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(CDOM,lambda)), total downwelling sunlight attenuation coefficients (K-d,K-tot,K-lambda), and light attenuation coefficients due to CDOM (K-d,K-CDOM,K-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(CDOM,lambda )at 305 nm (a(CDOM,305)) ranged from 23.3 to 65.2 m(-1), Kd,CDOM,305 ranged from 26.3 to 74.1 m(-1), and Kd,tot,305 ranged from 19.0 to 63.7 m(-1). The ratio of sunlight absorbed by CDOM relative to total sunlight attenuation (K-d,K-CDOM,K-lambda/K-d,K-tot,K-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.

The Mackenzie River Delta in Canada is a mediator of hydrological transport between the expansive Mackenzie River watershed and the Beaufort Sea. Within the delta, lakes frequently act as water and sediment traps, limiting or delaying the movement of material to the coastal ocean. The degree to which this filtering takes place depends on the ease with which sediment-laden water is transported from distributary channels into deltaic lakes, referred to as functional lake-to-channel connectivity, which varies both spatially and temporally. Tracking of connectivity has previously been limited to either small regions of the delta or has focused on a snapshot of connectivity at a single instance in time. Here we describe an algorithm that uses Landsat imagery to track summertime functional lake-to-channel connectivity of 10,362 lakes between 1984 and 2022 on an image-by-image basis. We calculate a total average connected lake area of 1400.7 km2 during the 2 weeks after peak discharge, 763.6 km2 higher than previous estimates, suggesting a larger influence of connected lakes on water movement through the delta than previously estimated. We also identify water level thresholds that lead to the initiation of high sediment river water movement into 5,989 lakes (908 lakes with uncertainty <=+/- 0.5 m), and identify an additional 2899 lakes whose connectivity does not vary at all. As the Arctic hydrological cycle responds to climate change, this work lays a foundation for tracking the movement of water, and the matter it carries, from the Mackenzie River watershed to the Beaufort Sea. The Mackenzie River Delta in Arctic Canada lies at the intersection between the Northwest Territories and the Beaufort Sea. The delta contains over 45,000 mostly small lakes, which are either isolated from nearby river channels or connected to channels via movement of floodwater over land or via feeder channels of their own. This connectivity influences the amount of sediment that is able to make it from the Mackenzie River to the coastal ocean and additionally controls plant and plankton productivity within lakes. In this research, we use satellite imagery to expand our understanding of connectivity in the delta from 141 lakes to 10,362 lakes. We find that between 1984 and 2022, connectivity is highly variable through both space and time. On average, connected lakes can store 7% of riverine floodwater, higher than any other studied Arctic delta. Additionally, we calculate how high river water needs to reach in order to flow into 908 lakes, and identify 2,899 lakes whose connectivity does not vary at all. As the Arctic hydrological cycle responds to climate change, this work lays a foundation for tracking the movement of water, and the materials carried within, from the Mackenzie River watershed to the Beaufort Sea. We track functional lake-to-channel connectivity in 10,362 lakes in the Mackenzie Delta between 1984 and 2022 using remote sensing Connected lakes have the potential to attenuate, on average, 7% of floodwater in the Mackenzie Delta, higher than any other measured delta We identify water levels required to connect 908 lakes and find 2,899 lakes whose connectivity is stable regardless of water level