Many seaweeds, like brown intertidal “rockweed,” provide important ecosystem services, such as habitat for many organisms, including juvenile oysters (see “Oysters”). However, eutrophication (i.e., excess nutrients loaded into our estuaries) can spur growth of nuisance seaweeds (usually red and green seaweeds) that in turn can block light and smother eelgrass. Too much of these unwanted seaweeds, like red Gracilaria or green Ulva, can also impact other desirable habitats such as oyster reefs. Because there is little or no eelgrass in the Hampton-Seabrook Estuary, seaweed is not actively monitored.

No increasing trends for nuisance seaweeds.

Intertidal 

In a healthy estuary, seaweeds are in balance with other estuarine life and are not a cause for concern. For example, attached brown seaweeds, like the image at the bottom, form important habitat and support overall productivity in the Great Bay Estuary. However, international shipping has allowed seaweeds to hitchhike from distant coasts to our estuaries and proliferate. In the Great Bay, south of Adams Point, we are seeing high abundances of these seaweeds competing with eelgrass.

Overall, data from eight stations in the Great Bay Estuary (Figure 10.1) suggest that red and green seaweeds expanded between 1980 and 2013 and since then, have decreased. The average cover of red and green seaweeds combined was generally less than 20% at Little Bay and Piscataqua River sites (Figure 10.2), and about twice as abundant in the Great Bay (Figure 10.2).

Nuisance red Gracilaria and green Ulva species composed nearly all the cover in intertidal areas. Both species can survive as drift or attached seaweed. Over the seven-year study period, only Depot Road and Adams Point had more than four years of data available for statistical analysis. Both red and green seaweeds declined significantly at the Depot Road site along the southern shore of Great Bay, but only greens were found to decline at Adams Point on the north shore of the Bay (Figure 10.3).

Subtidal

The intertidal transects established earlier in the Great Bay Estuary were extended in 2018 into the subtidal regions to examine seaweed beds and directly assess eelgrass-seaweed competition (Figure 10.4). The attached browns that dominated some intertidal areas were mostly absent from subtidal sites. Despite high variability in the abundance of drift seaweeds from year-to-year, some trends are evident. The highest seaweed abundance (dominated by Gracilaria and Ulva) and lowest eelgrass abundance was at the Depot Road location, whereas eelgrass was most dominant at the Sunset Hill Farm site (lower East Great Bay) for all three years of sampling (Figure 10.4). Eelgrass cover was stable or increased at three of four sites.

Acknowledgments and Credits

David Burdick (UNH), with contributions from Gregg Moore (UNH), Art Mathieson (UNH), Andrew Payne (UNH), Chris Peter (GBNERR/NHFG), Lara Martin (UNH), Natalie White (UNH), Elizabeth Cianciola (UNH), Grant McKown (UNH), and a host of UNH students. We appreciate the assistance of volunteers organized by the Coastal Research Volunteers.

Coming soon!