EFFECTS OF SIMULATED LAND-USE PRACTICE S
ON THE PRODUCTIVE CAPACITY OF STREAM S
BY
C. DAVID McINTIRE,
AND
DEAN M. DeNI COLA
WATER RESOURCES RESEARCH INSTITUT E
OREGON STATE UNIVERSIT Y
CORVALLIS, OREGON
WRRI-109
JANUARY 1992
EFFECTS OF SIMULATED LAND-USE PRACTICES ON THE PRODUCTIV E
CAPACITY OF STREAMS
by
C . David McIntire and Dean M . DeNicola
Department of Botany & Plant Patholog y
Oregon State University
Final Technical Completion Report
Project Number G1444-0 3
Submitted to
United States Department of the Interio r
Geological Survey
Reston, Virginia 2209 2
Project Sponsored by :
Water Resources Research Institut e
Oregon State Universit y
Corvallis, Oregon 9733 1
The activities on which this report is based were financed i n
part by the Department of the Interior, U .S . Geologica l
Survey, through the Oregon Water Resources Research Institute .
The contents of this publication do not necessarily reflec t
the views and policies of the Department of Interior, nor doe s
mention of trade names or commercial products constitute thei r
endorsement by the United States Government .
WRIU -10 9
JANUARY 1992
i
FORWAR D
The Water Resources Research Institute, located on th e
Oregon State University campus, serves the State of Oregon .
The Institute fosters, encourages and facilitates wate r
resources research and education involving all aspects of th e
quality and quantity of water available for beneficial use .
The Institute administers and coordinates statewide an d
regional programs of multidisciplinary research in water an d
related land resources . The Institute provides a necessar y
communications and coordination link between the agencies o f
local, state and federal government, as well as the privat e
sector, and the broad research community at universities i n
the state on matters of water-related . research . Th,e Institute
also coordinates the interdisciplinary program of graduat e
education in water resources at Oregon State University .
It is Institute policy to make available the results o f
significant water-related research conducted in Oregon' s
universities and colleges . The Institute neither endorses nor
rejects the findings of the authors of such research . It doe s
recommend careful consideration of the accumulated facts b y
thos e' doncerned with the solution of water-related problems .
ii
TABLE OF CONTENTS
FORWARD .
.
i
LIST OF FIGURES
ii i
ABSTRACT
v
1.
INTRODUCTION
1
Background
Project Objectives
1
2
2.
3.
EXPERIMENTAL STUDIES . .
A.
.
.
5
Introduction
Laboratory Facilities Experimental Design and Methods Experiment #1
Experiment #2
Experiment #3
Results
Experiment #1 Experiment #2
Experiment #3
Summary and Conclusions 5
6
7
8
9
9
9
9
11
13
15
MODELING
19
Approach
19
The McIntire and Colby Stream Model 20
A New Model of Herbivory
23
Biological Characteristics of the New Herbivory
Subsystem Model
26
Model Behavior
27
Summary and Conclusions
44
REFERENCES
47
LIST OF FIGURES
Figure 1
Page 1 0
Mean algal biomass, expressed as g m 2 ash-free dr y
weight, on the top and bottom levels of the largel and smal l
blocks . "Control" refers to the locations which corresponde d
to large and small blocks in streams with no relief . Each
point represents a mean of 3 observations, for each of thre e
treatments (large block, small block, and no relied) .
Figure 2
Page 1 2
Mean algal biomass, expressed as g in2 ash-free dr y
weight, on the top and bottom levels of large blocks a t
treatment irradiances of •400, 90, and 15 uE m- 2 sec' . Each
point represents a mean of three observations for each of th e
three irradiation treatments .
~
Figure 3
Page 1 4
Mean algal- biomass, expressed as g m- 2 ash-free dry
weight, on the top and bottom levels of large blocks a t
treatment irradiances of 400, 90, and 15 uE m-2 sec-1 . The
snail Juga silicula was introduced into each stream on day 1 3
of the experiment at a density of 375 animals m -2 . Each poin t
represents a mean of three observations for each of, the thre e
irradiation treatments .
Figure 4 Page 2 1
A schematic diagram representing the hierarchica l
structure of the McIntire and Colby Stream-Ecosystem Model .
Figure
A
of the
Stream
5
Page 22schematic diagram representing the detailed structur e
Primary Consumption Subsystem of the McIntire and Colb y
Ecosystem Model .
Figure 6
Page 2 5
A schematic diagram representing the detailed structur e
of the Herbivory Subsystem of the McIntire and Colby Strea m
Ecosystem Model before modification .
Figure 7
Page 2 9
Standard run of the Herbivory Subsystem Model for a
360-day year- showing the biomasses of the algal functiona l
groups with grazing .
Figure 8
Page 3 0
Relationship between irradiance level and the biomasse s
associated with the processes of primary production an d
grazing, as indicated by a standard run of the Herbivor y
Subsystem Model .
Figure 9
Page 3 1
Relationship between irradiance level and the annua l
production and turnover associated with grazing, as indicate d
by a standard run of the Herbivory Subsystem Model,.
iv
Page 3 2
Figure 10
Relationship between irradiance level and the annua l
gross primary production and algal turnover, as indicated b y
a standard run of the Herbivory Subsystem Model .
Page 33 '
Figure 11
Standard run of the Herbivory Subsystem Model for a
360-day year showing the biomasses of the, algal functiona l
groups• without grazing .
Page 3 5
Figure 12
Relationship between algal refuge and the annua l
production and mean biomass associated with the process o f
grazing, as indicated by simulation runs of the Herbivor y
Subsystem Model .
