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PLoS One
2017 Sep 05;129:e0184175. doi: 10.1371/journal.pone.0184175.
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Benthic community structure on coral reefs exposed to intensive recreational snorkeling.
Renfro B
,
Chadwick NE
.
Abstract
Chronic anthropogenic disturbances on coral reefs in the form of overfishing and pollution can shift benthic community composition away from stony corals and toward macroalgae. The use of reefs for recreational snorkeling and diving potentially can lead to similar ecological impacts if not well-managed, but impacts of snorkeling on benthic organisms are not well understood. We quantified variation in benthic community structure along a gradient of snorkeling frequency in an intensively-visited portion of the Mesoamerican Barrier Reef. We determined rates of snorkeling in 6 water sections and rates of beach visitation in 4 adjacent land sections at Akumal Bay, Mexico. For each in-water section at 1-3 m depth, we also assessed the percent cover of benthic organisms including taxa of stony corals and macroalgae. Rates of recreational snorkeling varied from low in the southwestern to very high (>1000 snorkelers d-1) in the northeastern sections of the bay. Stony coral cover decreased and macroalgal cover increased significantly with levels of snorkeling, while trends varied among taxa for other organisms such as gorgonians, fire corals, and sea urchins. We conclude that benthic organisms appear to exhibit taxon-specific variation with levels of recreational snorkeling. To prevent further degradation, we recommend limitation of snorkeler visitation rates, coupled with visitor education and in-water guides to reduce reef-damaging behaviors by snorkelers in high-use areas. These types of management activities, integrated with reef monitoring and subsequent readjustment of management, have the potential to reverse the damage potentially inflicted on coral reefs by the expansion of reef-based recreational snorkeling.
Fig 1. Maps of (a) Yucatan Peninsula showing major tourist cities, and (b) Akumal Bay showing areas sampled in the present study. In (a), the extent of the Mesoamerican Barrier Reef System (MBRS) is shown as a shaded area along the coastlines of 4 countries: Mexico, Belize, Guatemala, and Honduras. The study site at Akumal is underlined and marked with a rectangle to indicate the area shown in detail in (b). The detail map in (b) presents the 10 bay sections in which visitor abundances were quantified, each identified by letter in the upper left corner of the section (delineated by dashed lines): 4 sections on land along the beach, sampled for beach visitors (A-D), and 6 in the water, sampled for snorkeling visitors (E-J). G to J are inner bay sections that parallel the on-land sections A to D, and E and F are reef crest sections along the mouth of the outer bay. Shown also are patch reefs #1–10 sampled for coral reef benthic community composition: 2 reefs in each of the 4 inner bay sections, and 1 reef each in the 2 outer bay sections. The landward edge of the sandy beach is delineated by a solid grey line, the water’s edge (coastline) by a solid black line, and the coral reef crest by a dotted line. For details and site comparisons see text here and maps in [17,43,46].
Fig 2. Spatial and temporal variation in visitor abundance at Akumal Bay, Mexico, during July 2015.Shown are numbers of visitors (x¯ ± SE, N = 5 d) observed every 2 h between sunrise (06:00) and near sunset (18:00) in: (a) 4 on-land beach sections, and (b) 6 in-water sections (also referred to as snorkelers). See map (Fig 1) and text for details.
Fig 3. Variation in the percent cover of stony corals (left) and macroalgae (right) among patch reefs ranked by level of snorkeling in Akumal Bay, Mexico.N = 10 transects examined per reef.
Fig 4. Growth form (a,b) and species-level (c-f) variation in the percent cover of stony corals among patch reefs ranked by level of snorkeling in Akumal Bay, Mexico. N = 10 transects examined per reef.
Fig 5. Variation in the percent cover of non-coral sessile animals (a: Millepora fire corals, b: zoanthids [zoantharians], and c: gorgonians) among patch reefs ranked by level of snorkeling in Akumal Bay, Mexico. N = 10 transects examined per reef.
Fig 6. Growth form variation in the percent cover of macroalgae among patch reefs ranked by level of snorkeling in Akumal Bay, Mexico.CCA = Crustose coralline algae, CMA = Calcified macroalgae, FMA = Fleshy macroalgae. N = 10 transects examined per reef.
Fig 7. Variation in the abundance of Diadema antillarum and other sea urchin species grouped (Arbacia punctulata and Eucidaris tribuloides) among patch reefs ranked by level of snorkeling in Akumal Bay, Mexico.N = 10 transects examined per reef.
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