Effects of Total Suspended Solids on Photomineralization of Dissolved Organic Matter in the Peace-Athabasca Delta, Canada

Abstract

Northern deltas receive chromophoric dissolved organic matter (CDOM) from their watersheds, which can be oxidized to carbon dioxide upon absorption of sunlight (i.e., photomineralized). These deltas also receive total suspended solids (TSS), which may shade sunlight absorption by CDOM, thus limiting photomineralization. To quantify this interaction for the first time, we measured photomineralization rates at 11 sites in the Peace-Athabasca Delta (PAD), Canada. We sampled waters during a July 2022 field campaign for TSS concentration, CDOM concentration ($a_{\text{CDOM},\lambda }$), total downwelling sunlight attenuation coefficients ($K_{d,\text{tot},\lambda }$), and light attenuation coefficients due to CDOM ($K_{d,\text{CDOM},\lambda }$). TSS ranged from <1 to 112 mg/L with an average of 19 ± 34 mg/L (mean ± one standard deviation), an order of magnitude lower than TSS reported in rivers entering the PAD earlier in the open water season. $a_{\text{CDOM},\lambda }$ at 305 nm ($a_{\text{CDOM},305}$) ranged from 23.3 to 65.2 m⁻¹, $K_{d,\text{CDOM},305}$ ranged from 26.3 to 74.1 m⁻¹, and $K_{d,\text{tot},305}$ ranged from 19.0 to 63.7 m⁻¹. The ratio of sunlight absorbed by CDOM relative to total sunlight attenuation ($\frac{K_{d,\text{CDOM},\lambda }}{K_{d\text{,tot},\lambda }}$) was inversely correlated with TSS concentration across all wavelengths measured (305–412 nm). TSS thus limited photomineralization rates by shading CDOM from ultraviolet A and visible wavelengths of sunlight, reducing photomineralization rates by up to 56% compared to rates in the absence of TSS or other non-CDOM particles that attenuate sunlight. Results suggest that shifts in delta hydrology that affect TSS concentration likely influence photomineralization rates within TSS-rich northern deltas.