Document Type

Statistical Report

Publication Date

2002

Keywords

Water quality, Seagrass

Abstract

Historical (pre-1930s) seagrass meadows in Tampa Bay are believed to have covered 31,000 ha of the shallow bay bottom. Later, impacts to the bay from increasing population and industrial development of the Tampa Bay area have resulted in large seagrass losses. By 1982, approximately 8,800 ha of seagrass remained. Since 1982, Tampa Bay seagrass monitoring programs have recorded a reversal in the trend of seagrass loss, with the baywide seagrass cover increasing from 9,420 ha in 1988 to 10,890 ha in 1997. However, between 1997 and 1999 the trend again reversed with substantial losses recorded, specifically in the Old Tampa Bay segment. The 1999 baywide cover was estimated at 10,050 ha, thus eliminating most of the seagrass coverage gains recorded since the late 1980s. Similarly, in Hillsborough Bay, the segment of Tampa Bay that historically has had the poorest water quality, seagrass increased from near 0 ha in 1984 to about 56 ha in 1997. Following 1997, the seagrass expansion stagnated in this segment, with a slight reduction in cover recorded between 1998 and 1999. It is generally agreed that the Tampa Bay seagrass expansion observed since the mid- 1980s was triggered by water quality improvements during the late 1970s to the mid 1980s. These improvements included reductions in phytoplankton biomass and water column light attenuation. These improvements also followed a nearly 50% reduction in external nitrogen loading from domestic and industrial point-sources in the early 1980s. The loading reductions primarily affected point-source discharges to Hillsborough Bay. The reductions in the Tampa Bay seagrass expansion rate and areal cover realized since 1997, are most probably related to a recent period of high rainfall that began in 1995 and extended through the strong 1997–98 El Niño event. During this period, nitrogen loading and discharges of water with high color content increased, and subsequently, phytoplankton biomass, light attenuation, and color content increased in all major bay segments. High levels of these constituents are known to be detrimental to seagrass growth and it is not surprising that Tampa Bay seagrass monitoring programs recorded recent reductions in expansion and losses of seagrass cover, specifically in the upper bay segments. Although other factors may have contributed to the recent reductions in seagrass growth, it is likely that the high rainfall period created the major impacts. Rainfall in the Tampa Bay area was below normal in 1999 and 2000. With an extended period of relatively low rainfall it could be expected that the seagrass expansion rate again would increase. Baywide information for seagrass coverage in 2000 is not available; however, Hillsborough Bay seagrass coverage increased from about 56 ha in 1999 to about 69 ha in 2000.

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