Land-Water Interactions Research
Current Projects and
Results
Production and fate of
dissolved materials on land.
Microbial processing across
the landscape.
Controls on biogeochemical
processes and catchment export.
References
Introduction. The simple idea of a hydrological
catchment as a study ecosystem has provided a clear framework of biogeochemical
cycling within and between ecosystems for several decades. Yet it has
proved an extraordinary challenge to measure the outputs of energy and
biochemical elements and relate them back to the underlying processes
controlling the structure and function of the terrestrial ecosystem.
 We
still understand little about the complex dynamics and rates of production of
dissolved materials on land, and their delivery to surface waters. For
example, a review of 42 studies of DOC and DON concentrations and fluxes in
temperate forests found that lab and field studies differed greatly in their
results, and that site-specific controls such as temperature or C:N ratios were
rarely evident at regional scales (Michalzik et al. 2001). We do know that
across biomes the climate and vegetation influence the production of materials
moving from land to water, and precipitation and hydrology are strong drivers of
material exports and can govern the response of receiving surface waters.
Using our conceptual model of these controls (Figure LW-1), we are asking
research questions about each of them and synthesizing our observations in part
by determining how these processes scale in space and time across landscapes.
We have also begun to incorporate these concepts and measurements into
mathematical models, necessary for extrapolations and predictions of how the
arctic system operates and how it will respond to change. The sections
below outline some background and highlights of our major research questions and
results within the framework of land-water interactions.
In summary, the main conclusions are: (1)
Soil and especially root production of dissolved materials that can be exported
to surface waters is extremely high, but the net export is low and thus
processing by microbes must be substantial; (2) Microbial community
composition and activity are strongly linked, and there are distinct and
consistent patterns of microbial processing at key points in terrestrial and
aquatic ecosystems across the landscape; (3) Interactions between
different ecosystems along the toposequence of hillslopes are critical for
understanding how process-level knowledge can be scaled-up to answer questions
on catchment and regional biogeochemistry; and (4) Despite no
evidence of increasing thaw depth near Toolik Lake, observed changes in the
geochemistry of lakes and streams over time can only be explained by a melting
of permafrost and thus systematic changes in thaw depth in the basin due to
global warming.
(1) Production and fate of
dissolved materials on land.
Our initial research in land-water interactions for the Arctic LTER focused
on carbon cycling, and set the stage for current research directions. We
found that the C loss from the entire Kuparuk Basin via streams and lakes is
around 4 g C m-2 of land surface per year, with almost one third of
this loss as CO2 and CH4 released from surface waters
directly into the atmosphere (Figure LW-2). This lateral loss of C
is substantial, and prior to this was unaccounted for in terrestrial budgets.
Globally, this movement of gases to the atmosphere from lakes and streams is
about 25% of the total global C flux from land to the oceans through rivers.
On the basis of these findings we examined in
detail the production rates of dissolved C in soils, and applied a 14C
tracer to intact tundra ecosystems. The findings were
surprising in that the production of DOC from plant roots alone is extremely
high, from 1-4 g C m-2 per day, whereas DOC export is only 2-3 g C m-2
per year! Our conclusion is that microbial processing of this C
must be substantial, ~2 orders of magnitude higher than the net catchment export
(Judd and Kling 2002; Kling, Nadelhoffer, Sommerkorn, Rastetter unpublished).
And this importance of microbial processing as DOM moves from land to oceans is
not restricted to the Arctic; consider that in most terrestrial systems NPP is
very large, 100s of g C m-2
per year, and yet the NEP is usually near zero and dissolved export averages
only ~6 g C m-2 y-1 worldwide (Hope et
al. 1994). Thus the huge difference in terrestrial C production and
aquatic export must be due mainly to microbial processing in soils. Given
this strong biogeochemical influence across landscape scales, we can ask
questions about the spatial pattern of microbial processing – where are the
control points, and who is responsible.
(2) Microbial processing
across the landscape. The influence of land-water transfers of
nutrients and organic matter is a dominant aspect of aquatic ecology, and
microbial transformations of these materials underlie production, respiration,
and atmospheric gas-exchange in ecosystems. But consideration of how these
processes are linked and interact across the landscape is relatively new, and
requires an integration of concepts in microbial and
landscape ecology. For example, we must consider the congruence of
ecotones and spatial boundaries of ecosystems with the rates of microbial
activity, as well as the biogeographical diversity of microbes and the time
scales that microbial populations adapt physiologically and by changing
population frequencies. As described below, we found several distinct
patterns of microbial species and processing rates in terrestrial and aquatic
ecosystems across the landscape.
