Streambank legacy sediments in aquatic ecosystems: nutrient source or sink?

Historic erosion coupled with large number of milldams and other impoundments along creeks has resulted in large accumulation of legacy sediments in valley-bottoms of mid-Atlantic watersheds. Not surprisingly, many studies now report that streambank legacy sediments constitute a significant proportion of the sediment yield from watersheds. What is uncertain though is the contribution of legacy sediments to the nutrient (e.g., nitrogen and phosphorus) inputs to aquatic ecosystems. A key question remains - are streambank legacy sediments a nutrient source or sink for the stream and downstream aquatic systems?

We assessed the nutrient source-sink behavior for streambank legacy sediments through a series of targeted laboratory experiments. Legacy sediment samples were available from 15 streambank sites across varying land uses in the mid-Atlantic US. The first experiment determined the Phosphorus Sorption Index (PSI): the maximum sorption capacity of the sediments. In the second experiment, the Equilibrium Phosphorus Concentration (EPC₀) was calculated. Comparison of EPC₀ values to streamwater concentrations indicates whether sediments serve as sources or sinks once deposited in the fluvial environments. In a third experiment the sorption of P to legacy sediments was evaluated under oxic and anoxic conditions. Mean PSI values for coarse and fine legacy sediments were 292.65 ± 224.40 mg/kg and 652.01 ± 177.30 mg/kg, respectively. The PSI experiment demonstrated that the fine sediment sorbed more PO₄ from the solution compared to the coarse fraction. EPC₀ values ranged from 0.005 - 0.236 (Mean: 0.036 ± 0.05) mg/L across the 15 sites. Eight of fifteen sites had EPC₀ values greater than the baseflow stream water concentration indicating that the legacy sediment could be a potential nutrient source if deposited into the channel. The final solution PO₄ concentrations for the anoxic treatment group (mean 1.11 ± 0.62 mgP/L) were significantly greater than those measured for the oxic treatment group (0.22 ± 0.20mgP/L). The oxic/anoxic experiment results showed that the P-saturated sediment released more PO₄ under anoxic conditions than under oxic conditions. Thus, sediment with phosphorous will release it in anoxic conditions, effectively acting as a source.