Mangrove swamps are one of the most threatened ecosystems on the planet, having experienced a decline in area of 2% per year globally for several decades (Duarte et al., 2005). Recent estimates of the age and quantity of biomass stored in marine vegetation indicates that the loss of this habitat represents a significant loss of plant biomass in favor of atmospheric CO2. In the 1970’s and 80’s the Sahel drought in Africa produced massive alterations to river ecosystems in West Africa, including widespread mangrove forest mortality, in Guinea-Bissau, Gambia and Senegal (Conchedda et al., 2008).
Beginning in 2006, Oceanium Dakar, a Senegalese NGO began a mangrove recovery effort in the Casamance River mangrove estuary (16.4°N, 12.6°W). Their approach has been to mobilize the population in Casamance to plant mangrove seeds using a massive public service campaign that leverages both West African popular culture and the governing structure of individual villages to motivate replanting and stewardship. The Casamance Mangrove Replanting (CMR) movement is both unique and innovative because it exists as a homegrownsocietal response to climate change in West Africa: in 2009 35 million mangrove seeds were planted by over 2000 villages, and in 2010-11 Oceanium planted 65 million seeds. At this time, replanting effort is at a crossroads; the movement is trying to demonstrate that mangrove forests can be managed and restored, despite having experienced variable success in mangrove recovery in different villages along the banks of the Casamance River.
The focus of this is to initiate a program of environmental diagnostics and applied science in support of the CMR effort. Seasonal and spatial variability in the physico-chemical conditions of the estuary make mangrove recovery difficult. This project will work to overcome these limitations by mapping the distribution of ecosystem favorability for mangroves and by developing low-energy solutions for porewater remediation that can improve colonization rate of planted mangroves within the tidal delta of the Casamance River.
This project will proceed through two stages: diagnosis and monitoring of salinity, temperature and tidal wave penetration to map the quality of mangrove habitat. Next, porewater remediation methods will be developed around the available water resources so that hypersaline tidal deltas can be more rapidly restored to a lower salinity state.
Maps of ‘mangrove recovery favorability’. For the mangrove replanting effort it is critical to understand the variability of temperature and salinity. The regions with lower temperature and salinity are most likely to respond well to mangrove replanting efforts. Often these regions coincide with tidal deltas that receive two floods each day, which does not always coincide with the furthest upstream sections of the river. We have developed a design for a durable, low-cost probe that can be driven into the mangrove sediments and used to make time series measurements of porewater temperature, salinity and water depth in a mangrove tidal flat – the Casamance Realtime Estuary and Ocean (CREO) logger. To ensure complete data recovery, the CREO probes will be outfitted with a GSM telemetry modem and data will be relayed via text message over the cellular network on daily basis. This service is available with a subscription to the Senegalese Telecom, Sonatel ($360/yr). Initially a single prototype will be fabricated and deployed to capture the 28-day tidal cycle in different replanting sectors. GPS will be used to record the positions of each CREO logger. Maps of the mean seasonal salinity and temperature will be produced together with measurements of the tidal elevation and the number of flood tides received during a lunar cycle. These maps will be collated into an index of ‘mangrove recovery favorability’ that Oceanium can use to focus the replanting effort. The CREO logger is designed to be low-cost and to transmit data autonomously to minimize the impact of loss or damage of a single instrument. The longer-term goal of the CREO logger program will be to build a stock of ten or more instruments that will be deployed, recovered and maintained as monitoring tools by Oceanium in Senegal and its partners at the University of Ziguinchor, Senegal.
Solutions for porewater remediation. Mangroves exert a self-sustaining feedback on their surroundings. Many species, including Avicennia marina and Avicennia africana secrete salt through glands in the foliar structure of the plant onto the leaf surface (Drennan and Pammenter, 1982) and can become aerosolized as wind carries the salt crystals aloft. Mangroves further condition their surroundings by minimizing seawater evaporation through shade cover. Soil salinity exerts a direct feedback on the rate and number of seeds that germinate (Ye et al., 2005). Therefore porewater remediation may be necessary in some areas to restore salinity to a habitable level for more effective seed germination and growth. In Casamance, porewater remediation can occur using two sources: Semidiurnal flushing by 35 ppt seawater, and seasonal flushing by freshwater during the wet season.
Porewater residence time. The most effective means of flushing will depend on the time required to flush porewater from the sediments, which depends on sediment permeability. If porewater is renewed on a timescale of weeks or shorter, the effect of seasonal flooding with freshwater will be diminished as a result of tidal flooding with seawater. If the contrary is true, that the porewater flushing timescale exceeds one month, that argues in favor of remediation solutions that capture and percolate freshwater into the porewater environment. To determine the porewater flushing timescale, we will use Rhodamine dye as a tracer to measure the porewater renewal rate through a tidal delta. Dye will be added to a volume of ambient porewater that has been pumped from the region of interest. This water volume will be pumped to a depth of 10 cm below the mean low water tidal level (ca. 50 to 120 cm below the land surface). These tracers will be monitored by penetrating shallow porous tubes in a linear array that follows the mean hydraulic gradient. Water depth and water properties will be measured from these shallow wells by withdrawing samples using a tube and submersible pump. The time rate of change of dye tracer in the wells can be directly related to the permeability of the sediments and thereby to the flushing timescale. This experiment will be conducted once in the dry season and again in the wet season. Additional funds from the PI’s startup will be used to purchase Rhodamine dye and YSI in-situ fluorescence sensor.