Seaweeds

How has the amount of seaweed in the Great Bay Estuary changed over time?

At intertidal sampling sites, green and red seaweeds (combined) increased from approximately 8% cover in 1980 to 19% cover in 2016. At these same sites, invasive species now dominate the red seaweed category, which comprised approximately 15% of all seaweeds in 2016.


Seaweeds are an important and critical group of estuarine primary producers, but many of the factors affecting estuaries globally (e.g., climate change, sedimentation, nutrient pollution) also accelerate the growth of some seaweeds.33, 34 In these situations, seaweeds can grow so abundant that they shade eelgrass. Since they can “bloom”—that is, grow and die very quickly—they can also negatively impact sediment conditions by decomposing on the estuary floor.35 This can negatively impact shellfish and benthic invertebrates as well as eelgrass.
No increasing trends for seaweeds.
Great Bay Estuary seaweeds can be categorized as brown, green, and red. This indicator focuses on changes in the red and green seaweeds which are much more abundant in the subtidal areas (those areas always covered by water) and are more likely to compete with eelgrass. However, there are only a few data points in the subtidal areas of the Great Bay Estuary that allow for assessment of changes in the abundance of these seaweeds where impacts on eelgrass could also be assessed (Figure 6.2).

The mean percent cover of green and red seaweeds (combined) at a limited number of sampling sites in the Great Bay Estuary was 8% in 1980 but increased to 19% by 2016 (Figure 6.1). For green seaweeds, this increase includes the presence of both native and invasive species of Ulva. It is notable that no invasive species of Gracilaria (a red seaweed) were seen in 1980, but now two major invasive Asiatic red seaweeds (Gracilaria vermiculophylla and Dasysiphonia japonica) along with a native species (Gracilaria tikvahiae) dominate the red seaweeds.36

While the seaweed data are cause for concern, it is important to note that this dataset is not comprehensive in time and space; more research is required to verify these trends. In addition, these data are restricted to intertidal areas. While important steps to establish a baseline in the subtidal area have occurred, this work needs to be followed up by additional monitoring to better assess trends.

Figure 6.1 Percent cover of red and green seaweed at selected intertidal sites in the Great Bay Estuary.

Figure 6.2 Locations of the eight intertidal seaweed monitoring sites are designated by the black circles. Green areas indicate mapped eelgrass habitat from 2016.