Page 37
Figure 13
Relationship between algal refuge and the gross primar y
production of algal functional groups, as indicated b y
simulation runs of the Herbivory Subsystem Model .
Page 3 9
Figure l4
Relationship between herbivore food demand and annua l
gross primary production relative to four light schedules (se e
text for description), as indicated by simulation runs of th e
Herbivory Subsystem Model .
Page 4 0
Figure 15
Relationship between herbivore food demand and annua l
mean algal biomass relative to four light schedules (see tex t
for description), as indicated by simulation runs of th e
Herbivory Subsystem Model .
Figure 16
Page .41.
Relationship between herbivore food demand and the annua l
production associated with the process of grazing relative t o
four light schedules (see text for description), as indicate d
by simulation runs of the Herbivory Subsystem Model .
Page 4 2
Figure 17
Relationship between herbivore food demand and annua l
mean biomass associated with the process of grazing relativ e
to four light schedules (see text for description), a s
indicated by simulation runs of the Herbivory Subsystem Model .
Page 4 3
Figure 18
Relationship between herbivore food demand and th e
production ratio of algal functional groups relative to fou r
light schedules (see text for description), as indicated b y
simulation runs of the Herbivory Subsystem Model . The rati o
is the gross production of green plus blue-green algae divide d
by the gross production of diatoms .
v
ABSTRACT
This project was concerned with the potential effects o f
removing streamside terrestrial vegetation on biologica l *
processes in streams . The objectives of the _ research were, .( l
to investigate the effects of light energy inputs an d
substrate spatial heterogeneity on the productivity an d
species composition of plant assemblages in laborator y
streams ; and (2) to couple the results of these and othe r
experimental studies with the dynamics of the total strea m
ecosystem by mathematical modeling .
The experimental work investigated effects of substrat e
heterogeneity on the patch dynamics and productivity of plan t
assemblages in streams . Results of these experiment s
indicated that, in the absence of grazing, backwater area s
behind substrate blocks were colonized faster by algae an d
initially had plant biomasses 1 .5 to 2 times higher than area s
of faster current on top of the blocks . After 30 days, alga l
biomasses on top and behind blocks were similar . Patterns of
community development in the two areas were similar at a give n
light intensity, bu t ' the rate of development was faster behind
blocks . Patchiness and taxonomic heterogeneity in th e . algal
assemblages generated by substrate irregularity were mor e
pronounced at the higher light intensity (400 uE n-2 sec' )
than at lower intensities (20 and 1 .00 uE m-2 sec") . The snai l
Juga sil.icula preferentially grazed algae behind the substrat e
blocks, thereby reversing the pattern of plant growth observe d
in the absence of herbivory .
Moreover, the effect o f
herbivory was more pronounced at the higher light intensities .
vi
These results suggest that the flow of water over an irregula r
substrate in a streambed can generate heterogeneous patches o f
algae, and therefore, may account, in part, for differences i m
the diversity and biomass of periphyton in natural streams ..
. Output from a lotic ecosystem model indicated tha t
diatoms dominate the plant assemblages when the process o f
grazing is active' and the stream is . shaded by riparian ,
vegetation during late spring and summer . In this case, th e
model predicts that the algal biomass turns over about 6a
times each year, the annual gross primary production
g/m2 , the annual grazer production is 6 .6
is
10 6
g ' m2 , and the -
turnover for the grazer biomass is 3 .4 times per year. I f
light energy inputs are increased to simulate clearcu t
logging, mean grazer biomass and production and annual gros s
primary production increase, while the annual mean' plai t
biomass decreases slightly . Although the algorithm allows the
green algae to reach high biomasses at high light energy, thi s
does not happen in the presence of grazing, as high rates o f
algal consumption restrict the flora to a diatom assemblag e
which is more characteristic . of an early stage• of successicnx .,
Another set of simulations indicated that anima l
production in streams can be limited by overexploitation o f
food resources . In this study, an algal refuge parameter
protected the food resource from grazing to any desired level .
Model output from these runs indicated that animal productio n
was maximized when the algal assemblage was protected fro m
grazing at biomasses below 7 g m 2 ; protection ~ayr this level .
generated a rapid decline in animal production because of a
vi i
decrease in the 'quantity and quality of the food resource .
The results of these runs also indicated .that an increase in
light intensity can lower food quality by altering th e
taxonomic composition of the algal flora . This impact is
particularly noticeable as food consumption rates decrease ,
through the possible mechanism of substrate heterogeneity o r
a change in the taxonomic structure of the animal community .
1 . INTRODUCTION
Background
-
In the Pacific Northwest, the major land uses includ e
forest production (55%), range (35%), agriculture (12%), an d
urban development (3%) . These uses commonly generate
activities that have enormous effects on the bioenergetics and
nutrient dynamics of flowin g - water (lotic) ecosystems . I n
particular, the use of land for range and timber productio n
can result in the removal of streamside terrestrial vegetation
and the subsequent deterioration of riparian habitat . The
research described in this report relates to the responses o f
biological processes in streams to human-related changes i n
riparian zones . A better understanding of such responses i s
needed by managers of forest and range lands and by fisherie s
and wildlife biologists . While some earlier studies have been
concerned with effects of canopy removal on fish productio n
(e .g ., Aho 1976), the complexity of food chains in strea m
communities often prevents a clear understanding of th e
relationships upon which reliable predictions must be based .
