Coral Reefs in Honduras: Status after Hurricane Mitch

Figure 3. Photographs depicting various diseases observed throughout the Cayos region. (A) Black band disease on a brain coral (Diploria sp). This disease typically begins at one location and spreads out very quickly killing the coral as it grows and leaving only bare skeleton in its path. (B) Bleached Meandrina sp. coral. (C) Montastrea sp. that has been partially killed by white plague or white band disease and the remaining skeleton colonized by red algae. Click on the image for a larger version.

Physical damage to coral reefs from Hurricane Mitch may have been kept to a minimum because the reefs in this area are composed of robust head coral species such as brain coral (Diploria sp.) and star coral (Montastrea sp.). Sedimentation and freshwater runoff from the mainland most likely caused more damage to corals than did waves and currents. Divers observed widespread coral disease such as black band, white pox, and bleaching and an abundance of algae during the initial visit to Cayos in October 1999, one year after Mitch (Figure 3).

Figure 4. Color-enhanced satellite image (SeaWiFS) from November 1, 1998, showing the extent of the freshwater plume (yellow) that was induced by intense rainfall from Hurricane Mitch. SeaWiFS image provided by University of South Florida, St. Petersburg, Florida. Click the image for a larger version.

The occurrence of these diseases and algae are thought to be a result of stress induced from a combination of pre-hurricane high sea-surface water temperatures, and post-hurricane high sedimentation and nutrient influx from the mainland. SeaWiFS (Sea-viewing Wide Field-of view Sensor) satellite imagery (Figure 4) taken November 1, 1998, shows a large plume of sediment-laden, high-nutrient river water flowing from the engorged Aguán River Valley directly to Guanaja. Portions of this large plume eventually inundated the Cayos Cochinos region. Coral reef communities typically thrive in clear, low-nutrient oceanic water and therefore are affected when subjected to water that has lower-than-normal salinity, increased sedimentation, and additional nutrients. Cayos Cochinos is regularly influenced by all of these factors, especially during the rainy season (August - February).

Figure 5. Examples of data collected at Cayos Cochinos. (A) 11-month record of sea-surface temperatures taken in a water depth of 5 ft. (B) Surface salinity recorded approximately once a week at the biological field station dock (-2 ft) on Cayos Pequeño and from the reef (-17 ft) at Pelican Point, which was collected at 15-minute intervals during the summer of 2000. (C) Light (PAR) record for a seven-day period showing decreasing light intensities with pulses of 'fresher' water. Click the image for a larger version.

It is probable that Hurricane Mitch prevented further bleaching damage to the corals throughout the Bay Islands (Roatán, Guanaja, Utila, and Cayos Cochinos) during the fall of 1998. Upwelling of deep oceanic water lowered the surface temperatures by 4ºF as Hurricane Mitch passed over the Bay Islands (Jennifer Keck, pers. comm.). The drop in surface temperature most likely had a positive effect by reducing the severity of coral bleaching, thereby preventing further coral mortality. In addition, during the past two years (1999-2000), summertime sea-surface temperatures throughout the Caribbean have receded below coral bleaching threshold temperatures (~86ºF) (Hoegh-Guldberg, 1999). Temperature data at Cayos confirms a trend of lowered sea-surface temperatures (Figure 5A). In October 1999 there was a brief period of high temperature (88ºF) that did not cause severe widespread bleaching. A recovery period was continuing in 2000 as the high appeared to stay below 86ºF.

Figure 6. Photo depicting (A) a severe infestation of filamentous algae (species?) along the western side of Cayos Pequeño (photo taken October 1999). Infestation had disappeared by April 2000. (B) Fleshy algae (Lobophora sp.) are consistently present on all portions of the reef. Click the image for a larger version.

Coral reefs at Cayos Cochinos displayed fewer diseased corals in October 2000 than the previous year. This is an encouraging sign that the corals are recovering from the stresses induced by Hurricane Mitch. However, widespread turf and fleshy algae are still present throughout the Bay Island archipelago (Figure 6). Observations by previous researchers (Guzmán, 1998) suggest that coral diseases and algae were present before Mitch. Coral diseases, however, were not well documented prior to Mitch. On the other hand, algal persistence around Cayos has been well documented and may have resulted from repeated nutrification of surface waters, whether from land clearing, agricultural runoff on the mainland, or from local human sources (Guzmán, 1998). River discharge can be monitored by looking for low-salinity spikes (~29 parts per thousand, ppt; Figure 5B) or decreased light levels (Figure 5C). Continued monitoring of these parameters is essential to understanding the long-term impacts that sedimentation, nutrification and low-salinity events may have on coral reef health within the Cayos Cochinos Biological Reserve. Human impacts, such as over-fishing, poor sewage-disposal practices, and coral mining, though not investigated in this study, may also lead to the deterioration of coral reef environments.