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

during 1963 to 1967 and from Lake Sammamish
in 1968, by the Municipality o f
Metropolitan_ Seattle . The diversion remove d
about 55 percent of the annual external P
supply and 12 percent of the inorganic N
from Lake Washington . Phosphorus and N
external supply into Lake Sammamish was
reduced by about 65 and 22 percent, respectively.
Although the N supply to Lake Washington
is not greatly different than that t o
Lake Sammamish, Lake Washington receive d
about twice the supply of P than Lake Sammamish
(1 .06 vs. 0.56 g/m 2 ) before diversio n
and this difference is still maintained afte r
diversion (0 .48 vs. 0.20 g/m 2 ) (table 5). Th e
diversion brought the P supply to Lake Washington
to near Vollenweider's danger limit fo r
eutrophication and below the danger limit i n
Lake Sammamish (table 5). The prediversio n
N supply did not exceed the danger limi t
nearly as much as did P in either lake so N
diversion may be considered less significan t
than P. According to the alteration in P sup -
ply and if P is most significant in these lake s
as Vollenweider's relationship shows, the n
p h y t o p l ankton productivity and biomas s
should have been reduced in both lakes .
The mean winter (December to April) total
P concentration in the surface waters of Lak e
Washington decreased over 60 percent following
sewage diversion (Edmondson 1970) .
Although diversion was not complete unti l
1967, winter P concentrations began gradually
decreasing soon after the 4-year diversio n
process was initiated in 1963 . In contrast,
little difference can be seen in the 197 1
winter mean P concentrations in Lake Sammamish
3 years after diversion in 1968 (fig .
1) .
Phytoplankton biomass quickly responde d
to the reduction in mean winter P0 4 -P content
in Lake Washington as shown by Edmondson
(1970) (fig. 2) . Chl a decreased in
direct proportion to P0 4 -P, while the other
macronutrients, C and N, varied independent
of Chl a. A significant change in phytoplankton
biomass, production or water clarity ha s
not been observed in Lake Sammamish.' In
I R. M. Emery, C. E. Moon, and E . B. Welch ,
unpublished data .
one respect this is gratifying because winter
mean NO 3 -N and total P concentrations,
which should indicate available supply, also
have not changed . In another respect, the delayed
responses of winter mean P content to
diversion of over one-half the annual suppl y
to the lake suggests that factors controllin g
these winter levels in the two lakes are different
in either kind or magnitude .
Factors controlling winter P concentrations
in Lake Sammamish are not yet understood ,
but comparison of seasonal changes in total P
and morphological characteristics between the
two lakes offers a hypothesis . Winter P content
remained high until the spring diato m
pulse in Lake Washington following which a
moderate decrease was observed (see footnot e
1). In Lake Sammamish, total P content
normally increased to peaks as high as 70 t o
100 pg/1 following turnover in November . Instead
of remaining high until the sprin g
diatom pulse in April, as it does in Lake Washington,
total P decreased during the winter in
Lake Sammamish before the diatom pulse
(fig. 1). The surface water in Lake Sammamish
has been observed to become cloudy
with particulate matter during turnover an d
remain that way for a month or two before
clearing. Phosphorus may be sorbed by this
particulate matter and removed in shallower
Lake Sammamish (mean depth 17 .7 m), while
in deeper Lake Washington (mean dept h
37 m), particulate matter from the bottom is
not so readily mixed to the surface . In support
of this, iron content during and followin g
turnover is higher in Lake Sammamish than i n
Lake Washington particularly in the hypolimnion
. The lower residual P content in late
winter in Lake Sammamish is undoubtedl y
due to P sedimentation through interactio n
with relatively greater amounts of iro n
(Horton 1972, Shapiro et al . 1971) .
Recovery rate in Lake Sammamish may b e
slower than in Lake Washington because th e
former had never attained the enrichment o r
productivity level of the latter . Rate of recovery
might have also been rapid in Lak e
Sammamish if prediversion annual supply had
been as great as that in Lake Washington .
There may exist a control threshold level of P
in the lake, above which alteration by manip -
307