Molluscan fauna from core 25B, Whipray Basin, central Florida Bay, Everglades National Park

Molluscan assemblages preserved in an 80-cm core from Whipray Basin in
central Florida Bay, Everglades National Park, illustrate changes in the environmental
conditions within the basin over the last two centuries. Salinity remained polyhaline to
euhaline throughout the time of deposition (1800-1997), with alternating periods of
stability and increased fluctuations. Since 1800, a Brachidontes assemblage has
characterized Whipray Basin and the dominant faunal components have remained the
same in terms of presence and absence of species. However, patterns of dominance and
diversity within the Brachidontes assemblage have changed and these changes indicate
fluctuations in the environment.
The period from 1815 to 1857 was distinguished by an abundance of molluscs
dwelling on seagrass and sub-aquatic vegetation. Faunal richness and abundance were
high and stable, and epiphytic molluscs flourished. Polyhaline conditions existed,
although periods of slightly lower salinities occurred. The period from 1862 to 1894
appears unstable based on fluctuations in molluscan faunal richness, abundance, and
dominant species. The epiphytic molluscs experienced significant shifts (? >30%)
associated with changes in sub-aquatic vegetation. The changes in epiphytic molluscs
from 1871 to 1913 may be indicative of a seagrass die-off. The period from 1899 to 1950
was the most stable section of the core in terms of changes in the molluscan fauna.
Faunal richness and abundance reached highs of 31 groups and 726 individuals per
sample during this period and epiphytic molluscs were prevalent. Beginning in 1955,
faunal groups experienced high amplitude fluctuations in abundance; this pattern
continued through the second half of the 20th century. Fluctuating salinity, changes in
vegetation, and reduced water quality (low O2, increased nutrients and/or reduced clarity)
oxygen supply) have characterized the past 50 years. These changes preceded a seagrass
die-off in 1987-88 and may be related to the causes of the die-off. Whether the cause of
the changes seen in Whipray Basin is natural or a combination of natural and
anthropogenic factors, the amount of change in the molluscan fauna in the last 50 years
clearly exceeds the preceding 150 years.