The Effects of Varying Salinity on Ammonium Exchange in Estuarine Sediments of the Parker River, Massachusetts

Abstract

We examined the effects of seasonal salinity changes on sediment ammonium (NH 4 + ) adsorption and exchange across the sediment-water interface in the Parker River Estuary, by means of seasonal field sampling, laboratory adsorption experiments, and modeling. The fraction of dissolved NH 4 + relative to adsorbed NH 4 + in oligohaline sediments rose significantly with increased pore water salinity over the season. Laboratory experiments demonstrated that small (∼3) increases in salinity from freshwater conditions had the greatest effect on NH 4 + adsorption by reducing the exchangeable pool from 69% to 14% of the total NH 4 + in the upper estuary sediments that experience large (0-20) seasonal salinity shifts. NH 4 + dynamics did not appear to be significantly affected by salinity in sediments of the lower estuary where salinities under 10 were not measured. We further assessed the importance of salinity-mediated desorption by constructing a simple mechanistic numerical model for pore water chloride and NH 4 + diffusion for sediments of the upper estuary. The model predicted pore water salinity and NH 4 + profiles that fit measured profiles very well and described a seasonal pattern of NH 4 + flux from the sediment that was significantly affected by salinity. The model demonstrated that changes in salinity on several timescales (tidally, seasonally, and annually) can significantly alter the magnitude and timing of NH 4 + release from the sediments. Salinity-mediated desorption and fluxes of NH 4 + from sediments in the upper estuary can be of similar magnitude to rates of organic nitrogen mineralization and may therefore be important in supporting estuarine productivity when watershed inputs of N are low.

Effects of cyanobacterial-driven pH increases on sediment nutrient fluxes and coupled nitrification-denitrification in a shallow fresh water estuary

“…Our adsorption coefficient (K = 2.6 ± 0.4) were similar to the observations of 2.1-7.1 in Potomac River sediments (Simon and Kennedy, 1987) and in the upper Chesapeake Bay (Cornwell and Owens, 2011). The K value in this freshwater estuary is higher than other coastal sediments (1.0-1.7, Mackin and Aller, 1984) and marine sediments (1.1-1.3, Rosenfeld, 1979), which may result from different salinity influences on NH + 4 adsorption (Seitzinger et al, 1991b;Weston et al, 2010).…”

“…Exchangeable NH + 4 is weakly bonded to negatively charged particle surfaces, buffering pore water NH + 4 concentrations (Rosenfeld, 1979). In estuaries, fine grained sediment generally has a large pool of adsorbed NH + 4 (Wang and Alva, 2000;Weston et al, 2010), with freshwater sediments having considerably more adsorbed ammonium relative to saline sediments (Seitzinger, 1991). Once alkaline pH results in the conversion of NH + 4 to dissolved ammonia (NH 3 ), formation of non-ionized NH 3 may decrease NH + 4 cation adsorption on sediments and potentially alter the balance between pore water and exchangeable NH (Cornwell et al, 1999).…”

Fertilizer legacies meet saltwater incursion: challenges and constraints for coastal plain wetland restoration

“…This increase in salinity could lead to increased release of both nitrogen and phosphorus from soils and sediments. This occurs primarily in two ways: 1) high concentrations of divalent cations (Ca 2+ and Mg 2+ ) in saltwater displace NH 4 + from soil cation exchange sites (Weston et al, 2010;Ardón et al, 2013); and 2) the high concentration of sulfate in saltwater stimulates sulfate-reducing bacteria in anoxic sediments, resulting in the production of sulfide. Sulfide, in addition to being a potent phytotoxin (Lamers et al, 2013), suppresses nitrification (Joye and Hollibaugh, 1995), denitrification (Seitzinger et al, 1991) and outcompetes P (PO 4 3-) ions binding with iron (Caraco et al, 1989;Lamers et al, 1998).…”

The effect of increasing salinity and forest mortality on soil nitrogen and phosphorus mineralization in tidal freshwater forested wetlands

“…First, enhanced availability of sulfate in salt-impacted anoxic soils results in desorption of sediment phosphate through sulfide and Fe interactions as well as increased microbial metabolism and mineralization of N and phosphorus (P) by providing an alternative electron acceptor (Caraco et al 1993;Portnoy and Giblin 1997;Lamers et al 1998;Weston et al 2006Weston et al , 2011Geurts et al 2010). Second, increased ionic strength in porewater can desorb exchangeable soil and sediment ammonium (Rysgaard et al 1999;Weston et al 2010) and phosphate (Sundareshwar and Morris 1999). Finally, increases in the amount and nutrient content of detritus inputs associated with plant stress or mortality could stimulate microbial nutrient mineralization in soils (Scott and Binkley 1997), although this mechanism has not been documented in tidal freshwater wetlands.…”