Oceanogr., 41(4), 1996, 732-743
0 1996, by the American Society of Limnology and Oceanography, Inc.
Limnol.
Unbalanced particle flux budgets in Crater Lake, Oregon:
Implications for edge effects and sediment focusing in lakes
Jack Dymond, Robert Collier, and James McManus
College of Oceanic and Atmospheric
Sciences, Oregon State University,
Corvallis
9733 1
Gary L. Larson
College of Forestry, CPSU, Oregon State University,
Corvallis
9733 1
Abstract
Flux estimates show that upward mixing of the deep-water nitrate pool accounts for more than 85% of
the total new nitrogen input to the euphotic zone of Crater Lake. Because measured primary productivity
(360 mg C m-2 d-l) is lo-30 times higher than a level supported solely by the input of new nitrogen into
the euphotic zone, nitrogen must be recycled in the euphotic zone many times before it is transferred by
particles to the deep lake. Nitrogen recycling in the deep lake is also very efficient. Sediment trap measurements
of particulate nitrogen fluxes reveal a major imbalance in our estimates of the lake’s internal nitrogen budget.
We propose the imbalance reflects an “edge effect,” whereby enhanced biological production occurs near the
lake margin and the shallower portions of the lake. Our measurements also reveal that Al, an element carried
by refractory phases, is accumulating in basin sediments at a rate 15-30 times higher than the flux we measure
with sediment traps. This diRerence is maintained by the near-bottom transport of lithogenic particles from
the lake margins to the deep basins of the lake. These comparisons of nutrient and refractory element fluxes
reflect two important lake processes- enhanced productivity at the lake margin that may be due to greater
availability of macro- and micronutrients at the lake edges and focusing of particulate material into the deep
lake basins.
Crater Lake, Oregon, is a deep (590 m) lake formed by
caldera collapse following the explosive eruption of Mt.
Mazama, 6950 B.P. (Bacon and Lamphere 1990). The
spectacularly steep caldera walls surrounding the lake result in a very small watershed. The lake surface accounts
for 78% of the total watershed resulting in a system in
which external flows of nutrients to the lake are low and
dominated by precipitation and dry deposition from the
atmosphere. These conditions contribute to the oligotrophic nature and exceptional clarity of the lake. Dissolved
nitrate concentrations in the upper 200 m of the lake are
co.05 PM throughout most of the year, while phosphate
concentrations
are relatively high and nearly uniform
throughout the water column (see Fig. 4). Also, the lake
is well oxygenated, with deep-lake oxygen contents > 300
PM (McManus et al. 1996).
To characterize the nutrient cycles and particle fluxes
in the lake, we consider a budget for nitrogen-the
dominant factor limiting primary production. The first step
in developing the nitrate budget is to evaluate the allocthonous (external) fluxes into and out of the lake system.
These fluxes include nitrogen introduced to the lake
through snow and dry deposition and nitrogen transferred
out of the lake by seepage and permanent burial in the
sediments. The second step is to describe an internal lake
budget for the flow of nitrogen between the upper lake
(< 200 m) and the deep lake. The upper lake is that portion of the water column that is well-mixed annually
(McManus et al. 1993). We used moored sediment traps
to estimate the vertical fluxes of particulate matter in the
lake since 1983 and have determined the accumulation
rates of nutrients in the sediments. We have also drawn
together other data for fluxes and physical processes needed to complete the budget and demonstrate the remarkable efficiency of nitrogen recycling throughout the lake.
In our discussion of the nitrogen cycle, we draw on a
conceptual framework that distinguishes new and regenerated production from total primary production. This
model has been extensively applied to marine systems
(Dugdale and Goering 1967; Eppley and Peterson 1979;
Platt et al. 1992) and more recently in oligotrophic lakes
new production is that
(Caraco et al. 1992). Qualitatively,
portion of the total primary production supported by the
input of nutrients from outside the euphotic zone. For
Crater Lake, we consider the allocthonous sources plus
the nitrogen upwelled from the nutrient reservoir of the
deep lake as the new nutrients. At steady state, this must
also be equal to the exportjlux of nitrogen, which includes
sedimented particulate organic matter (POM) plus any
downwelled dissolved nitrogen in the lake. The regenerated production is the balance of the total primary production that must be sustained by rapid recycling of nitrogen via heterotrophic regeneration processes in the up-
Acknowledgments
We are grateful to the Crater Lake National Park personnel
who have given untold hours of their time for this project. The
exceptional efforts of Mark Buktenica, John Salinas, and Scott
Stonum are particularly appreciated. Geof Wheat provided porewater measurements. Sediment trap deployments and recoveries were coordinated and directed by Chris Moser. Bobbi Conard and Chi Meredith analyzed the sediment trap materials.
The research was supported by a grant from the U.S. National
Park Service.
732
733
Partic1eJlu.x in Crater Lake
per lake. In the oceans, regenerated production exceeds
new production by factors ranging from two to more than
ten (Eppley and Peterson 1979).
The conceptual framework for distinguishing new and
regenerated primary production, which best applies to
stable, oligotrophic
ecosystems (Platt et al. 1992), is a
useful descriptive model. However, attempts to set the
parameters of the general relationships between new and
total primary production (Eppley and Peterson 1979) are
complicated by diverse ecosystem functions that control
nutrient cycling. In this paper, we document the extensive
recycling of nitr,ogen in Crater Lake but do not consider
the specific processes or ecological relationships leading
to these observations. In parallel with oligotrophic marine
systems, the mw nitrogen in the lake is a very small
fraction of the total nitrogen used in primary production.
Our nutrient budget demonstrates that this new nitrogen
is dominated by upwelled nutrients efficiently recycled
from POM throughout the lake.
50m contours
Methods
”
1
To define the nutrient budget, we use measurements of
atmospheric sources of fixed nitrogen, analyses of watercolumn nitrate concentrations, sediment trap studies of
particle fluxes, and estimates of the accumulation rate of
nitrogen and oilher biogenic components in lake sediments. Most of’ our measurements and sampling have
been done in the deepest part of the lake in the north
basin (Fig. 1); however, sediment cores have been collected in other locations to define spatial variability
in
sediment accumulation.
