Abstract
One of the major stresses on corals is the settlement of suspended sediment on their surfaces. This leads to the blocking of light, the covering of the coral mucus surface and an increased risk of disease. For this reason sediment deposition on a reef is considered a highly important variable in coral reef studies. With the use of sediment traps and oceanographic sensors, the sediment deposition rate and water conditions during a rainy season (April-May 2009) on a Tropical Eastern Pacific coral reef (La Azufrada) at Gorgona Island in Colombia were investigated. To quantify sediment deposition, sediment traps were established in nine stations along the coral reef (three stations per reef zone: backreef, flat and slope). To minimize disturbance by aquatic organisms in the sediment traps these were filled with hypersaline borax-buffered 10% formaline solution before their deployment; animals found in the filter contents were fixed and stored in a 4% formalin solution, frozen and identified in the laboratory. To determine the water conditions, discrete samples of water from 1 m and 10 m depths were collected using a Niskin bottle. Oceanographic variables (temperature, salinity and dissolved oxygen) as well as turbidity, chlorophyll and nutrient concentration (nitrite, nitrate and phosphorus) were measured in the samples from both depths. Vertical records of temperature and salinity were carried out with a Seabird-19 CTD nearest to La Azufrada and water transparency was measured using a Secchi disk. We found a mean trap collection rate of 23.30±4.34gm-2d-1 and did not detect a significant difference in the trap collection rate among reef zones. The mean temperature and salinity in the coral reef depth zone (0-10m layer) were 26.98±0.19°C and 32.60±0.52, respectively. Fourteen taxonomic groups of invertebrates were detected inside the sediment traps with bivalves and copepods being the most abundant and frequen. The findings presented here constitute the first report of both the potential sediment deposition rates and the water conditions of La Azufrada coral reef. Rev. Biol. Trop. 62 (Suppl. 1): 107-116. Epub 2014 February 01.
Highlights
Coral reefs represent an important biological, economical and cultural resource, providing sources of food, building material and revenue through tourism and fisheries (Birkeland, 1997; Rees, Opdyke, Wilson & Fifield, 2005)
The mean sediment deposition rate estimated for La Azufrada reef is in accordance with the “normal” sedimentation rate expected on a coral reef (
Cantera et al (2003) reported a complex spatial pattern of sedimentation for Playa Blanca reef. These authors found a lower sediment deposition rate on the slope (10.28g m-2 d-1) than on the backreef (29.38g m-2 d-1); results that support the idea that terrestrial runoff from the island should be the main source of sediment in this reef (Zapata, 2001)
Summary
Coral reefs represent an important biological, economical and cultural resource, providing sources of food, building material and revenue through tourism and fisheries (Birkeland, 1997; Rees, Opdyke, Wilson & Fifield, 2005). The health and survival of reefs around the world are undoubtedly already under threat by both man-made and naturally occuring destructive forces These include terrestrial runoff, increased turbidity, pollution, siltation, coral bleaching and abnormal inputs of nutrients and organic matter (Acevedo & Morelock, 1988; Roberts et al, 2002; Rees et al, 2005). Gorgona Island, which houses the largest and most continuous fringing reefscape on the Pacific coast of Colombia (Zapata & VargasÁngel, 2003), has not been exempt from disturbance caused by sediment depostion. As this island was Colombia’s top security prison from 1960 to 1983, the reefs have suffered extensive coral death. We (1) evaluate the small scale variability of temperature and salinity around La Azufrada reef; (2) measure the potential short-term sediment deposition rate on the main physiographic zones of the reef: backreef, flat and slope, (3) relate those variables to the coral stress level and (4) describe the macroinvertebrate assemblage associated with sediment trap deployment at La Azufrada reef
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