Peat accretion and phosphorus accumulation along a eutrophication gradient in the northern Everglades

Recent rates of peat accretion (as determined by¹³⁷Cs) and N, P, organic C, Ca and Na accumulation were measured along a 10 km eutrophication gradient in the northern Everglades area of Water Conservation Area 2A (WCA 2A) that has received agricultural drainage from the Hillsboro canal for the past 25–30 yrs. Rates of peat accretion were highest at sampling locations closest to the Hillsboro canal, 1.6 km downstream, (5.67 ± 0.50 mm/yr) and decreased to 2.01 ± 0.31 mm/yr at distances of 7.1 to 10.7 km downstream. Phosphorus and Na accumulation were a function of both peat accretion and soil P and Na concentrations. The concentration and accumulation of P in peat deposited in the past 26 years was highest near the Hillsboro canal (1478 ± 67 ug/g, 0.66 ± 0.06 g/m²/yr) and decreased to 560 ± 20 ug/g and 0.10 ± 0.02 g/m²/yr at distances of 8.8 to 10.7 km downstream. Like phosphorus, the concentration and rate of Na accumulation was highest near the Hillsboro canal (3205 ± 1021 ug/g, 1.48 ± 0.53 g/m²/yr). Although sodium enrichment of the peat was limited to 1.6 km downstream of the Hillsboro canal, increased rates of Na accumulation penetrated 5.2 km downstream of the Hillsboro canal, the extent of the area of enhanced peat accretion. In contrast to P and Na, there was no difference in the concentration of soil organic C, N and Ca along the eutrophication gradient. However, there was a gradient of organic C, N and Ca accumulation corresponding to the area of enhanced peat accretion. The highest rates occurred 1.6 km south of the Hillsboro canal (212 ± 5 g organic C/m²/yr, 14.1 ± 0.4 g N/m²/yr, 22.1 ± 5.2 g Ca/m²/yr). Accumulation of organic C, N and Ca at distances of 7.1–10.7 km downstream averaged 87 ± 11, 6.3 ± 0.7 and 6.5 ± 0.9 g/m²/yr, respectively. The areal extent of enhanced peat accretion and organic C, N, Ca and Na accumulation encompasses approximately 7700 ha of the northern part of WCA 2A. The area of enhanced P accumulation is larger, covering 11,500 ha or 26% of the total area of WCA 2A. The 11,500 ha area has functioned as a sink for P for the past 25–30 yr removing 74% (49.3 MT/yr) of the 67 MT/yr that enters via agricultural drainage and rainfall. Moreover, P accumulation along the gradient was related to mean (1989–1990) surface water P concentration, decreasing as surface water P decreases. These findings suggest that P accumulation is dependent on the P concentration in the water column and that decreasing P loadings per unit area result in less P storage per unit area. The potential longterm equilibrium of the 11,500 ha area as a sink for P is based on a mean annual loading of 67 metric tons P/yr. Input rates exceeding this loading rate could result in an expansion of the 11,500 ha area until a new equilibrium size is reached.