Purpose
To quantitatively assess the benthic community.
Materials
Photoquadrat framer
Digital camera with underwater housing
Strobe
Slate
Waterproof data sheets
Ziplock bags
Mesh dive bag
Transect line
Camera Instructions
Settings depend on the type of camera being
used. This method was devised using an Olympus C3040Z digital camera in an Olympus
underwater housing with 128 Mb Smartmedia and an Ikelite substrobe DS-50 with TTL slave sensor.
Set the camera on automatic, forced flash to trigger the slave sensor, and SHQ (super
high quality) resolution which yields approximately 55 pictures per media card. When taking
pictures, press down part way on shutter for approximately 2 seconds to allow auto focus
to engage, then completely depress shutter to take picture.
Datasheet Instructions
The data sheet is very important for later photo analysis on the computer. Cryptic species
in holes or depressions or darker species may be difficult to interpret on the photographs.
Information noted on the data sheet will help decipher these hard-to-see areas. The data
sheet includes codes for the most common algal species, substrate types, invertebrates and
corals that may appear in quadrats. The data sheet also has a map rectangle for each quadrat
in which a map of the quadrat will be sketched. During data sheet completion, the second
diver "maps" the quadrat by drawing the areas of concern for algae, corals, substrate, etc.
and marking with codes.
Method
Determine number of quadrats per transect (minimum 10 per 25 m). Generate random
points (statistical program or Microsoft Excel) for placement of framer along transects. Lay out
two 25 meter transects parallel to the shore, at least 10 meters apart. If permanent transects
are desired, mark the beginning and end of each transect and record location and heading.
Before proceeding with the quadrats on the transect, a sample photoquadrat is taken
two meters off the transect in an area with similar topography and flora to that on the
transect.
The second diver maps the location of the algae and organisms in the quadrat. This
preliminary photoquadrat provides a photographic reference to facilitate analysis of digital quadrats
later in the laboratory. Algal specimens are also collected from the sample quadrat for
laboratory identification and voucher specimens.
The two divers move along the transect together with one diver operating the camera and the
other changing the framer number and marking the datasheet for the quadrat. For
each quadrat the photoquadrat framer's short edge is placed next to the transect with the
lower left hand corner of the framer positioned at the random point. The second diver corrects
the frame number located on the side of the bottom framer and a photograph is taken.
Before moving the photoquadrat, the second diver also fills out the data sheet by listing the
macro-algae and organisms within the quadrat by code, to species if possible. The relative
abundance of the 5 most abundant algae is noted by using a scale of 1 - 5 with 5 being the most
abundant. This number is placed next to or under the corresponding algal code on the data sheet.
The second diver also draws a quadrat map of dark areas or cryptic species that may be
difficult to identify in the photograph and marks the areas with the appropriate algal codes.
Once the data is recorded and the map completed, the photoquadrat framer is moved to the
next random point and the previous procedure repeated until all quadrats on each transect
are completed (Preskitt et al.).
Algae that cannot be identified in the field are collected for later identification. If
permanent transects are installed, minimize algal collections within the quadrats or collect
representative sample away from transect.
Photoquadrat Analysis
The digital photographs are downloaded to a computer and placed in a file with date,
location and project. Each photograph is reviewed in Adobe
Photoshop®. Rotate each photograph so framer material is even on all margins, then crop all the pictures evenly. It is
recommended that the framer be minimized so there is less chance of random points falling on the framer
in the photographic analysis program. If desired, color correct to lighten dark areas or
adjust color to aid in identification.
There are a number of programs that can be used to analyze digital photographs. Area can
be calculated with SigmaScan® or ImageJ (http://rsb.info.nih.gov/ij/), and percent cover
is calculated with randomly generated points with Sigma Scan® or Photogrid® (C. Bird,
University of Hawaii, Botany Dept.). Photogrid® was designed specifically for ecological application
and was used in the design of this method.
References
Preskitt, L.B., P.S. Vroom, and C.M. Smith. A Rapid Ecological Assessment (REA)
Quantitative Survey Method for Benthic Algae using Photo Quadrats with Scuba. Submitted to Pacific
Science.
