Coral mortality associated with dino-flagellate blooms in the eastern Pacific (Costa Rica and Panama)

Coral reefs at Cano Island, Costa h c a , and Uva Island. Panama, were affected during severe dinoflagellate blooms in 1985. In the second half of 1985. mass mortality of reef fishes and invertebrates, especially reef corals, occurred during blooms of the dinoflagellates Cochlodinium catenatum and Gonyaulax monilata. At Cano Island, up to 100% coral mortality was observed between the surface and 3 m depth, with pocilloporid species and Tubastrea coccinea most severely affected. At Uva Island, only 13 % pocilloporid mortality occurred and this was confined to the shallowest reef areas ('-3 m). The copious amounts of mucus associated with C. catenatum, present in the water column to 1-3 m depth, and adhering to coral colonies, suggested that coral death was caused by smothering. Other conditions that may have affected reef organisms were the presence of toxlc G. monilata, and possible oxygen depletion due to the hlgh densities of phytoplankton and the decomposition of dead organisms. These dinoflagellate blooms have interfered with recovery of reefs disturbed d u r ~ n g the 1982/83 El Nino warming event. There are only 2 reports of coral death associated with phytoplankton blooms. Baas-Becking (1951) reported on the death of corals in the shallow waters of New Caledonia, and attributed this to the putrefaction of thick masses of Trichodesmium thiebautii causing a marked reduction in dissolved oxygen. Smith (1975) commented on the impact of a 1971 'red tide' on reef coral communities off the west coast of Florida, USA, with near-complete mortality of shallow-water (< 40 m) biotas over extensive areas. Smith hypothesized these mortalities were caused by bacterial and fungal infec' Permanent address: Centro de Investigacion en Ciencias del Mar y Limnologia (CIMAR), Universidad de Costa Rica, San Pedro, Costa Rica O Inter-Research/Printed in F. R. Germany tions, oxygen depletion, and hydrogen sulfide poisoning rather than by toxins of Ptychodiscus brevis (Davis) that were present during the event. In this note we present observations of coral death associated with dinoflagellate blooms in the eastern Pacific at Cano Island, Costa Rica and Uva Island, Panama and discuss possible mechanisms that may have resulted in coral death. Study site and methods. Cano Island (8"43'N, 83"52'W) is situated in the southern region of the Pacific coast of Costa h c a , ca 15 km offshore (Fig. 1). Information on the coral community structure and climatic and oceanographic conditions is available in Guzman & Cortes (1989a, b). The percentage of dead colonies of all coral species encountered at Cano Island in July 1985 was recorded. Eight permanent plots of 10 m2 each at 4 localities from 0.5 to 12 m depth that have been surveyed since January 1984 (for a n assessment of coral recovery after the 1982/83 El Niiio event) were mapped in January 1985 before the dinoflagellate bloom and in late August 1985 near the end of the bloom. Water around the reefs was collected using 1 1 bottles in July 1985 and the species composition and relative abundances of the phytoplankters were determined using a Palmer-Maloney chamber (Steidinger 1979). Uva Island reef (7"49'N, 81°46' W) is located about 40 km offshore in the Gulf of Chiriqui, Panama, with similar climatic conditions to those at Cano Island. Detailed information on the oceanographic conditions and coral community structure are available in Dana (1975) and Glynn (1976). The same scleractinian corals occur in both study areas; however, at Uva Island the