The impact of Dictyota spp. on Halimeda populations of Conch Reef, Florida Keys

Abstract

Species of the brown alga Dictyota dominate the reef tract in the Florida Keys. In surveys during summer and fall months between 1994 and 2001, Dictyota occupied as much as 70% of the benthos on Conch Reef. Dictyota spp. were found growing epiphytically on Halimeda tuna, Halimeda opuntia, Lobophora variegata, Galaxura sp., fire coral, hard corals, soft corals, bryozoans and a variety of sponges on Conch Reef. From 1994 to 2001, the percent coverage of Halimeda spp. declined from 15% to 3% on the same reef. In Aug. 1999, 2000 and 2001, on average, 56% of two Halimeda species on Conch Reef had >50% of their thalli covered by Dictyota menstrualis and Dictyota pulchella. To address the impact of Dictyota on Halimeda, short-term growth of unepiphytized and heavily epiphytized (>50% Dictyota cover) H. tuna were compared with unepiphytized H. tuna to which a Dictyota mimic was attached. The number of new segments per plant ranged from 1 to 174 over 9 days. Halimeda thalli with >50% Dicytota cover and thalli covered with Dictyota mimic grew significantly slower than unepiphytized thalli. A second short-term experiment addressed the impact of neighboring Dictyota on the growth and metabolism of unepiphytized H. tuna. Augmenting or clearing epilithic Dictyota around but not in contact with H. tuna had no impact on growth or metabolism of H. tuna. Unepiphytized and heavily epiphytized H. tuna were also collected for studies of metabolism. This work indicated that epiphytic Dictyota negatively impacts metabolic rates of H. tuna in part by shading H. tuna thalli. This negative impact was also in part chemically mediated, as exposure to Dictyota-conditioned water elevated respiration rates in the same manner as seen in the metabolic studies of naturally epiphytized H. tuna.