Wet and dry ‘bulk inputs of nitrate were derived from
measurements of nitrogen in precipitation and runoff made
over an 18-month period. Nitrate, phosphate, primary
production ( 14C assimilation), and other limnological indices have been routinely measured as part of a lo-yr
limnological
study (Larson et al. 1993; McManus et al.
1996). Lake physics and vertical mixing rates have been
discussed elsewhere (Collier et al, 199 1; McManus et al.
1992, 1993).
Moored sediment traps were deployed in the lake from
September 1983 to September 199 1 (Table 1). We used
a conical trap based on a design by Soutar et al. (1977).
The relatively steep cones (2 : 1 height to diameter) are
equipped with a Hexcel baffle (l-cm opening by 5-cm
depth) at the cone mouth to reduce turbulence and produce consistent collection under diverse current conditions. Traps are made of fiberglass and plastic to minimize
the possibility of metal contamination
of the samples.
Comparison of this trap design with other cones and cylinders shows similar particle fluxes (Dymond et al. 198 1;
Honjo et al. 1992). Measurements of 230Th fluxes suggest
a collection efficiency of >80% (Mahannah 1984). Particles that enter the traps move downward through the
relatively still water encompassed by the cone until they
enter a sample cup at the base of the cone. For most
deployments, the sample cup was a 45 x g-cm acrylic tube.
The particulate materials accumulating within this single
2
”
3
km
4
”
5
6
Fig. 1. Bathymetric chart of Crater Lake. Location of sediment cores used to determine sedimentation rates and accumulation rates of nitrogen and aluminum in the three major
basins of the lake-O; location of the sediment traps deployed
in 1983-A; location of the sediment traps deployed between
1984 and 1991-D.
tube represent the integrated flux of settling particles over
the deployment period. Because of the inaccessibility of
the lake during the heavy snow months, sediment trap
moorings were deployed and recovered in July and September each year, thereby providing a 2-month deployment in summer and a lo-month deployment in winter.
The sediment trap cups contained saturated sodium borate-buffered formaldehyde as a bactericide. The cup solutions had a pH of 8.5 and a formaldehyde concentration
of 2%. For most years, sediment trap samples and flux
estimates were made at three water depths: 200 m, 385
Table 1. Particulate C, N, and Al fluxes measured by sediment traps. Fluxes are determined by multiplying the total particulate mass flux times the total C, N, and Al contents. Values
are for the trap located at a depth of 200 m (Dymond et al.
1993). Flux units are pg cm-2 yr-l.
Date
deployed
13 Sep 83
18 Sep 84
6 Sep 85
4 Sep 86
11 Sep 87
7 Sep 88
7 Sep 89
18 Sep 90
No.
of days
369
361
361
370
361
363
367
363
c flux
35
70
142
29
108
147
55
67
N flux
4.0
10.9
19.6
3.7
14.5
14.8
7.7
7.4
Al flux
23
24
70
6
18
44
8
23
734
Dymond et al.
m, and 10 m above the lake bottom. The trap mooring
was deployed in the north basin at a depth of -590 m
except during the first year, when it was deployed at 480-m
water depth in the south basin (Fig. 1).
Sediments were collected with a short gravity core that
sampled the sediment-water
interface without disturbance. This core uses a thin-walled acrylic tube as a core
barrel and rubber-coated scoop closures that snap shut
before the core is pulled from the sediment. The integrity
of the sediment-water interface was indicated by the presence of a “fluff
layer, rich in phytodetritus, recovered at
the top of each core. The cores were placed in ice upon
collection and later split in half and stored in sealed tubes
at the Oregon State University core-storage repository at
3°C.
Sediment trap samples and sediments were analyzed
for total C, N, P, and Al. Carbon and nitrogen were measured with a CHN analyzer. Nutrients were determined
with calorimetric
methods described by Strickland and
Parsons (1972) and modified for an Alpkem rapid flow
autoanalyzer. Al was measured by instrumental neutron
activation analysis. Sedimentation rates were measured
in four cores that represent the principal basins of the
lake and the large platform east of Wizard Island (Fig. 1).
Using both 210Pband 14Cmeasurements of the sediments,
we determined both the bioturbation rate and the rate of
sedimentation
at these sites (Dymond and Somayajulu
unpubl. data). The settling particle fluxes and sediment
accumulation fluxes were calculated by multiplying
the
concentration of each element in the trap or core sample
by the total particle flux measured in sediment traps or
the mass accumulation rates measured in cores.
Carbon flux (pg cm- 2 yr’ ‘)
0
50 100 150 200
O-
200
3
&
‘t
a
300
I
I
400
500
600
Nitrogen
0
0
flux (pg cm- 2 yr’ ‘)
5
10 15 20 25 30
I
I
I
I
I
I
- 00
Results
There is a fivefold variation in the annual particulate
carbon and nitrogen fluxes over the 8-yr time series of
sediment trap deployments (Table 1). Although our time
series is too short to define an interannual pattern, the
data show two cycles of increasing fluxes for 2 or 3 yr
followed by a year of very low flux. Al fluxes vary by a
factor of > 10 and show a pattern similar to those of C
and N fluxes. Molar ratios of carbon to nitrogen vary
between 7.5 and 11.6, values that fall within the range
reported for phytodetritus measured in other lakes (Stabel
1985; Kilham 1990; Hecky et al. 1993).
In general, the biogenic fluxes do not show consistent
depth dependence (Fig. 2). In contrast, the Al fluxes (Fig.
2c) increase more rapidly and consistently with depth
than do the fluxes of the biogenic components. Settling
particles are primarily composed of materials derived
from two major sources: biological production (organic
matter with varying amounts of opal from diatoms) and
lithogenic material eroded from the caldera walls or carried by winds from other continental sources. Mixing of
these two sources accounts for most of the variability in
composition observed in the sediment trap material (Dymond and Collier 1990). For the present discussion, total
organic carbon is considered to be a proxy for all organic
I
- 0a
100
i
I
600
Aluminum
op
;o
flux (pg cm- 2 yr’ ‘)
40
6;o 8,o 1po
n
fi: 200
3
2
300
600
Fig. 2. Particulate carbon, nitrogen, and aluminum flu.x
measured with sediment traps at three depths. The data represents the average (+ la) of all the annual flux measurements
for a given water depth.
