PE EIE[R-Rg RESEARCH ON - HJ Andrews Experimental Forest

greater than 7 .2:1 (16:1 by atoms) then P
should be limiting further growth and its addition
should increase production with th e
reverse case true for N . If the ratio is aroun d
7 .2 :1, then both nutrients would be require d
to increase production. Of course, this can
only be approximately true because th e
atomic ratio of 16N :1P in algal cells is an
average and interaction among nutrients ma y
occur such that ratios different than thi s
average may stimulate uptake of one or mor e
nutrients .
Lake Sammamish has one major phytoplankton
outburst which occurs in the sprin g
and is dominated by diatoms . In 1970, th e
outburst occurred during May (fig . 3), whic h
for unknown reasons was about 1 month later
than normal. Productivity followed a similar
trend as biomass (Chi a), but carbon assimilation
(productivity) per unit biomass wa s
noticeably greatest prior to the biomass peak .
During this outburst, NO 3 -N decreased by
more than 500 pg/l while ortho P04 -P decreased
by 8 µg/l at most, a removal ratio of
more than 60 :1 (fig. 4) . If this ratio of removal
represents uptake by phytoplankton ,
which it most likely does, then it is weighted
heavily in favor of N relative to the expected
requirements of cells. Inspection of figure 4
suggests that prior to the outburst, P was i n
shortest supply relative to needs, but after -
ward, N actually decreased at the surface to a
level less than P, and N should then have been
limiting. The great change in N relative to P, if
all a result of plankton uptake, suggests tha t
the supply of P0 4 -P was much greater than
indicated by the concentration decrease. This
supply could have come from rapid recyclin g
from the particulate phase and, thus, the relatively
constant P0 4 -P content only represented
the difference between supply and demand
of P .
The change in mean ratio of N :P in the
photic zone is compared to surface productivity
and biomass in figure 3 . Although it is
clear from the ratio that P was in shortes t
supply relative to needs prior to the outburst
and that the uptake was still apparentl y
weighted in favor of N, the mean ratio in th e
photic zone following the outburst remaine d
near the 7 .2 :1 required value . Thus, if th e
average values in the photic zone best reflect
the consequence of nutrient uptake by plank -
ton, then both N and P would appear almos t
1600 - 2 8
I 24
Q 1200
- 80
N E
U
rnE
12
800
8
4
ti
1
- 0
F
M
I I 1 I
A M J J A 5 0 N D
1970
Figure 3 . Primary productivity and chlorophyll a in the surface water and mean N/P ratio in the photic zone o f
Lake Sammamish during 1970 .
309