158
8. Impact of Dynamic Light and Nutrient Environments
higher concentrations of N, the population of dinoflagellates increases . This
is despite setting the growth rate higher for diatoms (1.05). This result is essentially being found in the Baltic Sea and coastal regions (Elmgren 1989).
Si concentrations have been shown to either remain the same, or to decrease, with the amount of N loading increasing, thereby changing the ratio
of nutrients and selecting against siliceous phytoplankton such as diatoms.
With Si concentrations higher (0.003 or 0.008 moles/m"), there is ample Si
to maintain maximal growth of diatoms and dominate over the year.
8.4. Conclusions
This model illustrates the use of STELLA in describing a competitive biological interaction between two phytoplankton species. Although simple in its
approach, the model incorporates some of the complexities inherent to the
processes resulting in phytoplankton growth, such as physical mixing and
the responses to in water light fields. As with any modeling exercise , it was
important in this model to begin the model formulation with a simple and
basic interaction, then build in the complexity. The center of the model is
the response of growth to nutrients and the differential response of the individual groups to the two nutrients. Light is added and used to regulate the
nutrient requirement for each species and light, in turn, is driven by the
mixing dynamics .
The response of the model is similar to the competitive interactions in
the marine environment, with one phytoplankton group dominating when the
nutrient required for growth is present. Although diatoms in this case have
ample supplies of nitrate, if the nutrient concentration for silicate is not adequate for maximal growth, this model shows that dinoflagellates will always
dominate . The increased occurrence and frequency of non-siliceous phytoplankton blooms, often dinoflagellates, have become a concern because they
are often toxic and can increase mortality in fish, shellfish, birds, and marine
mammals. Changes in phytoplankton communities in the coastal ocean will
directly impact both the local coastal food webs and pelagic species that depend on coastal habitats for reproduction and/or seasonal feeding. From a
larger ecological perspective, these model results, and the real changes that
are occurring in coastal phytoplankton assemblages, illustrate the significance
and far-reaching effects of nutrient runoff and coastal eutrophication.
8.5. Questions and Tasks
1. For the physical mixing part of the model , alter the parameters for mixing depth and period of mixing with time and look at the changing light
dynamics. What is the mixing depth at which the populations are lightlimited for growth during mid-July on Julian day 197?
8. Impact of Dynamic Light and Nutrient Environments
higher concentrations of N, the population of dinoflagellates increases . This
is despite setting the growth rate higher for diatoms (1.05). This result is essentially being found in the Baltic Sea and coastal regions (Elmgren 1989).
Si concentrations have been shown to either remain the same, or to decrease, with the amount of N loading increasing, thereby changing the ratio
of nutrients and selecting against siliceous phytoplankton such as diatoms.
With Si concentrations higher (0.003 or 0.008 moles/m"), there is ample Si
to maintain maximal growth of diatoms and dominate over the year.
8.4. Conclusions
This model illustrates the use of STELLA in describing a competitive biological interaction between two phytoplankton species. Although simple in its
approach, the model incorporates some of the complexities inherent to the
processes resulting in phytoplankton growth, such as physical mixing and
the responses to in water light fields. As with any modeling exercise , it was
important in this model to begin the model formulation with a simple and
basic interaction, then build in the complexity. The center of the model is
the response of growth to nutrients and the differential response of the individual groups to the two nutrients. Light is added and used to regulate the
nutrient requirement for each species and light, in turn, is driven by the
mixing dynamics .
The response of the model is similar to the competitive interactions in
the marine environment, with one phytoplankton group dominating when the
nutrient required for growth is present. Although diatoms in this case have
ample supplies of nitrate, if the nutrient concentration for silicate is not adequate for maximal growth, this model shows that dinoflagellates will always
dominate . The increased occurrence and frequency of non-siliceous phytoplankton blooms, often dinoflagellates, have become a concern because they
are often toxic and can increase mortality in fish, shellfish, birds, and marine
mammals. Changes in phytoplankton communities in the coastal ocean will
directly impact both the local coastal food webs and pelagic species that depend on coastal habitats for reproduction and/or seasonal feeding. From a
larger ecological perspective, these model results, and the real changes that
are occurring in coastal phytoplankton assemblages, illustrate the significance
and far-reaching effects of nutrient runoff and coastal eutrophication.
8.5. Questions and Tasks
1. For the physical mixing part of the model , alter the parameters for mixing depth and period of mixing with time and look at the changing light
dynamics. What is the mixing depth at which the populations are lightlimited for growth during mid-July on Julian day 197?