735
Particle flux in Crater Lake
Table 2. Percentages of total lake inputs accumulating in
various lake areas. Values in parentheses indicate the extent of
focusing implied by the percent of total accumulation divided
by the percent of total lake area. A value of 1.0 indicates no
focusing. Larger values indicate greater relative focusing.
Area
Wizard platform and
intermediate depth areas
South basin
North basin
% of
total
lake
area
Bulk
accumulation
rate*
(mm
yr-‘)
17
0.13
13(0.8)
3.5
22.6
0.94
0.47
21(6.0) 34(9.7)
66(2.9) 59(2.6)
% of total
accumulation
N
Al
7(0.4)
*From Dymond and Somayaj ulu unpubl. data.
matter and Al is used to represent the lithogenous fraction.
Sedimentation
rates defined by 210Pb show that the
Wizard Platform (east of Wizard Island; Fig. l), a relatively flat region with depths between 250 and 350 m,
has slower sedirnent accumulation than either of the deep
basins (Table 2). Our 210Pb-derived sedimentation rate
on the platform is within 20% of a previously determined
14C rate (Nelson et al. 1994). The south basin, which is
somewhat more enclosed by steep caldera topography,
has a sedimentation
rate seven times greater than the
platform and double that of the larger north basin (Table
2). Together, these sedimentation rates reveal the significance of sediment focusing, whereby particulate material
is transported downslope and accumulates in the deep
basins (Lehman 1975; Blais and Kalff 1995).
Due to the observed variability in the burial rates between the two basins and the Wizard Island platform, it
is difficult to integrate the accumulation of any component over the entire lake. Nonetheless, to provide a firstorder examination of the material budgets in the lake, we
assume that our measured rates of south basin accumulation apply only to the part of the south basin deeper
than 450 m. We also assume that the portions of the lake
that are outside the south basin and deeper than 400 m
have accumulation rates equivalent to the value measured
in our north basin sediment core. Observations made
from a submersible reveal that little sediment accumulates on steep slopes and at depths ~300 m (Collier et al.
199 1); thus, we assume there is no sediment accumulation
on the very steep topography, most of which lies at depths
~300 m. Areas of the lake with depths between 300 and
400 m are assumed to accumulate sediment at the rate
measured on Wizard Platform. We use this areal breakdown of accumulation rates and the measured concentrations of Al in the sediment cores from these areas to
compute a total burial of 1.7 x lo8 g of Al each year in
Crater Lake sediments. Of this, 34% is deposited in the
south basin, 59% in the north and northwest basins, and
only 7% on Wizard platform and the other intermediate
depth areas (Table 2). This distribution
of particulate
lithogenous burial in the lake basins reveals a pattern of
downslope transport.
Flux
evaluation
and modeling
Over appropriate time scales (-years), the allocthonous input of elements must be balanced by processes
that remove them from the lake (Natheson 1990). This
steady-state hypothesis requires that the sediment burial
flux plus the output from seepage must equal the allocthonous inputs from the atmosphere and runoff from the
caldera wall. The equation for this total lake balance is
(1)
El f Fr = Fb i- F,.
F, is atmospheric input, F, the input from runoff, Fb the
sediment burial rate of a given component, and F, the
loss by seepage through the lake floor.
Runoff and precipitation inputs of nitrogen can be estimated from measurements made over an 18month period of total dissolved nitrogen in precipitation.
A volume-weighted estimate of the total N (TN) concentration
in precipitation is 4.4 PM (Reilly et al. 1989). Ifwe assume
that this concentration estimate applies for runoff as well
as direct precipitation
on the lake, the total exogeneous
inputs of nitrogen can be computed from total water input, 1.3 1 x 10” liters yr-1 (Redmond 1990; Collier et al.
199 1). This results in an estimate of 5.8 x lo5 mol N yr-1
for the allochthonous inputs to the lake surface. Because
78% of the precipitation falls directly on the lake, differences between the nitrogen concentration in runoff and
that measured in precipitation
would not produce significant errors in our nitrogen budget.
Seepage of nitrogen can be estimated from the hydrologic budget of the lake, which indicates that seepage
accounts for 5 1% of the total water loss-the remainder
is lost by evaporation (Redmond 1990). This amounts to
a seepage loss of 6.8 x 1010liters yr-I. We have partitioned
this loss according to the lake areas deeper and shallower
than 200 m. Because 73% of the lake area is deeper than
200 m, we assume that 5.0~ lOlo liters yr-1 seep from
the deep lake and that 1.8 x 1010 liters yr-1 are lost from
the upper lake. Although we have no direct way of evaluating this partitioning,
it is believed that most seepage
occurs from deep within the lake (Redmond 1990) and
that the specific distribution
has little effect on the TN
budget. The TN concentration of the upper lake is roughly
0.05 PM, resulting in a very small seepage loss (< 1O3mol
yr-‘), whereas the deep lake has a seasonally averaged
nitrogen content of - 1.O PM, resulting in a whole-lake
loss of nitrogen of 0.5 x 1O5 mol yr-’ by seepage.
We use measurements of TN in the cores and the same
areal distribution of bulk sedimentation rate used for the
Al budget (Table 2) to compute that 4.5 x 1O5 mol of
nitrogen accumulate in the lake sediments each year. This
calculation demonstrates that the deep basins, which account for 26% of the lake area, accumulate 87% of the
nitrogen that accumulates in the sediments (Table 2).
Intermediate depth areas, such as the Wizard Island plat-
Dymond et al.
736
form and the Chaski slide area on the south side of the
lake, comprise 17% of the lake and account for 13% of
the buried nitrogen. The shallow areas and areas of steep
topography account for 55% of the lake area but, as mentioned, accumulate little sediment and essentially no nitrogen.
These estimates provide all the fluxes necessary to complete the whole-lake nitrogen budget described by Eq. 1.
