Ecological Assessment of Algae
Despite the nonflattering images of “pond scum” many people often associate with algae, marine macrophytes have proven themselves to be among the most diverse, most ecologically important, most prevalent, and most beautiful organisms present in tropical reef systems. Their importance to the ecosystem is staggering: algae form the base of the food chain, occupy much of the available substrate, and help to oxygenate the water for animal life to thrive. Additionally, without microscopic symbiotic algae living in healthy coral tissue, most corals would be unable to survive – a scenario that is becoming all too real as coral bleaching events (processes where stressed corals expel their algal symbionts) become more common.
Although large, fleshy algal forms are the most recognizable floral components on reefs to most divers, tiny turf algae and crustose coralline red algae are also extremely prevalent and play significant roles in the ecosystem. Turf algae are the first to colonize vacant substrate and cover essentially every nonliving hard surface on the reef. Turf algae are also among the most important food source for herbivorous fish and invertebrates. Relatively fast growing crustose coralline red algae act as a glue that cements together loose components of the reef system, and serve as a settling surface for larval invertebrates and other algae. Without crustose algae holding everything together, much of the reef would be washed into deep water or onto shore during heavy winter storms.
Clearly, without algae there would be no tropical reef ecosystem, yet marine algae are among the least studied and least understood organisms on the reef. Research is sorely needed to catalog and quantify the species that are present on reef systems around the Pacific, and ecological studies are necessary to examine the role of these critical plants in reef ecosystems.
To accomplish these objectives, CRED is studying tropical reef algae to address the following questions:
- What is the best way to quantify algal functional groups (macroalgae, crustose coralline algae, turf algae) in tropical reef settings?
- What species are present in each island ecosystem and in what quantity?
- Do changes in algal populations serve as a good environmental indicator of reef heath?
- How do algal diversity and abundance change over time?
- Can biogeographical hypotheses be formulated about algal dispersal and evolution using qualitative and quantitative data from island groups around the Pacific?
A modified Rapid Ecological Assessment technique that incorporates the use of digital cameras and photoquadrats is our primary field method.
Rapid Ecological Assessment (REA) Methods to Quantitatively Assess Algae
The goal of algal surveys is to quantitatively describe the algal community and prepare a comprehensive species list for each site. Working at depths of 3 to 16 m with teams using other existing rapid assessment methods, a method for algal assessment is needed that will minimize the time in the water yet yield the greatest amount of data possible. A high-resolution digital camera mounted on a 0.18 m² photo quadrat frame (Fig.1) with an Ikelite Substrobe digital slave strobe has been built to quantitatively assess marine algae.
In addition to photographs, data from each quadrat are recorded on a waterproof data sheet that includes space for recording the species found in the quadrat, a “map” area for identifying cryptic species in depressions that may prove difficult in computer analysis, and codes for the most common genera and species of macroalgae, corals, invertebrates, and substrate types to standardize and shorten note taking.
Two trained observers move along the transect together with one observer placing the framer and operating the camera and the other taking notes. (Fig.2). Photographs are taken at predetermined random points .
|Figure 2. Algal team quantitatively measuring algal percent cover with photoquadrat at Kure Atoll, NWHI.|
After a photograph is taken by the first diver, the second diver identifies algae within the photoquadrat, records the relative abundance of the 5 most abundant algae on a scale of 1 – 5 (with 1 being most abundant), draws a quadrat map locating species that may be hard to identify in a photograph, and collects representative samples of the algal species in the quadrats for later identification in the laboratory.
Once data are recorded, the photoquadrat is moved to the next random point and the procedure repeated. To prevent redundancy, only samples of new algal species found in subsequent quadrats are collected.
In the laboratory the photographs taken with the photoquadrat methods are downloaded to a computer, renamed with a unique location code, then cropped and color corrected in Adobe Photoshop using autocorrect.
Each photo is analyzed for percent cover using the software PhotoGrid (C. Bird, Dept. of Botany, University of Hawaii), a software program capable of random and stratified random point analysis on digital photography. Once all photos are analyzed, data are imported into Microsoft Excel for further statistical application.
Ultimately, field-collected specimens will be critically analyzed in the laboratory to ensure positive species identification, will be cataloged, and will subsequently be placed in research institutions where they can be accessed by researchers interested in a suite of topics ranging from taxonomy to bioprospecting.
After identification, provisions are made to ensure appropriate preservation and curation of each specimen, providing a historical record that will be available to future researchers.
|Figure 3. Scinaia huismanii, a constricted red alga named after Dr. John Huisman.|
Three species of algae new to science have recently been described by staff at CRED. All 3 are red algae from the Northwestern Hawaiian Islands: Acrosymphyton brainardii (Vroom & Abbott 2004a), Scinaia huismanii (Vroom & Abbott 2004b), and a species of Dasya (Vroom, in press). Additionally, reproduction was documented in one of the most ubiquitous algae in the Northwestern Hawaiian Islands, the green alga Halimeda velasquezii (Vroom & Smith 2003).
In addition to the discovery of new species and ecological processes, our field sampling photoquadrat method (dubbed the Preskitt method) was recently published (Preskitt et al. 2004) and has been used to successfully complete detailed benthic cover analyses at French Frigate Shoals, Northwestern Hawaiian Islands (Vroom et al., in press).
|Figure 4. Acrosymphyton brainardii, a gooey red alga named after Dr. Russell Brainard, PI of NOAA, Coral Reef Ecosystem Division.|
Preliminary results reveal most healthy reef habitats in the Northwestern Hawaiian Islands to be dominated by different algal functional groups rather than scleractinian coral species (Friedlander et al. 2004), breaking the paradigm that high abundances of macroalgae are always indicative of a deteriorating reef system. Using the field note component of the Preskitt method, relative algal abundance of macroalgae (RAM) was temporally assessed across the entire Northwestern Hawaiian Island archipelago, and suggested that algal composition may have been altered because of 2002 and 2004 coral bleaching events at Kure and Midway atolls (Vroom & Page, in review).
|Figure 5. A satellite image of the French Frigate Shoals, Northwestern Hawaiian Islands showing 7 a posteriori ecozones delimited by differences in benthic species composition of algae and coral.|