Seagrass beds: manatee habitat

Seagrass beds are important foraging habitat for manatees (Best, 1981, Lefebvre, 2000, Lefebvre and others, 2001) and cover substantial areas of Pine Island Sound, Matlacha Pass, and San Carlos Bay (Fig. 23), primarily in the shallow depths of the waterways (<1-6 ft [<0.3-2 m] Fig. 24). Most of the islands of Pine Island NWR, Matlacha Pass NWR, and Island Bay NWR are surrounded by or adjacent to seagrass beds. JNDDNWR waters also include significant seagrass beds. As of 1995, large sections of seagrasses in Lee County were rated as degraded with light to severe scarring from propeller cuts of boats operating in the shallow waters (Madley, and others, 2004, Sargent, and others, 1995). Areas of scarred seagrass were scattered throughout the region.

Figure 23. Seagrass coverage of Matlacha Pass and Pine Island Sound, Florida, from 1999. Figure provided by FWRI. [larger image]

Figure 24. Bathymetry of Matlacha Pass and Pine Island Sound, Florida. Figure provided by FWRI. [larger image]

The track of Hurricane Charley took the most destructive forces of the hurricane over North Captiva Island (Fig. 6) and across the northern sections of Pine Island Sound, Matlacha Pass, and into Port Charlotte. The worst damage to seagrass appears to have occurred at the breach on North Captiva. Wind and waves eroded sand into the sound burying the seagrass adjacent to the island (Fig. 25). Dr. Brad Robbins, Mote Marine Laboratory (pers. comm.), visited the area after the hurricane and estimated approximately 1 ha (~2.5 acres) of seagrass was lost due to the breach. Refuge personnel documented additional localized damage. Scarring occurred as wind and waves dragged many dislodged trees from the breach across the seagrass beds scouring the vegetation from the substrate. Sand inundation and scars from tree drags are clearly seen in aerial photos (Fig. 25). This area, however, has been repeatedly eroded by storms and undoubtedly will be again. Prior to the breach at North Captiva Island from Charley, two smaller channels were opened in the same area in 2001 from Tropical Storm Gabrielle (USGS Coastal & Marine Geology Program, Hurricane and Extreme Storm Impact Studies http://coastal.er.usgs.gov/hurricanes/charley/index.html, 16 Oct 2004).

Figure 25. Sand inundation of seagrass bed at the breach on North Captiva Island, Florida, and scarring from trees dragged by the storm. [larger image]

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Figure 26. Relative abundance of manatees within the warm season (Mar.-Nov.) based on aerial surveys of Lee County, Florida, conducted 1994-1995 and 1997-1998. Analysis and Figure provided by FWRI. [larger image]

Manatee population

Examination of aerial survey data from the late 1990s (Fig. 26) shows that during hurricane season the highest concentrations of manatees within the refuge complex occur in Tarpon Bay and around Buck Key in JNDDNWR; near Chino Island in Pine Island NWR; and near Big Island and Fisherman Key (San Carlos Bay), Manatee Bay, Master's Landing, McCardle Island (Matlacha Pass), and Bull Island and Brown Pelican Island (north of Matlacha Bridge) in Matlacha Pass NWR. Low abundances of manatees are seen along the west edge of Pine Island Sound with only a few animals found along the eastern edge. By far the greatest numbers of manatees occur throughout areas of Matlacha Pass and San Carlos Bay. The distribution of manatees (Fig. 13) correlates fairly well with the distribution of seagrass (Fig. 23). The low number of sightings along the east edge of Pine Island Sound is probably a factor of low water levels preventing manatees from easily feeding on the seagrass there (Fig. 24). The area where the breach occurred at North Captiva Island was characterized as a low-use manatee area prior to the hurricane. Within the cold season the distribution patterns are similar with the exception of fewer manatees sighted in Pine Island Sound (Fig. 27). A similar pattern is seen from the telemetry data (Fig. 28).

Figure 27. Relative abundance of manatees within the cold season (Dec.-Feb.) based on aerial surveys of Lee County, Florida, conducted 1994-1995 and 1997-1998. Analysis and Figure provided by FWRI. [larger image]

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Figure 28. Manatee places and corridors for Matlacha Pass and Pine Island Sound, Florida. Analysis and figure provided by FWRI. [larger image]