The phenomenon of "mangrove die-off" has been reported from Florida Bay periodically for decades. Botanical explorations of the Everglades, Florida Bay and the keys from the late 1800s and early 1900s by Small and others, reported salt barrens with standing dead stems on islands in the bay and along the northern mainland shoreline. Following the seagrass die-off in the bay in the 1980s, fishermen and scientists, reported a noticeable thinning of the mangroves on several bay islands and new interest was generated in mangrove die-off. Most recently Florida Bay-Watch reported renewed die-off in early 2001.

In the mid 1990s permanent vegetation plots were established at 11 sites (both island and mainland) in Florida Bay: Crocodile Point, Munroe Lake, North Dump Key, Roscoe Key, Samphire Key, Clive Key, Little Rabbit Key, Low Key, East Key, Little Butternut Key and Duck Key. At each site, two transects were established to overlay on a pre-existing network of sediment porewater sampling wells. Transects had been aligned to run from the shoreline into the interior salt barren at each location. The porewater well served as the center of the plot. All stems >1.4m in height and <3m from the center, were identified, measured for diameter at breast height (dbh), and mapped, using distance and bearing from the center. Stem density and basal area were calculated. Herbaceous vegetation and mangrove seedlings were sampled using three randomly placed 1m² quadrats within each plot. All species within the quadrat were recorded and the number of "hits" for each species on a 10x10 grid counted. This yielded a percent cover for each species. A subset of plots was resampled during the summers of 1999 and 2000. Change in a stem's dbh was used to calculate growth between the two samplings. Historical aerial photographs dating from 1927 were examined for each study location and the vegetation / cover was visually interpreted.

At almost all sites mangrove stem density decreased exponentially away from the shoreline. Rhizophora was most abundant towards the shore, with Avicennia dominant away from the shore. Laguncularia was seldom encountered. The number of living mangrove stems increased from the first to the second sampling. However, at many sites growth of individual mangrove stems is minimal. An exception was on Clive Key where significant growth occurred in all plots, particularly for Rhizophora (fig. 1). A pulse of recruitment (new mangroves reaching 1.4m in height) was also measured at this location.

Figure 1. Increase in average stem diameter for Rhizophora mangle at Clive Key from 1995 to 2000. [larger image]

Total cover of herbaceous vegetation changed dramatically over the short time period studied, with both increases and decreases observed. During the initial sampling in 1995, the interior portions of several sites were nearly devoid of vegetation, especially at Crocodile Point and North Dump Key. The tops of the porewater wells, which protruded above the sediment level, were easily seen. In 2000, these areas were covered with vegetation and the porewater wells obscured (to the point they were very difficult to find, fig. 2). Increase in herbaceous cover was due primarily to abundant growth by the succulents Batis maritima, Salicornia virginica, and S. bigelovii at the interior end of the transects. Herbaceous cover decreased shoreward at most locations, possibly due to increased growth of the mangrove canopy causing lower light at the forest floor.

Figure 2. Increase in herbaceous vegetation cover at Dump Key from 1995 to 2000. [larger image]

Interpretation of the aerial photographs revealed the presence of interior barren areas on numerous islands in Florida Bay in 1927, and their persistence through time. This record, combined with our short-term observations, highlight the dynamic nature of these coastal habitats and their ability to respond rapidly to short-term environmental change.

Contact: Smith, T.J., III. USGS, Center for Water & Restoration Studies, 600 Fourth St., South, St. Petersburg, FL 33701. Phone 727-803-8747 x 3130, FAX 727-803-2030, Email Tom_J_Smith@usgs.gov, Question 4.

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

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