Seagrass-Watch Magazine Issue 47 - March 2013 | Page 9

Documented vessel impacts
The most well documented example of
widespread vessel impacts to seagrass
meadows was derived from an aerial
survey of Florida's coastal waters (1). In the
1990s scientists and resource managers
began to recognise the value of using
aerial remote sensing to observe and
monitor shallow water benthic habitats at
much larger and more relevant scales than
previously attempted, yet still providing
sufficient detail for ecological
assessment(6). The authors of the Florida
study inspected aerial photography
covering nearly the entire state's coastal
waters and calculated that there was
approximately 704km 2 injured by
motorised vessels. This rather
extraordinary revelation of the magnitude
and extent of the damage in Florida
eclipsed all of the previous smaller-scale
examples(20). Of all the areas in Florida that
were impacted by motor vessels, the
damage was most extensive in the Florida
Keys where the U.S. National Oceanic and
Atmospheric Administration (NOAA) had
recently established a National Marine
Sanctuary (FKNMS), and a damage
assessment and restoration program(13). In
2007, 217 reported boat groundings
occurred in the FKNMS, with

approximately 80% of these in seagrass
meadows(13,18). These were only those
groundings reported to authorities and did
not account for the unreported incidents
which could easily double the total
number of injuries(13).
Funding provided by NOAA, the State of
Florida, and non-government
organisations led to a series of studies
which identified the ecological
consequences of vessel damage, rates of
seagrass recovery, and the efforts required
to physically and biologically restore
injured sites(8,10,11,12,13,15,17,18,21). Most notable
was the finding that large gaps in
meadows of the climax (late-successional)
species Thalassia testudinum made by
vessel excavations which exceeded 20 cm
in depth showed low probability
(approximately a 50% chance) of
recovery unless they were refilled to grade.
Essentially, without sediment re-grading
natural recovery was a coin toss, and if
seagrass recovery occurred at all it was a
slow process of species succession.
From this series of studies much was
learned about the impacts of motor vessels
in tropical seagrasses ecosystems, as well
as the response of the seagrass
communities to disturbance (15) . Our
knowledge about how important

seagrasses are in stabilising sediments
during severe storms was acutely revealed
when tropical cyclone Georges passed
through the Florida Keys in 1996. Gaps
formed by vessel disturbances expanded
significantly, previously stable seagrass
meadows that were injured by motor
vessels became vulnerable to further
degradation by erosion, and natural
recovery was interrupted(12,21).
These observations prompted studies
focused on developing and implementing
cost effective techniques for repairing
tropical seagrass meadows impacted by
motor vessels before they could expand
and cause further degradation. Thalassia
testudinum meadows are, by far, the
seagrass community most seriously
threatened by vessel impacts in the
subtropical/tropical Atlantic, Caribbean
and Gulf of Mexico region. Unfortunately,
T. testudinum is the slowest growing
species in the community, making it the
most difficult and expensive seagrass to
restore(8,22,23,24).
To improve the probability of recovery,
studies were conducted to test the
feasibility of utilising wild bird fertilisation
(i.e. bird faeces) to provide phosphorus,
the limiting nutrient in carbonate
sediments(25).

Strandings
Large vessel aground in a shallow Thalassia testudinum meadow in south Florida, USA.
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 damage than the original injury.

WK

MARCH 2013

9