Seagrass-Watch Magazine Issue 47 - March 2013 | Page 8

Vessel access to docks constructed in
shallow water may also encourage
propeller scarring and propeller dredging
while boaters navigate to and from the
structure. Motor vessels frequently
encounter seagrass meadows directly
when the water is as shallow as the vessel's
draft.
Much like mowing your lawn, propellers
shear off seagrass leaves and damage the
plant canopy by removing important
photosynthetic tissue and disrupting
productive epiphyte communities. Known
commonly as “prop scarring” these
injuries are most frequently associated
with relatively smaller motorised vessels;
however, when larger deep V-hulled
vessels with multiple propellers are
involved the injuries can be much worse.
The most severe damage occurs when
propellers dredge sediments and uproot
entire plants. Usually referred to as “prop
dredging”, this activity destroys apical
meristems (growing tips) and the clonal
integrity of the meadow, and frequently
buries adjacent seagrasses with displaced
sediments. When large vessels run their
hulls aground they can create deeply
excavated blowholes that scrape and
scour away the entire mat of seagrass and
the upper layers of sediment, leaving
behind a gap of unvegetated, exposed,
and unstable substrate. These injuries are
often exacerbated by the operator's
attempts to escape the grounding site by
powering off the seagrass meadow,
causing significantly greater physical and
biological harm than the original injury.
Studies of simulated and actual prop
dredging incidents in subtropical/tropical
seagrass meadows(6,7,8,9,10), as well as large
vessel hull groundings(12,13,14) indicate that

Prop scar
Propeller scar in a
shallow Thalassia
testudinum meadow in
south Florida:
approaches to mitigate
impacts involve boater
education, improved
channel marking,
increased enforcement,
and limited-motoringzones
WK

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WWW.SEAGRASSWA
TCH.ORG

WK

Prop dredging
The photograph above, of a large vessel excavation in a shallow Thalassia
testudinum meadow in south Florida, indicates (red dots) the perimeter of the
excavated blowhole created by prop dredging and the vessel hull collision with
the seagrass bank. The bright white areas surrounding the excavation are the
berms formed from displaced sediments that bury seagrass. Dark area
surrounding the grounding is the undamaged T. T
estudinum meadows.

these types of activities can be among the
most severe human-caused seagrass
impacts. Since most seagrasses are rooted
vascular plants, they depend on the
sediments for attachment and nutrition.
Disrupting the sediments destroys these
functions that in many subtropical/tropical
seagrass ecosystems have taken decades
or even centuries to develop ( 1 5 ) .
Furthermore, the injuries leave behind
unstable and nutritionally deficient
substrates, so that natural recovery may be
delayed if not completely impaired(12,14). In
some subtropical/tropical seagrass
meadows it may be necessary to perform
expensive and uncertain restoration to
physically and biologically remediate a
vessel grounding site to recover the lost
ecological services that the seagrass
provided before being damaged(13,14,16,17,18).
Most vessel injuries are localised around
the immediate area of impact(9), but in
places where there are consistently high
densities of boat traffic the damage can be
widespread and persistent (1,13,4,3). Seagrass
meadows associated with active
waterways, shipping channels, urban
areas and public access sites are
particularly vulnerable, especially where
channel marking and other aids to
navigation are inadequate. The problem is

further compounded by the lack of
operator training, limited operator
experience, and in many areas, poor
water visibility. Many groundings also
result from poor operator judgment, lack
of common sense, or even flagrant
disregard for the potential injury to natural
resources. Very few groundings are
deliberate because salvage costs and
damage to a vessel can be expensive.
However, some vessel damage is
intentional, especial