Comparing the nitrogen inputs (precipitation and runoff)
to the outputs (seepage and burial) reveals a balance to
within 15%:
(precipitation
Upper Lake
_jc
A
Fp (0.4-0.5)
v
Fd (~0.001)
Fu (2.0-4.0)
v
Fo, deep
(0.05)
Deep Lake
+ runoff)
Fo, upper
(<O.OOl)
= (burial + seepage)
t+
\(5.8 x lo5 mol yr-l)
= (4.5 x lo5 + 0.5 x 105) mol yr-l.
This budget shows that -90% of the nitrogen entering
the lake .from precipitation and runoff is eventually buried
in sediments; the rest is lost by seepage.
Internal
nitrogen cycling
For Crater Lake, the sources of nutrients that can support new production are precipitation, runoff, and mixing
of deep water with surface water. These sources can be
formalized by expanding the one-box, whole-lake budget
model (Eq. 1) to a two-box model that partitions the lake
into a shallow, nitrogen-depleted
euphotic zone (O-200
m) and a deep-lake reservoir (Fig. 3). The upper box
receives a flux of nutrients from runoff (F,), the atmosphere (F,), and upward mixing of deep-lake water (FJ.
Nutrients are removed from the upper lake by particle
settling (F,), downward mixing of surface water (Fd), and
). The nutrient fluxes into the deep
outflow seepage (FO,u,,,,er
box (Fp and Fd) are balanced by upwelling (FJ, particle
Over appropriate
burial (Fb), and outflow seepage (Fo,deep).
time scales, the system will be at steady state for the
bioactive elements (the inputs will equal the outputs). The
mass balance equation for the shallow box is
Fa +- Fr + Fu = Kw.,pper+ Fd f Fp’
(2)
whereas the steady state balance for the deep hypolimnion
box is
Fp + f-d = Fu + Fb + Fo,cieep.
(3)
These internal mass-balance equations can be combined
with the whole-lake budgets to evaluate the relative significance of different physical processes, quantified as
model parameters, on the particulate nitrogen fluxes.
The sediment burial term, Fb, in the whole-lake N budget (Eq. 1) can be transformed by assuming that a fixed
fraction of the particulate N (PN) that falls to the deep
lake is buried and the rest is recycled into the dissolved
nitrate pool of the deep lake (Broecker 197 1):
Fb = cy x Fp
where a is the fraction of nitrogen that is buried.
Substituting into Eq. 1,
(4)
Fig. 3. Box model showing three reservoirs of organic matter
and nutrients: upper lake (O-200 m), deep lake (>200 m), and
sediments. Exchange of dissolved and particulate materials into
the system and between the reservoirs are represented by the
various flux arrows: F,-atmospheric inputs; F,-inputs from
runop, F,-- nutrients transported by upward mixing of deep
water; F,-flux from the upper lake by downward mixing; F,loss from the lake by seepagethrough the lake floor (Fo,u,,per
is
seepagelosses from the upper lake and Fo,de.pis seepagefrom
the deep lake); Fp- nutrients removed from the upper lake by
particle settling; Fb- sediment burial rate of a given nutrient.
Our independent estimates of the individual fluxes of nitrogen,
which we believe to be the limiting nutrient, are shown by values
in parentheses (units of 1O6mol yr- l).
Fa + Fr = a (Fp + F,).
(5)
We estimate the amount of particulate recycling, 1- 01,
in the deep lake from comparison of sediment trap and
sediment data. If we assume that our particulate Al estimate is conservative, differences between the N : Al values observed in deep sediment traps and surficial sediments will be due to nitrogen recycling. The ratio of N :
Al values of deep traps with the N : Al in surficial sediments indicates that 79-90% of the nitrogen is recycled
in the deep lake (Dymond and Collier 1990). This degree
of recycling is equivalent to an a ranging from 0.1 to 0.2 1.
To compare our model results to sediment trap measurements of PN flux, we solve for Fp:
Because there is essentially no dissolved nitrogen in the
upper lake (Fig. 4), seepage (Fo,upper)
and downward mixing (Fd) account for a very Small fraction Of the nitrogen
removal. As a result, the flux of PN (F,) should balance
inputs from upward mixing (Fu) and allochthonous sources
(F, + F,), and Eq. 2 simplifies to
FP
= F, i- F, + Fu.
U)
737
Particle flux in Crater Lake
o 0.0
Phosphate and Nitrate (PM)
0.5
1.0
1.5
r
6’
---_
200
8
A
300
P
fi
400
600
Fig. 4. Dissolved nitrate and phosphate profiles for the north
basin.
Based on dissolved tritium measurements in Crater
Lake, Simpson (1970) estimated that the lake mixes over
time scales of l-2 yr. The oxygen budget of the deep lake
requires a deep-water residence time of 2-3 yr (McManus
1992; McManus et al. 1993). Furthermore, recent measurements of clnlorofluorocarbon
in the lake (Weiss unpubl.) suggest an average age of the deep waters (measured
in 1989) of -2 yr (Collier et al. 1991). Therefore, the
available data are consistent with a deep-water residence
time of 2-4 yr (McManus et al. 1993). Because 47% of
the lake volume is deeper than 200 m (8.1 x 1012 liters),
this range in mixing rates would cause an annual exchange
of 2.0-4.1 x lOI liters of upper and deep lake water. If
there is a nitrat#e concentration of 1.O PM in the deep lake
(Fig. 4), this would produce an upwelled flux of nitrogen
to the upper lake of 2.0-4.1 x lo6 mol yr-l.
Equations 6 and 7 place strong constraints on the particle flux, the internal and external N fluxes, and the extent
of nitrogen recycled in the deep lake. Qualitatively,
greater deep-lake nitrogen recycling (low a) requires greater
PN fluxes. Likewise greater allochthonous and upwelling
inputs of nitrogen to the euphotic zone must be balanced
by greater particle fluxes. The steady-state model, however, further constrains the particle fluxes because only
certain values are compatible with the observed concentrations of nitrogen in the upper and deep lake. In Fig.
