Regional and global elevational patterns of microbial species richness and ev...
FW Biology Publication Final
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Effects of agricultural land use on benthic macroinvertebrate communities
in Southern Illinois headwater streams
Nathan J. Boyer-Rechlin*†
Gregory L. Bruland, Ph.D.*
Michael A. Rechlin, Ph.D.*
*Biology and Natural Resources Department
Principia College
1 Maybeck Pl.
Elsah, IL 62028
†2519 Mountaineer Drive
Franklin, WV 26807
nathan.boyerrechlin@gmail.com
Land Use Effects on Illinois Benthic Macroinvertebrates
Keywords: biotic index, headwater streams, Illinois, land use, macroinvertebrates
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SUMMARY:
1) Headwater streams are important in buffering the downstream effects of
agricultural sedimentation and nutrient loading. In the Midwestern United
States, agricultural development has degraded the quality of freshwater
systems from headwaters to the Mississippi Delta. In order to assess the
effects of varying degrees of agricultural impacts on Illinois headwaters,
benthic macroinvertebrates were sampled from 15 perennial headwater
streams in Jersey County, Illinois.
2) Land-use/land-cover (LULC) distributions were calculated for each
watershed within three spatial scales: watershed, 150m radial buffer, and
50m radial buffer. Sampled watersheds ranged from 5-80% agricultural
cover (cultivated crops & pasture/hay lands).
3) Family diversity and richness were low across the region. A total of 20
taxa were identified, among which only four were Ephemeropteran,
Plecopteran, or Trichopteran (EPT) taxa. Hydropsychidae (Trichoptera)
was the most dominant family and had a strong positive correlation with
% agriculture. Additionally, a PCA was used to analyze community
structure trends among the macroinvertebrate functional feeding groups.
4) While community bioassessment metrics, including biotic indices and
various diversity metrics are commonly used methods of stream quality
monitoring and analysis, these metrics did not correlate well with LULC in
this study and they may not be effective metrics for agricultural streams
with low benthic macroinvertebrate diversity in this region.
5) Overall, none of the streams sampled were indicative of highly degraded
communities and most streams had similar macroinvertebrate
composition in spite of variability in LULC. Even within these streams,
however, % agriculture had a significant impact on community
composition and results suggest that % Hydropsychidae may be an
effective means of assessing small-scale impacts of agriculture in the
headwater systems of Southern Illinois.
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Introduction:
Agricultural development negatively impacts freshwater systems in a variety
of ways and often causes the following: (1) increases in sedimentation; (2) greater
flow variation, which can disrupt biological communities and processes; (3) loss of
high quality habitat; (4) contamination from pesticides and (5) increases in nitrogen
(N) and phosphorous (P) concentrations (Allan, 2004, Herringshaw et al., 2011).
Nutrient transport through headwater systems is complex due to the high rates of
uptake and transformation in these streams (MacDonald & Coe, 2006, Ohio EPA,
2003). However, it has been shown that in highly impacted agricultural watersheds,
headwater streams serve as the primary transport mechanism for excess nutrients
and sediments (Borah & Bera, 2003, Ohio EPA, 2003).
These nutrient and sediment stressors decrease biotic diversity and alter
trophic interactions, which in turn can have cascading impacts on functional group
distributions, organic matter transport, and overall stream function throughout the
entire continuum of lotic systems (Cummins, 1973; Vannote et al., 1980; Allan, 2004,
Burcher, Valett & Benfield., 2007). Studies suggest that agricultural development has
a net negative impact on invertebrate diversity and overall community structure
when agriculture is prominent, i.e. greater than 30-40% of watershed land use/land
cover (LULC) (Genito, Gburek & Sharpley, 2002; Stone et al., 2005; Kratzer et al.,
2006; Niyogi et al., 2007). The extent of agricultural impacts and stress is variable,
however, and can be largely influenced by the spatial distributions and types of
agriculture in the watershed, with cultivated croplands having much higher impacts
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than pasture and hay fields (Allan, 2004; Stone et al., 2005; Kratzer et al., 2006; King
et al., 2005)
Across Illinois, agricultural expansion has severely impacted the landscape.
