Seagrasses are marine flowering plants that live entirely underwater.
They reproduce both asexually and sexually, growing by rhizome extension.
Seagrass meadows are globally distributed in areas with high light levels
and clear water, within intertidal and shallow subtidal zones. Due to their
light constraints, growth is restricted to the photic zone and is not possible
in deeper waters. Seagrasses are ecological engineers, altering the environment
by changing the water flow, nutrient cycling, sediment stabilization, and
providing refuge for benthic organisms. In recent decades, global seagrasses
have dramatically declined in density, extent, and biodiversity. Multiple
stressors have directly contributed to these declines, including climate changes,
shifts in water quality, and increased contaminants entering coastal waters.
Conducting studies on why seagrass populations are declining can be complex,
time consuming, and costly. Designing a simulation model with data collected
from field and lab experiments allows us to mimic realistic environmental
changes and predict their impacts on a model organism. Models can predict how
populations should react based on stochastic stimuli such as the intensity of
light reaching seagrasses. Individual-based models (IBMs) are used to predict
population-level impacts by modeling how individuals react to abiotic and biotic
conditions in their local environment in terms of their survival, growth, and
reproduction, each resulting from the individuals’ unique behavior, genetics,
and experiences. Halophila johnsonii is a rare threatened seagrass located on
a 200km stretch of Atlantic Floridian coast. This species has relatively low
biomass, a high turnover rate, and is tolerant to fluctuations in temperature
and salinity. We constructed an IBM to predict how H. johnsonii should be impacted
by a variety of light conditions that resemble natural and anthropogenically-driven
reductions in light levels. The model predicts that seagrass patches will decrease
in density and extent when exposed to more severe and/or more frequent light
reduction. This suggests that without improvements in water quality, H. johnsonii’s
density and distribution should decrease. While this species is relatively
small-bodied, it provides ecosystem services in shallower waters than many
species are able to inhabit, increasing concerns about the impacts of its decline.
The java implementation of this model was written at Rowan University by Vincent Scavetta
under the supervision of Dr. Courtney Richmond. The code was initially written in FORTRAN
90 by Dr. Richmond in conjunction with Dr. Rose. The model uses real growth data collected
from field and lab based experiments on the seagrass H. johnsonii. Solar data was collected
from the NOAA database measured at Miami station #722030 (West Palm Beach Intl Arpt, FL).
The raw solar data was transformed to produce a value relative to the maximum solar irradiance
recorded. These transformed values were implemented into the mode to simulate typical
irradiance values throughout a given year.
In its current state, the model is very basic in its nature. There is currently only one
variable for light reduction that is adjusted thoughout the course of the simulation. By the
end of this project, we hope to have a more realistic simulation that takes into account the
- Shading due to other biomass in the area.
- Variable storm lengths.
- Water level calculations to account forlunar tides.
- Localized effects such as boat traffic and dock shading.
Storm intensity effect dampens as frequency decreases
These figures show that as storm frequency decreases the population size remains more stable.
Also the deeper the seagras live, the less effect the storms have. As storm frequency increases, the population size and density decreases more rapidly in shallow waters
Sequence diagram for the Individual Based Model
The model follows this sequence of events. The model starts by initializing the startup parameters, output files, ect.
Then for every day, the simulation will generate the surface lihgt for that particular day, assign it for each cell, then simulate the population.
After the simulation is finished, a final statistic analyis is preformed.
View of simulation after 365 days
Each blue "stick" represents a connection between two nodes. The orange "sticks" are
new connections that were created this cycle of the simulation
Halophila johnsonii growing in the wild
h.johnsonii are a small species of seagrass. This particular patch was growing off the eastern coast of florida.