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

Methods
The water column in Lake Sammamish wa s
sampled at 2-week intervals in spring, summer,
and early fall and monthly in winter during
1970 and 1971 . Phytoplankton biomas s
and productivity and nutrient content wer e
determined in samples collected from severa l
depths. Nutrient supply from surface water s
was estimated by monthly sample collectio n
and flow measurements from two major an d
11 minor streams entering the lake .
Primary productivity was determined i n
situ according to procedures described b y
Goldman (1961) . Water samples inoculate d
with C 14 were incubated at four depths for 4
hours and the results reported as integrate d
productivity in the photic zone extrapolate d
to daily rates assuming a 1 :1 relationship with
incident light. Data are reported from on e
centrally located station in each of the lakes .
Methods of Strickland and Parsons (1968 )
were followed for N, P, and Chlorophyll a
(Chl a) analyses in water . Total and orthophosphate
phosphorus were determined spectrophotometrically
as a phosphomolybdat e
complex. Reactive silicate was also deter -
mined from a silico-molybdate complex .
Nitrate and nitrite were determined spectrophotometrically
following reduction in a
cadmium-copper filled column and are reported
together as NO 3 -N. Chl a was determined
with a Turner Model 110 fluorometer .
Cations were determined by atomic absorption
techniques and anions by routine procedures
(American Public Health Associatio n
1971). All analyses except for total P wer e
performed on filtered (0.451.t poresize) water
samples .
Surface sediments (surface to 10-cm depth )
were collected with a Peterson dredge in the
four lakes . Surface sediments in Lake Sammamish
were sampled more extensively (2 6
samples from 26 different depths) than those
of the other three lakes where only four to si x
samples were collected . Analyses for total C ,
N, and P contents of air-dried sediments wer e
performed with procedures of Baker (1970 )
for C, Bremner (1960, 1965) for N, and
Delfino et al. (1969) for P. The phosphomolybdate-ascorbic
acid method of Strickland
and Parsons (1968) was used for the determination
of extracted P . Results are expresse d
on an ovendried (104°C) basis .
Bioassays to determine the limiting nutrient(s)
were conducted in large (0 .21 m z X5m)
plastic cylinders submerged in the lake for 7
days. Nitrogen, P, C, and Si were added t o
experimental bags separately and in combination
. Phytoplankton response was determine d
by daily measurements of productivity rat e
and Chl a concentration . Significance of
response was judged from results of analysi s
of variance using a factorial design and
Dunnett 's test at the 95-percent level of confidence
(Steel and Torrie 1960) . Data are
graphed as integrated values over time to indicate
total production .
Lake Sammamish
Trophic Status
Lake Sammamish is considered mesotrophic
judging from measurements of phytoplankton
productivity, biomass (Chi a), hypolimnetic
oxygen deficit, and concentration s
and loading of N and P . Guidelines for thes e
characteristics are suggested in table 1 fo r
judging the trophic status of a lake . With on e
exception, values for these characteristics i n
Lake Sammamish fall in between ranges typical
of oligotrophy and those of eutrophy . Th e
exception, mean winter ortho PO 4 -P, i s
greater than the level often considered indicative
of subsequent summer nuisance alga l
blooms (Sawyer 1952). Of probably mor e
significance than winter concentration, how -
ever, is annual supply of P . In this regard ,
Lake Sammamish lies clearly between safe
and danger limits of eutrophication, her e
construed to suggest oligotrophy an d
eutrophy, respectively (Vollenweider 1968) .
These ranges of measured characteristic s
are at best only guidelines for judging the
trophic status in lakes . Eutrophication o f
lakes is a complex process and is affected b y
climate, basin morphology and soil type . Differences
in these factors could lead to considerable
inconsistencies in judging trophi c
status using the limited number of characteristics
in table 1 . On the other hand, the
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