Almost three quarters of pre-settlement forests and 90% of the state’s prairies have
been converted to agriculture over the past 200 years (Iverson, 1988). It has been
estimated that 15% of the N and 10% of the P inputs leading to the hypoxic zones in
the Gulf of Mexico come from Illinois croplands (Gentry et al., 2000; Alexander et al.,
2008). Jersey County, in west-central Illinois, is unique in that it is home to highly
forested, highly cultivated, and mixed LULC drainages all within close proximity to
each other and without substantial urban impact. Because of its many protected
natural areas and rugged topography, characterized by steep ravines and riverside
bluffs, the southwestern half of Jersey County, has nearly twice as much contiguous
forest (proportionately) as rest of the state (Joselyn & Brown, 1996; Krohe, 1997). In
contrast, the northeastern half of the county resembles the typical Illinois landscape,
dominated by row-crop agriculture. (Fry et al., 2011). The county’s LULC continuum
from forested to agriculturally dominated watersheds makes this region
representative of the varying extents of agricultural impacts found across Illinois.
One common method used to monitor stream quality response to stressors
such as agriculture is bioassessment, including the use of various macroinvertebrate
community metrics (Hilsenhoff, 1987; Lenat, 1993; Barbour et al., 1999; Bode et al.,
2002; Rawer-Jost et al., 2000; Allan, 2004,). Bioassessment provides insights into the
health of riverine systems while avoiding the complexity, costliness, and dramatic
daily and seasonal variance that accompanies traditional water quality testing
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(Barbour et al., 1999; Bode et al., 2002,). Hilsenhoff’s macroinvertebrate biotic index,
which was developed to show stream quality response to organic and nutrient
pollution, and its various regional adaptations are some of the most commonly used
bioassessment metrics. These biotic indices, as well as other diversity metrics, have
seen widespread application over the past 30-40 years and have been used
extensively by the Illinois Natural History Survey to monitor stream condition across
Illinois (Hilsenhoff, 1982; Hilsenhoff, 1987; Lenat, 1993; Chirhart, 2003; Pond et al.,
2003; Miller, 2004). Additionally, simplified family and order level biotic indices have
become common in volunteer stream monitoring programs across the United States
(Miller, 2004; Marchal, 2005). Because macroinvertebrate communities reflect long-
term disturbances and stress trends, community metrics and biotic indices reflect
the effects of anthropogenic stressors on the function of stream systems as a whole,
including influences on food web dynamics, biodiversity, and organic matter
transport (Cummins, 1973; Vannote et al., 1980).
The purpose of this study was to investigate the relationships among
agricultural land use and benthic macroinvertebrate communities in Jersey County
headwater streams. Secondarily, we aimed to evaluate the use of various
macroinvertebrate community indices and metrics as means of assessing regional
stream quality. It was hypothesized that benthic community structure would be a
function of % agricultural land use. This was evaluated at three spatial scales:
watershed, 150m radial stream buffer, and 50m radial stream buffer. Additionally it
was hypothesized that diversity, evenness, and richness would be negatively
correlated with % agricultural land use and that Hilsenhoff’s (1988) family biotic
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index (FBI) scores would be positively correlated with agriculture for all spatial
scales. Because of the mosaic of forested and agriculturally developed watersheds in
the county, understanding the relationships between agricultural distributions and
benthic community structure, and the applicability of commonly used bioassessment
metrics in regional headwater streams would be valuable for stream quality
monitoring programs and trend assessment throughout Illinois and the greater
Mississippi River Basin.
Methods
Study area
Jersey County covers 979 km2 in west central Illinois, bordering the
Mississippi and Illinois rivers, and is split between three Illinois natural divisions
(Schwegman et al., 1973). Sample sites were all located in southern and western
Jersey County, within or directly adjacent to the Middle Mississippi Borderland
Division (MMBD). This region is characterized by highly dissected karst topography
with deciduous forests dominating the hills and ravines and cultivated croplands in
the floodplains and ridge top plateaus (Schwegman et al., 1973; Panno, Weibel & Li,
1997). The majority of the county’s springs and high gradient streams fall within this
region (Krohe, 1997; Wetzel & Webb, 2007; Fry et al., 2011).
