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Community Structure and Habitat Association of Reef Fishes on a Staghorn Coral (Acropora cervicornis) Reef in Broward Co., Florida, U.S.A.

Weaver, D. C.1, D. S. Gilliam 2, D. Anderegg 2, and R. E. Dodge 2

1 United States Geological Survey
7920 NW 71st Street, Gainesville, FL, USA.

2 National Coral Reef Institute, Nova Southeastern University
8000 North Ocean Drive, Dania, Fl  33004, USA.

Presented at the 9th International Coral Reef Symposium, Bali, Indonesia.  Oct. 23-27, 2001.

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ABSTRACT

The recent demise of Acropora cervicornis populations throughout the Florida Keys and Caribbean requires an investigation of the population dynamics and community ecology of this once dominant reef type.  Simultaneous surveys involving high-resolution coral mapping/ monitoring and fine-scale spatial reef fish censuses will not only lead to increased understanding of processes that drive colony dynamics, including mortality and subsequent erosion, but identify impacts on the diverse assemblage of reef fishes associated with these monotypic populations of A. cervicornis "thickets" or "haystacks".  Photoquadrat analysis and belt transect surveys were initiated in May 1999 on a broadly distributed, high-latitude A. cervicornis reef off Broward County, FL to document spatial distribution and percent cover of living coral colonies and associated reef fishes. Ongoing research efforts will document the food web structure of reef fishes associated with living and dead A. cervicornis reefs to identify associated shifts in species abundance, spatial distribution and trophic ecology of resident reef fishes to provide a more comprehensive understanding of the biological processes linking hard coral and reef fish community dynamics in this geographically unique biological assemblage.

Figure 1.  Aerial view of Fort Lauderdale Beach, 1996 - click to enlarge

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INTRODUCTION

       Stability of natural communities is a central question in community ecology and has become the focus of many coral reef monitoring projects.  Staghorn coral, Acropora cervicornis, forms reefs throughout the Caribbean that exhibit highly variable temporal patterns of community structure (Aronson and Precht, 1997; Knowlton et al., 1990; Porter et al., 1982). Episodes of A. cervicornis mass mortality due to hurricanes, low temperatures, or disease have resulted in dramatic declines in live coral cover, a pattern that may persist through time or lead to replacement by other species of hard corals (Knowlton, 1992; Aronson et al., 1998). A once-dominant reef building coral throughout the Caribbean, A. cervicornis, is a major contributor of three dimensional reef structure (Knowlton et al., 1990) as the interlocking network of branches in A. cervicornis forms "bafflestones" that can provide the foundation for rapid reef growth (Aronson and Precht, 1997) and provide habitat for associated reef fishes. The recent decline in Acropora throughout the western Atlantic is enigmatic, as it has persisted throughout the fossil record (Greenstein et al., 1998, Aronson and Precht, 1997; Aronson et al., 1998).  This project provides a preliminary description of the community structure and spatial distribution of a staghorn coral reef occurring in Broward County, Florida, at the northern extreme of distribution for the species.

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Figure 2.  Acropora cervicornis photoquadrat - click to enlarge

METHODS

       Living staghorn coral communities are distributed in discontinuous patches along a north-south belt along the nearshore waters off Fort Lauderdale Beach, Broward County, Florida (Fig.1).  Reef surveys and monitoring activities were initiated in May 1999 on a single patch, approximately 200 by 500 meters in dimension, to determine percent cover of living coral colonies and community structure of associated reef fishes.

       Percent coral cover was determined by deployment of a "floating grid" system to establish multiple transect lanes for digital still photography and videography (Fig. 3).  High resolution (2000 X 1600 pixel) photographs (Fig. 2) were taken in each square meter cell, using a Nikon Coolpix 950 housed camera. Video belt transects were made in each lane using a housed Sony VX-1000 digital video camera.  Photoquadrats and video censuses were made at three grids placed within the survey area. 

       Individual photoquadrats were analyzed by placing 10 random points over each photograph to determine percent coverage of live staghorn corals, other living organisms, and non-living substrata such as coral rubble and bare rock. A photomosaic was also produced from low-resolution stills captured from digital video.  Reef fishes were identified from videotape transects, noting species associated with staghorn reef communities and non-reef areas dominated by cobblestone rock and hard pavement.

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Acropora cervicornis community - click to enlarge
Acropora cervicornis photomosaic - click to enlarge (450kb, slow loading)

A. cervicornis photomosaic

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Figure 3.  "Floating Grid" system used for photoquadrat and transect surveys for visual census of hard corals and reef fishes. - click to enlarge
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RESULTS

       A total of 489 photoquadrats were analyzed for percent cover (Figure 4), and ten transects analyzed for preliminary estimates of habitat association of reef fishes (Figure 5).  Estimates of  percent cover of living A. cervicornis ranged from 10% to 32%, with a mean of 15.5%. Other living substrata (scleractinian corals, soft corals and sponges) exhibited low coverage (less than 1% in each category). Reef fish abundance was higher in areas with living staghorn coral and staghorn rubble thickets (4.9 individuals/m2) than interspersed cobble/rock habits (1.65 individuals/m2).

DISCUSSION

       The extensive growth of A. cervicornis found throughout this region appears to be at the northernmost geographic limit of distribution for this species, and represents a potential source of recruits for surrounding reef communities of south Florida. Located near a major urban area, and slightly north of a major shipping channel (Port Everglades Figure 6), this reef system presents an opportunity for research on the population dynamics and community structure in a heavily impacted location. Ongoing research and monitoring will determine annual influence of storms and other natural disturbances on the distribution of living staghorn corals, and identify the community structure and trophodynamics of  the associated reef fish community.

Figure 4.  Acropora cervicornis cover.  Mean and range given, based on average values estimated from each (1X25m) belt transect. - click to enlarge

Figure 4.  Acropora cervicornis cover.  Mean and range given, based on average values estimated from each (1X25m) belt transect.

Figure 5.  Fish Density associated with staghorn coral communities and interspersed cobblestone/bare rock habitat.  Mean and range given, based on ten (1X25m) belt transects. - click to enlarge

Figure 5.  Fish Density associated with staghorn coral communities and interspersed cobblestone/bare rock habitat. Mean and range given, based on ten (1X25m) belt transects.

Figure 6.  Port Everglades Inlet, Located directly south of the study Area. - click to enlarge


REFERENCES

Aronson, R. B. and W. F. Precht. 1997. Stasis, biological disturbance, and community structure of a Holocene coral reef. Paleobiology 23(3):326-346.

Aronson, R. B., W. F. Precht and I. G. MacIntyre. 1998. Extrinsic control of species replacement on a  Holocene reef in Belize: the role of coral disease.  Coral Reefs 17:223-230.

Knowlton, N., J. C. Lang, and B. J. Keller. 1990. Case study of natural population collapse: Post-hurricane predation on Jamaican staghorn corals. Smithson. Contr. Mar. Sci. 31:1-25.


Greenstein, B. J., H. A. Curran and J. M. Pandolfi. 1998. Shifting ecological baselines and the demise of Acropora cervicornis in the western North Atlantic and Caribbean Province: A Pleistocene perspective.  Coral Reefs 17:249-261.

Porter, J. W., J. F. Battey, and G. J. Smith. 1982. Perturbation and change in coral reef  communities.  Proc. Natl. Acad. Sci. USA 79:1678-1681.

ACKNOWLEDMENTS

We thank Captain L. Armstrong, S. Thornton, B. Hilkemann, B. Ettinger and L. Jordan of Nova Southeastern University Oceanographic Center and C. Bennett of USGS for providing assistance in the field and laboratory.

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