199
end of his career in science – he went on to earn his Ph.D. and is a well-respected
paleoecologist and professor. For some of my field assistants, however, the rigors of
our fieldwork sent them running to alternate professions.
A Picture of the Recent Past – More Freshwater
All the long hours and extensive field work paid off because the data we gathered on
living mollusks were essential to interpreting the past environments recorded in the
sediment cores collected on mud banks throughout Florida Bay. A core is collected
by inserting a hollow tube into the mud; when the tube is extracted, we have a record
of the sediment that accumulated at that site over time, with the oldest material on
the bottom. Remember that a primary goal of Everglades restoration is to restore
more natural freshwater flow through the ecosystem, so the goal of our work was to
establish what those pre-twentieth century flows were by using the information preserved in the sediment cores. Contrary to what you might think, the best place to
study the amount of freshwater flow in the past is not from cores in the freshwater
wetlands. It is in the estuaries, where freshwater from the land mixes with marine
waters. True freshwater, by definition, does not have any measurable salt mixed into
the water so the amount cannot be quantified. It will always be zero parts salt no
matter how much or how little water is flowing. Saltwater, however, has a range in
the amount of salt dissolved in the water so it is possible to quantify this amount.
The key is that when more freshwater flows through the Everglades wetlands, more
freshwater enters the estuaries, and the amount of salt (the salinity) of the bay water
is reduced. The reverse happens when freshwater flows drop, whether due to natural
climate variations or to human alterations of the flow path. Less freshwater in the
wetlands means less flow into the estuaries, so the salinity of the bay water increases.
These changes in salinity as more or less freshwater enters the estuaries are reflected
in the mollusks (and other organisms) that live there because every species has a
range of salinity which it can tolerate. When we cut our cores into thin slices and
examine all the species present in each slice, we get a snapshot of the salinity of the
water at the time the animals were alive. When we count the numbers of each species in each core sample, we can use simple math to relate their modern salinity
preferences to the core sample (“the present is the key to the past”) and calculate an
average salinity at the time the sediment was deposited.
As we examined cores from different parts of Florida Bay, a clear picture
emerged. In the several hundred years before canal construction and water management, salinity had been gradually increasing by very small amounts and the populations of mollusks were relatively stable in the different zones of the estuary. This
gradual increase in salinity was due to rising sea level (more on this later). The
influence of the freshwater flow entering the system along the mangrove transitional
zone could be seen all the way out to the western edge of Florida Bay where the bay
water mixes with the waters of the Gulf of Mexico (Fig. 12.1). After the beginning
of the twentieth century, however, salinity increased across all the zones of the bay
12 Climate, Sea Level, and People – Changing South Florida’s Mangrove Coast
end of his career in science – he went on to earn his Ph.D. and is a well-respected
paleoecologist and professor. For some of my field assistants, however, the rigors of
our fieldwork sent them running to alternate professions.
A Picture of the Recent Past – More Freshwater
All the long hours and extensive field work paid off because the data we gathered on
living mollusks were essential to interpreting the past environments recorded in the
sediment cores collected on mud banks throughout Florida Bay. A core is collected
by inserting a hollow tube into the mud; when the tube is extracted, we have a record
of the sediment that accumulated at that site over time, with the oldest material on
the bottom. Remember that a primary goal of Everglades restoration is to restore
more natural freshwater flow through the ecosystem, so the goal of our work was to
establish what those pre-twentieth century flows were by using the information preserved in the sediment cores. Contrary to what you might think, the best place to
study the amount of freshwater flow in the past is not from cores in the freshwater
wetlands. It is in the estuaries, where freshwater from the land mixes with marine
waters. True freshwater, by definition, does not have any measurable salt mixed into
the water so the amount cannot be quantified. It will always be zero parts salt no
matter how much or how little water is flowing. Saltwater, however, has a range in
the amount of salt dissolved in the water so it is possible to quantify this amount.
The key is that when more freshwater flows through the Everglades wetlands, more
freshwater enters the estuaries, and the amount of salt (the salinity) of the bay water
is reduced. The reverse happens when freshwater flows drop, whether due to natural
climate variations or to human alterations of the flow path. Less freshwater in the
wetlands means less flow into the estuaries, so the salinity of the bay water increases.
These changes in salinity as more or less freshwater enters the estuaries are reflected
in the mollusks (and other organisms) that live there because every species has a
range of salinity which it can tolerate. When we cut our cores into thin slices and
examine all the species present in each slice, we get a snapshot of the salinity of the
water at the time the animals were alive. When we count the numbers of each species in each core sample, we can use simple math to relate their modern salinity
preferences to the core sample (“the present is the key to the past”) and calculate an
average salinity at the time the sediment was deposited.
As we examined cores from different parts of Florida Bay, a clear picture
emerged. In the several hundred years before canal construction and water management, salinity had been gradually increasing by very small amounts and the populations of mollusks were relatively stable in the different zones of the estuary. This
gradual increase in salinity was due to rising sea level (more on this later). The
influence of the freshwater flow entering the system along the mangrove transitional
zone could be seen all the way out to the western edge of Florida Bay where the bay
water mixes with the waters of the Gulf of Mexico (Fig. 12.1). After the beginning
of the twentieth century, however, salinity increased across all the zones of the bay
12 Climate, Sea Level, and People – Changing South Florida’s Mangrove Coast