Sources of energy to flowing water ecosystems fall int o
two general categories : solar radiation and the allochthonou s
detrital inputs-from adjacent terrestrial systems . Therefore ,
the loss of riparian habitat associated with forest an d
rangeland streams can shift energy inputs from detrita l
materials to solar energy, thereby altering patterns of energ y
flow within the aquatic 'community . Unfortunately, because o f
the complex nature of lotic foodwebs, such changes often
produce counterintuitive behavior in the economically and
2
aesthetically important biological subsystems . The basi c
link between inputs of solar energy and the biologica l
components of a lotic ecosystem is the process of primar y
production . Consequently, our research concentrated on th e
taxonomic structure and production of algal assemblages i n
streams, and the close examination of the relationshi p
between plant and animal, production relative to simulate d
manipulations of the riparian zone . Results of this researc h
includes new experimental data relating inputs of light energ y
.to the dynamics of algal assemblages in streams . Moreover ,
the research has generated an updated, microcomputer versio n
of a stream ecosystem model which relates energy inputs to th e
bioenergetics of the important biological components of loti c
food chains .
Project Objectives
The general objectives of this research were (1) t o
investigate the effects of light intensity, current velocity ,
and substrate heterogeneity on the productivity and specie s
composition of . plant assemblages in laboratory streams ; and
(2) to couple the results of the experimental studies with th e
dynamics of the total stream ecosystem by mathematica l
modeling .
Research related to objective (1) examined the respons e
of algal assemblages to increases in light energy i n
laboratory systems exposed to different patterns of substrat e
heterogeneity . While dramatic increases in algal primar y
production have been observed in natural streams after remova l
of streamside terrestrial vegetation, mechanisms relating to
the corresponding changes in light energy are not wel l
understood, particularly in relation to other variable s
associated with lotic ecosystems (e .g ., shear stress ,
substrate roughness and irregularity, and activities of anima l
populations) . Objective (2) was primarily concerned with th e
complex ramifications that changes in the pattern of energ y
inputs can have on the trophic structure of lotic communities .
New information obtained from recent experimental work wa s
used to update and modify the structure of a stream ecosyste m
model, and model behavior was examined relative to changes i n
energy inputs and manipulations of selected physica l
processes . In this case, emphasis was placed on the capacit y
of lotic ecosystems to support animal production in relatio n
to simulated effects of canopy removal .
5
2 . EXPERIMENTAL . STUDIES
Introduction
Relationships between aquatic plants and animals in _
stream communities have been reviewed recently by Gregor y
(1983) . In general, benthic algae provide the most nutritiou s
' food source for stream grazers . Therefore, the responses o f
algal assemblages to changes in light energy .and nutrient
inputs have important manifestations i n . -the bioenergetics o f
herbivores and coupled predator populations (Sumner
and
McIntire 1982 ; Weigert and Mitchell 1973 ; Towns 1981 ; Elwoo d
et al . 1981) . In addition, changes in the cover and production of riparian vegetation can alter the relativ e
importance of allochthonous detritus and solar radiation a s
energy sources for -lotic communities, perturbations tha t
affect fish production as well as energy relationships i n
herbivorous and detritivorous invertebrate populations (Ah o
1976) . Some recent research on plant-animal relationship s
in streams was conducted at_the Oregon State Universit y
Fairplay Laboratory under the direction of S . V . Gregory an d
C . D . McIntire . This facility
y - was equipped in 1984 wit h
financial support from the National Science Foundation (NSF) .
Funding for this work ended in April 1986, and the laborator y
was closed in September 1986 until the initiation of WRR I
Project G1444-03 in July 1987 . Results from the NSF projec t
clearly indicated .
that aquatic invertebrates in loti c
ecosystems may reduce the biomass of benthic alga l
assemblages, stimulate primary production, and influence the
species composition and food value of the plant communities .
6
Variations in light intensity and grazing pressure can .
create a patchy distribution of algae in .streamS. However ,
the heterogeneity of algal assemblages within a channel uni t
has been hypothesized to' be,mainly a result of variation i n
microturbulence (shear stress) created by water flowing ove r
an irregular substrate (Blum 1963) . At small spatial scales ,
Stevenson (1984) and Munrteanu and Maly (1981) found that alga l
colonization on sheltered areas of a substrate was faster tha n
on more exposed' areas., and was responsible for the greate r
biomass accumulation on the more protected areas . The
experimental work supported by Project G1444-03 and describe d
in this report, used the laboratory streams at Fairpla y
Laboratory to examine effects of substrate heterogeneity o n
the spatial distribution and abundance of algal patches i n
lotic ecosystems . The design and spatial scale of these
experiments were more representative of processes'in a natura l
stream bed than in our previous studies . The effect o f
substrate heterogeneity on lotic algal assemblages also wa s
examined in relation to variations in light energy input an d
grazing .
Laboratory Facilities '
The Fairplay Laboratory consists of an
8-room ,
single-story building with floor space of approximately'175 0
ft2 . The largest room (600 f t2 ) is used as a "wet" laboratory
and contains 17 artificial streams . The streams ar e
constructed of fiberglass and are similar in design 'to th e
experimental systems described by McIntire et al . (1964). .
Each stream is 3 m in length, 0 .6 m wide, and 20 cm deep, and
7
consists of two parallel channels with openings in a center .
partition at each end . The streams are supplied with wel l
water which is circulated by•rotating paddle wheels connecte d
to variable speed motors . Each stream has an adjustabl e
inflow, and the entire water volume (150 1) is replaced onc e
every two hours . . Water depth is determined by the height o f
a standpipe which serves as an outlet to a central drain .
