Nitrogen Versus Phosphorus Limitation of Benthic Primary Production and the
Role of Epiphyte Grazers in Florida Bay
Thomas A. Frankovich, Amy Willman, and James W. Fourqurean
Florida International University, Miami, FL
Kenneth L. Heck
Dauphin Island Sea Lab and University of South Alabama, Dauphin Island, AL
Restoration of the greater Everglades ecosystem will increase freshwater input to
Florida Bay. Ecosystem health will be dependent upon the quality and quantity of
that water. Recent evidence suggests that, depending on location, either nitrogen
or phosphorus limitation exists for Florida Bay primary producers. Increased
freshwater flow, as envisioned by the Restoration Plan, may increase loadings of
both nitrogen and phosphorus into the Bay. The regional and temporal effects of
these increased loadings on benthic primary production are a focus of this
investigation. The grazing activities of indigenous epiphyte mesograzers may
ameliorate some of the negative effects of increased nutrient loadings. The role of
these associated consumer organisms as mitigating factors are a second focus of
this investigation.
N+P
P
N
C
N+P
N
C
N+P
P
N
N+P
N
P
C
C
N+P
P
C
P
P
N
C
N+P
N
Figure 1. Schematic of 2 X 2 factorial design employed at each site. Individual
plots are 0.25 m-2 and are separated by 1.5 meters. Treatments were assigned
randomly within each column and row.
To determine nutrient limitation for benthic primary producers, and to assess the
regional and temporal variability of N vs. P limitation, long-term replicated 2X2
factorial experiments (Figure 1) designed to assess the effects of nutrient
additions (N, P, N + P) are being conducted at six sites within Florida Bay (Figure
2). Sites were selected in northeastern, central and western Florida Bay along an
existing nutrient availability gradient (Fourqurean et al., 1993; Boyer et al., 1997).
Slow-release nitrogen fertilizer (Poly-on, 38-0-0, Pursell Technologies) and
phosphorus as granular phosphate rock (Multifos, IMC Phosphates) are being
applied to the sediment surface at rates of 0.62 g N m-2 day-1 and 0.09 g P m-2 day1
. The following response variables are being measured: seagrass biomass,
productivity, Braun-Blanquet abundance, and leaf CNP, total epiphyte load,
epiphyte chlorophyll-a, epiphyte accumulation rate, and benthic chlorophyll-a.
+
Duck
+
Nest
+
Bob Allen
+
Rabbit
+
9-Mile
+
Sprigger
Figure 2. Location map of long-term fertilization sites. Chamber experiment
conducted on banktop adjacent to Rabbit site.
To assess the role of epiphyte mesograzers under conditions of increased nutrient
loadings, a series of short-term field mesocosm experiments are also being
conducted. During August 2002, the abundances of 3 mesograzers (i.e., caridean
shrimp - Thor and Hippolyte spp, the hermit crab Paguristes tortugae, and the
gastropod Turbo castanea) and nutrient loading rates were manipulated inside
acrylic chambers set within a Thalassia testudinum meadow. Nutrients were
supplied to the water column within each chamber via slow-release fertilizer
(Osmocote, 18-6-12). A two-way ANOVA design consisting of 3 levels of
nutrient treatments and 8 levels of grazer treatments was employed. The 24
unique experimental treatments were replicated 3 times. The following response
variables were measured: seagrass short-shoot biomass and leaf CNP, watercolumn nutrients, total epiphyte load, and epiphyte chlorophyll-a.
Preliminary Results - In October 2002, at the long-term fertilization sites, initial
mean Braun- Blanquet Thalassia testudinum abundances measured on a scale
from 0 to 5 ranged from 1.5 (Sprigger Bank) to 5.0 (Rabbit Key Basin).
Syringodium filiforme was only abundant (BB = 3.1) at Sprigger Bank. Mean total
epiphyte loads ranged from 0.22 mg dw cm-2 at Rabbit to 12.16 mg dw cm-2 at
Bob Allen. Epiphyte chlorophyll-a ranged from 0.02 ug cm-2 at Rabbit to 2.63 ug
cm-2 at Bob Allen.
Preliminary analyses of the 2002 chamber experiments indicate fertilized
treatments, relative to unfertilized treatments, produced higher mean total
epiphyte load, epiphyte chlorophyll-a, epiphyte autotrophic index, water-column
chlorophyll-a, leaf nitrogen and phosphorus, water column dissolved inorganic
nitrogen and soluble reactive phosphorus. Grazer treatments, relative to ungrazed
treatments, produced higher mean total epiphyte load, epiphyte chlorophyll-a,
epiphyte autotrophic index, water-column chlorophyll-a, and Thalassia shortshoot biomass.
Further results from the long-term fertilization experiment and statistical analyses
from the 2002 chamber experiment will be presented.
Future Plans - The replicated nitrogen and phosphorus addition experiments will
continue through summer 2005. The response of the epiphytic diatom species
assemblages to the nutrient manipulations will also be assessed beginning in
February 2003. Further mesocosm experiments will also be conducted in other
areas of Florida Bay during different months to assess seasonality and the role of
different indigenous grazer organisms.
This research is being supported by a CESI grant from Everglades National Park,
Department of the Interior
Tom, Frankovich, Florida Bay Interagency Science Center, 98630 Overseas
Highway, Key Largo, FL 33037
Phone: 305-852-2668, FAX: 305-852-0325, frankovich@virginia.edu, Question 4