Site Selection
Benthic macroinvertebrates were sampled from 15 perennial headwater
streams (watershed area <10 km2). For the purposes of this study, streams were
classified as perennial if the sampled reach maintained surface flow throughout the
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semi-drought conditions in the fall of 2013. All streams had notable (>5m) riparian
vegetation on at least one stream bank. Substrates were primarily cobble and gravel
intermixed with silt and, at a few sites, bedrock.
Streams were selected by isolating possible watersheds in ArcMap (Version
10.2, ESRI, Redlands, CA) using a digital elevation model, an Illinois streams layer,
and the NLCD 2006 layer (Fry et al., 2011). Personal knowledge of regional
watersheds along with suggestions from local experts and site visits to potential
streams were also crucial in picking sample sites. Drainages were selected on a
gradient from low to high agricultural land use (Table 1) (Fry et al., 2011). ArcMap
was used to delineate watersheds using the Hydrology tool in Spatial Analyst, and to
calculate LULC distributions within the watersheds and the 50m and 150m radial
buffers for each stream.
Reach metrics
Riffle velocity was measured at a representative riffle in each reach. Discharge
was calculated at channelized areas with uniform flow, often transition zones
between riffle and pool habitats, by measuring the wetted width, average depth, and
velocity. Because of the extremely low flow, velocity was calculated by averaging the
time it took to float a stick one to two meters over three repetitions. Width was
measured at representative riffle, pool, and channel cross sections. Additionally, a
visual habitat assessment was performed at each reach following the rapid
bioassessment protocols outlined by the U.S. EPA (Barbour et al., 1999).
Temperature, pH, conductivity, total solids, and dissolved oxygen were measured
using the YSI 556 water quality probe (YSI Incorporated, Yellow Springs, OH).
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Measurements were taken in riffle areas and recorded after allowing the probe to
assimilate to stream conditions for 10 minutes.
Macroinvertebrate sampling
Sampling was completed during base-flow conditions between 15 September
and 16 October 2014. A Surber sampler was used to collect macroinvertebrates from
riffle habitats (stream flow rates >0.02 m s-1) within a 60-meter reach at each stream.
Between two and five samples were collected at each site by disturbing, kicking,
rubbing, and upturning the substrate for one minute. Large sticks and leaves were
removed and all macroinvertebrates and remaining detritus were transferred into
mason jars and mixed with 95% ethanol. The net was rinsed and picked for
remaining macroinvertebrates (>3mm) for roughly five minutes.
Upon return to the lab, jars were drained and filled with 70% ethanol. Each
sample was subsampled by spreading the invertebrates and remaining detritus
across two gridded pans and picking all invertebrates from randomly selected grids
until >150 invertebrates had been picked. Individuals were identified to family and
stored in 70% ethanol or isopropyl alcohol.
Macroinvertebrate metrics
Shannon Diversity (H’), family richness, evenness, and functional feeding
group (FFG) distributions were calculated and used to assess community structure.
For FFG analysis, families were grouped into five groups: shredders, scrapers,
collector-filterers, collector-gatherers, and predators (Cummins, 1973; Voshell, 2002;
Cummins, Merritt & Andrade, 2005). Hilsenhoff’s FBI was also calculated to estimate
the effects of organic and nutrient pollution (Hilsenhoff, 1982; Hilsenhoff, 1988).
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Statistical analysis
Pearson’s and Spearman correlations were run among the explanatory
variables, LULC distributions, macroinvertebrate abundances, and community
metrics using the SPSS statistical software package (Version 19, IBM, New York, NY).
Correlations were considered significant at p<0.1. A Principal Components Analysis
(PCA) was also run using PCOrd (Version 6, MjM Software, Gleneden Beach, OR) to
investigate multivariate patterns in the family-level and the FFG data as well as to
identify the strongest explanatory variables (McCune & Grace, 2002).
Results
Land use
Agricultural land use (including both cropland and pasture hay) of the
sampled watersheds ranged from 4.7% to 80.2% with cultivated crops composing
roughly two thirds of total agriculture (Table 1). Forest cover ranged from 8.6% to
92.3% and urban development was negligible. All streams had forested riparian
buffers with no stream having less 26% forest within its 50m buffer (Table 1). Forest
cover became more dominant at smaller spatial scales with the 50m buffers having
the highest proportion of forest cover (Table 1). Forest cover was also inversely
correlated with agriculture for the watershed (r2=0.99), 150m buffer (r2=0.99), and
50m buffer (r2=0.72) scales. The LULC distribution for Jersey County, including
sampled watersheds, is shown in Figure 1.
