Phosphorus Mass Balance and Internal Load in an Impacted Subtropical Isolated Wetland

Abstract

Internal loading is a critical component of the phosphorus (P) budget of aquatic systems that can control trophic conditions. While diffusion across the soil-water interface is generally considered the dominant process controlling P load to the water column, advection due to water table fluctuations can also be significant. Our objective was to evaluate the role of diffusive and advective fluxes in relation to the total P (TP) loads entering and exiting an impacted wetland in the Lake Okeechobee drainage basin. The average diffusive flux of TP was 0.32±0.14 mgm −2 day −1 and occurred for 240 days out of 314, while advective flux was 1.31± 4.03 mgm −2 day −1 and occurred for only 57 days. Phosphorus load to the wetland via internal modes was estimated to be 2.3 and 4.0 gday −1 from diffusion and advection respectively, accounting for 18% of the total P input, while overland flow (51%) was the major input pathway to the wetland. Ditch flow and groundwater outflow accounted for 49% (18.0 gday −1 ) and 14% (5.0 g day −1 ) of the total P output, respectively. This study shows the importance of advective flux in addition to diffusive flux and that the former should not be neglected when estimating internal P load of transiently flooded isolated wetlands. The monthly P budget-based retention and release patterns were consistent with previous findings, showing that intermittent flooding and drying cycling significantly reduces the P retention capacity of a wetland.

Estimating internal P loading in a deep water reservoir of northern China using three different methods

“…Fick's law is based on the principle that concentration gradients trigger the exchange between two interfaces. Diffusive flux of SRP (soluble reactive phosphorus) between water and sediment was calculated according to Fick's first law of diffusion (Bhadha et al 2011;Graca et al 2006)…”

“…Various methods can be used to assess internal P loading, such as in situ flux measurements, sediment core incubations, mass balance methods, regression equation related to sediment P content (Nürnberg 1988), and the methods based on Fick's law, most of which were widely adopted in the previous studies (Lavery et al 2001;Bhadha et al 2011;Koriyama et al 2013). In situ flux chambers result in minimal disturbance to the sediment and overlying water (Burger et al 2007), and to some extent, is the optimal way to get realworld results about the internal flux; However, it is also the most costly way to assess internal loading from sediment.…”

Phosphorus retention of forested and emergent marsh depressional wetlands in differing land uses in Florida, USA

“…However, because of P concentration gradients internal to GIWs, they could serve as P sources, rather than sinks, depending on soil characteristics, hydroperiod and P concentration in inflowing waters, amongst other variables (Hoffmann et al 2009). For instance, Bhadha et al (2011) reported that internal loading (i.e., P in the soils within the GIW) accounted for 18 % of the P ''entering'' two studied historically isolated wetlands in southern Florida; a shallow ditch draining these wetlands, which eventually connected to Lake Okeechobee, accounted for 49 % of the P outflow, thus creating a potential P source. Bhadha et al (2011) also found infiltration to the ground accounted for 14 % of the P loss from the GIW, suggesting that near-surface flow gradients are important to landscape-level P dynamics.…”

“…For instance, Bhadha et al (2011) reported that internal loading (i.e., P in the soils within the GIW) accounted for 18 % of the P ''entering'' two studied historically isolated wetlands in southern Florida; a shallow ditch draining these wetlands, which eventually connected to Lake Okeechobee, accounted for 49 % of the P outflow, thus creating a potential P source. Bhadha et al (2011) also found infiltration to the ground accounted for 14 % of the P loss from the GIW, suggesting that near-surface flow gradients are important to landscape-level P dynamics. P retention is a dynamic process, and these processes can be characterized through the calculation of multiple metrics, including the equilibrium P concentration (EPC 0 ), P buffering capacity (PBC), and the maximum P sorption capacity (S max ).…”

“…The EPC 0 is a measure of the ''P in solution that is in equilibrium with P in the solid phase'' (Belmont et al 2009, p. 988). Water entering a GIW with P in solution that is below the EPC 0 will cause the release of P bound to wetland soils (see Bhadha et al 2011), whereas water with P in solution[EPC 0 will be sorbed onto available sorption sites until a capacity is reached (Reddy and DeLaune 2008). The slope of the EPC 0 line is described as the P buffering capacity (Rayment and Lyons 2011), a measure of the ability of a soil to effectively sorb additional P. Loading of P into systems may decrease the ability of the wetland to sorb P, resulting in an increase in the EPC 0 .…”