Internal phosphorus (P) in sediments plays an important role in the nutrient dynamics of lakes, sometimes long after external loads have been reduced. Similarly, internal P sources may drive the nutrient dynamics of small agricultural streams that drain to larger rivers and lakes, despite best management practices intended to reduce external P loads from adjacent fields. Here, internal P concentrations were measured with sequential extraction on cores collected in spring and summer from two small agricultural streams in the drainage basin of Lake Erie, a large, eutrophic lake experiencing increasing SRP loads. Average total extractable P concentrations were similar to within 5% during spring and summer, but mobile P binding fractions nearly doubled in summer, possibly due to accelerated rates of organic matter mineralization or iron reduction beneath suboxic, stagnant surface waters. One site had chronically greater internal P concentrations by 25–75%, despite the implementation of best management practices such as grass buffers. The site also had more aquatic vegetation that restricted the flow, less dissolved oxygen in surface water, and greater organic matter in sediments during both seasons, suggesting that variations in hydrology, sediment composition, and vegetation influence hot spots of P retention throughout small agricultural streams.

Metal contaminated sediments can be toxic to aquatic organisms and are common in human-dominated ecosystems, which results in metals being a leading cause of ecosystem impairment. Bioavailability of metals is influenced by their affinity for dissolved and solid-phase ligands, including iron (Fe) oxyhydroxides, which have been hypothesized to reduce metal toxicity in sediments. The authors examined the adsorption kinetics of copper (Cu) and nickel (Ni) with goethite (α-FeOOH) and characterized the influences of solute metal concentration, pH, ionic strength, and humate concentration on steady-state partitioning of the metals with goethite under conditions representative of natural aquatic environments. Copper and Ni readily adsorbed to goethite, and steady-state partitioning was achieved within 2 h. Although ionic strength had no effect on metal partitioning, adsorption of Cu and Ni to goethite was enhanced by alkaline pH and reduced by competition with humate. Because distribution coefficient (KD ) values for Cu and Ni from the present study are comparable to values measured in natural systems, the authors hypothesize that goethite may contribute significantly to the adsorption of both Ni and Cu to particles in the environment. The authors suggest that incorporating binding by Fe oxides in metal bioavailability models should be a priority for improving risk assessment of metal-contaminated oxic sediments.

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Microcystin is one of the most common toxins associated with freshwater harmful algal blooms, but little is known about microcystin fate in the aquatic environment. Laboratory wave tank experiments were performed to determine whether exchange of surface water and pore water (benthic exchange) removes and dilutes microcystin‐LR (MC‐LR) at environmentally relevant concentrations in coastal waters overlying permeable sediments. Over the 100 h experiment, 60% of MC‐LR mass was removed due to interaction with sediment (via adsorption and/or biodegradation), while only 20% was removed in an experiment without sediment. The observed fate and transport of MC‐LR in sediments was adequately described with a one‐dimensional reactive transport model that uses an enhanced diffusion coefficient to represent benthic exchange of solutes. Numerical sensitivity studies showed that MC‐LR removal increases with hydraulic conductivity of sediment and wave height and decreases with water depth. For MC‐LR concentration at the WHO recreational guideline (20 ppb), sandy sediments can remove the equivalent MC‐LR mass in 1 m of surface water under typical nearshore wave conditions within tens of hours. In open water at large depths above a silty bed, removal times are much longer (on the order of weeks). Wave‐driven benthic exchange is therefore an important control on MC‐LR fate in energetic coastal areas but not in deep or calm settings where sediment–water interactions are greatly reduced. The nearshore fate of algal toxins is important to human health and socioeconomic vitality, since recreational activities and direct human exposures are concentrated along coasts.

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