Reach metrics
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Average discharge was 0.019 m3s-1. The wetted width at riffle sections ranged
from 0.5m to 2.7m and channel width ranged from 3.5m to 13.3m. Habitat
Assessment (HA) scores ranged from 110 to 167, out of 200. Out of all the sub-
categories, substrate embeddedness, channel flow status, and vegetative protection
had the most variation. HA scores had a significant Pearson’s correlation with forest
cover at all spatial scales, with the strongest correlation occurring for the 50m
buffers (r=0.72, p=0.002).
Water quality characterization
Dissolved oxygen (DO) ranged from 6.14 mg L-1 to 10.17 mg/L and had a
negative Pearson’s correlation with agricultural land use at the watershed (r=-0.45,
p=0.09), 150m buffer (r=-0.52, p=0.05), and 50m buffer (r=-0.45, p-0.01) scales. All
sites were mildly acidic, averaging a pH of 5.90. Site 5 had an abnormally low pH of
4.66. Conductivity ranged from 0.514 to 0.728 MS s-1. Stream temperature varied
substantially among sites, ranging from 11.9 to 19.27oC.
Community structure
The distribution of FFGs at all sites is shown in Figure 2. The community
structure varied from 100% shredders at site 2 to collector-filterer dominance at site
13 (Figure 2). According to the PCA, axis one accounted for 55% of the variance and
axis two accounted for 19.2% of the variance (Figure 3). Axis one covered a gradient
from shredder dominance on the right, to scraper and collector-filterer dominance
on the left. Axis 2 showed a gradient from collector-gatherers on the bottom to
predators on the top. The sites fell into three main groupings in the ordination space
with shredder dominated sites on the far right, mixed FFGs in the center and
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collector-filterer dominance on the left. Additionally, % agriculture and % forest
cover, at the watershed scale, also had strong loadings on axis 1 (Figure 3).
Overall, 18 different families, one sub-phylum, and one sub-class of benthic
invertebrates were identified. Hydropsychidae (Trichoptera), Gammaridae
(Amphipoda), Asellidae (Isopoda), Baetidae (Ephemeroptera) and Chironomidae
(Diptera) were the most common, accounting for 94% of all invertebrates.
Hydropsychidae and Gammaridae were co-dominant accounting for 39% and 28% of
invertebrates, respectively. Sub-phylum Turbellaria, sub-class Oligochaeta, Elmidae
(Coleoptera), Physidae (Gastropoda), Heptageniidae (Ephemeroptera), Veliidae
(Hemiptera), and Calopterygidae (Odonata) were all found in at least three streams.
Six families were only found at one site and in most cases were very rare in their
communities. Perlodidae (Plecoptera), which was only found at site 14, was the only
exception and made up 3.5% of invertebrates collected in that stream.
FFG Relationships
Pearson’s correlation coefficients and p-values are shown for the five FFGs
and LULC variables at all spatial scales in Table 2. Both shredders and collector-
filterers had significant correlations with forest and agriculture at a 95% confidence
level for all spatial scales except for forest in the 50m buffers (Table 2). Shredders
had a negative correlation with agriculture, which was strongest for the 50m buffers.
Collector-filterers had a positive correlation with agriculture, which was also
strongest within the 50m buffers (Table 2).
Scrapers were significantly correlated with forest and agriculture within the
watershed scale, at a 90% confidence level. Scrapers also had a significant
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correlation at a 95% confidence level with agriculture in the 50m and 150m buffers
and with forest in the 150m buffers (Table 2). Collector-gathers were significantly
correlated with urban land-cover in the 50m buffers at a 90% confidence level
(r=0.48). Predators were not correlated with LULC or any explanatory variables.
