Abiotic controls of nutrient dynamics

Precipitation and sorption onto sediments are physical-chemical processes that can have a strong influence upon phosphate and a lesser influence upon ammonium-N, whereas nitrateN apparently is little affected by physical-chemical removal. Sorption of phosphate ion onto charged clays and charged organic particles occurs when SRP concentrations in stream or sediment pore water are high relative to an equilibrium SRP value of no net exchange, and desorption occurs when SRP concentrations are lower (House 2003). Phosphorus sorption to suspended sediments is rapid, and is fastest with smaller particles such as silts compared with coarser particles. Because of the association of P with sediments, amounts of particulate and total P in streamwater often vary temporally in parallel with concentrations of suspended sediments (Jordan et al. 1997, Ekholm et al. 2000). The affinity of orthophosphate for sediments is also responsible for the link between sediment and P concentrations in surface runoff. Once this particulate inorganic P enters the river, lower concentrations of dissolved inorganic P in streamwater result in desorption and thus an increase in bioavailable P.

Sorption-desorption processes can act as a buffer on streamwater nutrient concentrations by removing them from solution when concentrations are high and releasing them when concentrations are low. In a study of N dynamics in Bear Brook, Richey et al. (1985) concluded that sorption of ammonium to sediments was a significant storage during summer and autumn.

FIGURE 11.6 Net uptake and release of soluble reactive phosphorus (SRP) by river sediments. Uptake (■) was measured after spiking the overlying water with orthophosphate. The greater release of SRP under anaerobic (O) compared with aerobic (•) conditions is attributed to the dissolution of phosphate that had precipitated with Fe hydroxide minerals. (Reproduced from House 2003.)

FIGURE 11.6 Net uptake and release of soluble reactive phosphorus (SRP) by river sediments. Uptake (■) was measured after spiking the overlying water with orthophosphate. The greater release of SRP under anaerobic (O) compared with aerobic (•) conditions is attributed to the dissolution of phosphate that had precipitated with Fe hydroxide minerals. (Reproduced from House 2003.)

Nutrients sorbed onto sediments can be released back into the water column after weeks or months (Peterson et al. 2001). In streams that receive high nutrient inputs such as from waste water treatment plants, storage of sorbed ions in the sediments can be substantial. In a highly enriched Arkansas stream below a wastewater treatment plant, P dynamics were dominated by short-term, abiotic storage and release processes, which acted to stabilize high P concentrations over time (Haggard et al. 2005). Stream sediments may be a temporary storage site for ammonium, as Triska et al. (1994) showed by periodically retrieving sediment-filled bags from a third-order gravel and cobble bed stream in California. Comparison of the amount of ammonium extracted from the sediments to that present in interstitital water showed that sorbed ammonium was highest during summer base flows and lowest during winter stormflow.

In addition to sorption-desorption processes, under aerobic conditions both SRP and dissolved organic P may complex with metal oxides and hydroxides to form insoluble precipitates, and under anaerobic conditions this phosphate is released back into the water (Figure 11.6). Because the extent of the anaerobic zone varies seasonally and spatially with the amount of organic matter in the sediments and thus the extent of microbial respiration, the availability of dissolved phosphate varies accordingly. Phosphorus can also coprecipitate with calcite under high pH conditions that result from decreases in CO2 or HCO^ levels due to the photosynthetic activity of macrophytes and ben-thic algae (Mainstone and Parr 2002, House 2003).

0 0

Post a comment