5, we use the steady-state model (Eq. 6 and 7) to generate
a field of value:s compatible with the best estimates of the
allochthonous
nitrogen fluxes, the observed nitrate distribution in the lake, and the estimated efficiency of nitrogen recycling in the deep lake. This figure provides a
visual tool that emphasizes the importance of vertical
mixing on particle flux. For example, if the deep-water
residence time is between 2 and 4 yr, the extent of recycling must be > 84% (CU< 0.16). Also, with this mixing
particulafe
recycling
nitrogen
efficiency
1
m--m__---.
i mixing
+---Trap
0
500
_--_
1
range of
possible
particle
fluxes
Measurements
I
0
rates i
I
2
4
Deep Water Residence
I
I
6
8
Time (years)
10
Fig. 5. The modeled relationship between deep-water residence time (vertical mixing rate) and the settling flux of PN.
The relationship is derived from Eq: 6 and 7 by independently
estimating values of the allochthonous nitrogen inputs (F, +
F,.) and the deep-lake recycling fraction (1 - CV).The observed
dissolved distribution of nitrogen is also imposed such that F,
is determined from the seepagerate and F, is determined from
vertical mixing rate. Setting this group of parameters to the most
reasonable values (seetext) requires a PN flux out of the euphotic
zone of 2.5-4.5 x lo6 mol yr-l -considerably greater than the
0.4 x lo6 mol yr- 1 collected by the sediment traps (shown by
arrow on nitrogen axis). Solid line is the curve generated by
assuming F, + F, = 0.44 x 1O6mol yr- 1 and the noted range of
recycling efficiencies (79-90% or cx= 0.2 1-O.10). Stippled field
includes the complete range of estimates for F, and F, that result
from the full range of model parameter estimates.
rate and (x = 0.1, the precipitation
and runoff input of
nitrogen must be somewhat smaller than our upper estimate of 5.8 x lo5 mol yr- l. Similarly, the annual PN
flux for a deep-lake residence time of 2-4 yr must lie
between 2.3 and 4.4 x lo6 mol, regardless of the extent of
recycling or precipitation
and runoff inputs.
The annual N fluxes to the euphotic zone (Fig. 3) are
5-9 times greater than the sum of burial and seepage
removals. This comparison implies that nitrogen atoms
entering the euphotic zone are typically recycled in the
deep lake and upwelled many times before being buried
in the sediments or removed by seepage.
Additional nutrient recycling is revealed by comparing
our estimated input of nitrogen with the integrated primary productivity
measured in the euphotic zone. Primary production in summer is - 360 mg C m-2 d-l (12-h
day, avg since 1983; Larson et al. 1993). Using the C : N
of our sediment trap material (7.5-l 1.6; Table l), our
estimate of the total new nitrogen introduced to the euphotic zone could support new production that is only
3-9% of the total primary productivity
of the lake. These
calculations further demonstrate that nitrogen is efficiently recycled within the euphotic zone (as well as in the deep
lake) and that regeneration supports a level of total primary production lo-30 times higher than could be sup-
738
Dymond et al.
ported without recycling. This extent of nutrient recycling
is much higher than that measured in many other lakes
(Baines et al. 1994) and resembles the efficiency of recycling measured in the most oligotrophic
areas of the
ocean (Eppley and Peterson 1979).
Comparison
calculations
of measured PN fluxes to model
The average flux of PN measured in the 200-m traps
is 10 pg cm-2 yr- l (Table II). Multiplying this value by the
area of the lake (53.2 km2) results in the total PN flux for
the lake: 5.3 x lo6 g yr-l or 3.8 x lo5 mol yr-l. Therefore,
the sediment trap estimate of TN flux leaving the euphotic
zone (through 200 m) is 2.5-10 times smaller than the
flux required to balance the upper lake nitrogen budget
(Figs. 3 and 5). This significant disagreement in flux estimates must be carefully considered because it may reflect errors in the direct measurements (traps, reservoir
concentrations, mixing rates) or in the assumptions of the
box models. Perhaps more significantly, this difference
may indicate processes that we have not yet considered.
First we will consider possible errors in the trap flux
estimates that might account for the low nitrogen fluxes
observed. The first question is whether the trap is collecting the vertical particle flux with 100% efficiency. Certain trap designs and hydrodynamic
conditions can lead
to under-trapping (Gardner 1980; Baker et al. 1988). However, the trap design we used has been shown to have an
efficiency near 100% (Bacon et al. 1985); the identical
cone design, deployed in the central Pacific, has been
shown to be ~80% efficient based on 230Th collection
inventories (Mahannah 1984). As shown by other studies,
most of the collection artifacts are created by hydrodynamic effects around the traps in high-current environments that do not exist in the deep-water column of Crater
Lake. Measured deep-water current speeds in the lake
average - 1 cm s-l (McManus 1992)-well
below speeds
that might cause inefficient collection of particles (Gardner 1980; Baker et al. 1988). Therefore, it seems very
unlikely that low trapping efficiency is the cause of discrepancies in the internal nitrogen budget.
A related possible trapping artifact is the change in flux
and particle composition that can result from substantial
zooplankton activity in the trap cone or cup. The problem
of “swimmers” is well known for ocean trap experiments
(Lee et al. 1988) and may play a role in Crater Lake.
Swimmer effects are more likely to be a problem in the
shallow traps, which are at depths where zooplankton are
more abundant. Zooplankton
invaders can result in a
decrease in the collected flux (mobile organisms feeding
within the cone) or in an increased flux (swimming organisms are poisoned by formaldehyde in the trap cup).
Most studies suggest that swimmers result in flux increases, not decreases (Silver and Gowing 199 1). Over the past
few years, we have observed an increased number of chironomid larvae in the cups, which may be swimmers, but
the overall chemical composition and systematics of flux
with increasing depth suggests that swimmers are not a
major problem.
Another possible trapping artifact relates to the chemical preservation of the particles once they enter the tralp.
