Effects of Macroalgal Zonation on a Reef Flat

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Introduction
Tropical island reef flats serve as habitats for a diverse range of organisms. Among these organisms is an abundance of macroalgae. Macroalgae may even be numerous enough to comprise most of the benthic area near a reef (Carpenter 1986, Villaça et al. 2010, Hay 1981). The large presence of macroalgae corresponds to a large role as primary producers (Carpenter 1986). Thus, these organisms can play an important part in marine food webs. Aside from the effects of herbivores, macroalgae distribution may be affected by other environmental factors (Marques et al. 2006, Villaça et al. 2010). Physical factors that affect macroalgae include desiccation, salinity, and temperature shifts (Tsuda 1977, Villaça et al. 2010). As autotrophic organisms, differing pH levels and oxygen, nitrate, and phosphate concentrations could possibly play an important role in their distribution. Their specific location on the reef flat might also be significant if varying wave actions affect algal growth. Aside from wave action, the lack of attachment structures that can be used to grow on sand may limit the distribution of some species (Tsuda 1977). Furthermore, distributions could be unique to differing groups of macroalgae. This study focuses on three important macroalgal groupings: Rhodophyta, Chlorophyta, and Heterokontophyta. Rhodophyta, or red algae, is a relatively uniform group that is generally characterized by red pigments and a lack of motile cells (Chapman 1964, Drew 1951, Trainor 1978, Solomon et al. 2008). The cell walls of Rhodophyta are commercially important in the production of gelling agents (Trainor 1978, Solomon et al. 2008). Chlorophyta, or green algae, contain pigments that are nearly identical to those of higher plants, use starch as their main storage agent, and have cellulose in species that have cell walls (Chapman 1964, Iyengar 1951, Trainor 1978, Solomon et al. 2008). Heterokontophyta mostly consists of the brown algae. These brown pigmented organisms comprise the largest of the algal species, many of which have “branched filaments; tufts; fleshy “ropes”; or thick, flattened branches” (Solomon et al. 2008). Similar to the red algae, Heterokontophyta are commercially important as thickening agents (Solomon et al. 2008). These three groups of macroalgae are considerably important in the fields of commerce and science. On Guam, there is a need for further study of macroalgal abundances and their influential factors (Tsuda 1977). Relatively little research has focused on the algae of the island’s fringing reefs. The purpose of this study is to observe how Rhodophyta, Chlorophyta, and Heterokontophyta are physically distributed on the Pago Bay Reef Flat. Also, the study will also assess environmental factors (namely temperature, salinity, pH, nitrate concentrations, and phosphate concentrations) which might influence the distributions.

Experimental design
The study was conducted on the windward reef flat of Pago Bay, Guam on September 14, 2012 during a 6 hour BI 410 Ecology lab. Three 50 m transects were run 30 m apart and 10 m perpendicular from the algal ridge. A global positioning system (GPS) was used to precisely determine the starting and ending point of each transect (Figure 1). A 0.5 m x 0.5 m quadrat frame was used to survey the abundance of red, green, and brown algae at 5 m intervals (total of 11 samples per transect line). The quadrat frame was divided into 9 squares, resulting in 16 points of intersection. The frame was randomly assigned to be laid on the right side of each transect line. Starting at 0 m, the frame was placed underwater and the abundance and group of algae (Rhodophyta, Chlorophyta, or Heterokontophyta) found under each intersection was recorded. The points that contained noalgae were quantified as bare substrate. Samples of each algal material were manually collected from each transect in 25m intervals beginning with 0m and ending with 50m. Representative algal groups that fell...
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