Isotopic fingerprinting of nutrient sources and biological sinks in Florida Bay: A
geochemical tool for evaluating ecosystem response to changing nutrient inputs
Ana M. Hoare, David J. Hollander, Cynthia A. Heil and Susan Murasko,
College of Marine Science, University of South Florida, St. Petersburg, FL 33705
Patricia M. Glibert, Marta Revilla, and Jeffrey Alexander
University of Maryland Center for Environmental Research, Horn Point
Laboratory, Cambridge, MD
The stable isotopic measurements of carbon (13C) and nitrogen (15 N) in
biological materials and dissolved species is a geochemical tool capable of tracing
varying nutrient sources and their assimilation into biomass, delineating different
biosynthetic pathways, and assessing trophic relationships among organisms
within an ecosystems. The proposed re-establishment of surface flow through the
Everglades is expected to change the amount, sources and ratios of dissolved
nutrients (organic and inorganic) delivered to the bay potentially inducing an
ecosystem response of changing structure and functions in both planktic and
benthic habitats. As part of a NOAA-funded program, we have initiated a spatial
and temporal study focused on determining the carbon and nitrogen isotopic
composition of nutrient sources and their biological sinks (e.g., seagrass, algae,
sediment) within the Everglade watershed and Florida Bay. The goal of this study
is to develop isotopic tools that can monitor nutrient changes and evaluate
ecosystem response associated with pending Everglade restoration.
The first of four sampling periods was conducted in November 2002. Waters,
sediment cores, seagrass and other aquatic vegetation were collected at 6
geographically and ecologically unique sites within Florida Bay (Little Maderia
Bay, Duck Key, Sunset Cove, Rankin Bight, Barnes Key, Rabbit Key), and a Gulf
of Mexico influenced western Florida Bay site. In addition, two transects, one in
the Everglades and the other along the bay/reef sides of Florida Keys, were also
sampled for water, sediments, seagrass, and aquatic and terrestrial vegetation.
Surface waters were filtered for particulate organic matter (POM) and filtrate, and
then, as with sediments and vegetation samples, were immediately frozen for later
stable isotope analysis. Initial efforts have focused on determining the carbon and
nitrogen isotopic composition of POM, aquatic vegetation (particularly Thalassia
testudinum), and surface sediment from the bay sites, and from the three
Everglade watersheds (Canal 111, Taylor Slough, and Shark River Slough),
representing the different source waters entering the bay.
Initial results indicate that the carbon and nitrogen isotopic composition of bay
organics (the biological materials isolated from the water column (POM) and the
benthic habitats, i.e. seagrass, and sedimentary organic matter) provides a
sensitive indicator of local to regional changes in nutrient sources, and variations
in carbon and nitrogen biosynthesis and cycling. Spatial trends in the isotopic
composition of bay organics mimic the spatial variations in the nutrient
stoichiometry (e.g. N/P) of bay waters. A pronounced 10‰ east-west gradient in
15 N values and a distinct 15‰ north-south gradient in 13C values results in a
well-defined northeast to southwest isotopic trend in organic materials that
defines regional controls on nutrient sources, their availability, and their
incorporation into biomass.
The range of 15N values for bay organics was extraordinarily large (10‰) with
values ranging from 13‰ in the eastern portion of the bay (Duck Key, Sunset
Cove, Little Madeira) to 3‰ in the western portion (Rabbit Key, Barnes Key,
Gulf Station). This large range and systematic distribution in the 15N values of
bay organics overlaps directly an observed 10‰ range and east-west gradient in
15N values for POM isolated from the three watersheds draining the Everglades.
The range and spatial similarity in the 15N composition of organics between bay
and Everglades samples is remarkable. This data infers that a wide array of
natural (e.g., terrestrial, oceanic, and recycled) and anthropogenic (e.g. urban,
sewage, agricultural) nutrient sources are being introduced into well-defined
regions of the bay and that nutrient sources originating in the Everglades directly
influence nitrogen cycling in bay waters and sediments. Furthermore, the spatial
trends in the 15N values of bay organics from 14 to 4‰ reflect regional
availability of nutrient and their control on the biogeochemical process of
nitrogen assimilation (N2 vs. NO3) and recycling (e.g., denitrification,
remineralization). For example, the isotopic data indicates an increase in the
relative importance of nitrogen fixation by planktic and benthic algae in the
western portion of the bay as waters become nitrogen-limited.
Carbon isotopic composition of bay organics display a strong north-south gradient
of over 13‰ with values of –7 to -11‰ in the northern sites (Duck Key, Little
Maderia, Rankin Bight) and more depleted values of –20‰ at more southern sites
(Barnes Key, Rabbit Key , Gulf Station). This broad range of carbon isotopic
compositions likely reflects the assimilation of different sources of carbon (HCO3
vs. CO2) during biosynthesis. Macrophytes, which are concentrated on the
northern margin of the bay, preferentially utilize bicarbonate that is 8 – 12‰
enriched relative to carbon dioxide. The more depleted 13C values occur in the
southern portion of the bay sites suggest the dominance of C3 photosynthesizing
algae (benthic and planktic) which selectively ultilize dissolved CO2. 13C values
of Everglade organics are depleted relative to bay samples with values between –
21 and –33‰ indicating inputs from C3 algae and terrestrial plants.
The nitrogen isotopic composition of seagrass species, Thalassia testudinmu,
measured at our 6 bay sites are significantly different than those measured by
Anderson and Fourqurean (in press) which focused on determining the inter-and
intra-annual variations at two sites significantly westward of the bay interior. In
our samples we observed a well-defined 9‰ gradient in the 15N of seagrass from
14‰ in the east to 5‰ at our westernmost site, similar to the isotopic range seen
for all bay organics and for POM from the different the Everglades watersheds.
In contrast, 15N values of T. testudinum sampled from the western margin of the
bay were relatively depleted, showing a narrow range between –1.2 and 2‰
(Anderson and Fourqurean, in press) and exhibiting no overlap with our current
measurement of T. testudinum in the bay proper. The differences in the 15N
values between the two studies are significant. Our study focuses on the interior
of the bay and may provide a truer representation of the nutrient dynamics and
biogeochemical processes occurring throughout the bay, whereas Anderson and
Fourqurean (in press) investigated sites at western edge of Florida Bay that reflect
a greater gulf influence on nutrient dynamics. Efforts in the future will be directed
at correlating variations in the health of seagrass beds to spatial and temporal
variations in their nitrogen and carbon isotopic compositions.
Future work will also include spatial and temporal analyses of the isotopic
composition of dissolved inorganic and organic nutrients (DIN, DIC, DON, DOC)
to accompany ongoing isotopic determinations of the biological materials.
Developing isotopic relationships between nutrient sources and biological
materials will allow evaluation of the ecological response in the bay to changing
nutrient inputs and biogeochemical cycling. Complementary work on the isotopic
composition of bioassay experiments with nutrient enrichments and alterations
will help define isotopic trends in organic materials resulting from changes in
abundance and sources of nutrient inputs associated with the restoration of surface
water flow through the Everglades. Finally, this research will incorporate isotopic
analyses of organics in well-dated sediment cores from the 6 bay sites in order to
evaluate the historical response of the bay ecosystem to documented changes in
hydrologic conditions and nutrient cycling. The extensive analytical approach
undertaken in this study promises a sensitive, cost-effective monitoring tool.
David J. Hollander, College of Marine Science, University of South Florida, 140
7th Ave. S., St. Petersburg, FL, 33701, Phone: 727-355-1019, Fax: 727-553-1189,
davidh@marine.usf.edu