Use of a dynamic, mechanistic simulation model to assess ecology and restoration
of the Florida Bay seagrass community
Christopher J. Madden, Amanda McDonald, Stephen P. Kelly
South Florida Water Management District, West Palm Beach, FL, USA
Marguerite Koch
Florida Atlantic University, Boca Raton, FL USA
A six-compartment dynamic simulation model was developed using STELLA
software on a desktop computer. The model has been translated into MATLAB
and FORTRAN for ease of incorporation into other models, such as
hydrodynamic, and geochemical. The model, current spatially averaged, will
form the kernel of a spatially explicit GIS-based landscape model. State variables
include Thalassia above ground (TAG), and below ground (TBG) material,
epiphytes (EPIPHYTE), Sediment Organic Carbon (OCS), Porewater Phosphate
(PPW), Porewater Sulfide (SPW). The governing equation for Thalassia is:
T (t) = T (t - dt) + (Tps – Tmort - Tresp – T trans - Tsl) * dt
Where the Thalassia biomass (T) at time t equals previous biomass plus the sum
of photosynthetic production minus mortality, respiratory, translocation and leaf
sloughing losses. The model runs with a timestep of 3 hr, and a simulation length
of 1 yr. During testing and calibration, the model is run for 3 or more years to
achieve stability and reduce initialization errors. Under base case conditions, the
model was stable for as long as 10 years. The model employs relationships
between nutrient and light resource availability, primary productivity rates in
Thalassia and epiphytes, and resulting uptake of nutrients from the sediment
porewater and water column pools. Releases to porewater pools occur from
decomposing plant material in the TAG, TBG and EPIPHYTE compartments.
Nutrient uptake of inorganic N and P is controlled by Michaelis Menten kinetics
via separate functions for DIN and DIP. Half saturation parameters are derived
from direct experimentation and literature values for Thalassia in subtropical
systems. The sulfide effect function is an empirical relationship developed from
mesocosm experiments on Florida Bay Thalassia under a range of H2S
concentrations (Erskine and Koch 1999, Kemp 2001). Mesocosm experiments
were also used to develop a salinity-productivity relationship for Florida Bay
seagrasses (Koch 2001).
We are utilizing the calibrated Florida Bay model to determine the tolerance
limits for the seagrass community to a variety of stresses, which occur either
naturally or as a consequence of human activities. Data from mesocosm
experiments on the effects of high salinity will be applied to the model to project
how water management effects on the salinity regime will impact the plant
community. The model is also being used to predict the mix of environmental
factors required to sustain or restore the seagrass community via management of
specific variables in the environment, particularly freshwater input, nutrient input
and water transparency. An index indicating the status of nutrient and light
limitation in the model was developed so that at any point in time the
instantaneous level of nutrient or light stress is known. This is useful in
determining the effects of seasonal or transitional stresses on the plants as well as
the effects of synergistic phenomena that may be incurred in combination with
salinity or temperature stresses.
In general, if light is sufficient, the plant will show signs of physiological stress
when nutrients are limiting. We are correlating field measurements of stress via
fluorescence (PAM) with the nutrient and light environments of the plants for
incorporation into the model. Measurements were taken in the field to determine
the level of stress of plants at five sites across a gradient of nutrient availability
from Duck Key to Rankin Lake. Because Florida Bay waters are shallow and
clear, it is generally accepted that it is nutrients that are limiting to plants in large
parts of the Bay. Indeed, experiments have shown that additions of nutrients
result in increased plant growth. However, the abundant growth of Thalassia in
many parts of the Bay, such as Rabbit Key and Twin Key Basin, beg the question:
what are the sources that provide nutrients in sufficient quantity to permit
luxuriant plant growth beyond that expected based on available nutrient
concentrations? Our model allows probing of this question by testing several
hypotheses related to nutrient supply and availability. First, we are running the
model using measured parameters of growth and tissue stoichiometry in the
model to develop projections of nutrient demand. Then, we are constraining
nutrient supply in the model from sources that are well documented- water
column and sediment pools. The free terms such as recycling rate and abiotic
processes are then being optimized to develop predictions of probable levels and
supply rates. These results will then be followed up with targeted verification
experiments.
In areas where seagrass growth is luxuriant, such as in Rabbit Key, and Barnes
Sound, there are indications that light limitation by self-shading, even in very
clear waters, may affect plant production. We are using the model to test this
hypothesis and to quantify the light levels that may be deleterious to plant
production. By manipulating the terms of the model to reflect expected changes
in nutrient and light resources due to both natural and managed effects, we hope
to predict the trajectory of the seagrass community in terms of biomass,
distribution and species composition, as restoration continues.
We are also directing model development toward addressing effects of the
interaction between seagrass plant morphology, bed configuration and ecological
rate processes. The physical structure of the bed and of individual short shoots
does affect the use of the resource environment. Light penetration through the
canopy and self shading are determined by length, density and number of leaves
on the short shoots; above to below ground biomass ratio determines the
allocation of fixed carbon, and rates of lateral versus vertical propagation.
We have developed the following general rules set to selectively apply to the
seagrass community in our model to test various hypotheses about how plant
behavior and form affect ecological processes:
In turbid environments, plants grow taller, and more widely spaced
In highly sedimentary or resuspended environments, more biomass is invested in
vertical rhizomes and in below ground material
In nutrient poor environments, leaves are thinner
In areas adjacent to bare areas, lateral growth is more rapid, decreasing with
increasing adjacent biomass
In areas with higher P, tissue concentrations of P are elevated and the uptake of P
is greater.
The model was adapted to incorporate a subset of these rules as a heuristic
exercise to determine the sensitivity of target variables biomass and production to
environmental conditions that invoke these rules.
This model has proved to be an effective organizing framework for information
about seagrasses in Florida Bay, demonstrating gaps in data and pointing to
research needs. It is also a valuable tool for hypothesis testing and a predictive
tool for testing management options.
Christopher J. Madden, Florida Bay and Lower West Coast Division, South
Florida Water Management District, 3301 Gun Club Rd., West Palm Beach, FL,
33406. Phone:(561)753-2400 x4647 Fax:(561)791-4077, cmadden@sfwmd.gov,
Question 4.