Limnol. Oceanogr., 55(5), 2010, 1918–1931
2010, by the American Society of Limnology and Oceanography, Inc.
doi:10.4319/lo.2010.55.5.1918
E
Sedimentation flux in a large subtropical lake: Spatiotemporal variations and relation to
primary productivity
Ilia Ostrovsky and Yosef Z. Yacobi*
Israel Oceanographic and Limnological Research, Yigal Allon Kinneret Limnological Laboratory, Migdal, Israel
Abstract
Spatial and temporal heterogeneity in sedimentation of particulate organic material (POM) was studied in a
large subtropical lake, and its export from the upper mixed layer was quantified. Sedimentation fluxes were
measured over 4 yr with traps deployed in pelagic and littoral areas, the benthic boundary layer (BBL), and the
lake interior. Analysis of fluxes and composition of collected material showed that traps deployed at the lake
periphery and in the BBL notably overestimate the export flux of newly produced POM. The best estimation of
the primary production (PP) export from the upper mixed layer was achieved when traps were deployed in the
quiescent hypolimnion, where the effect of lake boundaries is negligible. The proportion and composition of POM
exported from the upper productive stratum was dependent on lake thermal and chemical structure and on the
dominant phytoplankton species. The dynamics of photopigments (chlorophyll and b-carotene) collected in
hypolimnetic traps reflected the composition and abundance of phytoplankton in the upper mixed stratum.
Despite large variation in algal community composition, the ratio of POM sedimentation flux to PP (export ratio)
changed only slightly (average , 20%) throughout the stratified period. The observed temporal and spatial
variability of sedimentation flux in response to ambient conditions was influenced by secondary processes
(resuspension at the lake boundaries and oversampling under turbulent conditions) as intensely as by the actual
export of POM.
Sedimentation is a major process for removal of
particulate material from the water column, and it also
affects the rate of elimination of bioavailable nutrients and
pollutants from the upper productive layer. Thus, sedimentation may be a key determinant of the stability of
aquatic ecosystems (Håkanson and Jansson 1983; Bloesch
2004). The fluxes of particulate material are affected by
numerous physical, chemical, and biological processes,
such as turbulent and advective water motion, redeposition
and transportation of particles by currents, particle
breakage, aggregation, dissolution, decomposition of organic components, consumption by aquatic invertebrates
and fish, and so on (Bloesch 2004; Widdows et al. 2004;
Buesseler et al. 2007b). The resulting patterns of particle
deposition to bottom sediments depend on water column
thermal and chemical structure, bathymetry, and the
hydrological regime of aquatic ecosystems (Hilton et al.
1986; Hodell and Schelske 1998; Ostrovsky and Yacobi
1999). In addition to the innate complexity of processes
affecting the fate of suspended material, there is a
methodological caveat associated with sedimentation flux
assessments. Most measurements are acquired with trap
techniques that may be biased by local currents and
turbulence, which in turn affect the efficacy of particle
trapping (Buesseler et al. 2007a) and make quantification
of fluxes a challenging task. On the other hand, simultaneous deployment of sets of sedimentation traps in diverse
locations broaden the ability to acquire information about
factors influencing the measured sinking fluxes and
potentially may clarify the extent of real sedimentation
rates.
* Corresponding author: [email protected]
One of the important tasks in aquatic ecology is to
understand the fate of particulate organic material (POM)
produced in the system; that is, what part of this material is
recycled within the upper productive layer vs. what
proportion of this material sinks to the bottom? Export
of POM from the upper productive layer due to sedimentation is generally balanced by ‘‘new’’ production fueled by
import of limiting nutrients (Eppley et al. 1983; Ostrovsky
et al. 1996; Wassmann 2004). Thus, in the long term, POM
sedimentation rate should indicate the rate of new
production. In deep-water bodies, phytoplankton is the
major source of newly produced (autochthonous) POM
(Laws et al. 2000; Gehlen et al. 2006). However, intact
phytoplankton cells are seldom the major component of
sinking POM, as a high proportion of algae is typically
recycled within the upper mixed layer via the microbial
loop and/or utilized by grazers. As a result, part of the
settled POM found in traps is transformed algal material.
Still, massive disposition of mostly intact cells may follow a
bloom of fast-sinking algae, like many diatoms and large
dinoflagellates (Tilzer 1984; Lignell et al. 1993; Kiørboe et
al. 1996).
Phytoplankton, as a part POM sedimentation flux, may
be identified by direct microscopic examination or by
measurements of algal-specific cellular components, such as
photopigments. The concentration of chlorophyll a (Chl a)
is widely used in aquatic ecology as a proxy for
phytoplankton biomass estimation despite the variability
of cellular pigment content between different species and in
response to altering environmental conditions (Falkowski
and Raven 2007). Transformation and decomposition of
Chl a by algae occurs promptly following grazing or lysis;
hence, the presence of that pigment in water and trap
samples is a good measure of well-preserved algal material.
1918
Sedimentation in a subtropical lake
Therefore, simultaneous comparison of the relative content
of Chl a vs. other pigments, both in the water column and
in sedimentation flux, can trace the fate of algae in aquatic
ecosystems (Hurley and Armstrong 1990; Yacobi and
Ostrovsky 2008). In lakes, lateral heterogeneity of POM
and photopigments in traps and bottom sediments may be
high (Ostrovsky and Yacobi 1999; Scharf et al. 2009;
Ostrovsky and Te˛gowski 2010), while the spatial variation
of temporally averaged phytoplankton biomass is much
smaller because of horizontal advections of surface waters
by wind and currents. As a result, analysis of spatial
heterogeneity of measured POM fluxes may be used for
elucidation of site-specific processes that affect sedimentation and redeposition of particular material in water bodies
(Weyhenmeyer and Bloesch 2001; Bloesch 2004). Taking
into account the spatial heterogeneity of factors affecting
the flux of particulate material (newly produced, allochthonous, resuspended), measurement accuracy of autochthonous POM export rate from the upper productive stratum
will depend on sediment trap location (Bloesch and
Uehlinger 1986; Weyhenmeyer 1996; Bloesch 2004). Uncertainties pertaining to the effect of trap location on the
amount and quality of collected material make many data
sets reported in the literature difficult to interpret. As a
result, the linkage between primary production and
sedimentation of autochthonous POM is still poorly
understood in many natural systems and is a subject of
extensive discussions (Baines et al. 1994).
