The Impact of Boating

What can happen when a boat and eelgrass meet?

What is the direct damage caused by my boat?

  • Anchoring

    Anchors are designed to dig into the sediments on the seafloor to prevent your boat from slipping away with the movements of the current and tide. The elements that make anchors effective make it impossible to remove an anchor in eelgrass without ripping eelgrass out by the roots. This immediately kills the plants pulled up by the anchor. Eelgrass cannot replant itself after it is uprooted.

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Boats cause damages that are cumulative and feed off of each other. Some damages are immediately transparent. The eelgrass that an anchor pulls up is clearly damaged. But you might not readily understand what happens to the bare patch where eelgrass used to grow. The same goes for propellers or grounding. The feedback mechanisms put into motion lead to more eelgrass loss than you initially see.

Recreational boats are usually small enough to enter the shallow waters where eelgrass grows and accumulates fine sediments. This environment amplifies the effect of boating disturbance [1]. That gives recreational boaters the great responsibility of acknowledging the damage they can cause and understanding what they can do to prevent it. Removing eelgrass biomass decreases the biodiversity of the habitat, which has consequences for commercial and recreational fish, mollusks, and crustaceans [23]. The immediate impact of resuspended sediments is the decreased available light to eelgrass, leading to weakened or dead plants [4]. Beyond these foreseeable problems, the damages spiral to include -

  • Habitat that favors invasive species [5]

  • Change in eelgrass age and physical structure complexity leads to loss of species richness [6] 

  • Pollution of organic particles that eelgrass trapped in the sediments

  • Loss of erosion control

Feedback mechanisms continue to create more damage and hostile environments for eelgrass. To disrupt this cycle, we must understand the power of our actions.

Biodiversity relies on habitat continuity.

Biodiversity falls in fragmented habitats.

Eelgrass habitats are complex structures of canopy height, shoot density, and plant coverage. Anchoring and propeller damage disrupts the complex structures eelgrass creates [7]

  • Propellers chop off the seagrass blades, reducing the canopy height of the meadows. 

  • Propellers scar the seagrass beds, cutting deep into the sediments in shallow waters and ripping up large sections of eelgrass.

  • Anchors uproot seagrass to create 1-4 square meter bare patches within eelgrass beds [8]

This damage rarely happens in the same location, so the destruction they cause is amplified [98]. An individual propellor scar lasts almost three years before eelgrass can recover that area [10]. Regrowth may not be possible in areas with intense erosion due to the constantly shifting sediments [11].

Vegetative abundance and continuity support more macroinvertebrates and juvenile fish [12131415]. The bare patches created by these boating disturbances cannot support the abundance of fauna the seagrass could support [8]. Even small local losses of seagrass habitat can negatively impact the fisheries it supports [3]. Small-scale losses change how the fauna use the habitat and, based on species preference, can even change the species composition of the marine life the seagrass supports [16].

Disturbed Sediments

When boats meet shallow water, they disrupt the seafloor.

Habitat fragmentation doesn’t just create a problem for the fish; it also hinders seagrass meadow recovery [17]. Healthy eelgrass meadows gather sediments like silts and organic particles [8]. Dense, unfragmented eelgrass beds create more drag on the water flow, slowing the movement of particles and encouraging their settlement on the sea floor [18, 19, 20]

When recreational boats come through eelgrass beds with their propellers cutting into the seafloor and using anchors inside the meadow perimeter, they tear holes in the fabric of the seagrass rhizome mat [8]. These holes no longer have the eelgrass that held down the sediments, so they are free to move with the movement of the water. Sediment mobility increases as sections of eelgrass are lost.

Beyond anchors and propellers, boat-induced wake turbulence has the most significant effect on harming seagrass [4]. As boats use their motors to move through the water, they also move water far below the surface. This movement can be so powerful in shallow areas around 2.5 m - approximately 8 ft - that it can disrupt and erode the sediments [21]

The effects of wake turbulence are more significant in fine silts and organic particles since they are easier to stir up and stay longer in the water column [4]. Since that is precisely the kind of sediment frequently encountered in eelgrass meadows, they are at even greater risk of suffering the effects of suspended sediments in the water column.

How suspended sediments threaten eelgrass

The Cumulative Boating Damage to Eelgrass

Small actions make substantial differences. 

One anchor only causes 1-4 square meters of damage [8] in an eelgrass meadow that spans several hundred acres. That doesn’t sound like a big deal, right? 

Propeller scars start regrowing eelgrass within a year of the damage and fully regrow by three years on average [10]. That doesn’t seem like too long. What’s the worst that could happen?

The problem is that more than one boater creates this damage. Hundreds do them. This repetitive, cumulative damage is the most significant threat to eelgrass because the threat is underestimated. Your decision to anchor in the sand rather than the eelgrass or slow down in shallow areas profoundly impacts the ecosystems you regularly enjoy. Ensure your small actions make significant differences for the better, and do not underestimate your power over marine environments.