This blog post was written by Sierra Keyhoe as part of a project update newsletter about the Eelgrass Resilience Project, a collaborative research project involving partners from UNH, PREP and GBNERR.

Eelgrass is a cornerstone species of healthy estuaries including our Great Bay Estuary, and the status of eelgrass populations gives us insight into the system’s overall health. In 2021, the Piscataqua Region Estuaries Partnership, in collaboration with municipalities and partners such as GBNERR, launched an expanded monitoring program – the Eelgrass and Seaweed “Tier 2” Monitoring Program – to track long trends for this important habitat. Data from this monitoring data are an important component of the Eelgrass Resilience Project.

Why do we care about this aquatic grass? 

Eelgrass provides numerous water quality and resilience functions like stabilizing sediments, storing carbon, reducing wave energy, and providing habitat for numerous species including fish, crabs and lobsters! In a nutshell, the happier and healthier the eelgrass, the happier and healthier the estuary. Unfortunately, eelgrass abundance has declined significantly worldwide, and here in the Piscataqua Region, we have lost over half of our eelgrass acreage over the last few decades as a result of stressors including poor water quality and light conditions (Burdick et al. 2020).

How do we track trends?

The “Tier 2” monitoring program, piloted in 2021, visits 25 sites within the Great Bay Estuary. By visiting a spatially diverse set of sites, researchers are able to get an estuary-wide snapshot of eelgrass health each year. Divers collect eelgrass samples for analysis and take measurements such as biomass and density. Over 600 eelgrass shoots were collected for analysis between the two field seasons! 

In addition to collecting eelgrass for analysis, seaweed, sediment, and water samples are also collected from the sites. These water samples are the ones analyzed in the Eelgrass Resilience Project. Since the water quality samples are taken at the same location of the eelgrass that is analyzed, we can start to see how these two indicators are related. 

Back in the lab, scientists analyze these samples to calculate biomass and measure epiphytes (the stuff that grows or lives on the leaves of eelgrass). These metrics are all important indicators that give insight into the health of the eelgrass, and therefore, the health of the estuary!

The presence of some seaweeds can harm eelgrass growth, as they can be in competition with each other. Many seaweeds can prevent eelgrass plants from getting the light they need to thrive. Other seaweeds – like this massive piece of the brown seaweed that was brought in – don’t compete with eelgrass and perform important ecosystem functions.

What patterns do we see across the estuary?

Initial results reveal that eelgrass meadows look pretty different in different parts of the Great Bay  Estuary.  The biomass of eelgrass is higher at the coast by Portsmouth Harbor (180 g/m2) than it is by Dover Point (60.2 g/m2) and in Great Bay Proper (by Adams Point and the Great Bay Discovery Center, 28.7 g/m2) based on 2021 data.

In order to tell a more complete story, the Tier 2 monitoring program measures a number of different attributes about an eelgrass meadow, including the height of the canopy, the density of the shoots, percent cover, and the overall biomass of the plants. For example, while there is a large difference in biomass between Great Bay Proper and Dover Point (28.7 g/m2 vs. 60.2 g/m2), the percent coverage (44% vs. 38%) and bed density (77 plants/m2 vs. 73 plants/m2) are relatively similar. The factor that makes the biggest difference between sites is the canopy height; the plants in Great Bay Proper average at 66cm while the plants by Dover Point average at 98cm, almost a meter high! The taller stature may be due to the deeper water in the Piscataqua and the need for the plant to grow higher to reach light.

How is eelgrass changing over time?

While we know eelgrass populations in Great Bay are much lower than they were 20 years ago, this monitoring program will allow us to more carefully track trends over time. 

Although not all the data from 2022 have been analyzed yet, we are seeing some differences between the last two years. For example, we saw a 14% decrease in cover, a 44% decrease in canopy height and a 15% increase in the density of eelgrass shoots when looking across the eelgrass meadows sampled (including Great Bay, Dover Point, and Portsmouth Harbor). Although it’s hard to conclusively explain observed changes, we know there was record historic rainfall in July 2021, after that year’s Tier 2 sampling. Heavy rainfall can negatively impact eelgrass by delivering excess nutrients and sediments to the bay. It looks like the eelgrass meadows are still recovering in 2022, with many new shoots, increasing plant density relative to 2021, but the plants are smaller overall.

What’s next

The Eelgrass Resilience Project will be looking for relationships between environmental data and eelgrass populations to try to better understand what’s driving the differences we see across sites and time. 

Exploring these recent patterns in Great Bay’s eelgrass meadows highlights the importance of having robust data – including a number of different measurements collected year and year along with other environmental data – so we can carefully track what’s happening in this important habitat. Stay tuned for more eelgrass updates!

Want to learn more?

For more information about this 3-year collaborative research project or to read prior Project Updates, visit our project web page or reach out to a member of our Advisory Committee (see: List), which includes representatives from the municipalities and agencies that help protect Great Bay’s waters. Sign up here to receive future project updates via email.

Article contributed by Sierra Kehoe, Piscataqua Region Estuaries Partnership

For more information, contact: Lynn Vaccaro, Coastal Training Program Coordinator,