Variations in water clarity and bottom albedo in Florida Bay from 1985 to 1997

Following extensive seagrass die-offs of the late 1980s and early 1990s, Fl orida Bay reportedly had significant declines in water clarity due to turbi dity and algal blooms. Scant information exists on the extent of the declin e, as this bay was not investigated for water quality concerns before the d ie-offs and limited areas were sampled after the primary die-off. We use im agery from the Advanced Very High Resolution Radiometer (AVHRR) to examine water clarity in Florida Bay for the period 1985 to 1997. The AVHRR provide s data on nominal water reflectance and estimated light attenuation, which are used here to describe turbidity conditions in the bay on a seasonal bas is. in situ observations on changes in seagrass abundance within the bay, c ombined with the satellite data, provide additional insights into losses of seagrass. The imagery shows an extensive region to the west of Florida Bay having increased reflectance and light attenuation in both winter and summ er begining in winter of 1988. These increases are consistent with a change from dense seagrass to sparse or negligible cover. Approximately 200 km(2) of these offshore seagrasses may have been lost during the primary die-off (1988 through 1991), significantly more than in the bay, The imagery shows the distribution and timing of increased turbidity that followed the die-o ffs in the northwestern regions of the bay, exemplified in Rankin Lake and Johnson Key Basin, and indicates that about 200 km2 of dense seagrass may h ave been lost or severely degraded within the bay from the start of the die -off. The decline in water clarity has continued in the northwestern bay si nce 1991, The area west of the Everglades National Park boundaries has show n decreases in both winter turbidity and summer reflectances, suggestive of partial seagrass recovery. Areas of low reflectance associated with a majo r Syringodium filiforme seagrass meadow north of Marathon (Vaca Key in the Florida Keys) appear to have expanded westward toward Big Pine Key indicati ng changes in the bottom cover from before the die-off. The southern and ea stern sections of the Bay have not shown significant changes in water clari ty or bottom albedo throughout the entire time period.