Seasonal trends and relationships between surface and porewater dissolved organic matter in a disturbed semi-arid estuary

While many nutrient studies have shown large spatial and temporal variations in both inorganic and organic nutrient concentrations and sources, few studies have assessed the changes in the dissolved organic matter (DOM) molecular composition that may accompany these changes. This study aims to evaluate the seasonal changes in the DOM pool at the molecular level in a semi-arid estuary across different hydroclimatic conditions: from flood peak, to flood recession, to typical dry to semi-wet conditions. The PPL-solid phase extraction and HPLC Fusion Orbitrap mass spectrometry, in positive mode, was utilized to molecularly characterize surface water (n=60) and porewater (n=15) DOM across Nueces Bay, Texas. Dissolved organic carbon (DOC) exhibits non-conservative mixing between Nueces River and Corpus Christi Bay during summer flooding and up to 6 months post flooding likely due to flushing of the wetland at the head of the bay during flooding as well as due to benthic fluxes and groundwater discharge throughout flood recession. During summer and fall, 55% of the identified compounds are CHO but this increases to over 62% by winter as sulfur and phosphorous containing compounds decrease. However, CHON compounds comprise 25% of all identified compounds across all seasons indicating the organic nitrogen pool is more consistent than the organic sulfur and phosphorous pools. Volcano plots generated from predefined ratios (e.g., surface to porewater and season to season) were used to evaluate statistically significant (p-values ≤ 0.05) differences between samples/groups. Fall DOM is significantly different from all other seasons with over 1800 compounds significant to this season. This difference is likely due to enhanced benthic fluxes as the volcano plot for fall surface to porewater shows very few compounds significant to either group and the DOC mixing plot deviates toward the porewater endmember. Winter and spring samples are highly similar likely indicating that the influence of heavy precipitation had dissipated by 6 months post flooding. This study demonstrates temporal changes in DOM composition may be significant and should be considered in nutrient cycling and estuarine health assessments.