Preferential removal of dissolved carbohydrates during estuarine mixing in the Bay of Saint Louis in the northern Gulf of Mexico

Water samples were collected from the Bay of Saint Louis (BSL) estuary in the northern Gulf of Mexico to investigate the abundance, distribution, and mixing behavior of dissolved organic carbon (DOC) and total dissolved carbohydrates (TCHO). Laboratory experiments were also conducted to examine the removal mechanisms of DOC and TCHO during estuarine mixing. Both DOC and TCHO concentrations decreased with increasing salinity, showing a non-conservative behavior in the BSL with considerable removal of DOC (43%) and TCHO (63%). The ratio of TCHO to DOC varied from 0.31 in riverwater to ∼ 0.10 in coastal waters, decreasing with increasing salinity within the estuary. The TCHO was higher in the > 1 kDa colloidal fraction, ranging from 53% to 73% as quantified using an ultrafiltration permeation model compared to the bulk DOC that had a colloidal fraction of 40–63%. Coastal waters also had a slightly elevated CHO/DOC ratio in the colloidal phase, suggesting in situ production of CHO. Results from laboratory experiments showed that physical mixing resulted in a removal of 5–10% TCHO, while microbial processes accounted for a removal of up to 25–30% of TCHO during incubation experiments, suggesting that microbial degradation was the primary mechanism responsible for the removal of CHO in the BSL. However, physical removal combined with microbial degradation accounted for only 30–40% of the removal of DOC and TCHO in the BSL, indicating that the removal might be overestimated and/or other processes are also important in the removal of DOC and CHO in the estuarine environment. Both field observations and laboratory experiments demonstrated consistently greater removal of CHO compared to bulk DOC, supporting our hypothesis that carbohydrates are a more active organic component and could be preferentially removed during estuarine mixing.