Biogeochemical Cycling of Nutrients and Carbon in Subtropical Wetlands

As human development intensifies, ecosystems around the word are being exponentially destroyed and degraded. Wetlands have the capacity to mitigate some of the possible problems by retaining nutrients and carbon, keeping them from harming downstream ecosystems or being released into the atmosphere. This project focuses on the processes that make wetlands successful by studying two unique ecosystems: 1) a created urban stormwater treatment wetland and 2) mangrove wetlands in Florida and Puerto Rico that were affected by hurricanes in 2017. The first phase of this study investigates the role of sedimentation and vegetative and algal uptake of nutrients to retain nutrients within a 4.6-ha urban stormwater treatment wetland system in southwest Florida. Several years of studies suggest that the wetlands are significant sinks of both phosphorus and nitrogen, but total phosphorus retention efficiency might be decreasing with the age of the wetland. A combination of horizon markers and sedimentation traps used within the wetlands for 2.5 years suggested that net nutrient retention by sedimentation was 1.5 g-P m-2 yr-1 and 33.2 g-N m-2 yr-1 as well as show that resuspension of sediments is approximately 68% of all sediments that fall out of the water column. In the same wetlands, vegetative and algal uptake of nutrients are analyzed by using a combination of dawn-dusk-dawn dissolved oxygen measurements, aboveground biomass, aerial photography, and nutrient concentration data. Aboveground peak biomass was used to estimate macrophytic net primary productivity and nutrient concentrations within the plant tissue which in turn suggested approximately 0.11 g-N m-2 yr-1 and 0.09 g-P m-2 yr-2 being removed by emergent vegetation. Water column primary productivity accounted for a much larger flux of nutrients with approximately 52.4 g-N m-2 yr-1 and 3.1 g-P m-2 yr-1 retained in algal communities. Management suggestions are provided to help improve water quality by identifying vegetative species that are most effective at retaining nutrients. The results of both the sedimentation and vegetation studies suggest that there are large additional inputs of nitrogen that are not being accounted for from pumped inflow. The second phase of the study analyzes the role that anthropogenic and non-anthropogenic disturbances have on the carbon storage capabilities of mangrove wetlands. A study in Naples Bay, Florida analyzed pre- and post-hurricane soil cores and aboveground biomass to determine how carbon storage changed within the soils as a result of hurricane Irma (2017). During the study period, aboveground biomass increased at a rate of 0.72 kg m-2 yr-1 whereas approximately 2.7 kg-C m-2 was lost in the top 20 cm of the soil during the same 5.5 year period. Possible causes of carbon loss are explored. The increase of aboveground biomass points to the resiliency of mangrove swamps to tropical storms, but they could be threatened if tropical storm frequency and intensity increases. Additionally, a study was carried out in three mangrove wetlands in Puerto Rico after hurricane María (2017) to show how alterations of hydrology to each system effected overall carbon storage within the soils. Three sites were selected with high, medium, and low levels of historical hydrologic alterations. Belowground soil carbon is analyzed and shows that carbon storage is similar at each site despite hydrologic alteration. The highly-disturbed location averages 0.33± 0.02 g-C cm-1, moderate-disturbed location averages 0.33 ± 0.01 g-C cm-1, and the low-disturbed location averages 0.34 ± 0.02 g-C cm-1. The results of this study show that mangroves are adaptive and resilient and can, on average, retain their carbon storage capacities despite hydrologic alterations. Overall, this dissertation shows that wetlands are resilient and adaptive to disturbance, but the efficiency of their ecosystem services might be affected. In engineered stormwater treatment wetlands, the overall efficiency of nutrient retention may be decresased because of management strategies such as shortened hydroperiods and wet season conditions due to pumping and water storage problems, removal of “undesirable” but high nutrient vegetation species, and increased water flow leading to high rates of resuspension. Despite the lowered efficiency of the wetlands to retain nutrients, the studied wetlands are still resilient to storm-events and anthropogenic alterations and have adapted over the 10 years since construction to the conditions and continue to retain the capacity to retain nutrients. Additionally, the mangroves studied demonstrate the resiliency of these wetlands despite a decrease in the efficiency of belowground carbon storage. Although belowground carbon storage was negatively affected by hurricane and hydrologic disturbances in mangroves, the studied wetlands were shown to be resilient to hurricanes and hydrologic disturbance, retaining overall carbon storage functions and regrowing aboveground biomass after defoliation because these ecosystems have adapted to these disturbances over many years. Wetlands that are currently healthy should continue to be protected so that their overall ecosystem services are not affected, but wetlands that undergo disturbance may still have the ability to retain core functions and adapt to these changes