In addition to discovering these
patterns of community composition and activity, we tested an ongoing debate in
ecology that revolves around how species composition and ecosystem function are
related. To address the mechanistic controls of this relationship, we
manipulated the composition of DOM fed to aquatic bacteria to determine effects
on both bacterial activity and community composition. Sites along terrestrial
to aquatic flow paths were chosen to simulate movement of DOM through catchments
(Figure LW-3), and DOM was fed to downslope and control bacterial
communities. Bacterial production was measured and DOM chemistry and bacterial
community composition (using denaturing gradient gel electrophoresis of 16S rRNA
genes) were characterized following incubations. Bacterial
 production,
DOC-specific bacterial production, and DOC consumption were greatest in
mesocosms fed soil water DOM; soil water DOM enhanced lake and stream bacterial
production by 320-670% relative to lake and stream controls (Judd et al. 2006).
But the really novel finding was that adding upslope DOM to stream and lake
bacterial communities resulted in significant changes in bacterial community
composition relative to controls. In these experiments the bacterial community
composition converged based on DOM source regardless of the initial inoculum (Figure
LW-5). In other words, when lake bacteria were fed soil or stream DOM,
the lake community assemblage shifted to resemble the species present in the
soil or the stream (green and red arrows in Figure LW-5). Clearly the soil and
stream bacteria were already present in the lake in undetectable numbers, but
when exposed to soil or stream DOM these populations had a metabolic advantage
and grew to replace the originally-dominate lake bacteria. These results
demonstrate that shifts in the supply of natural DOM were followed by changes in
both bacterial production and community composition, suggesting that changes in
function are likely predicated on at least an initial change in the community
composition. In similar experiments we also examined how photo-oxidation of DOM
affected microbial activity and DOM processing along these dominant hydrological
flow paths. The impacts of DOM photo-oxidation depended in part on DOM
source, but also due to the relatively rapid shifts in bacterial community
composition to groups better able to consume photo-products or tolerate harmful
radicals (Judd et al. 2007). Overall, these results indicate that variation in
DOM composition of soil and surface waters influences bacterial community
dynamics, and in turn different communities control rates of carbon processing
in set patterns across the landscape.
These landscape-level
interactions were also observed between different aquatic ecosystems. In
earlier work of the LTER,
 we showed that in a connected series of lakes and
streams there was consistent and directional (downslope) processing
of materials that produced spatial patterns in many limnological variables, and
these patterns were coherent over time (Kling et al. 2000). That is, the
interactions of material processing in both lakes and rivers are critical for
understanding the structure and function of surface waters, especially in a
landscape perspective. In recent research we have expanded these ideas to
show that the processing of DOM by microbes, and the species of microbes
present, vary consistently as water moves through a network of streams and lakes
in the Toolik catchment. For example, in Toolik Lake itself the rate of
bacterial activity was related to shifts in the source (terrestrial versus
phytoplankton) and lability of DOM, and that bacterioplankton communities were
composed of persistent populations
present throughout the year and transient populations that appeared and
disappeared (Figure LW-6; Crump et al. 2003). Shifts in community
composition, measured by denaturing gradient gel electrophoresis (DGGE) of 16S
rRNA genes, were associated with an annual peak in bacterial productivity driven
by the large influx of labile terrestrial DOM associated with spring runoff. A
second shift occurred after the terrestrial DOM flux declined and as the summer
phytoplankton community developed.
Bacterioplankton community composition was also compared across 10 lakes and 14
streams within the Toolik catchment. Both lake and stream systems shared
bacteria species (OTUs from DGGE analysis), and stream communities changed with
distance from the upstream lake, suggesting both dispersal of species between
lakes and streams as well as inoculation and dilution with bacteria from soil
waters or hyporheic zones (Crump et al. 2007). At the same time, similarity in
lake and stream communities shifted gradually down the catchment (Figure LW-7).
We found evidence that dispersal influences bacterioplankton communities via
advection and dilution (mass effects) in streams, and via inoculation and
subsequent growth in lakes, and that the spatial pattern of bacterioplankton
community composition was strongly influenced by interactions among soil water,
stream, and lake environments. Overall these results reveal large differences
in lake-specific and stream-specific bacterial community composition over
restricted spatial scales (< 10 km), and suggest that geographic distance and
connectivity influence the distribution of bacterioplankton communities across a
landscape.
(3) Controls on
biogeochemical processes and catchment export. One of the most
critical issues in ecosystem research today is understanding how, exactly, do we
apply our mechanistic or process-based knowledge of ecosystem function generated
at small scales such as a m2 plot, to larger scales such as an
entire catchment, region, or biome. There are myriad concerns and
approaches related to issues of “ecological scaling”, but in our LTER we have
focused on hillslopes as the “missing scale” required to transfer detailed
process information to larger and larger areas (Figure LW-8).
