Review: Preskitt, Vroom, Smith (2004) A Rapid Ecological Assessment (REA) quantitative survey method for benthic algae using photoquadrats with scuba. Pacific Science, 58(2):201-209.

Feature Paper: DOWNLOAD * Preskitt, Vroom, Smith (2004) A Rapid Ecological Assessment (REA) quantitative survey method for benthic algae using photoquadrats with scuba. Pacific Science, 58(2):201-209.

Author Abstract: The challenge of assessing seldom-visited, benthic substrates has created the need for a method to describe benthic communities quickly and efficiently. Macroscale rapid ecological assessments (REAs) of algal assemblages provide managers of coral reefs and other benthic ecosystems with the fundamental descriptive data necessary for continued yearly monitoring studies. The high cost of monitoring marine communities, especially remote sites, coupled with the time limitations imposed by scuba, require that statistically valid data be collected as quickly as possible. A photoquadrat method using a digital camera, computer software for photographic analysis, and minimal data collection in the field was compared with the conventional method of point-intersect (grid) quadrats in estimating percentage cover in subtidal benthic communities. In timed studies, photoquadrats yielded twice the number of quadrats (and an almost infinite number of data points) as conventional methods, provided permanent historical records of each site, and minimized observer bias by having only one observer identifying algae in the field. However, photoquadrats required more post-collection computer analyses of digital photographs than conventional methods. In the manual method, observer bias in algal identification can occur depending on the degree of experience of individual divers. On the other hand, photoquadrats rely on one observer in the field and one observer in the laboratory, standardizing algal identification. Overall, photoquadrats do not yield the finer resolution in diversity that was found using point-intersect quadrats but do provide a more precise estimate of percentage cover of the abundant species, as well as establishing a permanent visual record in the time allowed by work with other teams.
 

Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.

Review: Since algae are often considered capable of overgrowing corals and precipitating phase shifts on coral reefs that are under nutrient or other environmental stress, it is important to know how to survey them in the field. This week's paper compares various methods used to collect algal field data with the aim at discovering a rapid, scientifically valid method. Yet while algae are the focus of this week's paper, the authors describe photoquadrat techniques that are applicable towards many benthic organisms, including corals. 

Photoquadrats are uniform-sized rectangles or squares (often made of PVC) that are placed over the substrate with a camera attached to them through a frame meant to keep the camera at a set height and angle above the bottom.

The authors designed the protocol as part of remote US-territorial Pacific island surveys conducted regularly by the National Oceanic and Atmospheric Administration (NOAA). The surveys had to "describe community structure and prepare a comprehensive species list for each site [surveyed]." 

The authors compared "conventional methods using point grid, point-intersect methods, or visual estimation" with "photographic and video quadrates." Yet while there were benefits of conventional methods (typically more taxa identified and canopy effects able to be assessed), the negatives (higher training required, more field time required) were enough to veto their use for algal data, which were considered of lower value than coral or fish data. As a result of these constraints, the authors developed a rapid field method that would still allow high-quality data to be collected.

In summary, the method involved laying a transect using fiberglass tape measures, and then placing photoquadrats at pre-determined random places along each transect, taking a high-resolution photograph and then having a second diver identifying "algae within the photoquadrat, recording the relative abundance of the five most abundant algae on a scale of 1 to 5 (with 1 being most abundant), and collecting representative samples of the algal species in the quadrates from outside the framer for later identification in the laboratory."

In the field, researchers were able to collect nearly three times as many photoquadrats as conventional measurements and statistical analyses showed that photoquadrat data were more consistent (in terms of data quality and quantity) than conventional methods. As the authors point out, "photoquadrats are not new; they have been compared with a myriad of point-intersect, visual estimation, and grid quadrate methods and have often been found wanting in scale and in diversity measurements." Their purpose was not to reinvent the wheel, so to speak, but rather to "refine the standard photoquadrat method by adding a two-observer team, note taking, collection of samples, and quadrate mapping to address known concerns with cryptic species."

The authors also found that photoquadrats helped produce more flexible statistical data. "Grid quadrates are static: the number of random points are fixed, usually with too few points due to time limitations, and sometimes the observer must quickly identify algae underwater in less than ideal conditions. In comparison, the photoquadrat method is more flexible in laboratory analysis."

Finally, while photoquadrats may not be appropriate where finer-resolution of taxonomic data and canopy effects are desired, they are appropriate for "quantitatively describing an ecosystem at a macro-community level. The photoquadrat method provides adequate quantitative data, analysis flexibility, and permanent specimens that enable the investigators to determine the patterns in distribution and abundance [of taxa] in remote, inaccessible regions. A standardized rapid ecological assessment protocol not only provides the quantitative data needed to establish baselines for these communities but also ensures that comparable data are collected during the ongoing monitoring needed for management decisions."

Next we'll look at algae in the Indian Ocean and how they differ to the better-surveyed Pacific and Caribbean.