Abundance of Octocorals at Stations Selected from the Florida Keys National Marine Sanctuary Coral/Hardbottom Monitoring Project.
Florida Fish and Wildlife Conservation Commission, St. Petersburg, FL
Stations analyzed for octocoral abundance were selected from Coral Reef Monitoring Project (CRMP) sampling stations. A total of 107 stations have been sampled annually in the interval 1996 to 2002. At each station, three parallel video transects (approximately 22 meters long and 60 cm apart) are filmed. Filming is conducted from a standard height of 40 cm, perpendicular to the benthic under-story. Visible width of imagery filmed from this height is 40 cm. Total average area sampled by video at one station is 26.4 m2.
Octocoral abundance was measured at 28 stations each year, 1996, 1998, 1999, and 2002. The stations were equally distributed by habitat type: 4 patch, 4 offshore shallow, and 4 offshore deep reef stations in the Upper keys and 4 hardbottom, 4 patch, 4 offshore shallow, and 4 offshore deep reef stations in the Middle keys. Stations were selected according to a stratified random scheme. Video-derived octocoral abundance was determined, from the 1996 video data set, for all stations where average octocoral percent cover was greater than 5%. From each habitat type, one station was randomly selected from each of the maximum and minimum abundance quartiles. Two stations were randomly selected from the middle abundance group, 25% to 75%. These selection criteria provided 28 stations for collection of octocoral abundance. Abundance data were collected from 1996 and 2002 video, to maximize the temporal span of data. Years 1998 and 1999 were also selected to assess the impact of hurricane Georges. Thus, octocoral abundance was measured at 28 stations each year, 1996, 1998, 1999, and 2002.
Video was played on a color monitor and the number of octocorals in view was tallied. Only colonies with their holdfast clearly visible within the field of view were counted. Each station was counted twice. The census counted the number of Scleraxonians, the number of Gorgonia ventalinain three size classes, and the number of “other octocorals” in three size classes. Size classes were defined as <10cm(short), 10-40cm (medium), and >40cm (tall). The delineations of size classes are estimates, based on scaling items in the video image (e.g. chain link size and 40cm average height of camera lens).
The total abundance of all octocorallia surveyed was surprisingly stable from 1996 to 2002, increasing 6%. Scleraxonia abundance decreased 38%, from 961 in 1996 to 597 in 2002 (sum of 28 stations). Total abundance of G. ventalina decreased 19%, from 869 to 702 colonies. Decline in the previous two categories was offset by an 18% increase in the total abundance of “other octocorals”, from 5730 to 6734 colonies. This increase was driven by a 191% increase in the abundance of short “other octocorals,” from 545 to 1585 colonies.
From 1996 to 2002, the total abundance of G. ventalina decreased 19%, from 869 to 702 colonies. This decrease was driven by a 39% decline in the abundance of medium G. ventalina, from 627 to 386 colonies. The magnitude of this decline obscured increases in the abundance of short and tall G. ventalina. The abundance of short G. ventalina increased 120%, from 52 to 115 colonies. Tall G. ventalina increased 6%, from 191 to 202 colonies.
Changes between 1998 and 1999 reflect, in part, the effects of hurricane Georges. 1998 field video data collection was completed in late August, and hurricane Georges crossed the Keys one month later on September 25, 1998. Between 1998 and 1999, the total abundance of all octocorallia decreased 11% from 7277 to 6454 colonies. The total abundance of G. ventalina fell 41%, and the total “other octocoral” abundance decreased 8%. G. ventalina and “other octocoral” abundance declined in nearly every size class. The drop in G. ventalina abundance was driven by declines in medium (-43%), and tall (-55%). The decrease in “other octocoral” abundance was driven almost entirely by a 41% decline in tall colonies, from 1090 to 646. When all erect octocorals are pooled by size, the 1998 to 1999 change is striking. Short octocorals declined 9%, from 812 to 742 colonies. Medium octocorals declined 4% from 4750 to 4559 colonies. Tall octocorals declined 43% from 1314 to 747 colonies. This differential mortality implies that tall octocoral colonies are selectively removed by storm energy. Scleraxonia abundance reinforces this suggestion. The abundance of these encrusting octocorals was essentially unchanged, from 402 to 407 colonies.
A copy of the poster may be obtained by requesting the following reference: Lybolt, M. 2003. Abundance of Octocorals at Stations Selected from the Florida Keys National Marine Sanctuary Coral/Hardbottom Monitoring Project. Florida Marine Research Institute, St. Petersburg, FL. IHR#2003-004.
Lybolt, Florida Fish and Wildlife Conservation Commission, 100 8th Av SE, St. Petersburg, FL 33701, Voice 727-893-9860 x1134, Fax 727-893-1270, email@example.com