Current velocity in each stream is controlled by adjusting th e
speed of the paddle wheel . Light energy for the streams•i s
provided by sixteen 10 .00-watt Metalarc lamps (Sylvania Corp .) ,
each mounted in a symmetrical Maxigro reflector . A system of
timers controls the photoperiod to• produce alternatin g
light-dark periods appropriate for the experimental design .
Substrate in the streams consists of a grid of cla y
tiles, which closely resemble the textural characteristics o f
stream rocks, but provide a more uniform surface for samplin g
algal assemblages . Each stream channel is paved with abuttin g
7 .5 X 7 .5 cm tiles, which are the primary sampling units . In
the experiments, tiles were stacked in various ways to produc e
patterns of substrate heterogeneity and roughness .
Experimental Design and Method s
At the beginning of each experiment, each stream wa s
inoculated with one liter of an algal suspension obtained by
scraping rocks from four local natural streams (Steinman an d
McIntire 1986) . During an experiment, algal biomass wa s
estimated in the stream channels as ash-free dry weight ai d
chlorophyll a . Methods related to the measurement of thes e
variables and the evaluation of taxonomic structure have been
described by McIntire and Phinney (1965) and Steinman an d .
McIntire (1986) . The Results section below reports th e
patterns of biomass accumulation relative to experimenta l
treatments . The microscopic examination of samples fob th e
analysis of taxonomic composition and successioma l
trajectories is still in progress and is not .yet ready fo r
presentation .
Experiment # 1
This experiment was designed to examine effects o f
substrate heterogeneity on the biomass and species compositio n
of algal assemblages at a light intensity above saturation for
photosynthesis (400 uE m -2 sec') . The substrate treatmen t
patterns were blocks 22 .5 cm wide X 22 .5 cm long X . 4 cm hig h
(the"large block" treatment) and blocks 22 .5 cm X 7 . 5 ..cm X 4
.cm (the "small block 's treatment) . Each pattern contained equa l
areas of elevated and recessed substrate in a laborator y
stream . Areas on top and sunken areas between blocks ("top "
and "bottom")--were sampled separately to examine within strea m
heterogeneity in the algal assemblages . In addition, stream s
with no relief were established . Areas corresponding i n
location to the top and bottom of the two block treatment s
were sampled separately from these flat bed streams fo r
comparisons to streams with the different block
configurations . Each of the three treatment patterns - larg e
blocks, small blocks and flat bed - were represented by thre e
replications, i .e ., three different streams for eac h
treatment . Algal samples were taken from all nine streams o n
days 4, 8, .16, 24 and 32 of the experiment .
9
Experiment # 2
This experiment was designed to examine effects o f
substrate heterogeneity and light intensity on biomass an d
species composition of algal assemblages . Shading screen was
placed over streams to create treatments of low (15 uE m-2
sec'), medium (90 uE m-2 sec') and high
(400 uE m-2 sec' )
light intensity . Three different streams were rand m1 y
assigned as replications for each treatment intensity . ' Al l
nine streams had the large block pattern to create substrat e
heterogeneity . The tops and bottoms of blocks were sample d
separately on days 8, 16, 24, 32 and 42 .
Experiment # 3
This experiment was designed to examine effects o f
substrate heterogeneity, light intensity and grazing on th e
biomass and species composition of algal assemblages .
' The
light intensity and substrate configuration for thi s
experiment was identical to Experiment #2 . In addition, each
stream was stocked with 750 snails (Juga silicula) on day 1 3
of the experiment . This stocking density was-equivalent t o
375 animals m-2 .
Algal samples were taken on days 9, 20, 3 3
and 56 . To examine patterns of animal behavior, the number o f
snails on the top and bottom of blocks was counted each day .
Results
Experiment # 1
Algal biomass accumulation in all . streams during thi s
experiment followed an exponential pattern until day 24 (Fig .
1) . Streams with no relief had faster rates of biomas s
accumulation after day 16 than streams with large or small
10
Figure 1
O
r
N
r7
o
co
N
O
N
07
N
O
(Z- w b) M04
V
a
11
substrate blocks . However, mean biomass accumulation wa s
similar for the two sizes of blocks . After day 24, alga l
biomass in streams with blocks exhibited little change ; whil e
biomass continued to increase in the flatbed streams . Algae
colonize d. the backwater areas on the bottom level of block s
more quickly than the areas of faster current on top o f
blocks . A decrease in biomass on top of the large block s
occurred between days 24 and 32 when the algal mats began t o
slough off the tiles .
Preliminary data on taxonomic composition indicated tha t
the filamentous green alga
Stigeoclonium
tenue
became
established more quickly on the bottom level of blocks than o n
top of blocks . The algal assemblages on the top of block s
were dominated by the blue-green alga Oscillatoria sp . and
diatoms . However, by the end of the experiment Stigoeclonium
tenue
also was present on top of the blocks, and th e
assemblages on the lower and upper levels were more similar i n
species composition .
Experiment # 2
At light intensities of 400 and 90 uE m- 2 ,sec", algal
biomass increased exponentially until day 32 ; at 15 uE m-2
sec exponential growth continued to the end of the experimen t
(Fig . 2) . By the end of the experiment, biomasses in streams
receiving the highest input of light energy (400 uE m-2 sec-s )
were approximately two times and six times higher than i n
streams subjected to 90 and 15 uE m-2 sec_' respectively . As
in Experiment #1, biomass accumulated faster on the bottom
Figure 2
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5)
maw
O
M)
O
N
(Z-w 5)
(z-w 5) MO4d
C
maw
'1 3
level of the blocks early in the experiment . • By day 42 ,
biomasses on the top and bottom levels were approximatel y
equal in streams receiving 15 uE
m-2
sec' .