Productivity in costal zones of large aquatic ecosystems
is usually higher than in pelagic zones because of a higher
supply of allochthonous nutrients or larger internal loads
caused by greater diapycnal mixing in the inshore zones of
vigorous hydrodynamics (Kratz et al. 2005). However,
POM sedimentation rates measured in zones of high
turbulence, resuspension, and lateral advective transport
of particles potentially lead to an overestimation of the
actual downward flux. The coincidence of high biological
productivity and measured sedimentation rates may result
in an overestimation of the relative importance of POM
export in such productive zones.
Spatial and temporal variability of primary production
and POM export may perhaps be best studied in lakes with
simple morphology and energetic hydrodynamics where
boundary processes play a conspicuous role in material
transport and biogeochemical cycling. In this work, we take
advantage of the extensive sampling program occurring in
Lake Kinneret (Israel), a deep subtropical lake, for
investigation of the relationship between primary production and POM sedimentation fluxes measured in different
seasons and locations. Specifically, this study was focused
on understanding the reasons accounting for spatial and
temporal heterogeneity in sedimentation of POM. Determination of proper location for trap deployment in the lake
was an important task for accurate estimation primary
production export from the upper mixed layer by
sedimentation. We hypothesized that thermal stratification,
which constrains turbulence and vertical advective water
motions below the thermocline, should minimize the bias of
POM flux measured by the trap technique. Further, we
studied the seasonal dynamics and composition of POM
1919
Fig. 1. Seasonal dynamic of (a) temperature and (b) oxygen
in the water column of Lake Kinneret. Temperature (uC) and
oxygen (mg O2 m23) were sampled weekly for the period of
study (2004–2008).
accumulated in traps, with special emphasis on the most
common thylakoid-bound pigments as markers of algal
biomass. We used the difference in degradability of Chl a
and b-carotene (b-car) (Leavitt and Hodgson 2001) to
follow the fate of newly produced algal material on sinking.
Finally, we quantified the proportion of new POM
exported from the upper productive layer by sedimentation
and discuss the factors affecting material sedimentation
over the annual cycle.
Limnological background
Lake Kinneret is a warm, monomictic lake with a surface
area of 164 km2, an average volume of 4100 Mm3, and an
average annual recharge of about 450 Mm3. The mean and
maximum depths are 23 and 43 m, respectively, when the
mean lake level is 2209 m above sea level (masl). The
Jordan River is the main water source for the lake; it
delivers , 80% of suspended solids from the watershed
(Kinneret Limnological Laboratory database). Water
inflow fluctuates widely in response to precipitation
(Samuels et al. 2009). Water provision and withdrawal in
the lake is asymmetrical and is the reason for the waterlevel fluctuations. Homothermy occurs between January
and March with minimum water temperatures of 13–15uC.
Lake Kinneret is stratified from about April to December
(Fig. 1a). The depth of the upper mixed layer deepens
slowly from 8 to 9 m in March–April to 15 m in
September–October. Following declines in surface temper-
1920
Ostrovsky and Yacobi
ature in November, the thermocline deepens fast until
complete disappearance. With the onset of thermal
stratification, oxygen concentration rapidly declines in the
hypolimnion (Fig. 1b), becoming fully anoxic in late May
or early June. Subsequently, hydrogen sulfide concentrations increase until complete overturn of the lake. Algal
biomass usually peaks in the lake in April–May, followed
by a rapid decline usually occurring in May–June.
The lake is forced by a powerful sea breeze (Shilo et al.
2007). Diurnal wind patterns and summer stratification
result in large-scale internal waves, an energetic benthic
boundary layer (BBL), and strong horizontal and vertical
mixing processes (Serruya 1975; Lemckert et al. 2004; Shilo
et al. 2007). In summer, when strong winds prevail, vertical
oscillations of the metalimnion can reach up to 10 m at the
lake periphery (Serruya 1975) and up to 5 m at the lake
center (Antenucci and Imberger 2003). These displacements
of the metalimnetic isotherms decrease notably at the end
of summer. The energetic hydrodynamics in nearshore
areas and at the BBL induce diapycanl mixing, resuspension, and lateral transportation of particulate material in
the lake (Ostrovsky et al. 1996; Ostrovsky and Sukenik
2008), which affect the distribution of organic material in
the uppermost layer of bottom sediments (Ostrovsky and
Yacobi 1999).
Prior to 1993, the winter–spring phytoplankton community was consistently dominated by the large dinoflagellate
Peridinium gatunense, with peak Chl a concentrations of
hundreds of mg m23. Since 1994, this species has appeared
only in some years, and the winter–spring phytoplankton
has became dominated by other species with lower peak
Chl a concentrations (Zohary 2004). The lower boundary
of the euphotic zone (1% of incident light) is at a depth of
9 m 6 2 m (Yacobi 2006) such that in spring and early
summer this zone is shallower than the upper mixed layer
(Fig. 2a). With the deepening of the upper mixed layer in
summer and fall, the euphotic zone becomes progressively
shallower than the upper mixed layer. Annual mean
primary production is , 1600 g C m22 d21 (Yacobi 2006).
Spatial distribution of phytoplankton biomass and total
suspended solids in the upper productive part of the water
column is uniform during most seasons (Yacobi and
Schlichter 2004), although conspicuous algal patches may
be found during and immediately after a flood event when
dinoflagellate bloom occurs. However, even in the latter
case, prominent wind-driven horizontal motions of the
surface stratum (Stocker and Imberger 2003; Shilo et al.
2007) result in small lateral variations of phytoplankton
density and photosynthetic activity when these parameters
are averaged over a period of several weeks. Specifically,
primary production and Chl a concentrations from stations
positioned remotely from the Jordan River inlet area
(where algal productivity is usually above seasonal means)
were, on average, within 6 30% of the values measured at
the lake center (I. Ostrovsky and Y. Z. Yacobi unpubl.).