The toposequence of a hillslope represents the major ecosystem types and landscape
morphology of an entire catchment, yet can be studied in depth and cohesively
(e.g., Giblin et al. 1991). For example, we can monitor soil water
chemistry from the hilltop to valley bottom through time, and relate the
observed patterns to soil, plant, and microbial processes. The pattern of DOC
concentration in soil waters on the hillslope in 2005 (Figure LW-9) shows
early and late summer peaks at mid-slope, slightly elevated concentrations at
the footslope near the valley floor throughout the summer, and no evidence of
major transport of DOC from upslope to downslope during the summer. Our
interpretation of this pattern is that most DOC production and consumption
occurs in situ, which is consistent with the idea presented earlier that
large amounts of DOC processing occur before DOC leaves the catchment.
Preliminary data suggest that the same patterns (and interpretation) occur for
other dissolved materials such as nitrogen and phosphorus, and the next step is
to examine the specific processes and rates at the landscape points where
concentrations are high or they change rapidly.
Although the mass of C or nutrients processed on the
hillslope may be much greater than that transported downslope and into streams
and lakes, the materials transported have both great impacts on the functioning
of receiving surface waters, and can be substantial relative to the net C
storage on land. Modeling of these landscape interactions based on a
spatially linked, transect model indicates that hillslope interactions such as
the downslope movement of nutrients and water may account for a 30% increase in
C sequestration in tundra ecosystems over the next century (Figure LW-10;
Rastetter et al. 2004).
Through monitoring and experimental manipulation, we are
collecting the data needed to better model these interactions. And
through this research we are developing the methodology to link plot-scale
biogeochemical models with realistic hydrological models, in order to better
simulate hillslope processes. In the future, we hope to scale this
hill-slope model so it can be driven with coarse-scale spatial data to project
our knowledge of hill-slope processes to the Pan Arctic and over the next
century and beyond.
(4) Permafrost melting and biogeochemical impacts on
terrestrial and aquatic ecosystems. Despite clear evidence of arctic
warming,
 from
the loss of sea ice to shifts in vegetation and species ranges, many
measurements throughout the Arctic, including at Toolik Lake, show a surprising
lack of permafrost melting in the soil (Figure LW-11). At the same
time, we have measured substantial increases in the alkalinity of Toolik Lake
since 1975, and these changes are unrelated to processes in the lake and appear
instead to be caused by increased weathering of mineral soils in the catchment (Figure
LW-11). This is puzzling given the fact that the depth of summer thaw
has not increased over time to expose more mineral soil. However, these
measurements of the maximum depth of summer thaw in soils are traditionally made
using steel probes, which are limited in their use to upland terrestrial
environments. To overcome this limitation we used a new approach to show
that changes in the geochemistry of surface waters must be related to changes in
thaw depth, although the thawing may be confined to the unfrozen zones
underneath streams and lakes rather than to the entire upland catchment.
The first piece of evidence we have to build this conclusion
is that the carbonate concentrations in the soils increase with depth because
the deeper soils are more mineral rich, and they have been frozen so that less
weathering has occurred (Keller et al. 2007). The second piece of evidence
is that the ratio of
strontium isotopes (87Sr/86Sr) in soils decreases with
depth near the Arctic LTER (Figure LW-12).  This means that as
rainwater flows through deeper and deeper soils it will pick-up the soil
signature and the strontium isotopic ratio in the water will decrease.
Finally, we observed just such a decrease in strontium isotope ratio in the
stream entering Toolik Lake over the last 10 years (Figure LW-12).
The implication is that water flowpaths in the basin have progressively deepened
and are now in contact with previously frozen soils of different chemistry.
Because thaw depths of terrestrial sites have not changed during that time
period, it is likely that the unfrozen thaw bulb found underneath streams and
lakes has actually expanded, and the deeper thaw here contributes most to the
altered chemistry. Our results also suggest that increasing thaw depth
will lead to increasing Ca supply to soils and streams, as well as spatially
variable increases in P and K supply (Keller et al. 2007). It may be that
such changes in stream chemistry caused by permafrost melting are more
widespread in the Arctic than currently believed, which can be tested using this
new geochemical method at other sites in arctic and boreal regions with
permafrost.
REFERENCES
Cole, J. J., N. Caraco, N., G. W. Kling, and T. Kratz.
1994. Carbon dioxide supersaturation in the surface waters of lakes.
Science 265:1568-1570.
Crump, B. C., G. W. Kling, M. Bahr, J. E.
