P-IIIA: Stream Ecosystems
Summary: The stream research is centered on the observation that there are biotic assemblages associated with habitat types within the stream.
Most streams in the eastern U.S. begin in forests and are dominated by allochthonous inputs. As width increases downstream, light to the streambed increases, allochthonous inputs are lower, and the stream becomes more autochthonous based (Vannote et al. 1980). Over the
past 5 years, much of
our effort in the LTER has been directed towards study of the first to
fourth order continuum within the Coweeta basin. Our results have not shown
strong changes along this continuum that can be attributed to increasing
stream size. Instead, specific geomorphological characteristics
of a site (e.g. pool, riffle, run) are more important in determining
functional and community characteristics. Further downstream, stream-size
characteristics become much more important (Grubaugh et al., in press);
however, even in the Little Tennessee River, there are areas of riffle that
have macroinvertebrate communities more typical of riffles in headwaters
than mid-order streams. Over a 60-km stream continuum, where annual degree
days increased by 50% and discharge increase 3000-fold, production of benthos increased from < 10 to > 150 g AFDM/m2/yr,
among the highest values ever recorded (Grubaugh 1994). In addition to large
changes in production, there were large changes in taxa over the gradient.
These stream size trends can be overridden by anthropogenic disturbance;
e.g., Ward and Stanford (1983) showed that a dam can reset a stream to
conditions more similar to those found upstream. In the Little Tennessee
River, human land use seems to have the opposite effect; effects of sediment
inputs, elevated nutrients, and lack of riparian shading create conditions
more typical of larger streams.The stream research is centered on the observation that there are biotic assemblages associated with habitat types within the stream (Huryn and Wallace 1987, Naiman 1988, Gregory et al. 1991). We recognize several habitat types within any reach of stream: riffles, runs, depositional zones, bedrock chutes, and channel expansion zones (i.e. areas wet only during storms) that characterize small headwater streams. Downstream, areas of specific habitat types get larger, depositional zones become pools, and channel expansion zones become floodplains. New habitat types, such as macrophyte beds, may become increasingly important, and the habitat types found within a reach of stream will also reflect land use in the watershed.
Our current research is designed to answer two questions:
(1) How do stream size, in-stream habitat types, and riparian land use vary along a 1st to 7th order stream gradient?
(2) How are organic matter sources, in-stream organic matter dynamics and invertebrate communities affected by stream size, habitat types, and riparian land use along this gradient?
In addition to this research, we are continuing our ongoing studies that address long-term recovery of stream processes and invertebrate communities from watershed (clear-cutting) and site specific (log addition) disturbances. These studies address the question: what is the long-term (decadal) pattern of recovery from disturbance for organic matter dynamics (inputs, storage and decomposition) and invertebrate communities in southern Appalachian headwater streams? In combination with the regional stream research, these studies will enable us to analyze the long-term response of stream ecosystems to anthropogenic disturbance in the context of longitudinal gradients of change along a stream continuum.
For more information:
P-IIIA1.
Characterizing Stream Ecosystems
P-IIIA2.
Long Term Stream Studies
P-IIIA1: Characterizing
Stream Ecosystems
Summary: In 1996, we continued surveying geomorphological
measurements with the total data set spanning over 50 km, beginning at Ball
Creek to the confluence of Coweeta Creek down the Little Tennessee River to
Fontana Reservoir.
At 100 m intervals, we measured width, bankfull
width, flood plain width, mean depth, extent of riparian vegetation,
substrate composition, and coverage of aquatic macrophytes.
Stream geomorphology and biology changed in response to experimental
additions of woody debris: depth increased, current velocity decreased,
cobble substrate was covered by sand and silt, and benthic FPOM and CPOM
standing stock increased (Wallace et al. 1995). Invertebrate
community structure changed dramatically, although solute uptake lengths did
not.
Secondary production of aquatic invertebrates
within dense growths of Podostemum ceratophyllum at downstream sites in the
Little Tennesee River are among the highest reported for aquatic
invertebrates.
We are investigating the importance of
Podostemum to
invertebrate community structure by comparing seasonal benthic abundances
and biomass of taxa in functional groups on cobble and bedrock substrates
where Podostemum has previously been manually removed (by scraping and
brushing) with substrates containing unmanipulated Podostemum. We predict
distinct shifts in benthic abundances and biomass of taxa (i.e., from
filterers to predominantly scrapers) and lower abundances and biomass of
benthos in areas where the Podostemum has been removed.