Experiments show that formaldehyde is an effective particle preservative and that it eliminates ?significant microbial degradation (Lee et al. 1992). However, extremely
labile organic fractions can become dissolved in the preservative solution after the particles enter the cup. Unless
these solutions are carefully collected and analyzed as part
of the sample, a significant fraction of the total flux can
be lost. Furthermore, a small fraction of this loss diffuses
out of the traps while they are still collecting and cannot
be recovered. Because it is impossible to measure dissolved organic C (DOC) in formaldehyde, we have monitored this loss to solution indirectly by the analysis of
dissolved P in the cup solution. This approach assumes
a Redfield ratio of the soluble P to determine a worst
estimate of the amount of total organic matter (TOM)
lost to the cup solutions. The soluble P measurements
indicate a potential soluble N release that averages 25%
of the measured PN flux. Our experience with marine
particulate matter suggests, however, that the loss predicted on the basis of P is an overestimate of the TOM
lost, because P seems to be the most labile organic component. Therefore, our N fluxes are probably no more
than 25% low on average, which would increase our estimate of the trap-defined fluxes to 0.5 x 1O6 mol N yr- l.
Therefore, this process cannot account for the discrepancy
in our nitrogen budget.
The differences between the models and the trap flu.x
observations could also result from errors in model parameters or structure. The upper 200 m of the lake are
consistently depleted in dissolved nitrate (Larson et al.
1993), suggesting that all new inputs of nitrogen must be
quantitatively
removed by settling particles. Relatively
high detection limits for ammonia and organic nitrogen
analyses precluded our consideration of this reduced N
pool as part of the budget. However, we were unable to
detect any DOC above a blank of 1 PM and consider it
very unlikely that a significant bioactive pool of reduced
nitrogen exists in this extremely oligotrophic and wellloxygenated system. Therefore, we will examine the other
parameters that make up the estimate of new nitrogen
inputs- the allochthonous inputs and the vertical mixing
flux from the hypolimnion.
Although the estimate for the primary source of nitrogen to the whole-lake system (atmospheric input and runoff) is based on just an 18-month record of precipitation
compositions and monthly precipitation records, this term
is < 15% of the upper lake budget. Therefore, even errors
greater than the factor-of-two
range used in Fig. 5 and
shown in Fig. 3 would not produce a balance between
model and trap measurements.
The extent of particle recycling has a significant impact
on the range of possible particle fluxes that are compatible
with observed distributions
of lake nitrogen (Fig. 5).
However, the model shows that for a deep-water residence time of 2-4 yr the fraction of nitrogen recycled
must lie between 85 and 90%. Therefore, it seems unlikely
that the extent of recycling can lie outside the range indicated by a comparison of N : Al values in the sediments
and sediment trap materials (79-90%). This range is also
Particle flux in Crater Lake
compatible with our independent estimate for the burial
rate of nitrogen in the whole lake. If we assume that only
lo-20% of the nitrogen carried by particle settling is buried, we can use our estimated burial rate of nitrogen
(0.45 x lo6 mol N yr-l) and Eq. 4 to define the particulate
flux of N:
0.45 x lo6 mol N yr-l
0.1-01.2
739
Nitrate
(PM)
2.0
+c----‘-_r’---
4.0
’
3.0
----
1,
= 2.3-4.6 x lo6 mol N yr-l.
This estimate is 5-9 times higher than the PN flux measured with sediment traps, but it matches our estimates
of total fluxes to the upper lake (Fig. 3). A burial rate of
0.45 x lo6 mol N yr-l could be compatible with our sediment trap N flulies only if 90% of the PN flux were buried.
Such high preslervation is incompatible
with measured
N : Al values in sediments and deep traps (Dymond and
Collier 1990) and is simply counterintuitive.
The vertical rnixing flux is computed from the product
of the average deep-lake nitrate concentration (- 1 PM,
Larson et al. 19!23) and the vertical mixing rate. Although
the mixing rate is perhaps the most indirect and modeldependent estimate used, it has been independently derived from various tracers, including the rate of oxygen
consumption and ventilation in the deep lake (McManus
et al. 1993) as well as the observed penetration of surfacederived tritium (Simpson 1970) and freon (Weiss unpubl.). These estimates, which range between 1 and 3 yr
for the average ventilation age of the deep lake, are supported by a set lof limnologically
consistent observations
related to the rates of hydrothermal processes (Collier et
al. 199 1). Therefore, this parameter seems to be well constrained and supports the model calculation for the introduction of new nitrogen from the deep lake. Note,
however, that the residence time determined by McManus et al. (1’293) and that determined by Weiss (unpubl.) were both derived during the same year; thus further estimates will be required to better refine any interannual variability in deep-water residence time. Although
the model-based estimates of PN flux depend strongly on
ventilation
rates (Fig. 5), even a deep-water residence
time of 10 yr (Fig. 5) is not consistent with the PN fluxes
measured with sediment traps.
Because of the oligotrophic nature of Crater Lake, we
have assumed that denitrification
has no significant impact on the nitrogen budget, but given the mismatch between PN fluxes measured by sediment traps and the
estimates of nitrate input to the upper lake, we must
evaluate this prclcess as well. Oxygen contents of the water
column are always ~300 PM. These high concentrations
are incompatible with denitrification
in the water column.
In addition, sediment pore-water measurements reveal
that denitrification
occurs only below lo-cm depth in the
sediments (Fig. 15),indicating that sediments cannot be a
major sink for nitrate in the lake. Although areas of the
lake with hydrothermal venting through the sediments or
higher deposition rates do have less oxic sediments that
could be sinks lor nitrate, McManus et al. (1996) concluded that the area of the lake floor influenced by hydrothermal activity is too small to contribute significantly
to either the dissolved oxygen or the nitrate budgets. Rough
- C+ - north basin
-*
- Wizard Island
platform
i
Fig. 6. Dissolved nitrate profiles in sediment pore waters
from two sites. The Wizard Island platform is at a depth of
-300 m. The lower sedimentation rate at this site (Table 2)
results in more oxic sediments that do not achieve denitrification
above a depth of 10 cm. The north basin site has a higher
sedimentation rate (Table 2). The less oxic conditions here indicate some denitrification by 10 cm. This core is more representative of average conditions in the deep lake than is the
Wizard Island core.
estimates based on expected nitrate gradients, estimated
diffusion coefficients (Berner 1980), and areal distribution
of higher sedimentation rates suggest that the potential
loss due to denitrification
is CO.2 x 1O5 mol yr- l. This
value is small compared to the TN budget. Moreover,
the mismatch in the N budget requires an additional sink
for the upper 200 m of the lake. Because this part of the
lake is essentially barren of sediments and well oxygenated, denitrification
cannot be a sink for nitrogen introduced into the upper lake.
