Nature-based Solution to Restore Nutrient Impaired Seagrass Meadows

Carl Persson/Ocean Solutions Inc

Land-based runoff of nutrients to freshwater, estuaries, and the ocean produces eutrophic conditions in these waters. Here we concentrate on estuaries and nearshore ocean environments where suitable habitat for seagrass meadows once existed. Over time na

Loss of seagrasses worldwide due to a combination of nutrient impairment and warming water produced by climate change is one third of all seagrasses and is increasing at an annual rate of 7 percent. We start by addressing the loss of Zostera marina, otherwise known as eelgrass, a globally important seagrass and one that is local. Loss of dissolved oxygen in the bottom water and upper sediment resulting from environmental stressors, mostly occurring during the summer, are the more specific causes of degradation and loss. It leads to lower rates of decomposition of organic matter thereby resulting in its accumulation on the sediment in both organic particulate and dissolved forms. As oxygen demand rises so does its consumption rendering biogeochemical processes inefficient, thereby increasing the level of reactive nitrogen that facilitates algal blooms. A healthy ecosystem stores phosphate in the sediment through adsorption of ferric oxides. A lack of sufficient oxygen in the top layer of sediment chemically reduces ferric to ferrous oxide releasing phosphate to the water as an additional nutrient stressor, encouraging algal blooms. The high levels of organic matter accumulation led to their being buried by the sediment. Decomposition under anaerobic conditions is slower than that under oxic conditions. It produces both methane and nitrogen in the form of ammonium that rises to the sediment surface through porewater. The level of oxygen penetration into the sediment is a measure of its health. Solution In both biological and biogeochemical terms oxygen or the lack thereof is likely the major factor for ecosystem health. Seagrass habitat is in shallow coastal waters with a depth of less than 10 meters with requisite salinity. It grows in inshore waters such as estuaries as well as the nearshore. This ecosystem is in protected coastal waters. Locations that are suitable for seagrass are also places of high organic matter accumulation. Prior attempts at using dissolved oxygen for restoration have failed do so because they are not able to supply enough oxygen to the sediment to meet its consumption. For the past 20 years most of these attempts have used aerators (20% oxygen, 80% nitrogen) that are not efficient in transferring oxygen into soluble form. About 10 years ago oxygen nanobubble technology was introduced to commercial markets. These products have now been scaled to enable scaled capacity that makes them of interest for our applications. Nanobubble generators have an oxygen transfer efficiency up to 85 percent as opposed to traditional aerators at about 2.8 or about 30 times as much. Organic matter accumulation is more intense inshore. It lessens with distance seaward. The ability to adequately supply dissolved oxygen to the sediment is a necessary condition for restoration success. In shallow coastal areas where seagrass meadows are found to solve the restoration problem, we need to do more than meet the supply criterion we must deliver dissolved oxygen into the sediment. We will use a combination of methods based on diffusion, advection, and velocity of current across and parallel to the surface of the sediment. Due to current ability to deliver oxygen through nanobubbles, the need to supply target areas, the ability to dispatch resources, and the capacity to speed the process while penetrating the surface sediment with oxygen we will Insall our system on a floating and moving vessel. While the vessel holds the process equipment, distribution can take on several forms depending upon the vegetation state of the sediment surface. For a bare surface, for example, distribution piping will be attached to a frame on wheels to ensure delivery at a height about 20 centimeters above the sediment surface. The use of oxygen is pervasive in a seagrass ecosystem. Assuming a nutrient impaired location where 100% of seagrass (eelgrass) have been lost here is a sampling of such uses: • Immediate to air breathing species of all kinds • Acceleration of organic matter mineralization • Acceleration of nitrogen removal from the sediment through coupled nitrification-denitrification, especially during summer • Some improvement in sediment reoxidation should positively help air breathing species that are members of the benthic community. For example, air breathing worms burrow the sediment, enabling the penetration of oxygen to go deeper into it. Faunal communities provide mixing to the sediment so that pockets of buried organic matter can be mineralized. • Loss of seagrass (eelgrass) in a nutrient impaired ecosystem is caused by the increase in sulfide concentration directly related to chronic and usually seasonal (summer) low oxygen concentrations in the bottom water. Our solution is based on loss of eelgrass due to nutrient enrichment and caused by the mechanism described. We intend to optimize the efficiency of our process in many ways. These include: • Oxygen penetration into the sediment is

Milton, Massachusetts, United States
Milton, Massachusetts, United States
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