8.2. Model Formulation
145
atom species (Humborg et al. 1997). These non-siliceous phytoplankton are
predominantly flagellates, often dinoflagellates, forming dense blooms along
the coast. The occurrence of these blo oms has raised concern as they are increasing in frequency and are often toxic (see Anderson and Garrison 1997).
These toxic blooms are associated with massive mortalities in fish (both
farmed and wild) , shellfish, birds , and marine mammals. Although there are
no global estimates of economic losses due to harmful algal blooms, estimates from isolated individual events provide some indication of the scale of
the problem. In Alaska alone , it is estimated that losses from the shellfish industry are $50 million annually (Neve and Reichardt 1984). The Gulf coast of
Florida experiences frequent red tides , often accompanied by dead fish
washing up on beaches, contaminated shellfish, and human respiratory
problems owing to toxics aerosolized by the surf. Habas and Gilbert (1974)
estimated a loss of $20 million for each event in 1974.
Increased sediment loading and changing bloom composition also modify the relative light fields to which phytoplankton are exposed. As with nutrients, different phytoplankton groups and species respond differently to
light intensity with respect to photosynthesis and growth. This exerts an additional selective mechanism on the phytoplankton community structure
that may influence the coastal food web. Recent significant changes in environmental conditions select against species that have narrow tolerance
ranges with respect to nutrients and light.
8.2. Model Formulation
Here we demonstrate the dynamics and multiple feedback mechanisms between phytoplankton abundance, light and nutrients. The model integrates
the concepts of nutrient uptake kinetics, light utilization and photosynthesis
with respect to changing environmental conditions. Given these interactions , the impacts of changing nutrient ratios, the effects of water-column
mixing and light propagation on phytoplankton abundance and community
structure can be predicted. The model is designed to help elucidate some
potential anthropogenic impacts on phytoplankton communities in coastal
ocean environments.
8.2.1. Physical Environment
The physical portion of the model follows a 1 m 3 parcel of water (Lagrangian) in one dimension (vertical) over time. Following a single parcel
of water is appropriate because the objective of the model is to examine a
large-scale phenomenon, where it is assumed that similar processes are occurring in adjacent water masses . This parcel of water is then mixed, expressed as a simplified description of circular motion, which leads to a sinshaped variation of the depth (2) similar to Pahl-Wostl (992).
145
atom species (Humborg et al. 1997). These non-siliceous phytoplankton are
predominantly flagellates, often dinoflagellates, forming dense blooms along
the coast. The occurrence of these blo oms has raised concern as they are increasing in frequency and are often toxic (see Anderson and Garrison 1997).
These toxic blooms are associated with massive mortalities in fish (both
farmed and wild) , shellfish, birds , and marine mammals. Although there are
no global estimates of economic losses due to harmful algal blooms, estimates from isolated individual events provide some indication of the scale of
the problem. In Alaska alone , it is estimated that losses from the shellfish industry are $50 million annually (Neve and Reichardt 1984). The Gulf coast of
Florida experiences frequent red tides , often accompanied by dead fish
washing up on beaches, contaminated shellfish, and human respiratory
problems owing to toxics aerosolized by the surf. Habas and Gilbert (1974)
estimated a loss of $20 million for each event in 1974.
Increased sediment loading and changing bloom composition also modify the relative light fields to which phytoplankton are exposed. As with nutrients, different phytoplankton groups and species respond differently to
light intensity with respect to photosynthesis and growth. This exerts an additional selective mechanism on the phytoplankton community structure
that may influence the coastal food web. Recent significant changes in environmental conditions select against species that have narrow tolerance
ranges with respect to nutrients and light.
8.2. Model Formulation
Here we demonstrate the dynamics and multiple feedback mechanisms between phytoplankton abundance, light and nutrients. The model integrates
the concepts of nutrient uptake kinetics, light utilization and photosynthesis
with respect to changing environmental conditions. Given these interactions , the impacts of changing nutrient ratios, the effects of water-column
mixing and light propagation on phytoplankton abundance and community
structure can be predicted. The model is designed to help elucidate some
potential anthropogenic impacts on phytoplankton communities in coastal
ocean environments.
8.2.1. Physical Environment
The physical portion of the model follows a 1 m 3 parcel of water (Lagrangian) in one dimension (vertical) over time. Following a single parcel
of water is appropriate because the objective of the model is to examine a
large-scale phenomenon, where it is assumed that similar processes are occurring in adjacent water masses . This parcel of water is then mixed, expressed as a simplified description of circular motion, which leads to a sinshaped variation of the depth (2) similar to Pahl-Wostl (992).