. In stream s
developed at 400 and 90 uE m-2 sec', the top of blocks ha d
greater mean biomasse s . than the lower level by day 42, bu t
these differences were small and within the range of variatio n
among the replications . A conspicuous sloughing of the alga l
mats did not occur in this experiment . The relative
differences in biomass between the top and bottom level of th e
blocks were larger at the higher light intensities . Th e
taxonomic composition in streams receiving 400 uE m2 -sec'' wa s
similar to that found in Experiment # 1-., with filamentous gree n
algae developing initially on the bottom level areas and late r
on top of the blocks . In streams with 90 and 15 uE m '2 sec' ,
the top . and bottom levels of the blocks supported simila r
floras which consisted mainly of blue-green algae and diatoms .
However, by day 42 filamentous green algae were present on th e
bottom level in streams with 90 uE m-2 sec' .
Experiment # 3
At 15 uE m-2 sec', algal biomass decreased initially whe n
snails were introduced into the streams on day 13 . Although
there was a- increase in biomass between days 20 and 56, mean
values were extremely low at this light intensity (Fig . 3) .
At 400 and 90 uE m'2 sec', algal biomass increased between the
time snails were introduced and day 32 (Fig . 3) . In contras t
to the results from the previous experiments, . alga e
accumulated faster and had higher biomass on the top levels of
14
Figure 3
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15
the blocks than on the bottom levels at the two higher light •
intensities . This difference was related to the grazin g
behavior of the snails . The mean numbers of snails observe d
on the top and bottom levels of the blocks for all stream s
were 22 and 76 individuals, respectively . These values were
significantly different (p < . .01, n=279) and indicated tha t
the snails preferred the slower current velocity at the lowe r
level . The decrease in algal biomass between days 32 and 56 o n
the top of blocks at 90 and 400 uE nti-2 sec-1 was related t o
periodic sloughing of the algal mats . Apparently, sloughin g
was accelerated by the feeding activity of Juga .
At 15 uE m-2 sec', the flora on both top and botto m
levels of the blocks• consisted of diatom assemblages . In
streams with 90 uE m-2 sec-', tops of blocks exhibited a mat of
Stigeoclonium tenue and diatoms by the end of the experiment ,
and the more heavily grazed bottom level supported a n
assemblage of mostly diatoms and S . tenue basal cells . By day
32 streams with 400 uE m- 2 sec' had a thick mat of bluegree n
algae and S . tenue on the top of the blocks ; assemblages o n
the bottom level were composed of a thin covering of diatom s
and blue-green algae mixed with tufts of S . tenure filaments .
Summary and Conclusions
Results from the experimental work indicated tha t
roughness and irregularity of the stream channel can generat e
a patchy distribution of biomass and species of the algal food
resources in the absence of grazing . Specifically, backwate r
areas at the lower level behind the elevated substrate s: were
colonized faster by algae, and during early stages of
16
succession, had plant biomasses 1 .5 to 2' times higher tha n
areas of faster current on top of the substrate blocks .
However, after about 30 days the biomasses of these two area s
were similar . Successional patterns of community developmen t
in the two areas were similar at a given light intensity, bu t
the rate of development was faster at the lower level behin d
the blocks . Biomass and taxonomic heterogeneity in the alga l
assemblages generated by substrate irregularity were mor e
pronounced at higher light intensities (400 and 90 uE m-2
sec-1 ) than at the lowest intensity (15 uE m-2 sec') . I n
accordance with predictions of the stream model (Section 3) ,
in the absence of herbivores, diatoms were dominant at th e
lowest light intensity and green filamentous algae wer e
prominent only at the higher light intensities .
The snail Juga silicula preferentially grazed algae a t
the low level behind the substrate blocks, thereby reversin g
the pattern of community development observed in the absenc e
of herbivory . Moreover, the effect of herbivory on th e
difference in plant biomass between the two 'block levels was .
more pronounced at the . higher , light intensities . Also ,
diatoms dominated the flora in the more heavily grazed area s
behind substrate blocks as predicted by the stream mode l
(Section 3) . On the less heavily grazed tops of blocks ,
filamentous green algae developed at the higher ligh t
intensities . If the stream model predictions are correct, th e
development of high biomass and the growth of filamentou s
green algae on the top of blocks may have reduced food qualit y
and hence the impact of grazing by Jucia, in these areas .
17
In summary, the results of the experimental work sugges t
that water flow over an irregular substrate can interact wit h
variations in light energy input to generate patches of alga e
which differ in biomass and species composition .
Consequently, possible changes in substrate roughness an d
light energy inputs from riparian management practices ma y
affect the quantity, quality, and distribution of algal foo d
resources available 'to'
herbivores . Results from the
experiment with Juga indicated that substrate irregularity ma y
also determine the distribution and behavior of herbivores, a n
effect that can generate different patterns of alga l
heterogeneity . Therefore, the capacity of lotic ecosystems t o
support the production of populations at the higher trophi c
levels (e .g ., fish) is strongly coupled to factors influencin g
the spatial and temporal distribution of algal resources and
the availability. of these resources to herbivores .
19
3 . MODELING
Approach
Ecosystems can be conceptualized as hierarchical system s
of biological processes with physical and chemical processe s
as driving or control variables (McIntire 1983) . Depending o n
the resolution levels of interest, the various biologica l
processes are the systems, subsystems, and suprasystems unde r
consideration . This view of ecological systems is consisten t
with FLEX, a general ecosystem-model paradigm developed b y
Overton (1972, 1975) and based on the general systems theor y
of Klir (1969) .
