Carbon and Nitrogen Stable Isotope Composition of Sedimentary Organic Matter from
Florida Bay: Evidence of Historic Seagrass Distribution and Paleoproductivity.
Samantha Evans, William Anderson, James Fourqurean, Rudolph Jaffe, Evelyn Gaiser,
Laurel Collins, Suzie Escorcia
Florida International University, Miami, FL
The shallow marine waters of Florida Bay provide an ideal environment for seagrasses,
which are the most common benthic community in the region. However, seagrass
communities are susceptible to a variety of anthropogenic disturbances, particularly
changes in water quality, and environmental conditions in Florida Bay have become a
concern due to recent increases in salinity, algal bloom frequency, and seagrass mortality.
These changes have been attributed to 20th century alterations in freshwater discharge
from the Everglades to Florida Bay, deteriorated water quality, and changes in exchange
between Florida Bay and the Atlantic Ocean. In order to better understand environmental
change and impacts on the seagrass community over long time scales, sediment cores
were collected in Summer 2002 from four locations in Florida Bay (Ninemile Bank, Bob
Allen Bank, Russell Key, and Trout Creek) for multiple proxy analyses of seagrass
abundance. All cores were collected to bedrock, with sediment depths ranging from 94 to
244 cm, potentially representing a 5000-year time series. 210Pb dates for the Bob Allen
Bank and Trout Creek cores indicate that the upper 30 and 50 cm of sediment represent
the past 100 years, respectively. Basal peat evident in the Bob Allen Bank core has been
constrained with 14C-dating at between 2770 and 4190 b.p. Cores were sampled in 2-cm
increments, representing an average of 2-10 years for bulk isotopic analysis of sediment
organic content.
In two cores analyzed, Bob Allen Bank and Trout Creek, significant differences are
evident in the carbon and nitrogen isotopic signatures both between sites and over time
within site, with 15N values ranging between +3.2 and +7.6‰ and 13C between –24.3
and –8.6‰, both oscillating over time. These patterns may reflect changes and mixing in
source material between terrestrial and estuarine organic matter, as terrestrial material is
more depleted in terms of both 15N and 13C than estuarine organic matter due to
differences in nutrient supply and recycling. Fluctuations in 15N and 13C signatures
may also reflect changes in nutrient availability and primary production in the marine
environment. Enrichment trends in nitrogen and carbon isotopic signatures may result
from increases in productivity levels, limitation of carbon and/or nitrogen, or a
combination of both factors, as changing production demand and nutrient supply
influence processes of isotopic discrimination. Finally, there is evidence of decoupling
between the carbon and nitrogen isotopic systems, although values throughout suggest
that buried organic matter is predominantly seagrass-derived over time. Further bulk
isotopic analyses of remaining cores, together with organic biomarker analyses, diatom
and foraminiferal community analyses, and completion of age models for remaining
cores will allow more definitive interpretation of the isotope patterns with implications to
seagrass productivity levels over long time scales in Florida Bay.
Samantha Evans, Department of Earth Sciences, Florida International University, 11200
SW 8th St., PC 344, Miami, FL, 33199
Phone: 305 348-3147
Fax: 305 348-3877
Sevan002@fiu.edu, Question 4