Assessing Environmental Stress in Mature Mangrove Stands: Linkages to Nutrient Loading

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WARC Researchers are comparing tree and root growth, soil CO2 flux, and surface elevation change between fertilized and unfertilized mangrove forests to assess the potential impact of increased nutrient loading and to help rate mangrove stand vulnerability.

leaf photosynthetic water use efficiency

Measuring leaf photosynthetic water use efficiency in fertilized mangrove forests

The Science Issue and Relevance: Mangroves occur in tropical, sub-tropical, and warm temperate climates, and take up large areas of U.S. Department of Interior lands in South Florida, USA, where they are mostly managed passively (through protection). Mangroves are habitat for wildlife and provide ecosystem services, but are subject to a number of threats, including nutrient loading and hydrological alterations that compromise their capacity to adjust to rising seas and recover from major hurricane events. At J.N. “Ding” Darling National Wildlife Refuge (NWR) on Sanibel Island, Florida, managers are concerned that the refuge’s mangroves will be affected by increased nitrogen (N) and phosphorus (P) loading in the nearby Caloosahatchee River. Among the concerns are the impact this may have on habitat persistence and whether the mangroves will continue to provide ecosystem services to residents and tourists (e.g., carbon storage, atmospheric CO2 uptake, water conservation, recreational fisheries, sediment retention). Mangrove persistence is often a subtle balance between environmental stress and their natural capacity to respond to change.

Rhizophora stylosa seedling

Lone Rhizophora seedling

 

Methodologies for Addressing the Issue: To assess the potential impact of increased nutrient loading on mangrove forests in Ding Darling NWR, multiple mangrove forests have been fertilized with N and P and will be compared to unfertilized forests. Fertilizer concentrations were established using current river nutrient data and projected increases in nutrient concentrations. Comparing tree and root growth, soil CO2 flux, and surface elevation change will help rate mangrove stand vulnerability. Additionally, sap flow and leaf gas exchange will be used to identify whole-tree physiological processes that may be causing differences among stand nutrient exposures. Furthermore, data are being scaled to stand water use, and water use efficiency values are being used to convert to net ecosystem uptake of CO2 among forests as a measure of environmental stress from nutrient amendment.

Future Steps: To assess mangrove stand water use and net stand uptake of CO2 across basin mangrove forests at Ding Darling NWR. We will also use ecosystem nutrient loading assays to model and project sea-level rise vulnerability using the Wetland Accretion Rate Model for Ecosystem Resilience (WARMER) platform (with Drs. Karen M. Thorne and Kevin J. Buffington, USGS). We will also expand modeling and vulnerability assessment to mangrove island environments of Ding Darling NWR and Ten Thousand Islands NWR, both of which are found within the U.S. Fish and Wildlife Service’s Southwest Florida Refuge Complex.