The mathematical representation of biological processe s
for lotic ecosystems was organized into a structure compatibl e
with FLEX5, a model processor for a model written in the FLE X
paradigm . The processor is programmed in Pascal ; and i s
available for an IBM PC or compatible microcomputer .
A
particular model is implemented by representing the syste m
variables in a series of short procedure files according to a
standard format . This code is-compiled jointly with FLEX5 t o
generate a working model which can be used to simulate syste m
dynamics in relation to user- manipulated changes in the input
variables, parameters, and initial conditions . FLEX5 als o
allows for a hierarchical representation through a series o f
stacked modules (maximum of 5 levels) which couple togethe r
while running at different time resolutions . An earlier ,
mainframe version of the processor was use d - for the loti c
ecosystem model described by McIntire and Colby (1978) .
ti
20
,The McIntire and Colby Stream Mode l
Results of recent experimental studies at the Fair Pla y
Laboratory (this prokect and NSF grant No . BSR-83183.86 )
provided the basis for? an updated version of the McIntire an d
Colby model . The original version of this model was developed
for scientific purpose : to generate hypotheses, to synthesiz e
the results of field and laboratory research, to evaluate a
data base, and to se t '; priorities for future research . More
!
recently, the model was used specifically for the integratio n
and evaluatio n , of ourlatest research with laboratory stream
communities and for the generation of new hypotheses relate d
to the processes of primary production and grazing in•loti c
ecosystems .
Briefly, the McIntire and Colby model has a hierarchica l
structure and represents biological processes that are usuall y
active in most lotic ecosystems (Figs . 4 and 5) . From thi s
perspective, stream ecosystems are conceptualized as two
coupled subsystems, the processes of primary consumption an d
predation . Primary consumption represents all processe s
associated with direct consumption and decomposition of bot h
autotrophic organisms and detritus, including th e
autochthonous production dynamics of the autotrophic organisms.
collectively . Predation includes processes related to th e
transfer of energy among primary, secondary, and tertiar y
macroconsumers . The subsystems of Predation are the processe s
of invertebrate and vertetiate predation, while Primary .
Consumption is represented by the processes of herbivory and
21
Figure 4
.22
Figure 5
23
detritivory . Herbivory consists of all processes associate d
with the production and consumption of autotrophic organism s
within the system, whereas Detritivory includes th e
consumption and decomposition of detrital inputs . For the
research described in this report, emphasis was placed on th e
dynamics of the Herbivory subsystem, as the processes o f
primary production and grazing are tightly coupled t o
human-related manipulations of riparian zones .
A New. Model of Herbivory
The McIntire and Colby stream model served as th e
conceptual basis for our experimental studies of plant-anima l
relationships in streams . Modeling activities supported by
this project involved the isolation, modification, an d
investigation of the Herbivory subsystem of the model i n
relationship to new information from our recent experimenta l
work . The general approach included :
1.
identification of the driving (input) variable s
related to the Herbivory subsystem ;
2.
generation of input tables for the Herbivor y
subsystem from a standard run of the McIntire . and
Colby model ;
3.
isolation of the Herbivory subsystem fo r
modification and study ;
4.
a standard run of the Herbivory subsystem i n
isolation using the input tables compiled from ste p
(2) ;
5.
comparison of output from step (4) wit h
corresponding output from the total ecosystem mode l
to assure identical tracking of state variables ;
6.
utilization of new experimental results to modif y
and update the structure of the Herbivor y
subsystem ;
24 '
7.
investigation of the updated subsystem mode l
relative to selected parameters and inputs ;
8.
replacement of the old Herbivory subsystem with th e
updated version in the total ecosystem model ; and
9.
investigation of the updated total ecosystem mode l
relative to selected parameters and inputs .
The research supported by this project progressed through ste p
(7) . Activities related to steps ( .8) and (9) were initiate d
in July 1988 and will continue into 1989 . Consequently ,
results presented below are confined to a behavioral analysi s
of the updated Herbivory subsystem in isolation and associated
implications relevant to project objectives .
In the early version of the stream model, the Herbivory
subsystem contains subsystems which represent the processes o f
primary production and grazing (Fig . 6) . The state variable .
in each of these subsystems represents the biomass that i s
involved in the corresponding process at any time . New data
from the experimental work (Steinman and McIntire 1986, 1987 ;
Steinman et al . 1987) allowed the state variable inside the
Primary Production subsystem to be partitioned into three ne w
"state variables which are related to the taxonomic compositio n
and successional stage of the algal biomass . In this case ,
the state variables represent the collective biomasses o f
filamentous and coenobic chlorophytes, . diatoms, and blue-gree n
algae along with epiphytic heterotrophic microorganisms .
addition, recent feeding experiments by .Lamberti {unpublishe d
data) provided a -preliminary basis for establishing
a
quantitative relationship between the relative abundance o f
the three algal functional groups and the food consumption
25
Figure 6
26 '
rates and assimilation efficiencies associated with th e
process of grazing .
In summary, the new version of, the Herbivory subsyste m
model now tracks the successional trajectory and productio n
dynamics of the algal assemblage as well as the response o f
the process of grazing to corresponding changes in foo d
quality and quantity . This representation also expresses th e
feedback control that the process of grazing has o n
successional changes within the algal assemblage . Therefore,.
the new Herbivory subsystem model provides an opportunity t o
examine the responses of lotic plant assemblages an d
associated heterotrophic processes to simulated changes in th e
canopy structure of riparian zones .