Benthic macroinvertebrates are also collected seasonally at each site and foregut contents mounted on slides, identified, digitized and summarized as described by Wallace et al. (1987). We will examine how resources (seston composition, benthic organic matter, and primary production) vary seasonally along the stream size and elevational gradient. With gut analyses, we can determine how food resources (i.e., detritus, fungi, algae, and animal) ingested by the dominant taxa within various functional groups vary seasonally over the gradient. Using the procedure of Benke and Wallace (1980) and our measures of secondary production at these sites (Grubaugh 1994), we will be able to assess how the trophic basis of production varies over the gradient.
During 1997, litterfall inputs to small streams
were sampled by suspending litter traps over the stream. At the larger
river sites we placed litter traps in the riparian area and use the
fraction of the annually inundated area covered by riparian vegetation to
estimate allochthonous
input. Other than direct allochthonous inputs
and in-stream primary production, the other source of organic matter to
heterotrophic organisms in a stream is organic matter transported from
upstream. During 1999, we estimated seston transport at each site by collecting
samples every two weeks for non-storm transport. Seasonally we
also collected samples during storms and determined both organic and
inorganic particle transport. Streamflow at non-gaged sites will be
estimated from drainage area and flow at gauged sites.
We synthesized breakdown and transport of allochthonous detritus from many Coweeta stream studies, and demonstrated that leaves generally break down near where they enter streams at a rate predicable from litterbag measurements (Webster et al. 2000). Fine particles of organic matter, however, travel long distances before being metabolized (Webster et al. 1999). We also found that small streams are very efficient in retaining dissolved inorganic nitrogen (Tank et al. 2000, Peterson et al. 2001). Flood entrainment of floodplain detritus is a measurable source of organic matter in the middle reaches of the Little Tennessee River, but is nevertheless small compared to leaf fall and in-stream primary production (Neatrour 1999). For more information and photos please visit part of the Coweeta slide show.
And finally, in 1998, we measured primary production and community respiration at each site using the upstream-downstream diurnal oxygen change technique (e.g., Bott 1996) five times per year. This technique has been extensively used in larger streams in eastern US (reviewed by Webster et al. 1995) and has recently been modified for use in smaller streams (Marzolf et al. 1994).
P-IIIA2:
Long Term Stream Studies
Summary: The length of our long-term data record is critical to our understanding
of fundamental processes in the southern Appalachian Mountains.
We analyzed 23 years of data for trends and dynamics in inorganic N
deposition and loss for 6 reference and 8 disturbed watersheds at Coweeta
(Swank and Vose 1997). Reference watersheds are in a
transition phase between stage 0 and stage 1 of N saturation that is
partially attributed to significant increases in NO3 and NH4 in bulk
precipitation and/or reduced biological demand due to forest maturation.
Disturbed watersheds were in stages 1, 2, and 3 of N
saturation.
The following ongoing studies address long-term
responses to disturbance. We have been following recovery of a stream from
clear-cut logging for over two decades.
Hydrologic and solute responses were analyzed for
Watershed 7 (WS 7) at
regular intervals since it was clearcut and cable-logged in 1977 (Webster
et al. 1992, Swank et
al. 2001). Explanations for rapid water yield recovery and nutrient
retention were linked to long-term process level studies (Knoepp and Swank
1997, Elliott et al. 1999). We used time series analysis of WS 7 and its
reference (WS 2) to examine long-term stream water NO3 concentrations
(Worrall et al. 2001, Worrall et al. 2002). We determined that the
hydrological pathways and N reserves of WS 7 are in metastable
equilibrium, but in dynamic equilibrium with respect to long-term
temperature
change. These results are guiding ecosystem management in the southern
Appalachian Mountains (Meyer and Swank 1996).
In addition, we have conducted a
long-term woody debris manipulation (Wallace et al., in press) and
monitored the long-term recovery of a WS 7 stream by periodically measuring
litter content, leaf decay rates, benthic organic matter, stream
geomorphology, nutrient and dissolved organic carbon concentrations, and
invertebrate community structure and production. Annual sampling for
benthic invertebrates at 3 experimental sites (large woody debris addition)
and 3 reference sites (cobble riffles) on Cunningham Creek (Wallace et al.,
in press) has also continued. These sites have been sampled seasonally from 1988 until 1992,
when an annual sampling regime was initiated. These data are providing
valuable long-term records of invertebrate abundances and biomass at
manipulated and reference sites.
Investigators and Collaborators:
Fred Benfield
Judy Meyer
Bruce Wallace
Jack Webster