Seaweed Monitoring in the Great Bay Estuary: 2019 Annual Report

As global warming increases temperature and nitrogen inputs change—either due to greater inputs associated with growing populations in the Great Bay or with nitrogen reductions at wastewater treatment plants—it is important to understand how these changes are impacting the estuary. To that end, the abundance and taxa of intertidal seaweeds have been assessed at fixed locations throughout the estuary since 2013. Seaweed abundance may be influenced by environmental conditions such as nutrient levels, water temperature, light availability, and invasive species. Therefore, seaweed communities can provide insights into the overall health of the estuary and signal ecological change. In 2019, abundance data (percent cover and biomass) were collected from five of the eight intertidal sampling locations and four subtidal locations. Two more sampling arrays were established at each subtidal site, making three replicates per site. Data from 2013-2019 show appreciable cover and biomass of nuisance seaweeds (reds and greens), including several introduced species. Green seaweeds decreased in cover at the two intertidal sites that are sampled annually (Depot Road and Adams Point), and cover of red seaweed decreased at one site (Depot Road). However, there were no decreases at the other six sites, and results from 2019 still show high levels of nuisance seaweed at the lowest intertidal elevations. In subtidal areas, percent cover assessments by snorkel appeared successful based on strong correlations between cover and biomass. Percent cover of seagrass measured by snorkel was very similar to independent measurements from underwater photos. The abundance of seaweed in association with eelgrass beds was ecologically significant and may have impacted eelgrass density and productivity. Further monitoring of seaweed and eelgrass is required to determine potential impacts to the estuary from emerging threats of increased nutrients from impervious surfaces and rising water temperatures due to global warming, as well as reduced nutrient threats from improvements to wastewater treatment plants and stormwater management. For example, the 2019 eelgrass survey showed an increase in area of eelgrass beds within Great and Little Bays which co-occurred with declines in nuisance seaweed at two of our stations in Great Bay