of DOM and AA (e.g., in proteins, peptides) to the sediment-water interface. AA represented 1.4 to 6.6% of bulk DOC in pore waters and thus most of the DOM compounds diffusing out of the sediments do not contain AA. Estimated AA pore water fluxes were between 32 and 141 μmol C m−2d−1 with lower values at the downstream locations. The correlations measured between AA concentrations in pore waters and deep waters, the compositional similarities between pore water DOM and deep-water DOM and their relatively altered state (measured with different diagenetic markers) suggest that a large fraction of the DOM released from the pore waters, including some AA-containing compounds, is not rapidly mineralized in the water column. Pore water DOM and deep-water DOM were the two sample types having the most similar composition when 31 parameters were considered. This similarity steadily increases downstream. Local conditions, such as POM inputs, redox conditions, and sediment mineralogy, seem to control the sediment's capacity for producing pore water AA and DOM. The C-normalized yields of the specific bacterial biomarkers and the correlations with AA yields suggest that bacteria are the major contributors to AA and to changes in POM and pore water DOM composition. In addition to the direct diffusion of altered and recalcitrant DOM out of the sediment, pore waters also provide less altered compounds, such as AA-containing structures, that can survive in the water column when the conditions are unfavorable to degradation (e.g., hypoxia) or be transformed into more recalcitrant DOM. This study suggests, based on different molecular and bulk parameters, that sediments have an important impact on the concentration and composition of the DOM persisting in deep waters.