Family-level relationships
Pearson’s correlation coefficients and p-values are shown for the five
dominant families and LULC variables at all spatial scales in Table 3. Gammaridae
were positively correlated with forest cover and negatively correlated with
agricultural land use at most spatial scales. For both agricultural land use and forest
cover, the strongest correlations occurred when LULC was calculated at the
watershed scale. Hydropsychidae were negatively correlated with forest cover and
positively correlated with agricultural land use at all spatial scales, excepting forest
cover within the 50 m buffers (Table 3). Chironomidae had significant Spearman
correlations with watershed agriculture (r=0.57, p=0.03) and forest cover (r=-0.60,
0.02). The strongest family-LULC correlation was between Hydropsychidae and
agriculture in the 50m buffers (Figure 4).
Community metrics
Hilsenhoff’s FBI scores ranged from 4.0 to 6.0 (Table 4). FBI was not
correlated with any the LULC data or explanatory variables. Shannon-Weiner family
diversity (H’) ranged from 0.07 to 1.57 (Table 4) and had a positive Pearson’s
correlation with channel width (r=0.54, p=0.04). Additionally, H’ had positive
Spearman correlation with watershed size (r=0.46, p=0.08) and riffle width (r=0.45,
p=0.01). H’ was not correlated with LULC or any other of the explanatory variables.
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Family richness ranged from 2 to 12 with an average richness of 7. Family evenness
ranged from 0.10 to 0.72 (Table 4). Evenness also had a significant Pearson’s
correlation with channel width (r=0.47, p=0.08).
Discussion
Land use/land cover
Even though agricultural cover was as high as 80% in some sampled
watersheds, none of these watersheds had over 60% cultivated croplands, which are
responsible for the majority of agricultural based nutrient loading and sedimentation
(Allan, 2004; King et al., 2005; Kratzer et al., 2006). Although there are some almost
completely cultivated parts of Jersey County, we were unable to sample watersheds
in these areas because we found no perennial headwater streams in these regions.
This is explained in part by the spatial extent of the county’s karst geology, where
most of its springs are located. The most cultivated watersheds were found in the
flatter, glacial till plain of northeastern Jersey County. While better adapted for
agricultural development, this area lacks the karst geology characteristic of the hillier
and more forested MMBD in the southern and western portions of the county
(Schwegman et al., 1973; Panno et al., 1997).
Because of its karst geology, subsequent groundwater systems and numerous
springs, the majority of perennial headwater streams, including all streams sampled
in this study, were located in the MMBD (Schwegman et al., 1973; Webb et al., 1995;
Wetzel & Webb, 2007). However, because of the dissected topography in this region,
agricultural development is limited to the ridge tops and floodplains (Figure 1),
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limiting the extent and impact of agriculture across the region. Due to this tendency
for agricultural development to occur in the uplands, away from steep ravines,
agriculture is less prominent, proportionately, close to the streambeds than it is
across whole watersheds. Among our sites, the ratio of agriculture to forest cover
was the lowest in the 50m buffers (0.32) and the highest across the watersheds
(0.84). The comparatively high proportion of forest cover within the 50m and 150m
buffers may have helped reduce the impacts of upland agriculture (Weigel, 2003;
Allan, 2004; Vondracek et al., 2005; Song et al., 2009).
The limited range of agricultural impacts among sampled watersheds,
evidenced by the lack of completely cultivated watersheds and the tendency for
agriculture to occur away from immediate stream buffers, is a potential limitation of
this study. Resampling both perennial and non-perennial streams with a multi-
habitat sampling approach would facilitate a more comprehensive survey of the
county’s streams, allowing for sampling in the most heavily cultivated regions.
Community structure
The dominance of shredders and collectors observed in this study matches
the predictions of the river continuum concept for heterotrophic headwater streams
(Vannote et al., 1980). However, in contrast with Sangunett (2005) and Stone et al.
(2005) who found collector-gatherers to be the dominant FFG in agriculturally
impaired watersheds and small streams, we found mostly collector-filterers. One or
two families dominated both the shredder and collector-filterer FFGs with
Gammaridae and Asellidae being the only shredders and Hydropsychidae accounting
for all but 10 individual collector-filterers. This indicated that Gammaridae and
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Hydropsychidae are key taxa in fulfilling the ecological rolls of streams in the region.
Additionally the dominance of single, tolerant taxa in these groups suggested that
across all streams, persistent stress and degradation might have impacted the
viability of less tolerant taxa.
