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 [1, 2]. 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 [5, 6] 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 [2, 6]. The damage lessens as the eelgrass gets further from the central anchor but does not typically exceed 18% coverage within an 8 m radius [2, 8]. 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 [9, 10, 11]. 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?

  • In healthy eelgrass beds, water current speeds are reduced by 70-90% and wave heights by 45-70% compared to unvegetated regions [16]. Decreasing the speed and sizes of the waves is an essential service eelgrass provides to prevent sediment resuspension.

    Greater shoot densities and distance into the eelgrass beds create more drag on water flow [17, 18, 19]. When moorings destroy parts of the meadows, mean elevation can drop by 13.6% [20].

    The increase in erosion will also trigger a vicious feedback loop where eelgrass will continue to lose vital habitat. Soft sediments that eelgrass grows in and then accrues are particularly at risk [21]. These sediments are easily stirred up and stay in the water column for a long time [1]. Repeated resuspension and erosion pressure can inhibit the regrowth of the rhizomes and lead to permanent seagrass loss [9].

  • Eelgrass is an efficient filtration system; it draws carbon-rich particles out of the water column and settles them into the sediments in the meadows [22, 23]. Sediment accumulation increases when the meadows are healthier, denser, and unfragmented.

    Eelgrass limits the resuspension of particles by dissipating waves and turbulence [24]. Without high current speeds, the sediment retains the carbon better.

    Thinning or lost eelgrass cannot retain the carbon it stored when it was healthy [25, 26]. The sediments will resuspend, and with it, the carbon. Sediments in mooring scars are less cohesive and have less carbon than in eelgrass meadows [6]. This change in sedimentation leads to a coarse type of sediment on the seafloor [27], which is less hospitable to the recolonization of eelgrass into the mooring scars

  • Eelgrass beds reduce water current speeds, which promote nitrogen-rich particles to settle on the seafloor [28, 23]. Mooring chains inhibit eelgrass from settling and retaining the nitrogen in the water column.

    Sediments in mooring scars have less nitrogen than those in the eelgrass bed [6]. As moorings resuspend the nitrogen in the water column, it will fuel increased algal cover [29], creating hazardous water conditions and shade over eelgrass beds.

    Macroalgae will take the opportunity to grow where eelgrass cannot [2] due to the shade and low oxygen caused by the blooms. These conditions will limit the recovery potential of eelgrass [30].

  • Vegetative abundance and continuity support more macroinvertebrates and juvenile fish [31, 32, 33, 34]. Higher canopy heights add to the structural complexity of the eelgrass meadow, which increases the amount of biodiversity it can support.

    Seagrass has more marine fauna than mooring scars [6, 35]. When the chains scrape along the bottom of the seafloor, it damages the microhabitat complexity of the benthic habitat [36]. Furthermore, the damaged eelgrass beds lead to a decline in macroinvertebrates [37, 38] and juvenile fish [7, 39].

    Finally, the chains from moorings also decrease the canopy heights on the seagrass beds [2]. This decrease in blade height reduces the complexity and biodiversity of the meadow. The cumulative effect is that the eelgrass cannot support the fisheries that depend on eelgrass as essential habitat [40].

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].