Seaweed Accumulation on Maine’s Coastal Beaches

In the last two years, there’s been a rise in reports related to an increase in seaweed on Maine’s coastal beaches. Municipalities have been challenged and swamped with public comments and media attention related to the stench and overall aesthetics of the seaweed mounds that are affecting their beach experience. Last August, York’s beaches were “taken over” by seaweed and it took several weeks to clear out.

There are also water quality concerns. Detached seaweed cast onto the beach warms in the sun, allowing potentially unsafe bacteria to persist and even multiply in the seaweed mats and in the neighboring sand.  Typically, beach bacteria levels are high during and following rainfall events as rain washes the land, picks up various pollutants along the way, and eventually transports them to the shoreline. However, this summer has been relatively dry and elevated bacteria levels have aligned with field reports of loose seaweed on the beach or in the water column. Last August, while York waited for the mounds of seaweed to subside, it also took weeks for bacteria levels to wane.

However, reports of seaweeds mounds are occurring outside of the typical “big storm” events that detach large quantities of biomass that can be left stranded on the beach.

So what’s likely causing the apparent increase?

According to Stephen Dickson, with the Maine Geological Survey, there are likely four major physical oceanographic factors at play:

Wind direction – a prevailing summer wind can bring seaweed (wrack) ashore, especially when a beach’s orientation faces the sea breeze, which develops because of the contrast in air temperatures over the ocean and land.

Spring tides – these are tides with the greatest range or with a very high and very low watermark. High water levels can liberate seaweed from rocky shores, making drifting wrack available for stranding on beaches (with the appropriate onshore wind). Tides are driven by the lunar cycle and overall, July 2015 experienced high water levels with many days in July with tides over the mean high water level.

Neap tides – these are the tides with the smallest range and can allow seaweed to stay stranded on the beach. When this occurs in the upper few feet of the beach profile it can create a wide wrack zone.  This reduction in high tides tends to leave wrack behind on the berm where it dries out, attracts insects, and competes for beach blanket space.

Surf – or breaking waves can push seaweed onto the beach when the force of the surf is greater than the backwash that might pull the seaweed off of the beach. With onshore sea breezes, the loose wrack floating in the water is more like to be cast ashore with wave action than not.  In mid-July 2015, a period of moderate waves may have aided in stranding seaweed high on the beach profile just as the tide range was declining.  This combination of wave action and falling tidal range could be partly responsible for placing seaweed high on the beach where it could linger for days or longer.

There are also important biological considerations. Seaweed is critical to the ecology of the system as it enriches adjacent areas, provides food and shelter, increases biodiversity, helps prevent erosion, etc. Seaweed can also be easily composted as the materials break down quickly and can be used as a valuable resource for restoration and enhanced gardening. According to phycologists Clinton J. Dawes, University of South Florida and A. C. Mathieson, Jackson Estuarine Laboratory/UNH that study large seaweeds, the apparent increase in accumulation may also be related to:

Warmer waters in the Gulf of Maine during the spring and summer

Increasing numbers of introduced Asiatic seaweeds (e.g. Dasysiphonia japonica, previously referred to as Heterosiphonia japonica) are found within the Gulf of Maine, and these are opportunistic, fast growing species. Detached pieces can also regrow rapidly.

Excess nutrients (pollution) entering the system that can lead to coastal imbalances. For example, fertilizers, detergents, sewage, etc. entering the water causes eutrophication, a process where algae grows rapidly (blooms), resulting in water quality and habit degradation.

Remediation can require more than simply raking the seaweed to the low tide mark for the tides to wash it away as the seaweed may return to the beach on the next tidal cycle. Before delving into seaweed management, there are rules and support to develop a site-specific management plan.

Site-specific considerations:

  • Does the area of concern technically fall within a coastal sand dune system? If so, there are rules linked to the Natural Resources Protection Act
  • If it’s a coastal sand dune system, communities can move seaweed but not remove it from the system. Wholesale removal requires formal approval and some level of permitting by Maine DEP. Here’s the applicable language from the Coastal Sand Dune Rules:

4.   Review not required. This section clarifies when certain activities in coastal sand dune systems do not require approval pursuant to the NRPA. Permits are required for all other projects.

A.  De minimis activity. The following activities have minimal impacts and are not considered to be included in the listed activities requiring a permit at 38 M.R.S.A. §480-C(2).

(8) Removal of seaweed from the beach by hand or mechanical means provided the seaweed is not removed from the coastal sand dune system and does not disturb dune vegetation.

  • Maine DEP can provide guidance and is willing to work with towns on a beach-specific seaweed plan. Contact Marybeth Richardson, Tel. 207-592-1692. Individual landowners should also check with the DEP to determine what is allowed for seaweed management on private property. NOTE: In the York situation, the seaweed was determined to be an especially noxious, invasive type that DEP regarded as “debris,” not seaweed.  Under the Coastal Sand Dune Rules, removal of debris from the beach is allowed without a permit provided little or no sand is removed.

When seemingly excessive mounds of seaweed cover valued coastal beaches during Maine’s short summer season, there are critical factors to consider when mapping the course of action: water quality, public health, aesthetics, tourist economies, ecosystem health, and the laws that are meant to preserve the integrity of these environments. During the busy summer months, beach management approaches may also consider phenology (seasonal changes of plants and animals) while balancing the potential risks to public health and tourist economies. Managing seaweed accumulation on Maine’s beaches is a beach-specific weighing game with no one-size-fits-all solution.