Sunda and Huntsman (1998) suggested that the
cellular metal concentrations in phytoplankton
under steady-state conditions equal to the net
uptake rate divided by the cell- speci®c growth rate.
Thus, algal cells which grow at a faster rate may
have a lower metal concentration due to growth
biodilution. In this simple bioaccumulation model,
cell growth rate was treated as a growth biodilution
term. Because the uptake rates of Cd and Zn were
related to the cell growth rate in the diatom cells,
the metal concentration may change with a change
in cell growth. For Se, its uptake rate was not
dependent on the growth rate, thus its concen-
tration in the cells would decrease with increasing
In¯uence of nutrient on metal uptake 319
growth rate due to growth biodilution. Interest-
ingly, Cd concentrations in the suspended particles
during the spring phytoplankton bloom in San
Francisco Bay increased by more than 2 com-
pared to pre-bloom Cd concentration, whereas the
concentrations of Cu, Ni and Zn were relatively
constant during the bloom period (Luoma et al.,
1998). An increase in growth rate coupled with an
increase in uptake rate may be responsible for the
change in metal concentration observed in natural
phytoplankton (Luoma et al., 1998). Of course,
many other factors such as metal speciation and
metal concentration may also contribute to the vari-
ation in metal concentration during the phytoplank-
ton bloom.