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Evaluation of an Artificial Rock Reef
for Colonization by Macroalgae
Tina Wyllie-Echeverria, Anacortes, WA, David
O. Duggins and Sandy Wyllie-Echeverria, University of Washington, Seattle,
WA, Margaret Schwertner, Port of Anacortes, Anacortes, WA
| Abstract: We
set out to measure the colonization and percent cover of a newly
constructed bare rock reef by macro-algae. Rocks 1-2 feet in diameter
were placed on a subtidal (-15 to -20 MLLW) mud bottom in order
to mitigate for loss of macro-algal habitat. Diver video surveys
were completed in the spring and summer following construction of
the rock reef. After 1 month the rocks remained bare, after 3 months,
small tufts of sporophytes were evident, after 6 months approximately
70% of the rock surface was covered by a variety of macro-algal
species and this level of cover continued through the following
year (September 2002). Cover was estimated from randomly selecting
30 individual frames from the underwater video surveys and assessing
the percent cover in each frame. An average cover for the 1000 square
foot rock reef was obtained. This method is a quantitative, archival
and verifiable method of estimating cover, and can provide a reliable
and cost effective method for documenting habitat. |
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Introduction: A 1000 square
foot layer of granite boulders were placed over soft sediment. This
is a mitigation action that was aimed at establishing habitat for
kelp and macroalgae that would be disturbed by dredging activities.
In order to satisfy permitting requirements the rock reef needed
to maintain a minimum cover of 25% of the surface by kelp and macroalgae
for five years. Methods: The entire reef was recorded on video in
the course of the single underwater excursion. The very homogeneous
nature of the substrate (boulder size, depth, siltation, and water
current) made such a random sampling design (rather than following
predetermined and repeatable transects) statistically appropriate.
Video graphic analysis has the distinct advantage (relative to in
situ measurements of percent cover) of requiring a relatively short
investment of time gathering images underwater and is thus logistically
and financially economical. In the laboratory, video footage was
transferred to the computer and saved in imovie format From each
imovie sequence, 30 random frames were selected for percent cover
analysis. The selected frame was viewed on a computer screen and
an algal percent cover estimate generated. On occasion a frame was
rejected and another randomly selected if the image was blurred
or otherwise difficult to interpret |
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February 8, 2001-
Rock deposited at site.
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August 29, 2001 – Colonization begins
with 70% cover.
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May 22, 2002 –
After one year there is 99.1% cover.
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September 23, 2002 -
More rock visible and an increase in red and green algae.
Percent cover reduced to 77%.
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Results and Discussion: Visual estimation
of percent cover is an accepted method of rapidly and accurately
generating two-dimensional cover data (Dethier, 1993). However,
it must be acknowledged that these cover estimates are under estimates
in that this method cannot distinguish greater than 100% cover
(as when kelp blades are layered on top of each other). Also,
while total algal percent cover is easily obtained, it is difficult
to distinguish between algal species.
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CENSUS
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TOTAL COVER (%)
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KELP (%)
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OTHER ALGAE (%)
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BARE ROCK (%)
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5/17/01
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6.3 (1.3)
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6.3 (1.3)
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0
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97.7 (1.3)
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8/30/01
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70.7 (4.4)
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70.7 (4.4)
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0
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29.3 (4.4)
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5/23/02
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99.1 (0.3)
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99.1 (.3)
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0
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0.9 (0.3)
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9/13/02
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87.9 (4.4)
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77.2 (6.5)
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10.7 (4.6)
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12.1 (4.4)
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References:
Dehtier, M. N., E. S. Graham, S. Cohen, L. M. Tear. 1993. Visual
versus random-point percent cover estimation: ‘objective’
is not always better. Marine Ecological Progress Series 93:92-100. |
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