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178
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J.M. THEBAULT, M.J. SALENÇON
Factors in disappearance : prédation by zooplankt o n a n d sinking a r e the factors for d i s a p p e a r a n c e
o f t h e first g r o u p of algae. A t least two causes m a y
b e f o u n d for m a j o r variations in sinking speed. O n e
is physical, related to turbulence in the m e d i u m , and
c a n n o t b e simulated here ; t h e other d e p e n d s on
n u m e r o u s factors such as cell size, t e m p e r a t u r e , age
o f the p o p u l a t i o n or lack of n u t r i m e n t . W e chose
this latter factor for o u r model, linking sinking speed
t o g r o w t h limitation d u e t o n u t r i e n t s .
N a t u r a l m o r t a l i t y is considered to be a negligible
f a c t o r in c o m p a r i s o n to p r é d a t i o n a n d sinking.
C y a n o b a c t e r i a c a n generally not develop in a turb u l e n t m e d i u m a n d at low t e m p e r a t u r e s (Steinberg
& H a r t m a n 1988, Z o h a r y & Breen 1989). A s we d o
n o t s i m u l a t e t u r b u l e n c e , we have linked m o r t a l i t y
in this g r o u p of algae to t e m p e r a t u r e ; field
o b s e r v a t i o n s at P a r e l o u p indicate t h a t , generally
speaking, agitation in the w a t e r mass is at its maxim u m level in winter a n d spring, when the temperature is the lowest.
Zooplankton
T h e rate of ingestion depends primarily in the
short term on t e m p e r a t u r e a n d nutrient concentration. W e have simplified the model p r o p o s e d by
Thébault (1985) which permits simulation of their
c o m p o u n d e d effect. T h e function linking ingestion
to the quantity of available prey has been replaced
by a hyperbolic function (Mullin et al. 1975). T o
this, we have added a threshold below which feeding is nil. Only the « m a x i m u m feeding » coefficient is a function of the t e m p e r a t u r e . This simplification permits a considerable reduction in the number of coefficients needed.
Rates of assimilation a n d n a t u r a l mortality are
constant. P r é d a t i o n by fish, the principal cause of
Phytoplankton growth
Grazing
F i g . 1. D i a g r a m o f t h e d y n a m i c s o f t h e b i o l o g i c a l m o d e l .
F i g . 1. D i a g r a m m e d e f o n c t i o n n e m e n t d u m o d è l e b i o l o g i q u e .