The generally low spatial variability allows extrapolation of
primary production measured at the lake center to the
entire lake. The apparent similarity of mean phytoplankton
biomass and production at various stations is in contrast
with the high lateral heterogeneity in composition of
Fig. 2. Monthly averages of (a) upper mixed layer thickness
(UML), ratio euphotic zone to the UML thickness (EZ : UML),
and (b) particulate phosphorous (Ppart) concentration in the upper
mixed layer. Ppart was calculated as the difference between total
phosphorous and total dissolved phosphorous. Dashed lines show
temporal trends over specific time spans. Monthly means over
the period of this study are shown. Vertical bars show 6
standard error.
organic material and photopigments in the uppermost
layer of bottom sediments (Ostrovsky and Yacobi 1999;
Ostrovsky and Te˛gowski 2010), which is caused by
resuspension and redistribution of settling particles by
vigorous water motions at the lake boundaries (Ostrovsky
et al. 1996; Ostrovsky and Sukenik 2008).
Methods
Spatial variability of sedimentation fluxes was studied
using five sets of sediment traps, which were deployed at
three stations positioned along an offshore transect that
connects the northwestern corner of Lake Kinneret with its
center (Fig. 3). This transect was positioned far away from
the Jordan River outlet zone, where most of the particle
load is deposited (Serruya 1974; Markel et al. 1994). Trap
locations were chosen to represent the littoral (Sta. M, 10-m
depth), sublittoral (Sta. F, 20-m depth), and pelagic areas
(Sta. A, 38-m depth) of the lake. The average water level
during the period of this study, which lasted from January
2005 throughout December 2008, was 2211.3 masl. At each
station, a set of ‘‘lower’’ traps was positioned near the
bottom (, 2.5 m above the bottom at Sta. A and Sta. F and
Sedimentation in a subtropical lake
Fig. 3. Map of Lake Kinneret with locations of trap
deployments and 5-m isobaths. For details, see Methods.
, 1.5 m above the bottom at Sta. M) and designated as
Mlow, Flow, and Alow, respectively. At the two deeper
stations, an additional set of traps was deployed at a higher
position, which, based on temperature and turbulence
profiles in summer, should have been located in the
hypolimnion just above the BBL (Lemckert et al. 2004). In
Sta. F, the ‘‘upper’’ traps were deployed 3 m above Flow and
in Sta. A 9 m above Alow and were designated as Fup and
Aup, respectively. Thus, the combination of ‘‘upper’’ and
‘‘lower’’ traps was aimed to investigate the trap performance
under different turbulent regimes.
Chemical and physical conditions at trap locations
changed seasonally (Fig. 1). At the deepest station, traps
were in the anoxic hypolimnion from June to November
(Aup) or December (Alow). Sediment traps at Sta. F were
exposed to anoxic conditions from July to September–
October, assuming an unvarying level of seasonal thermocline. However, in summer during the period of strong
seiches, peripheral traps were in a zone influenced by
diurnal oscillations of the metalimnion such that they may
have been periodically exposed to metalimnetic water and
in early fall exposed to epilimnetic water. The shallowest
traps (Mlow) were positioned in the upper mixed stratum,
which was permanently oxygenated.
Each trap set consisted of four plastic cylinders (inner
diameter of 5 cm, height of 50 cm). See Koren and Klein
(2000) for a detailed description of the traps and their
setup. Traps were deployed usually from 1 to 2 weeks. The
four subsamples of collected material in each trap set were
1921
pooled prior to further analysis. The amount of material
accumulated in traps was determined by filtration of a
known aliquot of suspended material onto GF/F filters and
dried for 24 h at 80uC. This estimate was used for
calculation of gross sedimentation rate (GSR), that is,
the daily sedimentation flux of particulate material per
square meter in a specific location. Organic matter content
(OMC) of the particulate material was measured as loss on
ignition, that is, following combustion at 530uC; carbon
content was assumed to be 50% of OMC (Eckert and
Parparov 2006). Organic matter sedimentation rates
(OMSR) in traps were calculated as products of the
respective GSR and OMC.
Chl a concentration and primary productivity (PP) were
monitored at Sta. A on a biweekly basis using standard
protocols (Yacobi 2006). Pigment analyses of particulate
material, suspended water, and material accumulated in
traps were done using the protocols described by Yacobi
and Ostrovsky (2008). In brief, samples for pigment
analysis were prepared by filtration of suspended trap
material, or lake water collected from the depth of 1 m,
onto GF/C filters. Samples were immediately frozen and
stored in the dark at 218uC. Particulate material collected
on filters was processed within 1 week, following collection;
frozen filters were ground in 3 mL of cold 90% acetone; an
additional 3 mL of acetone were used to flush leftovers, and
the pooled extract was left overnight in the dark at 4uC.
Subsequently, the acetone extract was filtered through a
GF/F filter and separated by a reverse-phase high-pressure
liquid chromatography. In this study, we relate only to Chl
a, its degradation products (sum of all phaeopigments), and
b-car.
Chl a sedimentation flux was used as a proxy for the
estimation of phytoplankton-based organic carbon flux.
Conversion of Chl a to organic carbon was done using the
ratio of community carbon to Chl a (R) specific for each
sampling date. The ratio was calculated taking into account
the relative contribution of each algal taxa to total
community biomass (pi) and taxa-specific ratio of carbon
to Chl a (ri; Yacobi and Zohary 2010), as follows:
X
ð1Þ
R~
pi r i
Although such an approach is widely used for evaluation of
algal biomass, it may potentially overestimate the abundance of intact phytoplankton, as algal debris may also
contain Chl a (Roy et al. 1989).
