Data: The chemical characteristics and mixing behaviors of particulate organic matter from small subtropical rivers in coastal Gulf of Mexico.

Abstract.Rivers transport a substantial amount of dissolved and particulate organic matter (DOM and POM) to the oceans. In estuaries, the transition between DOM and POM due to dissolution or flocculation and their chemical compositions are key in determining their remineralization potential and thus influence the biogeochemical cycling of carbon and nitrogen. While much effort has been focused on trace metals, minerals, and nutrients in estuaries, the mixing behaviors of POM have been understudied despite their biogeochemical and ecological importance. In this work, water from several rivers from the northwestern Gulf of Mexico was mixed with seawater in different proportions, and the amount as well as the chemical characteristics of POM in the mixtures were measured. POM was characterized by measuring organic carbon and nitrogen contents, carbon and nitrogen stable isotopes (δ¹³C and δ¹⁵N), and total hydrolyzable amino acids (THAA). The results showed that POM in most cases behaved conservatively after the mixing of river water with seawater.However, rapid POM formation was observed in one of the seven experiments, which was conducted in 2021 with Mission River water. The amount of particulate organic carbon formed in this experiment (41 µM) was 12% of the dissolved organic carbon at a salinity of 6. The formed POM was more depleted in ¹³C and had a higher POC/PON ratio than expected for conservative mixing, suggesting that terrestrial DOM, likely sourced from lignins and/or lipids, may have contributed to the flocculates. The short duration of these laboratory experiments under dim or dark conditions excludes the potential effects from biotic or photochemical processes, and thus the observations should be attributed to the rapid physical transformation of DOM to POM or flocculation, particularly when the riverine OM is contributed more from terrestrial sources than in situ production. Overall, although POM mostly behaved conservatively, substantial POM can be produced from DOM under right conditions, and these results shed new light on the biogeochemical cycling of organic matter in subtropical estuaries.