Biological Characteristics of the New Herbivory Subsyste m
Model
The algorithm that introduced the new information int o
the Herbivory subsystem has the following characteristics :
1.
Primary production is modeled according to th e
mathematical relationships described by McIntire
and Colby (1978) . (Calculations of photosynthesis ,
respiratory expenditures, and export losses ar e
based on the total periphyton .biomass -- not the
biomass of individual algal functional groups . )
2.
The update increment from primary production i s
partitioned among the algal functional group s
according to these rules :
(a) if the irradiance is < 30 uE m-2 s- 1 or the alga l
biomass is < 2 g m- 2 , the update increment is 1.00%.
diatoms ;
(b) if irradiance is > 30 and < 100 uE m-2
ther e
is a linear relationship between light energy and
the proportions of diatoms and blue-green algae i n
the update increment, reaching a maximum of 19 %
blue-green algae at 50 uE m-2 s' when the alga l
biomass is 5 g m-2 ;
27 .
(c) if the light intensity > 100 uE m-2 s-' ,
chlorophytes, diatoms, and blue-green algae are al l
part of the update increment ; and
(d) at light saturation (> 300 uE m- 2 s'), the alga l
biomass will eventually assume a composition of 48 %
diatoms, 48% chlorophytes, and 4% blue-green alga e
when the algal biomass is > 45 g m 2 .
3 . A food quality limiting factor and the assimilatio n
efficiency .associated with the process of grazin g
is a function of the proportion of diatoms in the
algal assemblage according . to these rules :
(a) assimilation efficiency is a linear function o f
the proportion of diatoms in the assemblage ,
varying between 0 .53 (48% diatoms) and 0 .73 (100 %
diatoms) ; an d
(b) a food quality limiting factor expressed as a
proportional adjustment of the food demand (i .e . ,
the food consumption-rate with an optimum diet and
unlimited food supply) is a linear function of th e
proportion of diatoms in the assemblage, varying
between 0 .28 (48% diatoms) and 1 .00 (100% diatoms) .
' Model Behavio r
Behavior of the new Herbivory subsystem model wa s
examined first by obtaining output from a standard run, wit h
and without the process of grazing . This output then wa s
compared to runs designed to investigate the sensitivity o f
selected variables to changes in the light energy 'inpu t
schedule and to parameters that control the rate of foo d
consumption . Input tables for a standard run were the same a s
inputs for the Standard Run of , the total ecosystem mode l
(McIntire nd Colby 1978) . Such tables provide for th e
simulation of a small, low-order stream receiving annua l
allochthon l us organic inputs of about 470 g m-2 .
The
cobrespondijng light schedule generates maximum energy input s
in the spring, with very low inputs during the summer month s
when the stream is presumably shaded by a dense canopy o f
riparian vegetation . The parameters under investigation were
28
b[43], a multiplier that controls the light input schedule ,
b[29], an algal refuge parameter, and five parameter s
(.b[38 :42]) that control the food demand for the process o f
grazing .
Output from the standard run indicates that diatom s
dominate the algal assemblages when the process of grazing i s
active (Fig . 7) . In this case, the model predicts that th e
algal biomass turns over about 63 times each year, and tha t
annual gross primary production is 106 g m-2 of which only 3 %
of this total represents contributions from green an d
blue-green algae . 'With the standard set of inputs, the annua l
production associated with the process of grazing is 6 . 6 . g m-2 ,
and the corresponding turnover for the grazer biomass is 3 . 4
times per year . If . light energy inputs are increased fro m
shaded conditions to full sunlight, mean biomass and annua l
production associated with the process of grazing and annua l
gross primary production increase, while the anal mean alga l
biomass decreases slightly (Figs . 8 - 10) . Although the
algorithm allows the green algae to reach high bi©masses at
high inputs of light energy, this does not happen in the
presence of grazing, asp high rates of algal consumptionrestrict the flora to a diatom assemblage which is mor e
characteristic of an early stage of succession .
%.
In the absence of grazing, the standa-rd run predicts that
the annual mean algal biomass is 20 g m-2 , and that all thre e
algal groups are prominent in the spring and fall of
the
year
(Fig . 11) . Without grazing, annual gross primary production i s
530 g m-2 of which the diatoms, chlorophytes, and blue-green
29
Figure 7
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34
algae account for 73 .3%, 19 .3%, and 7 .4% of this total ,
respectively . Corresponding annual turnover numbers for thes e
groups are 14 .4, 16 .1, and 19 .4 times per year . Furthermore ,
annual energy losses from the algal assemblage without grazin g
partition into 41 .8% respiration, 41 .9% particulate export ,
and 16 .3% DOM leakage . In contrast, the standard run wit h
grazing indicates that 62 .6% of the annual algal production i s
lost to the process of grazing . In the latter case, the
diatoms lose 62 .8%, while chlorophytes lose only 49 .5%,
a
manifestation of the effects of food quality on consumptio n
rates .
-The parameter b[29] controls the algal refuge level for
the Herbivory subsystem . The algal refuge is defined as th e
algal biomass below which the consumption rate by the proces s
of grazing is equal to zero . The biological basis for thi s
parameter is related to (1) differences in feeding
efficiencies among consumer organisms with different pattern s
of behavior and mouthpart morphologies ; and (2) differences i n
food availability which result from substrate heterogeneity .
For the standard run, b[29] is set at 0 .7 g m2 , a value
roughly compatible with algal biomasses observed on flat til e
substrates subjected to heavy grazing pressure .
