Deon Knights

Vassar College

Subject Areas: Hydrology, Hydrogeology, Water Research, Modeling

 Recent Activity

ABSTRACT:

The Arctic Ocean is the smallest and shallowest of the world’s oceans but receives the largest riverine input per basin volume. Thus, the flux of nitrate from land to sea through rivers in the circumpolar Arctic can have large impacts on coastal life and global biogeochemical cycles. To assess the impact of terrestrially derived nitrate on Arctic Ocean chemistry under current and future scenarios, accurate accounting of nitrate loading is needed. Deltas act as filters that reduce nitrate loads, but current estimates of flux exclude the effects of Arctic deltas on nitrate export. We use a novel approach based on satellite imagery to estimate the potential impact of deltas on nitrate retention on five Arctic deltas, representing a range of morphologies and sizes. Our analysis shows seasonal and morphological differences in nitrate processing rates in each delta with retention rates ranging from 2.9-15 percent of incoming load and most retention occurring during winter months. Under future climate scenarios, the efficiency of nitrate retention decreases by up to 10 percent. Our models suggest that Arctic deltas alter the magnitude of nitrate entering the ocean and future predictions of loading or current earth system models could be improved by incorporating their effect. The dataset includes input parameters and model results for nitrate retention on the Kolyma, Lena, Mackenzie, Yenisei and Yukon deltas.

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ABSTRACT:

River deltas display a wide range of morphologic patterns that influence how nutrients interact in channels and wetlands on their way to the coast. To quantify the role of delta morphology on nitrate fate, we simulated reactive nitrate transport over steady base flow conditions for six synthetic, morphologically unique river-dominated deltas created in Delft3D by varying incoming grain size distributions. We parameterized nitrate removal kinetics using an observed relationship with elevation from Wax Lake Delta (Louisiana, USA). Total nitrate retention across the six synthetic deltas and Wax Lake Delta ranged from 1.2-13%, suggesting that these river-dominated deltas have limited ability to remove nitrate from incoming river water. Nitrate removal is constrained by limited hydrologic connectivity with the areas of greatest nitrate demand, which are found at higher elevation. In these synthetic numerical experiments, the efficiency of nitrate removal is greatest for deltas with more topologic complexity and greater proportions of the delta plain at higher elevation, which are both common characteristics of coarser-grained deltas. The positive relationship between grain size and nitrate removal may help guide land reclamation projects if project goals include minimizing nitrate export to the sea.

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ABSTRACT:

To understand the spatial distribution of nitrate removal in a river-dominated delta, we deployed 23 benthic chambers across ecogeomorphic zones with varying elevation, vegetation and sediment properties in Wax Lake Delta (Louisiana, USA) in June 2018. Regression analyses indicate that NDVI, a remotely sensed vegetation index, is a useful predictor of summertime nitrate removal. On average, potential removal rates were approximately three times greater on a vegetated submerged levee (13 mm hr-1), where NDVI was greatest, compared to other locations (4.6 mm hr-1). To protect coastal waters against high nitrate loads, management policies should aim to reduce upstream nutrient loads.

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ABSTRACT:

To understand the spatial distribution of nitrate removal in a river-dominated delta, we deployed 23 benthic chambers across ecogeomorphic zones with varying elevation, vegetation and sediment properties in Wax Lake Delta (Louisiana, USA) in June 2018. Regression analyses indicate that NDVI, a remotely sensed vegetation index, is a useful predictor of summertime nitrate removal. On average, potential removal rates were approximately three times greater on a vegetated submerged levee (13 mm hr-1), where NDVI was greatest, compared to other locations (4.6 mm hr-1). To protect coastal waters against high nitrate loads, management policies should aim to reduce upstream nutrient loads.

Show More
Resource Resource

ABSTRACT:

River deltas display a wide range of morphologic patterns that influence how nutrients interact in channels and wetlands on their way to the coast. To quantify the role of delta morphology on nitrate fate, we simulated reactive nitrate transport over steady base flow conditions for six synthetic, morphologically unique river-dominated deltas created in Delft3D by varying incoming grain size distributions. We parameterized nitrate removal kinetics using an observed relationship with elevation from Wax Lake Delta (Louisiana, USA). Total nitrate retention across the six synthetic deltas and Wax Lake Delta ranged from 1.2-13%, suggesting that these river-dominated deltas have limited ability to remove nitrate from incoming river water. Nitrate removal is constrained by limited hydrologic connectivity with the areas of greatest nitrate demand, which are found at higher elevation. In these synthetic numerical experiments, the efficiency of nitrate removal is greatest for deltas with more topologic complexity and greater proportions of the delta plain at higher elevation, which are both common characteristics of coarser-grained deltas. The positive relationship between grain size and nitrate removal may help guide land reclamation projects if project goals include minimizing nitrate export to the sea.

Show More
Resource Resource

ABSTRACT:

The Arctic Ocean is the smallest and shallowest of the world’s oceans but receives the largest riverine input per basin volume. Thus, the flux of nitrate from land to sea through rivers in the circumpolar Arctic can have large impacts on coastal life and global biogeochemical cycles. To assess the impact of terrestrially derived nitrate on Arctic Ocean chemistry under current and future scenarios, accurate accounting of nitrate loading is needed. Deltas act as filters that reduce nitrate loads, but current estimates of flux exclude the effects of Arctic deltas on nitrate export. We use a novel approach based on satellite imagery to estimate the potential impact of deltas on nitrate retention on five Arctic deltas, representing a range of morphologies and sizes. Our analysis shows seasonal and morphological differences in nitrate processing rates in each delta with retention rates ranging from 2.9-15 percent of incoming load and most retention occurring during winter months. Under future climate scenarios, the efficiency of nitrate retention decreases by up to 10 percent. Our models suggest that Arctic deltas alter the magnitude of nitrate entering the ocean and future predictions of loading or current earth system models could be improved by incorporating their effect. The dataset includes input parameters and model results for nitrate retention on the Kolyma, Lena, Mackenzie, Yenisei and Yukon deltas.

Show More