Molecular-weight-dependent mixing behavior of organic phosphorus and carbon in a subtropic river-estuary-bay...

Organic phosphorus (OP) and organic carbon (OC) are significant components of potentially bioavailable organic matter reservoir and key drivers of eutrophication in aquatic environments. However, the dynamics of their chemical speciation, transformation, and coupling mechanisms along the river-estuary continuum remain poorly understood. To bridge this knowledge gap, we quantified dissolved, colloidal, and particulate OP and OC in the Qinjiang River Estuary in the northern Beibu Gulf during July 2023. Dissolved OP (DOP) and dissolved OC (DOC) were further fractionated into different size-fractions using ultrafiltration devices equipped with membranes with different molecular weight cutoffs of 10 kDa, 30 kDa, and 50 kDa. The >10 kDa colloidal OP (COP) constituted 63 ± 17 % of the bulk DOP, indicating that most DOP occurred in colloidal form, whereas colloidal OC comprised 44 ± 14 % of the bulk DOC, suggesting that DOC predominantly partitions in the <10 kDa dissolved phase. In addition, DOP and DOC had distinct estuarine mixing behavior. DOP was rapidly removed, attributable to its strong particle reactivity and high biological uptake, while DOC exhibited largely conservative mixing due to continuous riverine inputs and the presence of refractory components within the bulk DOC pool. The percentage of particulate OP in the total OP decreased sharply from 69 % in river water to 14 % in the lower estuary, while the <10 kDa low-molecular-weight DOP (LMW-DOP) increased dramatically from 9 % to 64 % along the river-estuary transect, reflecting a dynamic transfer from particulate to dissolved phases, likely related with in situ production of bioavailable LMW-DOP from biological activities in the bay. Riverine colloidal organic matter (COM) was dominated by medium- to large-sized fractions with higher C/P ratios indicating sources from P-limited terrestrial/river systems or contributions from highly degraded terrestrial COM. In contrast, river-derived small-sized COM was diagenetically fresher with lower C/P ratios and greater potential bioavailability. Preferential biological removal of bioavailable small-sized COP (10-30 kDa) further increased C/P ratios in estuarine and open bay waters. These findings highlight the distinctive role between OP and OC and among different colloidal size-fractions in regulating biogeochemical cycling of dissolved organic matter and nutrients in the river-estuary-bay continuum.