It is also instructive to apply our N-budgeting approach
to P. Phosphorus is not strongly depleted in the surface
lake and is probably not limiting to primary production.
Although we do not have as much sediment trap flux data
for this nutrient (Dymond et al. 1993), we can still apply
the same box model used for N. The lake surface is depleted in dissolved P with respect to the deep lake by
-0.1 PM. Given our estimate of the vertical mixing rate,
this difference results in an average vertical flux of 24X lo5 mol P yr- l. The estimates of allochthonous
P
inputs (Reilly et al. 1989) add -0.6 x lo5 mol P yr-l. The
sum of the upwelled and allochthonous
sources (2.64.6 x lo5 mol P yr-l) must equal the particulate P flux.
A representative trap flux is roughly 2 x 1O4 mol P yr-l
and N : P ranging from 10 : 1 to 20 : 1. As was the case for
N, this measured flux is - 10% of that required by the
observed distribution
of dissolved P (box model result).
This additional mismatch lends further support to the
argument that the traps undersample all components of
the settling POM. It also provides confirmation that denitrification is not an explanation for the missing nitrogen
revealed in our budgeting.
Although Al in lake sediments has a very different source
than either N or P (lithogenic rather than biogenic), a
comparison between the sediment trap and burial fluxes
740
Dymond et al.
of Al helps characterize lake sedimentation
processes.
Because of the low aqueous solubility and the association
with refractive minerals, we would expect that nearly all
of the particulate Al reaching the lake will become part
of the sediment record. This expectation is supported by
observations that sediment trap fluxes and burial rates of
Al in the ocean show good agreement (Dymond and Lyle
1985, 1994). We used the sedimentation rate (0.94 mm
yr-l), dry bulk density of the sediment (0.23 g cm-3, and
Al concentration (7.90 %) measured for the north basin
sediment core to calculate an Al burial rate of 850 pg
cm-2 yr-1 near the sediment trap mooring site. In contrast, the average Al flux estimated from the particulate
flux measured at 200 m during the 8 yr of sediment trap
deployments is 27 hg cm-2 yr-l (Table 1). Thus, there is
a factor of 30 mismatch between trap and burial fluxes
of Al.
The lake margin is the predominant source of Al-bearing particles, and sediment focusing processes can move
Al-bearing particles from the lake edges, down the caldera
walls, and toward the center of the basins, thus creating
horizontal and vertical gradients in particle flux. The Al
flux in our deep-moored trap (10 m above bottom) is
higher than that measured at 200 m, although the difference averages a factor of only two (Fig. 2~). Therefore, if
focusing of caldera wall material is to account for the
mismatch between particulate Al flux and Al burial, the
downslope transport must primarily occur in the bottom
10 m of the lake (i.e. the distance of the deep trap to the
lake floor). Alternatively,
the Al flux mismatch may be
due to intermittent
sedimentation processes such as turbidite deposition that could dominate the long-term sediment accumulation but do not occur frequently enough
to be sampled during the 8 yr of our measurements. We
do not see sand layers in the central lake sediment core
that suggest deposition there is predominately
through
turbidity currents; however, we cannot rule out the possibility that the central lake is a site of distal turbidite
input.
Comparison between the inventory of 210Pbin the north
basin core and the flux of 210Pb measured in the sediment
traps provides another revealing comparison. Because Pb
is a particle-reactive
element and behaves conservatively
in sediments (Moore and Dymond 1988), we expect the
input of 210Pb required to support the inventory of this
isotope in the sediments to match the flux measured in
the sediment traps. Our measurements, however, show
that this is not the case. Measurement of the 210Pb sedimenttrapfluxesare0.23+0.10dpmcm-2yr-1
(Dymond
and Somayajulu unpubl. data) -a factor of - 4 lower than
the 0.98 dpm cm-2 yr- l of 210Pbinput required to support
the inventory measured in the core from our central lake
site. Unlike Al, 210Pb is predominately
atmospheric in
origin and should have roughly the same input for all
areas of the lake. The mismatch between trap flux and
burial- smaller than that of Al-can be explained by sediment focusing processes. There is also roughly double
the inventory of 210Pb in our deep basin cores than in the
Wizard Island platform core (Dymond and Somayajulu
unpubl. data) -a further indication of sediment focusing.
All our results suggest a process that transfers POM to
the deep lake without interception by midlake traps. The
process is reflected in the internal lake N and P mismatch
as well as an imbalance between the observed rate of
oxygen consumption in the deep lake (McManus et ad.
1996) and the flux of organic carbon measured by the
traps. Oxygen consumption in the deep lake, which must
be supported by a steady supply of organic matter from
the euphotic zone, is 4-10 times larger than supported
by measured trap fluxes. Likewise, Al and 210Pb, although
of different sources, must be transferred to the sediments
without being intercepted by either the upper or the deep
traps. Although the budgets for all components of the flux
are unbalanced, the differences in the sizes of the observed
mismatches may indicate both distinct sources of the
different components and lake processes.
Significance
of lake margins
in the nitrogen
budget
We hypothesize that edge-effects support significant
production that is not collected by the traps and that the
resulting particulate flux is focused into the deep basins
where it is remineralized, thereby consuming oxygen and
regenerating nitrate. Sediment focusing processes have
received extensive attention within the limnologic literature (e.g. Lehman 1975; Blais and Kalff 1995). If production is horizontally homogeneous, however, sediment
focusing alone cannot account for the nitrogen budget
problems we have identified. Because only 27% of the
lake area has depths ~200 m, particulate C and N, which
settle in this zone and are transported along the bottom
to the deep basins, could only produce a comparable 27%
error in predicted settling flux. There must be both greater
production and greater particulate export of carbon and
nutrients along the margins of the lake for this process to
account for our estimated flux mismatch. If this is the
case, it may imply that deep-lake mixing-the
dominant
source of N to the upper lake-does not result in an areally
uniform input of new nitrogen to the euphotic zone.