Comparison of algal pigment composition in the upper
mixed water stratum (or in the euphotic zone) with that in
sedimentation flux is a useful approach for understanding
the fate of phytoplankton in water bodies (Hurley and
Armstrong 1990; Yacobi and Ostrovsky 2008). We used
Chl a and b-car to characterize the entire phytoplankton
community, as these pigments are harbored by all oxygenic
photosynthetic organisms, with the exception of cryptophytes, which lack b-car (Rowan 1989). To characterize the
fate of newly produced algal material as a whole, we used
the difference in degradability of these most common algal
pigments (Leavitt and Hodgson 2001) as the basis for
comparing euphotic zone pigment composition to that of
1922
Ostrovsky and Yacobi
recently settled material, as previously suggested by Yacobi
and Ostrovsky (2008). First, for sake of comparison of
pigment abundance in the water column and in the traps,
the following pigment indices (PI) were calculated:
PIwater ~CChl a =Cb-car , mg L{1 (mg L{1 ){1
PItrap ~FChl a =Fb-car , mg m{2 d{1 (mg m{2 d{1 ){1
ð2Þ
ð3Þ
where PIwater is the ratio of pigment concentrations in the
water column, PItrap is the ratio of pigment fluxes in
sediment traps, CChl a is the concentration of Chl a in the
water column, and FChl a is Chl a flux in traps. Cb-car and
Fb-car are the concentration and flux of b-car, respectively.
b-car, being characterized by its high stability in aquatic
environments (Leavitt 1993), was chosen as the basis for
Chl a normalization in Eq. 2 and Eq. 3. Combination of the
ratios indicated in Eq. 2 and Eq. 3 led to the development
of the trap-to-water ratio (TWR):
TWR~PItrap =PIwater
ð4Þ
Thus, TWR reflects the ‘‘freshness’’ of the newly settled
algal material relative to that in the euphotic zone (Yacobi
and Ostrovsky 2008). The advantage of the use of
photopigments for investigation of the fate of autochthonously produced organic material is also associated with
the practical absence of these substances in the allochthonous POM in Lake Kinneret.
To evaluate the relative trap performance with respect to
POM, two flux indices (FI) were calculated—FIb-car and
FIOMSR—which define ratios between the flux of b-car and
OMSR, respectively, measured at a given trap to the flux
measured in Aup. The reason for choosing Aup as a
reference is derived from our finding that the flux of
organic particles measured with Aup provides the best
possible estimate of particulate material export from the
euphotic zone during stratification (see Discussion).
Results
Primary productivity and Chl a dynamics—The mean Chl
a concentration (considered a proxy of algal biomass) in
the upper mixed layer increased from December to April
and then rapidly declined following algal bloom collapse
(Fig. 4a). This variable remained low and almost stable
between July and December. PP showed a gradual increase
from January, a peak in May, and subsequent decrease
toward December (Fig. 4b). A consequence of these
seasonal dynamics of productivity and algal biomass was
a low PP : Chl a ratio during holomixis (Fig. 4c), when the
euphotic zone (EZ) consisted only 30% of the upper mixed
layer (UML). In contrast, this ratio was high in July–
September, when small species dominated the algal
community and EZ made up 70–80% of the UML
(Fig. 2a). A significant positive correlation (r 5 0.87, p ,
0.001) between PP and the EZ : UML ratio underlines the
fact that light availability throughout the entire UML is a
key factor influencing seasonal variation of algal productivity in the lake.
Fig. 4. Monthly averages of phytoplankton variables, based
on water column integrated data (0–15 m). (a) Chlorophyll a (Chl
a) content; (b) primary production (PP); (c) PP : Chl a ratio. Other
details as in Fig. 2.
General characterization of sedimentation—GSR decreased gradually from the shallowest Sta. M to the deepest
central Sta. A. In the lower traps at Sta. A and Sta. F,
measured GSRs were slightly higher than those in upper
traps at the same locations (Table 1). The smallest GSR
variation (shown as coefficient of variation [CV]) was
found at Mlow (57%), while the highest GSR variation was
detected at the Fup (111%). Average OMC of trapped
particles at Mlow was approximately half of the OMC of
material collected in the lake center. Differences in OMC
between the lower and upper traps in Sta. A and Sta. F
were small. The CV of OMC, like that of GSR, was lowest
at Sta. M and highest at Sta. F. OMC variations were
slightly higher in lower traps than in their upper
counterparts. OMSR continuously declined from Mlow
Sedimentation in a subtropical lake
1923
Table 1. Trap positioning, sampling summary, and means (6 SE) of sedimentation fluxes in Lake Kinneret from January 2005
throughout December 2008.
Trap position
Unit
Alow
Aup
Flow
Fup
Mlow
g m22 d21
g m22 d21
%
mg m22 d21
mg g21
mg m22 d21
mg g21
38
2.5
141
3.64(0.24)
1.41(0.07)
45.3(1.21)
4.22(0.60)
2.55(0.16)
0.85(0.13)
0.51(0.04)
38
11.5
137
2.48(0.17)
0.95(0.04)
46.6(1.31)
2.52(0.32)
2.49(0.19)
0.45(0.06)
0.44(0.04)
20
2.5
147
9.24(0.74)
2.28(0.12)
31.8(1.02)
3.97(0.30)
1.89(0.09)
0.92(0.08)
0.45(0.03)
20
5.5
144
8.46(0.77)
2.06(0.13)
34.1(1.16)
3.98(0.44)
2.02(0.11)
0.90(0.11)
0.45(0.03)
10
1.5
140
11.77(0.58)
2.45(0.09)
23.1(0.57)
3.66(0.25)
1.80(0.11)
0.73(0.06)
0.32(0.02)
Variable
Mean station depth
Trap height above bottom
Number of sampling periods
GSR
OMSR
OMC
Chl a sedimentation rate
Chl a OM21
b-car sedimentation rate
b-car OM21
m
m
toward the lake center. OMSRs were consistently higher in
lower traps compared to upper traps, with this difference
most pronounced at Sta. A. The lowest average Chl a
sedimentation rate was in Aup (Table 1). b-car sedimentation rates were, on average, lowest in Aup and highest in
traps at Sta. F. Chl a and b-car average sedimentation rates
at Sta. F were almost identical between lower and upper
traps. Organic matter–normalized Chl a content gradually
increased from Mlow to Sta. A, and differences between
upper and lower traps at Sta. A and Sta. F were small. The
organic matter–normalized b-car content decreased from
Mlow to Sta. A, but relative changes were smaller than those
for POM-normalized Chl a content.
