Abstract
Nitrogen enrichment of coastal waters due to anthropogenic alterations in the watersheds has become a world-wide, pervasive environmental problem. Past work suggests that fringing marshes may be one of those solutions when nitrogen is delivered to receiving open coastal waters mainly through groundwater discharge; as the groundwater plume traverses the rhizosphere of the marsh, a significant fraction of the nitrogen carried by the groundwater can be taken up by plants and stored as plant biomass or denitrified and gassed out to the atmosphere, thereby reducing the quantity of nitrogen entering the receiving coastal waters. Experimental demonstration of this hypothesis, however, is missing, particularly for black needlerush (Juncus roemerianus) marshes. These marshes, which are abundant in the Gulf of Mexico, have suffered and continue to suffer substantial losses due to human development and, as a result, numerous efforts to restore them are under way.
The need to restore black needlerush marshes, along with their potential role as effective filters of groundwater nutrient delivery and the importance to demonstrate such a role, provide an excellent opportunity well within the priorities of MASGC. Namely, we present two new designs of black needlerush restoration, and we intend to demonstrate that restored marshes following those designs are effective filters of nitrogen inputs into open coastal waters via groundwater. We also intend to characterize the processes responsible for nutrient filtration in the marsh rhizosphere (i.e. plant uptake, denitrification, incorporation into bulk sediment organic matter, and ammonification) to improve our understanding of how that filtration occurs and implement potential applications to environmental management needs.
To do that, we propose a series of isotope (15NO3) enrichment experiments where the accumulation of the isotope into the different targeted pools will be assessed as groundwater travels through the rhizosphere of the restored marsh.
To scientists, these results will reveal for the first time whether black needlerush marshes can indeed effectively filter groundwater nitrogen inputs and, if so, the processes responsible for that filtration. To managers, if we are successful in demonstrating our hypotheses, these results will help develop environmentally-friendly tools to palliate the intensity of anthropogenic eutrophication in coastal waters where that eutrophication mostly occurs through nitrogen-rich groundwater delivery. Furthermore, since one of our restoration designs is tailored to channel overland runoff into groundwater flowing through the restored marsh, our results may also prove applicable to reduce eutrophication in coastal waters where nutrient inputs through overland runoff are prominent.
To reach out efficiently to interested end-users and managers, we have formed a coalition of different environmental agencies (NEERs, Mobile Bay NEP and CPC at DISL) that will be instrumental in disseminating our results to those parties. Finally, as outreach education activities, we will first, through customary routes at the two NEERs involved, team up with local high-school students to plant the restored black needlerush marshes and, second, we will use our research to implement existing environmental courses for high school teachers at DISL.
Objectives
The main objectives of the project are:
- to plant the marsh restoration plots and start follow-up plant measurements
- to evaluate the cost-yield ratio and applicability of our restoration designs.
Some preliminary experiments will be done to determine subsurface groundwater flow rates. The isotopes experiments in the unrestored
plots will be done.
Methodology
We will recruit high school students and volunteers via the education coordinators at Weeks Bay and Grand Bay NERRs to plant the restoration plots. Follow-up plant measurements will include shoot density, shoot size and growth, and the dynamics of shoot recruitment and mortality, which will be done following customary procedures. To do the isotope enrichment experiment, we will spike a 15NO3-labeled groundwater at the upland plot edge and sample porewater, sediment organic matter and plant tissue at different locations and depths across the plot. The different 15N signatures accumulated in the pools examined will allow us to calculate how much of the spiked isotope is attenuated in the marsh through denitrification, ammonification, plant uptake and absorption by organic matter.
Rationale
Black needlerush restoration is becoming prominent in the Gulf of Mexico. However, the environmental benefits that restored black needlerush marshes bring about are hardly assessed. In particular, although much supporting evidence exists, we do not know if restored black needlerush marshes can act as effective filters of anthropogenic eutrophication via groundwater delivery. The information generated with this proposal will be novel and valuable for the scientific and management communities because it will (1) reveal for the first time the processes that sequester incoming nitrogen via groundwater in restored black needlerush marshes, (2) document how black needlerush marsh restoration can be used to palliate anthropogenic eutrophication of coastal waters by reducing groundwater nitrogen delivery into the receiving coastal waters, and (3) provide an example of efficient management towards environmentally-sustainable development by using black needlerush marsh restoration to reduce anthropogenic eutrophication of coastal waters.