Relationships between the algal refuge parameter and th e
production dynamics of the Herbivory subsystem wer e
investigated by a series of 12 simulation runs . With the
standard set of inputs, annual production and annual mea n
biomass associated with the process of grazing are maximize d
at an algal refuge between 5 and 7 g m` 2 (Fig . 12) .
35
Figure 12
D
N
0)
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In
00
(NI
0)
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as}aw a~DnbS/suJ WO
36
These results suggest that under-some circumstances secondar y
production can be limited by overexploitation of foo d
resources . The model also predicts that at refuge value s
above 7 g m -2 green algae become a significant part of th e
community,
a factor that lowers .food quality and furthe r
contributes to a decline in secondary production (Fig . 13) .
For the inputs and parameters of the standard run, secondar y '
production in the Herbivory subsystem goes to zero and alga l
production reaches its maximum annual rate as b[29] approache s
a value of 15 g m-2 .
Model behavior was also investigated in relation to th e
interaction between inputs of light energy and the rate o f
food consumption . Light energy inputs were controlled by, a
multiplier (b[43]) that adjusted the standard table of input s
to. the desired level . For the simulation runs reported here ,
the light schedules included the standard table of inputs, 3 X
the standard table, 5X the standard table,,and a constan t
input above th e , saturation intensity for photosynthesis .
Daily mean values in all tables were < 2000 uE m2 sec' . The
food consumption rate associated with the process of grazing
was controlled by parameters that adjusted the functiona l
relationship between food demand and temperature . Here, food
demand is defined as the consumption rate when food is i n
unlimited supply and the quality of the resource is optimal .
After the demand is calculated, the model determines th e
realized food consumption rate by multiplying the demand b y
food density and food quality limiting factors .
37
Figure 13
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)
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a)
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0
0
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0
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38
The simulation runs generated output for the standard deman d
{i .e ., the demand set up for the standard run), and for
a
series that included 90%, 80%, 70%, 60%, and 50% of th e
standard demand .
At the standard demand and 90% of the standard demand ,
increases in the inputs of light energy are accompanied b y
corresponding increases in gross primary production and th e
biomass and production associated with the process of grazing ;
the annual mean algal biomass remains low and virtuall y
constant under these conditions, between 1 .0 and 1 .8 g m-2
(Figs . 14 - 17) . When demand is decreased to 80% of th e
standard or below, there is a pronounced increase in alga l
primary production and annual mean biomass, and increases i n
light energy inputs bring about corresponding increases i n
both of these variables . At 80% demand or below, the syste m
does not support the process of grazing at the highest leve l
of light energy inputs . On the basis of strictly bioenergeti c
considerations, this behavior is counter- intuitive, as th e
lowest level of light energy supports some grazing at 60% o f
the standard demand . The explanation for this behavior i s
related to a shift in the taxonomic structure of the alga l
assemblage (Fig . 18) . At the highest light energy inputs and
a consumption rate of 80% of the standard demand or below, th e
model predicts that green and blue-green algae becom e
prominent members of the assemblage, a change that has a
negative effect on food quality . When consumption is reduce d
to 50% of the standard demand, the process of grazing is no t
supported at any of the light energy levels .
39
Figure 1 4
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Figure 15
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44
Summary and Conclusions
Output from the M & C Stream Model and the new Herbivor y
Subsystem Model predicts that the removal of riparian zon e
vegetation may have pronounced effects on patterns of energ y
flow in lotic ecosystems . In particular, it is likely tha t
such impacts will cause a shift from systems characterized b y
detrital-based food webs to systems that are more dependent o n
autochthonous primary production . With this change, th e
process of herbivory becomes a more prominent coupling betwee n
the primary food supply and the production of top predators .
In other words, in the absence of allochthonous inputs o f
terrestrial detritus, the capacity of a stream ecosystem t o
support fish production is determined in part by inputs o f
light energy and nutrients and the complex interaction s
between plant communities and herbivores, a principal foo d
resource of predatory fishes .
The Herbivory Subsystem Model contributed som e
interesting hypotheses which relate to the understanding o f
plant-animal interactions in streams relative to changes i n
light energy inputs . Specifically, the model predicts :
1.
Diatoms dominate lotic plant assemblages when the
process of grazing is active and the stream i s
shaded by riparian vegetation ; annual turnove r
numbers for algae and herbivores under thes e
conditions .are about 60 and 3 .5, respectively .
2.
If light energy inputs are increased to simulat e
clearcut logging, annual gross primary production
and mean grazer biomass and production increase ,
while the annual plant biomass decreases .slightly .
3.
In the absence of grazing, blue-green and gree n
algae can become prominent constituents of alga l
assemblages, particularly when the mean input of
light energy is greater than 300 uE m2 sec' .
454.
t
5.
Animal production in streams may be limited under
some circumstances by the overexploitation of foo d
resources . In a series of simulation runs, anima l
-production was maximized when the algal assemblag e
was protected from grazing at biomasses below 7 g
m`2 ; protection above this level generated a rapi d
decline in animal production because of a decreas e
in the quantity and quality of the food resource .
These relationships may be particularly noticeabl e
in natural streams when food consumption rate s
decrease, through the possible mechanisms o f
substrate heterogeneity (see Chapter 2) or a chang e
in the taxonomic structure of the animal community .
At a high rate of grazing, an increase in the inpu t
of light energy generates a corresponding increas e
in the production and biomass of herbivores ; as
food consumption rates decrease, an increase i n
light energy may actually lower herbivor e
production when food quality is affected negativel y
by the increase in filamentous green algae .
l
V.
ff.
47
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48
Overton, W . S . 1975 . The ecosystem modeling approach of the
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in
laboratory .
streams .
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