Seasonal dynamics of sedimentation rates—A seasonal
pattern of GSR was identified in Sta. A and Sta. F, with
similar findings from lower and upper traps, but no clear
seasonal pattern of GSR was observed in Sta. M. The
peaks and troughs in Sta. A and Sta. F were observed at
different times of the year. In Sta. A, peaks appeared in
February–April, while at Sta. F, they were observed in
November–December; the minima of GSR were measured
at Sta. A in October–November, while at Sta. F it took
place in August–September (Fig. 5a–c). Seasonal variation
of OMC in Sta. A and Sta. F followed a similar pattern in
the lower and upper traps, where values were relatively
stable from January throughout April, subsequently
Fig. 5. Monthly averages of sedimentation variables in Lake Kinneret. A, F, and M represent trap areal position as shown on
Fig. 3; indices ‘‘low’’ and ‘‘up’’ depict trap position in the water column (for explanations, see Methods). Other details as in Fig. 2.
1924
Fig. 6.
Ostrovsky and Yacobi
Monthly averages of photosynthetic pigment sedimentation rates in Lake Kinneret. Other details as in Figs. 2 and 5.
increased until September–October, and then gradually
declined to January (Fig. 5d–e). As in the case of GSR,
OMC at Sta. M showed low variation and did not display
an identifiable seasonal pattern (Fig. 5f). OMSR seasonal
variation (Fig. 5g–i) resembled that of GSR, but the
difference between extremes of OMSR was less distinct
than GSR. The peak of OMSR occurred in February at
Sta. A and in December at Sta. F. The patterns of OMSR
in lower and upper traps at Sta. A and Sta. F were similar.
Seasonal patterns of Chl a sedimentation were similar at all
three stations (Fig. 6a–c). The highest values of Chl a
sedimentation rates in all traps were in February, the only
period when Chl a sedimentation rates in traps Alow were
more than twice the sedimentation rates in other lower
traps. Peaks and troughs of Chl a sedimentation rates
usually did not correspond to temporal changes in Chl a
content in the water column (Fig. 4a), and only occasionally was a peak in Chl a density in the water column
followed by Chl a sedimentation peak and vice versa.
Temporal patterns of b-car sedimentation rates resembled
that of Chl a (Fig. 6d–f).
Using Chl a as a proxy for algal biomass, we estimated
the relative contribution of phytoplankton-based organic
material to total OMSR. This variable had similar
dynamics in various trap locations (Fig. 7a) with annual
means progressively decreasing from the lake center to
nearshore zones, that is, 35% 6 3% at Sta. A to 25% 6 2%
at Sta. F and 19% 6 2% at Sta. M. No differences in this
variable were found between upper and lower traps. The
observed changes reflect an increase in resuspended
fraction toward the lakeshore. The notably higher mean
contribution of phytoplankton to water column POM (53%
6 5%, A. Parparov unpubl.) than to traps located in the
lake center indicates that most of the downward POM
export was made up of organic matter transformed because
of its passage through the trophic chain to detritus.
The ratio between Chl a–degraded pigments and Chl a
has similar seasonal dynamics in all traps regardless of their
location, with maximum values of 0.4–0.6 during holomixis
and nearly zero in August–October (Fig. 7b). This ratio
reflects the trophic efficiency by which algal material in the
water column is utilized. Values of nearly zero were
detected during the period of stable stratification, when
algal community consisted of small species that possess
small settling velocity (, tens of centimeters per day;
Reynolds 2006) and are easily consumed by zooplankton.
Sedimentation in a subtropical lake
Fig. 7. Seasonal changes in (a) relative contribution of
chlorophyll-based organic matter to organic matter sedimentation
rate (Chl a–based OMSF : OMSF) and (b) mass ratio of
chlorophyll degradation product sedimentation rate to chlorophyll sedimentation rate (Chl a–degr : Chl a) in different traps.
Trap locations are shown in Fig. 3.
In contrast, maximum values were characteristics for the
period of holomixis, when large algae, not available for
consumption by zooplankton, populate the water column,
and a high proportion of algal fragments may be
maintained for a long time in the well-mixed turbulent
water column.
1925
The similarities (or degree of coherence, covariance) in
seasonal variations of sedimentation parameters in different locations were examined by correlation coefficients (r),
which were calculated on the basis of actual (not averaged)
measurements. The sedimentation variables measured with
upper and lower traps located at the same station showed
high r values, while the variables measured at different
spatial locations showed a weaker correlation (Table 2).
Among trap stations, correlations of sedimentation parameters were notably weaker between the most distantly
positioned Sta. A and Sta. M than between Sta. A and Sta.
F or between Sta. F and Sta. M, and that difference most
pronounced with respect to GSR.
Distinct seasonality and synchronicity in variation of
Chl a : b-car ratio was observed in material suspended in
the water column and material collected at Aup (Fig. 8a).
Similarity of the PItrap and PIwater changes on a monthly
time scale (which frequently exceeds the duration of a single
algae bloom) indicates that settling algal material closely
mirrored phytoplankton dynamics in the water column
despite possible time lags between periods of algal bloom
and their sedimentation. The highest values of Chl a : b-car
ratio took place during January–March and November–
December, when the upper mixed water layer exceeded the
width of the euphotic zone and thus contributed to lower
mean light available for algae (Yacobi 2006). Relatively
high cellular Chl a content is a typical response to light
limitation (Falkowski and Raven 2007).
Despite the general similarity of seasonal dynamics of
PItrap and PIwater, the TWR still displayed seasonal
variability (Fig. 8b). The highest TWR (, 1) was detected
when spring algal blooms collapsed (April–May) and algae
could be rapidly delivered from the euphotic zone to
sediment traps. From late fall to the end of holomixis,
despite water column vertical convective mixing, material
accumulated in traps was more degraded than material in
the water (TWR was 0.85–0.92). This portrays segregation
between living phytoplankton and degraded material (e.g.,
feces and resuspended particles) due to differences in
sinking velocities, even under turbulent conditions. The
lowest TWRs were typical in late summer to early fall,
when the smallest algae dominated the algal community
and their fragments containing Chl a–degradation products
were nearly absent in traps (Fig. 7b). This indicates that
Table 2. Correlation coefficients (r) between sedimentation variables in different traps. GSR, gross sedimentation rate; OMC,
organic matter content of trap material; Chl a POM21, chlorophyll a content of POM; b-car POM21, b-carotene content of POM.
