The Impact of Boating
What can happen when a boat and eelgrass meet?
What is the direct damage caused by my boat?
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 [2, 3]. 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 [9, 8]. 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 [12, 13, 14, 15]. 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
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Eelgrass is a plant that requires light. Without light, it cannot photosynthesize, which will eventually starve the plant. If you were to encounter eelgrass at a plant nursery, the “how to care for me” tag would tell you to plant it in direct sunlight.
Light does not travel far, even in clear water, so suspended sediments cut eelgrass off from this life-giving resource. Because of the light limitation, suspended sediments significantly impact eelgrass at the deeper end of its range [22]. Since these sediments are unlikely to settle immediately after their disruption, the effects on light availability are longer term than the time it takes to disrupt them.
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Eelgrass meadows are natural filters for nutrients from the land [23]. Fertilizers and septic systems frequently result in nitrogen runoff that can fuel algal and phytoplankton blooms. Eelgrass prevents these blooms by pulling nutrients into the sediments and preventing their resuspension by dampening wave energy.
Boat wake turbulence resuspends these nutrients that are made vulnerable by poor anchoring and navigational practices by recreational boaters. The water column suddenly has an excess of nitrogen previously unavailable to the phytoplankton [4]. The nitrogen fuels a bloom that will lead to additional shading of eelgrass beds, low oxygen zones, and potentially hazardous conditions for recreation.
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Eelgrass efficiently draws carbon-rich particles out of the water column and settles them in the seafloor sediments [24, 25]; this term is called blue carbon. Because eelgrass meadows that are healthy and intact dissipate the power of waves and currents [8], the carbon-rich particles remain settled on the seafloor.
Eelgrass destruction disrupts blue carbon stores [26]. Once the eelgrass is ripped up, the sediments are loosened and more susceptible to propeller wash and boat wake turbulence. This results in the destroyed meadows becoming sources of carbon pollution rather than carbon sinks.
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Healthy eelgrass beds significantly reduce wave heights and current speeds [27]. The decrease in wave power results in slower water that is less effective at eroding the coastline by resuspending sediments.
When boat traffic results in resuspended sediments, erosion is a natural conclusion. Unfragmented, healthy eelgrass beds with high shoot density are most effective at preventing erosion [18, 19, 20]. When boat traffic results in resuspended sediments, erosion is a natural conclusion. In studies where eelgrass was lost, mean elevation decreased by 0.1 m [28].
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.