Hobbie. 2003. Bacterioplankton
community shifts in an arctic lake correlate with seasonal changes in organic
matter source. Applied Environmental Microbiology 69:2253-2268.
Crump,
B.
C., H. E. Adams, J. E. Hobbie, G. W. Kling. In Press. Biogeography of bacterioplankton in lakes and streams of an Arctic tundra
catchment. Ecology 88: .
Giblin, A.E., K.J. Nadelhoffer, G.R. Shaver, J.A.
Laundre and A.J. McKerrow. 1991. Biogeochemical diversity along a
riverside toposequence in arctic Alaska. Ecol. Monogr. 61:415-435.
Judd, K. E. and G. W. Kling. 2002.
Production and export of dissolved C in arctic tundra mesocosms: the roles of
vegetation and water flow. Biogeochemistry 60:213-234.
Judd, K.E., B.C. Crump, and G. W. Kling. 2006.
Environmental drivers control ecosystem function in bacteria through changes in
community composition. Ecology 87:2068-2079.
Judd, K. E., B. C. Crump, and G. W. Kling.
2007. Bacterial responses in activity and community composition to
photo-oxidation of dissolved organic matter from soil and surface waters.
Aquatic Sciences 69:96-107.
DOI 10.1007/s00027-006-0908-4.
Keller, K., J. Blum, and G. W. Kling. 2007.
Geochemistry of soils and streams on surfaces of varying ages in arctic Alaska.
Arctic, Antarctic, & Alpine Research 39:84-98.
Kling, G. W. 1995. Land-water linkages: the
influence of terrestrial diversity on aquatic systems, pp. 297-310. In: F.
S. Chapin and C. Korner (eds.), The Role of Biodiversity in Arctic and Alpine
Tundra Ecosystems, Springer-Verlag, Berlin. 320pp.
Kling, G. W., G. W. Kipphut, and M. C. Miller.
1991. Arctic lakes and rivers as gas conduits to the atmosphere:
implications for tundra carbon budgets. Science 251:298-301.
Kling, G. W., G. W. Kipphut, and M. C. Miller.
1992. The flux of carbon dioxide and methane from lakes and rivers in
arctic Alaska. Hydrobiologia 240:23-36.
Kling, G. W., G. W. Kipphut, M. C. Miller, and
W. J. O'Brien. 2000.
Integration of lakes and streams in a landscape perspective: the importance of
material processing on spatial patterns and temporal coherence. Freshwater
Biology 43:477-497.
Rastetter, E.B., B. L. Kwiatkowski,
S. Le Dízes, and J.E. Hobbie. 2004. The Role of Down-Slope Water and Nutrient
Fluxes in the Response of Arctic Hill Slopes to Climate Change.
Biogeochemistry 69:37-62.
Reeburgh, W.S., J.Y. King, S.K. Regli, G.W. Kling, N.A.
Auerbach, D.A. Walker. 1998. A CH4 emission estimate for
the Kuparuk River Basin, Alaska. J. Geophysical Research
103:29,005-2,9013.
Zak, D. R. and G.
W. Kling.
2006.
Microbial Community
Composition and Function across an Arctic Tundra Landscape. Ecology
87:1659-1670.
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Arctic ecosystems differ consistently in landscape position,
plant species composition, litter biochemistry, and biogeochemical cycling
rates. To test the idea that these ecosystems contain distinct microbial
communities that differentially transform dissolved organic matter (DOM) as it
moves downslope from dry upland to wet lowland tundra, we studied soil microbial
communities in upland tussock, stream-side birch-willow, and lake-side wet sedge
tundra (Figure LW-3). Using phospholipid fatty acids and 16s-rRNA analyses,
we found that microbial community composition was distinct among tundra
ecosystems, with tussock tundra containing a significantly greater abundance and
activity of soil fungi (Judd et al. 2006; Zak and Kling 2006). We also
added compound-specific 13C isotope tracers and made measurements
of extracellular enzymes involved in cellulose, chitin, and lignin degradation to
examine rates of microbial activity. Although the majority of 13C-labeled
substrates rapidly moved into soil organic matter in all tundra soils (i.e., 50
to 90% of applied 13C), microbial respiration of labeled substrates
in wet sedge tundra soil was lower than in tussock and birch-willow tundra (Figure
LW-4 ; Zak & Kling 2006). Despite these differences, wet sedge tundra
exhibited the greatest extracellular enzyme activity. Thus it is apparent
that topographic variation in plant litter biochemistry and soil drainage shape
the metabolic capability of soil microbial communities, which, in turn,
influence the chemical composition of DOM across the arctic tundra landscape.