Number of compared pairs of measurements varied from 106 to 138. The significance of r for GSR: * p , 0.001, ** p , 0.01, *** p ,
0.05; significance of r for all organic matter variables is p , 0.001.
Pair of traps
Variable
Alow-Aup
Alow-Flow
Alow-Mlow
Flow-Mlow
Flow-Fup
Aup-Fup
Sedimentation flux
GSR
0.77***
0.19*
0.03
0.23**
0.79***
0.34***
Organic matter characteristics***
OMC
Chl a OM21
b-car OM21
0.89
0.73
0.82
0.54
0.74
0.80
0.25
0.63
0.66
0.44
0.55
0.62
0.76
0.69
0.89
0.61
0.68
0.79
1926
Ostrovsky and Yacobi
Discussion
indication of greater contribution of organic-poor resuspended sediments from the lake center toward its periphery
(Ostrovsky and Yacobi 1999; Bloesch 2004).
In locations where the upper mixed water layers overlay
the lake bottom, surface waves may generate resuspension
of particles and their advection in the water column (Evans
1994; Liu and Huang 2009). In stratified water bodies
where internal seiching is common (Fricker and Nepf 2000;
Goudsmit et al. 2002; Lemmin et al. 2005), seiche motions
cause resuspension of sediments at the basin periphery
(Gloor et al. 1994; Ostrovsky and Yacobi 1999; Jordi et al.
2008). The interaction between internal waves with the
sloped bottom produces a turbulent BBL, where large
amount of resuspended particles may occur (Gloor et al.
1994; Boudreau and Jørgensen 2001; Lorke et al. 2005).
Resuspension and advective transport of particulate
material via the turbulent BBL and along metalimnetic
isotherms by seiche-induced jets (Ostrovsky and Sukenik
2008) is the apparent reason for high accumulation of
resuspended particles in traps positioned close to the
thermocline-bottom interface (Sta. F). In particular, lateral
transport of resuspended particles through the metalimnion
and BBL, in conjunction with immense vertical metalimnetic oscillations in peripheral areas (up to 10 m; Serruya
1975), explains the similarity of measured sedimentation
fluxes and composition of material in Fup and Flow. Thus,
Sta. F depicts a typical sedimentological regime at the
peripheral parts of water basins characterized by energetic
hydrodynamics (Huthnance 2005).
The exposure of POM accumulated in traps to oxic
conditions may, in part, explain lower OMC in shallower
traps during the period of chemical stratification because
decomposition of organic material is generally faster in the
presence of oxygen (Harvey et al. 1995; Nguyen and
Harvey 1997). Yet this factor could play only a secondary
role relative to the effect of resuspended particles since
mean loss of accumulated POM during a 2-week period
was as low as 13–18% under oxic conditions and 4–10%
under anoxic conditions (I. Ostrovsky unpubl.).
Lateral variations in sedimentation—Discrepancies in
seasonal dynamics of sedimentation fluxes and in the
composition of collected trap material, as determined by
trap-paired r, indicate that factors affecting sediment
accumulation may have different intensity and seasonality
in various locations. The lowest r values were obtained for
pairwise comparisons between GSR at the shallowest and
deepest locations, underlining the discrepancy of sedimentological processes in these zones. On the other hand, the
seasonal covariance in POM-normalized content of photopigments in different traps was rather high, and r was
higher for b-car, which is less degradable than Chl a
(Leavitt and Hodgson 2001). This finding suggests similar
seasonal dynamics of settling POM, especially algal-based
organic particles, across the water body and confirms
lateral uniformity of temporally averaged distributions of
algae in the upper productive stratum.
The prominent decrease in proportion of various organic
components in trap material, in conjunction with the
simultaneous increase in measured GSR, is a clear
Sedimentation in the BBL vs. the hypolimnion interior—
The remote location of deep pelagic Sta. A, away from
zones of intensive provision of resuspended particles to the
water column, allows elucidation of the performance of
traps positioned above and within the turbulent BBL. GSR
measured at Alow in the BBL was 1.5 6 0.7 times higher
than that measured at Aup in the quiescent hypolimnion,
while composition of the collected material (OMC, Chl a
POM21, and b-car POM21) was nearly identical in both
sets of traps. Similarity of POM composition implies that
sinking POM could not be markedly degraded as it settled
between upper and lower traps and also excludes the
possibility that resuspended organic matter–depleted particles played an important role in the makeup of the
material collected in the turbulent BBL. The latter supports
our previous findings (Ostrovsky and Yacobi 1999). It is
notable that during most of the year, upper sets of
hypolimnetic traps were separated from the BBL, which
may reach a thickness of , 9 m in the lake center
(Lemckert et al. 2004) by weak but still detectable
Fig. 8. Seasonal changes in photosynthetic pigment ratios in
the upper mixed water layer (water) and in sedimentation flux at
Aup (trap). (a) Ratio between chlorophyll a and b-carotene (Chl
a : b-car); (b) trap-to-water ratio (TWR). Dashed line shows
equality of Chl a : b-car ratios in water and trap (TWR 5 1).
algal material was efficiently recycled within the epilimnion
during late summer and early fall.
