Protecting the Florida Everglades wetlands with wetlands: Can stormwater phosphorus be reduced to oligotrophic conditions?

摘要

The Florida Everglades is being threatened by high-nutrient stormwater coming from agricultural runoff. The main nutrient problem is phosphorus, which causes the highly oligotrophic sawgrass (Cladium jamaicense) communities in the northern Everglades to become eutrophic Typha latifolia/T. domingensis communities. Current government directives require that the total phosphorus concentration of storm water drainage into the Everglades be limited to approximately 10 ppb (mu g-P/L). Over 23,000 ha treatment wetlands, referred to locally as stormwater treatment areas (STAs), have been created from farmland to treat the stormwater. They are generally effective in removing 60-80% of the total phosphorus; however, the 10 ppb goal has rarely been achieved. A three-year experiment, involving mesocosms planted with Everglades-native wetland plants was conducted in the Florida Everglades from March 2010 to March 2013. Eighteen flow-through mesocosms (6 m x 1 m x 1 m with 40-cm water depth) received about 2.6 cm/day inflow. The eighteen mesocosms were randomly assigned with six different plant communities with three replicates of each treatment, consisting of sawgrass (C. jamaicense); waterlily (Nymphaea odorata); cattail (Typha domingensis); submerged aquatic vegetation (SAV) including Najas guadalupensis, and Chara sp. and a Nymphaea-Eleocharis sp. mixed community; and soil without vegetation as a control. Total phosphorus (TP) in the inflow water was 25 +/- 1 mu g-P/L (n = 55) over the 3 years. Through 2012 the average outflow of all of the treatments was 34 +/- 1 mu g-P/L, a 51% decrease from the average outflow of 69 +/- 6 mu g-P/L for 2011. Outflows began to be routinely lower than the inflow in the 3rd year of the study. The average total phosphorus concentration decreased overall to 19 +/- 1 (n = 5) at the end of the study in 2013 suggesting that the suspected phosphorus reflux from the mesocosm soils into the water column slowed after 2-2.5 years of mesocosm operation. Comparing outflows of the individual treatments for 2013, the Nymphaea, control/Chara, and Typha treatments were lower (p < 0.05) than the inflow with average outflow concentration of 11 +/- 1, 15 +/- 3 and 16 +/- 1 mu g-P/L respectively. When the 2013 data are isolated, 4 of the 6 vegetation treatments showed total phosphorus removal, ranging from mixed community (17% removal), to Typha (28% removal), to the control (34% removal) and to Nymphaea (51% removal). We conclude that any treatment wetland constructed with local Florida soils and designed to achieve low (similar to 10-15 ppb P) concentrations would probably take a minimum of 2 years to become sinks of phosphorus. We also conclude that wetlands can be created to achieve these low thresholds if low TP loading and self-design strategies are incorporated into the project design. (C) 2014 Elsevier B.V. All rights reserved.