Estimating the Effects of a Hurricane on Carbon Storage in Mangrove Wetlands in Southwest Florida.

Tropical and subtropical mangrove swamps, under normal conditions, can sequester large amounts of carbon in their soils but as coastal wetlands, they are prone to hurricane disturbances. This study adds to the understanding of carbon storage capabilities of mangrove wetlands and explores how these capacities might change within the scope of a changing storm climate. In September 2017, Naples Bay, FL, USA (28°5' N, 81°47' W) encountered a direct hit from hurricane Irma, a Saffir-Simpson category 3 storm. By comparing carbon storage, forest community structure, and aboveground productivity collected in 2013 and in 2019, we estimated the effects of hurricane Irma on mangrove functions. Aboveground biomass increased during the study period at a rate of approximately 0.72 kg m-2 yr-1, significantly less than the average found in undisturbed mangrove forests. Soil carbon storage decreased at all study sites. On average, 2.7 kg-C m-2 was lost in the top 20 cm between sample collections. Carbon loss in belowground pools could point to a feedback of mangrove swamps on climate change as they lose their ability to store carbon and increase net atmospheric carbon. Nevertheless, mangrove swamps remain resilient to tropical storms in the long term and can recover their carbon storage capacity in the years following a storm.

Above- and Below-Ground Carbon Storage of Hydrologically Altered Mangrove Wetlands in Puerto Rico after a Hurricane.

Mangrove wetlands are important ecosystems, yet human development coupled with climate change threatens mangroves and their large carbon stores. This study seeks to understand the soil carbon dynamics in hydrologically altered mangrove swamps by studying aboveground biomass estimates and belowground soil carbon concentrations in mangrove swamps with high, medium, and low levels of disturbance in Cataño, Jobos Bay, and Vieques, Puerto Rico. All three sites were affected by hurricane María in 2017, one year prior to the study. As a result of being hit by the Saffir-Simpson category 4 hurricane, the low-disturbance site had almost no living mangroves left during sampling. There was no correlation between level of hydrologic alteration and carbon storage, rather different patterns emerged for each of the three sites. At the highly disturbed location, belowground carbon mass averaged 0.048 ± 0.001 g-C cm-3 which increased with increased aboveground biomass. At the moderately disturbed location, belowground carbon mass averaged 0.047 ± 0.003 g-C cm-3 and corresponded to distance from open water. At the low-disturbed location, organic carbon was consistent between all sites and inorganic carbon concentrations controlled total carbon mass which averaged 0.048 ± 0.002 g-C cm-3. These results suggest that mangroves are adaptive and resilient and have the potential to retain their carbon storage capacities despite hydrologic alterations, but mass carbon storage within mangrove forests can be spatially variable in hydrologically altered conditions.

Nutrient retention via sedimentation in a created urban stormwater treatment wetland.

Nutrient removal by a 4.6-ha urban stormwater treatment wetland system in a 20-ha water/nature park in southwest Florida has been investigated for several years, suggesting that the wetlands are significant sinks of both phosphorus and nitrogen although with a slightly decreased total phosphorus retention in recent years. This study investigates the role of sedimentation on changes in nutrient concentrations and fluxes through these wetlands. Sedimentation bottles along with sediment nutrient analyses every six months allowed us to estimate gross sedimentation rates of 9.9 ± 0.1 cm yr-1 and nutrient sedimentation rates of approximately 7.8 g-P m-2 yr-1 and 81.7 g-N m-2 yr-1. Using a horizon marker method to account for lack of resuspension in the sedimentation bottles suggested that net nutrient retention by sedimentation may be closer to 1.5 g-P m-2 yr-1 and 33.2 g-N m-2 yr-1. Annual nutrient retention of the wetland system determined from water quality measurements at the inflow and outflow averaged 4.23 g-P m-2 yr-1 and 11.91 g-N m-2 yr-1, suggesting that sedimentation is a significant pathway for nutrient retention in these urban wetlands and that resuspension is playing a significant role in reintroducing nutrients, especially phosphorus, to the water column. These results also suggest that additional sources of nitrogen not in our current nutrient budgets may be affecting overall nutrient retention.