Sedimentation in a subtropical lake
1927
stratification. Such a separation was sufficient to dampen
advective motion and upward transport of particles from
the BBL. Thus, the difference in fluxes measured above and
within the BBL could have been imposed only by particle
overtrapping under turbulent conditions in the BBL. In
contrast, traps deployed in the interior of the quiescent
hypolimnion, where the diapycnal turbulent diffusivity is
lowest (I. Ostrovsky unpubl.), apparently do not overestimate the POM flux when the flow around is laminar
(Bloesch and Burns 1980). Minor amounts of particles,
resuspended at the lake periphery, could potentially be
transported laterally via the metalimnion (Ostrovsky and
Sukenik 2008) and reach the central location, but this
would not influence the observed difference between the
two sets of traps. While the last effect cannot be completely
excluded, GSR and OMSR measured in the central
hypolimnion provide the best possible estimates of actual
fluxes of particulate material from the euphotic zone during
the period of stratification. These findings stress the
importance of adequate vertical installation of pelagic
traps—they should be positioned below the thermocline
but high enough to avoid any influence of the turbulent
BBL. In deep marine systems, sedimentation flux declines
with depth because of decomposition and dissolution of
particulate material after leaving the upper mixed layer
(Buesseler et al. 2007a). In such cases, drifting traps, with
buoyancy adjusted to float just below the oscillating
thermocline, can be the best solution for accurate
measurements of POM fluxes from the upper mixed layer.
Trap performance with respect to POM—b-car is the
least degradable photopigment (Leavitt and Hodgson
2001); therefore, FIb-car seems an appropriate index of trap
sampling efficiency of algae-associated particles in comparison to Aup and can also indicate particle overtrapping
with values . 1. During the period of relatively stable
thermocline depth (between April and October), FIb-car
calculated for different traps displayed surprisingly similar
seasonal changes (Fig. 9a) and comparable mean values of
1.9–2.3. FIOMSR (index of relative sedimentation of organic
matter in comparison to Aup) in traps deployed at Sta. F
and in Alow varied also within a narrow range, from 1.6 to
2.1 (Fig. 9b), indicating that sampling efficiency of those
traps was also similar with respect to POM in general,
whether of direct algal origin or otherwise. The situation was different in Mlow, where FIOMSR was larger than
FIb-car, indicating that nonalgal material played a much
greater role in the littoral POM flux than in all deeper
locations. The similarity of the indices in Alow, where only
turbulence could be a reason for POM overtrapping, with
indices calculated for Flow and Fup, where additional
factors (e.g., resuspension) could contribute to overestimation of POM fluxes, suggests that turbulence was probably
the key factor affecting the accuracy of POM downward
flux assessment in Sta. F. The critical role of turbulence is
highlighted by the rapid increase in measured OMSR : PP
ratio (export ratio; see below) in Aup, following the
seasonal collapse of thermal stratification and exposure
of the entire water column during holomixis to turbulent
conditions. Although the relative contribution of various
Fig. 9. Seasonal dynamics of various flux indices (FI) in
different traps. See explanations in the text. Trap locations are
shown in Fig. 3.
constituents (newly produced, imported, or resuspended
particles) may be reliably measured using various tracers
and assuming that these constituents have similar trapping
efficiencies (Bloesch 2004; Buesseler et al. 2007a), the
appraisal of corresponding absolute fluxes under turbulent
conditions is nonetheless complex because trapping of all
constituents may be affected irrespective of their origin.
Furthermore, when large differences in settling velocities
occur, fast-sinking and slow-sinking components may be
affected differently (Bloesch and Burns 1980; Hawley
1988), and such a situation could account for the
discrepancy between FIOMSR and FIb-car in Mlow.
The influence of allochthonous input—Assuming that the
fluvial load of particulate material from the watershed is
distributed uniformly over the lake, annual mean contribution of allochthonous particles to GSR would be
0.47 g m22 d21 for 2005–2007 and 0.1 g m22 d21 for the
unusually dry 2008, that is, 21% and 4% of GSR measured
at Aup, respectively. During the winter–spring flood period,
loading of larger particles from the watershed may
1928
Ostrovsky and Yacobi
Fig. 10. Seasonal dynamics of the ratio between the monthly
averages of organic matter sedimentation rate and primary
production (OMSF : PP) in center of Lake Kinneret. OMSR was
measured at Aup. Other details as in Fig. 2.
contribute 40–80% to GSR. Considering that half the
riverine particles settle before reaching the lake center
(since the smallest clay particles are , 50% of the total load
of suspended solids; Inbar 1982) and that the OMC of
particulate material delivered from the watershed is , 5%
(Avnimelech 1980), allochthonous material contributes
, 13% of GSR and , 2% of OMSR measured at Aup on
an annual basis. Consequently, the flux of organic particles
measured at the lake center is mainly of autochthonous
origin.
POM sedimentation and primary productivity—Considering OMSR measured in the lake interior with Aup (mostly
positioned in the middle of the quiescent hypolimnion) as
the most reliable assessment of POM flux from the upper
productive layer, one can evaluate the export ratio
(OMSR : PP ratio), that is, the proportion of PP that
settles to the bottom. Changes in the OMSR : PP ratio
display clear seasonality (Fig. 10), which may reflect the
changes in natural processes or/and artifacts associated
with overtrapping.
During holomixis (January–March), most phytoplankton species are large species capable of forming dense
populations (e.g., the diatom Aulacoseira granulata, the
dinoflagellate P. gatunense, and the chlorophyte Mougeotia
gracillima) and cannot be consumed by zooplankton
(Zohary 2004). The fast-sinking velocity of large algae
could be an apparent reason why a high proportion of the
phytoplankton reaches the lake bottom, especially during
still, warm winter days when turbulent mixing is restrained.
Temporal asymmetry between algal production and sedimentation determine the highest proportion of POM
settling in February. This is typical for large negatively
buoyant algae that possess high settling velocities. For
instance, large peaks of sedimentation are frequently
observed at the time of diatom bloom collapse (Poister
and Armstrong 2003). Increased turbulence that prevails
throughout the well-mixed water column is especially
important for sedimentation measurements. Advective
and turbulent motions easily entrain large algal cells and
circulate them between the euphotic zone and trap
locations and thus may lead to an overestimation of
POM sedimentation flux of negatively buoyant particles in
the turbulent nonstratified water column (Buesseler et al.
2007b). Resuspension of POM material from the bottom
(specifically at the lake periphery) caused by surface waves
on windy days, together with loading of large amount of
clay particles from the watershed, may be the main reasons
for the occurrence of organic-poor particles in the water
column, lower OMC of trap material, and higher GSR and
OMSR in winter and early spring. All these factors could
account for the enlarged OMSR : PP ratio during holomixis, either full (January–March) or partial (December)
(Fig. 10).
