Monitoring seagrasses essential to managing coastal habitats

Seagrasses
Figure 1. Pictures of submerged aquatic vegetation, also known as seagrass, and a quadrant used to groundtruth the areas sampling for seagrass.

Seagrass requires good water quality to survive, so it is an excellent indicator of the overall health of an estuary. For this reason, the Division of Marine Fisheries and other agencies of the Department of Environmental Quality are monitoring seagrass in coastal North Carolina.

Submerged aquatic vegetation (seagrass or underwater grass), is a critical fish habitat for many important recreational and commercial species including spotted seatrout, red drum, bay scallops, blue crabs, and many other species. Additionally, this habitat improves water quality by cycling nutrients, stabilizing sediment, and releasing oxygen into the water. Seagrasses have also been shown to help reduce greenhouse gases such as carbon dioxide from the air, helping to offset the impacts of climate change and sea level rise. North Carolina has seagrasses in low salinity waters like Albemarle Sound and high salinity waters like Core Sound. These spatially distinct communities support different seagrass species and are adversely impacted by different sources. Sometimes referred to as the “canary in the coal mine”, seagrass requires good water quality and is therefore a good indicator of the overall condition of the estuary.

Traditionally, seagrass mapping is accomplished by taking aerial photos, and delineating (drawing polygons around the seagrass) habitat from the photos using computer software. Within a few weeks of the flights, staff travel by boat to numerous randomly selected sites to “groundtruth” the areas mapped. Groundtruthing involves taking samples from an area to confirm what the images from the air depict, including whether seagrass was present, density and species of seagrass, and environmental conditions.

As part of Coastal Habitat Protection Plan implementation, the N.C. Division of Marine Fisheries has participated in an interagency partnership to collaboratively map, monitor, and protect seagrass. The Department of Environmental Quality and the Albemarle-Pamlico National Estuary Partnership provided the majority of the funding for imagery acquisition. Division of Marine Fisheries, with staff and vessels located all along the coast, was well positioned to lead the groundtruthing effort. Staff from the division’s Habitat and Enhancement and Fishery Management sections, as well as staff with the Albemarle-Pamlico National Estuary Partnership, the National Oceanic and Atmospheric Administration, the U.S. Fish and Wildlife Service, and the N.C. Coastal Reserves and National Estuarine Research Reserve have assisted with collecting the groundtruthing data.

Coastwide mapping using aerial surveys was done from 2006 to 2008. In low salinity waters such as the Tar-Pamlico River and Albemarle Sound, seagrasses were visible in photos but underestimated due to the water clarity. Mapping of the high salinity seagrasses was repeated between 2012 and 2015 and in 2019. Based on the 2007-08 mapping, there was an estimated 138,100 acres of seagrass, with 101,670 acres in high salinity waters. Between 2012 to 2015, an estimated 96,900 acres of high salinity seagrass were mapped from Roanoke Sound through Mason’s Inlet. These acreages exclude Core Sound because of inadequate imagery. National Oceanic and Atmospheric Administration scientists are currently doing analysis to further assess trends between those two mapping periods. Preliminary findings indicate some decline of coverage in the high salinity waters.

To address underestimation of seagrass coverage from aerial photos in low salinity waters, an alternative method was developed by the partnership using acoustic sonar. Mapping has been done in the Tar-Pamlico and Neuse rivers and in the Albemarle Sound at least once since 2010 with a collaboration of partners and funding. While the exact amount of seagrass in these regions has not been quantified yet, sampling indicates there has been a significant decline in coverage, particularly in Albemarle Sound. The increase in frequency of extensive algal blooms in Albemarle Sound drastically reduce light penetration to the seafloor, making light conditions unsuitable for seagrass survival.

By understanding how seagrass distribution is changing over time, the division can protect these important habitats and better manage seagrass-dependent fishery species since changes in habitat will affect fish populations and improve the resiliency of the entire coastal ecosystem.