Responses of Benthic Primary Production to Nutrient Enrichment in the Upper
Florida Keys
Meredith Ferdie, and James W. Fourqurean
Florida International University, Miami, FL
Eutrophication of some coastal systems has resulted in shifts of primary producer
dominance, where slow-growing macrophytes such as seagrass and macroalgae
are replaced by fast-growing microalgae. Stoichiometric analysis of seagrass
tissue from the Florida Keys suggests a spatial pattern of nutrient limitation:
benthic coastal communities may be phosphorus limited close to shore and
nitrogen limited toward the offshore reef tract. This implies that benthic
community responses to eutrophication may differ along a spatial gradient from
nearshore to offshore habitats. A 14 month in-situ fertilization experiment was
carried out to assess the consequences of coastal nutrient enrichment on benthic
primary production and to determine the limiting nutrient of Thalassia testudinum
growth in the oceanside waters of the upper Florida Keys.
A factorial (nitrogen, phosphorus) design was conducted at 6 sites that were
divided into 2 groups, where each group represented the end points of the
stoichiometric gradient: nearshore (< 1km from the shoreline) and offshore (<
2km inside the reef tract). At each site, 24 plots (0.5 m2) were established in a 50
m x 50 m grid with 6 replicates of each treatment (N, P, N+P, Control), for a total
of 144 experimental plots. Sediments were fertilized monthly. Nutrient loading
rates of 0.77 N and 0.12 P g m-2 d-1 were based on current estimates of dominant
nutrient sources in the Florida Keys. This experiment was analyzed using a
mixed model ANOVA with a split-plot design and repeated measures. The
experiment began in May 2001 and terminated in July 2002.
Sampling was conducted on those benthic species considered most sensitive to
changes of nutrient availability in coastal marine systems: seagrass, macroalgae,
epiphytes and sediment microalgae. The following response variables were
determined every 4-5 months: Sediment nutrient and organic content, Thalassia
testudinum abundance, morphology, biomass, leaf growth rates, leaf and rhizome
nutrient content; macroalgae abundance; epiphyte total load and chlorophyll load;
sediment microalgae abundance. Syringodium filiforme morphology, biomass and
leaf nutrient content was analyzed at the end of the experiment.
The responses of benthic communities to nitrogen and phosphorus enrichment
varied appreciably between nearshore and offshore habitats. There were
significant increases in T. testudinum length, abundance, biomass and growth
rates in response to N addition, but not P addition, at offshore sites. This suggests
that T. testudinum in offshore habitats in the upper Florida Keys are nitrogen
limited. Significant increases in abundance and biomass of non-seagrass primary
producers were found exclusively at nearshore sites, and most often encountered
with +NP treatments. Macroalgae abundance (rhizophytic calcareous greens) and
total epiphyte loads increased with +NP at nearshore, but not offshore sites.
Increases in autotrophic epiphyte loads and sediment microalgae abundance were
also found only at nearshore sites.
-2 -1
Thalassia Areal Productivity (mg m d )
-2 -1
Thalassia Areal Productivity (mg m d )
The relative allocation of nitrogen and phosphorus to various seagrass system
compartments (sediment, seagrass, algae, water column) was similar between
nearshore and offshore sites. However, the net retention of added nutrients varied
considerably based on nutrient
4
and location. Although total
Inshore
system nitrogen doubled with
C
+N
nitrogen addition, less than 10 %
+NP
3
+P
of the added nitrogen was
retained
in
that
system.
Phosphorus
enrichment
2
dramatically increased total
system phosphorus and up to 82
% was retained in the system.
1
The differences in nitrogen and
phosphorus retention suggest
Sep 01
Mar 02
July 02
that
long-term
phosphorus
enrichment, unlike nitrogen
enrichment, may be important as
4
a driver of long-term community
Offshore
changes because most of the
C
+N
phosphorus is retained within
+NP
3
+P
the system, while much of the
nitrogen is lost from the system.
Nearshore systems may be
2
particularly
vulnerable
to
phosphorus enrichment because
the retention efficiency of
1
phosphorus at nearshore sites
(82 %) was nearly twice that of
Sep 01
Mar 02
July 02
offshore sites (49 %).
Seagrass leaf nutrient content increased at all sites with the addition of nutrients,
although the magnitude of these responses varied by time and location. Increases
in T. testudinum leaf N content were apparent quickly (3 months of fertilization),
whereas significant increases in P content were not observed at nearshore and
offshore sites until 10 months and 14 months of fertilization, respectively.
Relative changes in seagrass leaf and rhizome elemental content support strong
nitrogen-limitation of offshore seagrass, and suggest that phosphorus might be
more limiting at nearshore sites.
Growth responses in this experiment did not consistently follow the nutrient
content responses. Increases in T. testudinum leaf nutrient content, in the absence
of a positive biomass or growth response, implies that nutrient availability in the
study area was either insufficient to satisfy all plant metabolic demands for
nitrogen and/or phosphorus, or that T. testudinum was efficient at luxury
consumption.
This study demonstrated that eutrophication of the coastal waters of the Florida
Keys has the potential to affect benthic communities. Taxa-specific responses to
nutrient enrichment varied by location: macro- and micro-algae responded at
nearshore sites while seagrass species responded at offshore sites. The increase in
macroalgae and microalgae abundance at nearshore sites followed the predicted
sequence of community responses during eutrophication. This experiment also
demonstrated that seagrass communities in oceanside waters of the Florida Keys
are indeed nutrient limited, but the nature of that limitation is different for
nearshore and offshore sites. Thalassia testudinum responded to N+P addition at
nearshore sites, and N addition at offshore sites. The largest increase in seagrass
biomass and growth was found at offshore sites, demonstrating that the offshore
T. testudinum is strongly nitrogen limited.
This investigation increases our ability to predict the effects of nutrient
enrichment in seagrass communities and emphasizes the importance of nutrient
supply in determining benthic community composition. The response of benthic
communities in the oceanside waters of the upper Florida Keys to nitrogen and
phosphorus enrichment varied significantly between the nearshore and offshore
environment. Results may be used to model changes that anthropogenic
eutrophication may cause in the subtropical coastal marine waters of the upper
Florida Keys.
Fourqurean, James W., Florida International University, 11200 SW 8 th St.,
Department
of
Biological
Sciences,
OE
167,
Miami,
FL
33199
Phone: 305-348-4084, Fax: 305-348-1986, Jim.Fourqurean@fiu.edu,
Question 4