Many of the taxa found in this study, notably Hydropsychidae, Baetidae,
Chironomidae, Elmidae, Simuliidae, and sub-class Oligochaeta have been found to be
common across the state (Heatherly et al., 2007). However the high proportion of
Amphipoda and Isopoda observed has not been consistently documented (Sangunett,
2005; Stone et al., 2005; Heatherly et al., 2007). High proportions of these taxa are
often associated with spring communities and their dominance in our streams is
likely related to the numerous hard water springs found in the MMBD (Webb et al.,
1995; Wetzel and Webb, 2007). Additionally, a study of forested streams in northern
Illinois also reported a high proportion of amphipods and isopods, suggesting that
these taxa may play a pivotal role in linking the terrestrial and aquatic systems in
wooded Illinois streams (Vinikour & Anderson, 1984).
The dominance of Hydropsychidae found in this study indicates nutrient
stressed conditions (Voshell, 2002). Cheumatopsyche (Trichoptera: Hydropsychidae),
a common hydropsychid found across Illinois, has been shown to become hyper-
dominant in response to organic pollution and nutrient enrichment. Other genera
display similar responses (Pond et al., 2003; Heatherly et al., 2007). The general lack
of Ephemertopteran, Plecopteran, and Trichopteran (EPT) taxa, other than
Hydropsychidae and Baetidae which are generally more pollution tolerant, also
suggested degraded stream conditions and stressed benthic communities (Lenat &
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Penrose, 1996; Voshell, 2002). EPT taxa tend to be more intolerant of pollution and
habitat disturbance and EPT richness has been shown to decrease with increased
stress (Barbour et al., 1999; Griffith et al., 2001; Chirhart, 2003; DeWalt, 2003;
Sangunett, 2005). EPT richness varies across Illinois regionally and with stream size,
but a comprehensive survey showed most Illinois streams to have higher EPT
richness than those sampled in this study (DeWalt, 2003; Miller, 2004).
Still, in comparison with other statewide benthic macroinvertebrate surveys,
the streams in this study only appear to be moderately degraded. A similar study
conducted on Chicago area urban headwater streams found lower family richness
than we did (Gorman, 1987). Both that study and a study on agricultural streams in
southwestern Illinois reported significantly different community structures,
dominated by oligochaetes and highly tolerant non-insectan families along with a
few tolerant insectan families, namely Chironomidae and Ceratopogonidae (Gorman,
1987; Stone et al., 2005). These midge and non-insect dominated communities are
common in sediment- and nutrient-stressed agricultural streams (Voshell, 2002;
Stone et al., 2005; Heatherly et al., 2007; Herringshaw et al., 2011). Though most
streams in our study were less diverse than those in some other similar regions of
the state, the communities in the streams in this study are more characteristic of low-
medium impact systems than they are of highly degraded streams (Vinikour &
Anderson, 1984; Sangunett, 2005; Stone et al., 2005)
Despite the similarities in community structure among most sites, the
presence of Heptageniidae at four sites and Perlodidae (Plecoptera) at site 14
suggests that area streams are capable of supporting greater EPT diversity and
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higher quality communities. Located within an almost completely forested
watershed, site 14 lacked the obvious environmental stressors present across much
of the region and was expected to exhibit higher quality benthic communities.
Plectopterans are some of the most intolerant EPT taxa and the absence of this order
from all other sites suggests that even minimal agricultural disturbance significantly
impacted the viability of Plecopteran populations in regional headwater systems
(Chirhart, 2003).
However, the usefulness of site 14 as a reference site is suspect due to high
number Asellidae, of which some species are often found in urban and other
organically polluted streams (Williams, 1976; Heatherly et al., 2007). Asellids were
only common at one other site and the dynamics influencing the dual presence of
large numbers of isopods and a notable plecopteran population are not well
understood. It is possible that upstream septic systems or hard-water spring
dynamics may help explain the community dynamics in this stream. In spite of this
issue, the presence of Perlodidae at this site still shows that Plecoptera are not
completely extirpated from the region and may serve as a good indicator of higher
quality streams across the county.