Depending on the assumed lake ventilation
rate, external sources and deep mixing introduce 2- 10 times more
N to the upper lake than we measured with central lake
sediment traps. The edge-effect calls for an equivalent
higher new production at the lake margins. The hypothesis suggests that most of the upward mixing of nutrients,
perhaps > 80%, takes place at the edges due to enhanced
mixing or convective exchange through thermobaric instability or cabbeling (McDougal
1987; Carmack and
Weiss 199 1; Crawford and Collier in prep.). Alternativel.y,
nutrient upwelling could be relatively uniform across tlhe
basin, but physical conditions, enhanced micronutrients
(such as Fe), or the presence of substrates along the caldera
wall provide conditions conducive to higher production.
In this scenario, upwelled nutrients must be transported
to the sides of the caldera, where the production occurs.
Perhaps production by periphyton and mosses attached
to the walls of the caldera are important in the overall
nutrient and carbon budget. These plants have been shown
to account for the dominant fraction of the productivity
Particle Jlux in Crater Lake
in the littoral zone of Lake Tahoe and Crater Lake (Loeb
et al. 1983). However, much of the littoral periphyton
productivity
in these lakes is due to N-fixing species (Loeb
and Reuter 198 1) and would not be a major sink for new
nitrogen. Crater Lake, with its steep subaqueous caldera
walls and great clarity, is perhaps highly suitable for substantial growth of attached macrophytes to great depths.
At water depths ~60 m, N-fixing species are not significant (S. Loeb pers. comm.). Transects to define the plant
growth on the caldera wall were made with a manned
submersible (Collier et al. 199 1) in 1988 and 1989. Although these plants were most abundant between 30 and
80 m, moss was also collected and observed at depths
> 100 m (McIntire et al. 1993). Detrital patches of moss
were observed as deep as 545 m, and clumps of moss are
common in sediment cores collected in the deep basin.
These deep occurrences of moss indicate that macrophytes become detached from the rocks and move downslope, where they decompose in the deep lake or are buried in the sediments. Although the growth and transport
of these plants is apparent, it is difficult to quantify their
impact on the overall nutrient cycle of the lake.
If the new production in the lake and resulting POM
settling is 2-10 times greater than that measured by central lake sediment traps, it is unclear whether the observed
variations in trap-based C and N fluxes reveal anything
about whole-lake productivity
variations. Because our
edge-effect hypothesis implies that most of the biological
activity occurs :in the shallow areas around Wizard Island
and elsewhere along the caldera walls, the variability
in
central lake seldiment trap fluxes may be only weakly
linked to the predominate biological activity at the margins. We have measured a factor of five variability
in
annually averaged particulate C and N during the period
between 1983 and 199 1 (Table 1). It is possible that this
interannual variability reflects variable horizontal transport away frorn lake margins to central lake traps by
storms or processes other than nutrient dynamics. Alternatively, the variability
may indicate that interannual
differences in the upward mixing of deep-lake nutrients
influences both the margins and the central lake areas. If
this is the case, sediment trap measurements, although
not absolute measures of whole-lake productivity,
serve
as relative indicators of interannual biological production. Future studies, which define the horizontal gradients
in total primary production and link independent measures of the ext’ent of annual deep-lake mixing with measured particle fluxes, could lead to a much clearer definition of these important lake processes and budgets.
The comparison between the mismatch of Al trap and
burial fluxes also supports the edge-effect hypothesis. Although the lake edge is the unequivocal source for most
of the Al reaching the lake basin, 15-30 times more Al
accumulates in the basin sediments than we collect in our
sediment traps. Thus, there is little doubt that some particulate material is transported from the lake edge to the
central lake floor without being intercepted by sediment
traps. Furthermore,
measurements of 210Pb, which has
an atmospheric source and therefore an areally homogeneous input, also reveals a mismatch between burial
741
and trap fluxes, but the discrepancy is significantly smaller-a factor of four. Despite the uniform input of 210Pb
to the lake surface, the removal process may also be linked
to edge effects. Ocean studies reveal that scavenging by
organic matter is the primary removal process for 210Pb
(Moore and Dymond 1988). If the lake edges dominate
the biological fluxes as we hypothesize, these sites could
also be the predominate loci for 210Pb removal. In other
words 210Pb enters the lake uniformly over the surface
but is advected to the edges where it is removed by scavenging on setting POM. This edge effect has been described in the oceans as boundary scavenging (Bacon 1988;
Lao et al. 1992).
Conclusions
We cannot unequivocally define the source of the mismatch in the internal nitrogen cycle for Crater Lake. The
agreement in the budgets of nitrogen, phosphorus, and
oxygen suggest this discrepancy does not result from errors in any of our measurements or flux estimates but
rather is a consequence of important, undocumented lake
processes. Although the differential accumulation of Al
and 210Pb in sediments from different lake depths provides strong evidence for sediment focusing, this process
alone cannot account for the mismatch we have defined
for biological components of the particulate flux. Only if
focusing is combined with enhanced productivity
near
the caldera walls and in the shallow regions around Wizard Island, could this process produce the discrepancy we
observed between sediment trap fluxes and nutrient inputs to the euphotic zone. This enhanced productivity
may reflect the importance of phenomena such as sieching, edge waves, or general enhancement of turbulence at
the margins of the lake. Alternatively,
enhanced lake margin productivity
may be caused by greater availability
of
micronutrients,
the presence of substrates, or other poorly
understood factors. Although our existing data cannot test
these ideas, our results show that one-dimensional
approaches are inadequate for defining nutrient budgets in
this type of system. If sediment traps are to quantify
nutrient dynamics, experimental designs that incorporate
transects will be more definitive than single, central lake
stations.
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Submitted: 12 March 1995
Accepted: 29 January 1996
Amended: 23 March 1996