The prominent drop in the OMSR : PP ratio (from 43%
to 47% in January–February to 25% to 28% in March–
April) occurred rapidly after thermal stratification was
established and the low part of the lake was separated from
the upper productive stratum. After this, POM sedimentation rate gradually decreased throughout the stratified
period and was accompanied by an increase in OMC of
settling material. The latter coincides with the development
of anoxic condition in the hypolimnion, which may slow
down the decomposition rate of sinking organic particles
(Harvey et al. 1995; Nguyen and Harvey 1997). Despite
large changes in the composition of POM and the entire
algal community from the beginning of stratification to
October, the OMSR : PP ratio declined slightly (Fig. 10).
This decline could be associated with a shift in dominance
in algal community from large to small slow-sinking
phytoplankton species throughout the development of
strong thermal stratification (Zohary 2004). In summer
and early fall, small algae could have been efficiently
recycled mostly within the upper stratum such that their
fragments containing Chl a–degradation products were
practically absent in traps (Fig. 7b). High turnover rates
(PP : Chl a ratio; Fig. 4c) characteristic of small algae also
enhance nutrient recycling within the euphotic zone and
may be the reason for the lowest rate of phosphorous loss
from the epilimnion between July and November (Fig. 2b).
The seasonal timing of minimal POM export and the
corresponding retention of limiting nutrients in the upper
productive layer could be the result of universal adaptation
of planktonic communities to stratification, when nutrient
losses could not be effectively replenished by internal load
or from external sources.
During the period of rapid thermocline deepening in fall
to early winter, large areas of the lake bottom that were
previously part of the hypolimnion and where fresh organic
particles accumulated now become part of the metalimnion. The interaction between internal waves, which are
continuously present in the metalimnion, with exposed
sediments causes massive resuspension of recently deposited organic particles and their lateral transportation
toward the deepest part of the lake via the BBL and
metalimnion (Ostrovsky and Yacobi 1999; Ostrovsky and
Sukenik 2008). Such a mechanism of POM redeposition to
Sedimentation in a subtropical lake
deeper locations explains the prominent increase in the
OMSR : PP ratio at the end of stratified period (Fig. 10),
and that effect was overlooked in other studies. The
exposure of Aup to the metalimnion and then to the
turbulent epilimnion in December would also contribute to
overestimation of the OMSR at that time.
As was shown previously, the contribution of resuspended material and allochthonous particles to OMSR
measured in the lake interior is negligible between March
and October. Therefore, the average OMSR : PP ratio of
20%, computed for that period, indicates accurately the
proportion of PP that reached the bottom. This value is
typical for productive lakes, where the downward carbon
sedimentation flux was measured just below the upper
mixed layer (Bloesch et al. 1977; Tilzer 1984; Bloesch and
Uherlinger 1986). Baines et al. (1994) showed a fairly stable
export ratio (OMSR : PP) for lakes (average , 20%) within
a broad range of primary productivity. Notably higher
ratios were reported by Wassmann (1990) for coastal
boreal areas of the North Atlantic and further discussed in
terms of differences in food webs, ratios between the
euphotic zone and upper mixed layer, advective POM
import, and so on (Baines et al. 1994; Wassmann 2004).
Still, it seems that the difference is associated with various
scales of turbulence in small and large aquatic ecosystems
(Baumert et al. 2005) that may affect the accuracy of POM
export flux measured by traps, as discussed previously. The
role of advective transport of allochthonous POM and
material resuspended from the continental shelf may also
account for overestimates of export ratios in costal regions.
Laws et al. (2000) showed that the export ratio measured in
the open ocean has strong negative correlation with
temperature and is close to 20% of net PP on the global
scale. Significant negative correlation between OMSR : PP
ratio and surface water temperature was also observed in
this study for Lake Kinneret, showing a value of 20.78 (p
, 0.05) for the stratified period or 20.85 (p , 0.001) for
year-round, including holomixis, when the OMSR : PP
ratio could be overestimated.
This study demonstrates that the best estimation of the
export flux of the newly produced POM from the upper
mixed layer is achieved when traps are deployed in the
quiescent hypolimnion and where the effect of lake
boundaries is negligible. Such a deployment scheme
minimizes the contribution of resuspended material and
the biases associated with particle overtrapping under
turbulent conditions. Our measurements in Lake Kinneret
showed that the temporal variation of photopigments
collected with traps positioned in the lake interior follow
the dynamics of phytoplankton in the upper mixed stratum.
Since decomposition of organic matter in such traps was
small, the measured OMSR is a reliable representation
of POM exported from the upper mixed layer. Despite variation in algal community composition and density throughout the stratified period, the export ratio
(OMSR : PP) changed slightly and was on average , 20%.
The stability of this ratio is apparently the result of
adaptation of the planktonic community (e.g., decrease in
size of dominating species) to changing ambient conditions
(e.g., stratification, available light, and nutrients). The
1929
mean export ratio found in this work was close to values
measured in many other lakes and in pelagic areas of
marine systems; greater values were reported for costal
areas. In the last case, turbulent and advective water
motions around traps might bias the estimation of POM
fluxes. This is especially important for measurements in
bays, estuaries, and fjords and near the continental shelf,
where contribution of resuspended and allochthonous
material may strongly contribute to the measured downward flux of the POM. It is, therefore, still unclear if trap
performance is satisfactory for accurate assessment of
primary production export in nonstratified aquatic environments, where vigorous advective and turbulent motions
prevail.
Acknowledgments
We thank Semion Kaganovsky and Nir Koren for field and
laboratory assistance and Dr. Paul Blanchfield for lingual
corrections of the manuscript. We are grateful for the constructive
criticism of two anonymous reviewers whose useful suggestions
greatly improved the manuscript. This work was partially
supported by Lake Kinneret Monitoring Program funded by the
Israeli Water Commissioner and by a research grant from the
Israeli Science Foundation (ISF grant 627/07).
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2004.01271.x
Associate editor: Robert R. Bidigare
Received: 10 January 2010
Accepted: 26 May 2010
Amended: 15 June 2010