Primitive Equation Modelling ofPlankton Ecosystems
383
lated by a one-dimensional model. 1 shall describe the first attempt to simulate
patchiness with a three-dimensional primitive-equation model in 18.14.
At scales greater than the Rhines limit we have geographical variation, which is
the traditional concern of biological oceanography (Longhurst 1998). One-dimensional modelling can be used to simulate this large-scale horizontal variation, provided we use the appropriate surf ace fluxes to drive the model at each geographical
location. This raises technical problems, which can be solved by a method called
geographicalIy-Lagrangian integration (18.11). It also raises scientific questions of
population dynamics in a non-stationary environment, which 1 shall discuss in
18.12 and 18.16.
18.4.2 Zooplankton
Most plankton falI into the category of mindless micro-organisms for which we
can establish biological primitive equations. Many common zooplankton, such as
calanoid copepods, do have brains (Mauchline 1998), which have evolved to control their bodily functions in fixed ways, hard coded in their genes. The brains of
these zooplankton are like the computers in motorcars, which manage the smooth
functioning of the engine, gears and brakes, but which do not have the capability to
change their prescribed behaviour arbitrarily (a rather daunting prospect for motorists!). Some "neural network" computer programs are designed to learn from experience. However, such programmed learning follows formal rules, which can be
included in PE models. There is no reason to exclude plankton species that have
evolved to include pre-programmed adaptation in their portfolio of primitive equations.
18.4.3 Fish larvae
So where do we draw the line between mind1ess plankton, which obey biologic al
primitive equations, and higher marine organisms that do not? The line must be
drawn between plankton and fish, but where exactly? 1 suspect that newly-hatched
fish larvae falI into the category of mindless plankton, but that as they grow they
soon pass the line to become young fish with minds of their own. Some fisheries
biologists draw the line much higher, and assume that it is possible to make useful
models of the migration of adult fish (Steele 1977, Cushing 1995). Perhaps. The
test is not whether such models describe observed pattems of migration, but
whether they can predict how the migration will respond to a changing climate
(Laevastu 1993). Even if the line is drawn while the fish larvae are quite small,
they may still behave like mindless plankton during the early weeks of life when
most of their mortality occurs. If so, we can design primitive equation models to
compute the relationship between such losses and ambient weather. That would
make a significant contribution to introducing more science into fisheries recruitment (Cushing 1996).
There are many other problems of practical importance that depend on the ecology of plankton, and which can be addressed with greater scientific confidence
through the application ofbiological primitive equations. To give just a few exam-
383
lated by a one-dimensional model. 1 shall describe the first attempt to simulate
patchiness with a three-dimensional primitive-equation model in 18.14.
At scales greater than the Rhines limit we have geographical variation, which is
the traditional concern of biological oceanography (Longhurst 1998). One-dimensional modelling can be used to simulate this large-scale horizontal variation, provided we use the appropriate surf ace fluxes to drive the model at each geographical
location. This raises technical problems, which can be solved by a method called
geographicalIy-Lagrangian integration (18.11). It also raises scientific questions of
population dynamics in a non-stationary environment, which 1 shall discuss in
18.12 and 18.16.
18.4.2 Zooplankton
Most plankton falI into the category of mindless micro-organisms for which we
can establish biological primitive equations. Many common zooplankton, such as
calanoid copepods, do have brains (Mauchline 1998), which have evolved to control their bodily functions in fixed ways, hard coded in their genes. The brains of
these zooplankton are like the computers in motorcars, which manage the smooth
functioning of the engine, gears and brakes, but which do not have the capability to
change their prescribed behaviour arbitrarily (a rather daunting prospect for motorists!). Some "neural network" computer programs are designed to learn from experience. However, such programmed learning follows formal rules, which can be
included in PE models. There is no reason to exclude plankton species that have
evolved to include pre-programmed adaptation in their portfolio of primitive equations.
18.4.3 Fish larvae
So where do we draw the line between mind1ess plankton, which obey biologic al
primitive equations, and higher marine organisms that do not? The line must be
drawn between plankton and fish, but where exactly? 1 suspect that newly-hatched
fish larvae falI into the category of mindless plankton, but that as they grow they
soon pass the line to become young fish with minds of their own. Some fisheries
biologists draw the line much higher, and assume that it is possible to make useful
models of the migration of adult fish (Steele 1977, Cushing 1995). Perhaps. The
test is not whether such models describe observed pattems of migration, but
whether they can predict how the migration will respond to a changing climate
(Laevastu 1993). Even if the line is drawn while the fish larvae are quite small,
they may still behave like mindless plankton during the early weeks of life when
most of their mortality occurs. If so, we can design primitive equation models to
compute the relationship between such losses and ambient weather. That would
make a significant contribution to introducing more science into fisheries recruitment (Cushing 1996).
There are many other problems of practical importance that depend on the ecology of plankton, and which can be addressed with greater scientific confidence
through the application ofbiological primitive equations. To give just a few exam-