Moorings

Traditional swinging chain moorings damage eelgrass by scraping up the seafloor within the chain’s radius. That scaring is localized damage, but the consequences are far-reaching.

Seagrasses prefer soft sediments in shallow areas that are sheltered from the wind and waves; these areas often overlap with ideal locations to place moorings [12]. This overlap makes eelgrass susceptible to damage from the heavy chains and associated shading from boats [2].

The heavy chain of the mooring pivots and drags on the seafloor around the central anchoring point. Boats exacerbate the dragging since the wind catches and pulls them around easily [2]. The damage to eelgrass occurs as the chain destroys what is in its path [3], leading to a decline in seagrass [4].

Moorings create significant local damage to eelgrass beds; they scour the seafloor, reducing the eelgrass coverage by up to 100% [1]. Swinging chain moorings produce circular scars in seagrass meadows [56] and are more damaging in increasing numbers [7].

The worst damage occurs in the center of the circular mooring scars. 80% of moorings within eelgrass beds have decimated eelgrass coverage to 0% at the anchoring point [26]. The damage lessens as the eelgrass gets further from the central anchor but does not typically exceed 18% coverage within an 8 m radius [28]. At a 20 m radius, you can expect to see an average of 30% coverage. The chain also damages seagrass canopy height. Blade lengths increase further away from the anchoring point of the mooring.

Mooring infrastructure considerably impacts eelgrass [91011]. The physical damage to eelgrass stirs up the sediments [2, 9, 12], reducing the light available to eelgrass; the boats attached to the mooring cause additional damage by shading eelgrass [2]. The lack of light inhibits photosynthesis and growth [13] and significantly hampers recovery in deep areas [14]. Furthermore, the excessive physical disturbance has a significant impact on eelgrass recovery. Seedlings experience increased mortality rates in areas of high physical disturbance [15], such as the areas within mooring scar radii.

So how does this affect you?

How much damage do swing chain moorings cause seagrass?

The size of the scar depends on the length of the bottom chain [11, 12]. The average mooring scar radius is 5.4 m [2]. However, impacts extend to 20 m from the anchoring point of the mooring. Each swinging chain mooring is responsible for 122 square meters of eelgrass loss.

Cumulatively, moorings are directly responsible for 6 ha of eelgrass loss in the UK. [2]. 6 ha may seem small, but remember that mooring scars are just the beginning. The full-scale loss caused by mooring is much more significant than just the 6 ha that accounts for the mooring scars themselves. Eelgrass loss from moorings leads to resuspended sediments that start feedback loops that eventually lose even more eelgrass.

Eco-moorings: A solution to eelgrass damage

Eco-moorings fit a specific context considering the sediment, biodiversity, hydrodynamics, and boat size it needs to accommodate [3]. They all aim to reduce seafloor abrasion, which is problematic in traditional swinging chain mooring. Eco-mooring achieves this by elevating the chain away from the seafloor using ridged or buoyant parts [41].

Because eco-moorings are new technology and challenging to implement for insurance and financial reasons, scientists have researched ways to modify current moorings to make them less damaging to eelgrass. One proposed modification technique attaches trawler floats midway up the chain length [41]. This method successfully keeps the chain from dragging on the seafloor and allows the eelgrass to recover. During the study, eelgrass was recovering; shoot density and blade length increased. The meadow showed signs of filtering finer particles than it had pre-modification.

Another modification asserts that most swing chain moorings in shallow eelgrass habitats are mainly there because of precedent [42] especially given that the boats that moor there are usually small vessels. This study suggests that a simple switch from a heavy chain to a rope will prevent the damage moorings create without straining your wallet. The rope moorings significantly reduced the chains’ impact on the benthic habitat and increased seagrass cover by 44%.

These eco-mooring trials tell us another exciting thing. When mooring damage stops and the water quality is good, eelgrass can recover well [2, 43].