Another factor influencing the comparison of streams within Jersey County is
the possibility of similar reference quality streams having different unique
community structures. Sites 2, 10, and 15, were home to an overwhelmingly non-
insect community dominated by Gammaridae and, at site 10, Asellidae. Together,
these two families accounted for over 95% of invertebrates and only one individual
insect, a Baetidae, was found across all three sites. The PCA, which analyzed
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community structure based on FFG distributions, grouped these three sites, along
with site 3, together in a stand-alone cluster (Figure 3). These were classified as the
shredder-dominated communities. This classification also supports the observation
of a few unique, non-insectan communities because the only shredders found in this
study were Asellidae and Gammaridae.
A study on benthic spring communities in Illinois found that Amphipod
dominated non-insectan communities were related to the presence hard-water
springs (Webb et al., 1995; Wetzel & Webb, 2007). We hypothesize that southwest
Jersey County’s karst geology, and the corresponding hard-water springs might help
explain the differences between some of the benthic communities in our streams.
The possible differences in community structure among nearby or even adjacent
regional streams may prove problematic when using community metrics and biotic
indices to compare water quality and these factors must be considered when
applying these methods.
FFG and family relationships
A decline in shredders is an expected community response to disturbance
(Rawer-Jost et al., 2000). The negative correlation between shredders and
agricultural land use observed in this study supported the hypothesis that upstream
agriculture influences community structure and suggests that the loss of shredders
in stream communities may indicate habitat degradation. However, the family level
correlations with LULC were stronger than those with the FFGs. In streams with
similar functional quality, the distribution of individual families may serve as a better
indicator of fine-scale LULC impacts.
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The strong correlation between Gammaridae and forest cover in the 150m
buffers and watersheds explained most of the variance in shredder populations and
their correlation with LULC. Due to the possible complications with Gammaridae-
dominated hard-water spring communities discussed earlier, which in this study
often occurred in highly forested watersheds, this correlation with forest cover could
be skewed. Most importantly, our results show that in this region % Hydropsychidae
may be one of the most useful indicators of agricultural disturbance, especially
among low and medium impact streams with similar upstream environmental
conditions.
Community metrics
Along with community composition analysis, Hilsenhoff’s index and
community diversity metrics, including Shannon-Weiner diversity, species richness,
and EPT richness, have all been successfully used to differentiate between high
quality and stressed streams in agricultural regions (Griffith et al., 2001; Genito et al.,
2002; Stone et al., 2005; Song et al., 2009). Although Stone et al. (2005) found a
modified Hilsenhoff’s index useful in differentiating among agricultural streams in
Southern Illinois, our results support the findings of DeWalt (2003) and Sangunett
(2005) who found that Hilsenhoff’s index (and derivative indices) scores varied little
across the state and may not be effective tools for bioassessment in Illinois. Although
some of these metrics may not be consistently effective individually, the lack of
correlation among the LULC variables and any of the bioassessment metrics suggests
that stream quality varied little among sampled sites and was not significantly
impacted by increases in % agriculture, up to 80%.
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Although these metrics are often successful in depicting water quality
variance among highly dissimilar streams or in regions with a large range of
anthropogenic impacts, they are less successful when applied to areas with a smaller
range of environmental conditions (Kratzer et al., 2006). The higher proportion of
forest-cover within the riparian zones and limited range of agricultural LULC
distributions (<60% cultivated crops) likely helped minimize the effects of stream
quality differences among sites. Additionally, the low diversity found among even
the most pristine watersheds might have limited the ability of diversity metrics to
model the impacts of agricultural pollution in the region. The changes in community
composition in relation to LULC demographics, evident among the family and FFG
distributions, suggest that agricultural impacts had at least a moderate effect on
stream condition and macroinvertebrate communities. However, because of the
inherent low-diversity found across the county’s headwater streams, diversity and
richness metrics may not model these changes as well as community composition
data. Similarly, the lack of diversity likely decreases the resolution of Hilsenhoff’s FBI
and its ability to model water quality fluctuations among moderately degraded
streams.
It is important to note that our use of family-level identification may have had
an impact on the resolution and effectiveness of these metrics. Family level
taxonomic identification can significantly reduce the resolution of diversity and
richness metrics (Song et al., 2009). Although a North Carolina study comparing
order level and genus level macroinvertebrate biotic indices showed no significant
differences between the two, few comparative studies have been conducted in other
21. 21
regions and Hilsenhoff (1988) documented that his FBI tends to produce values
closer to four due to prevalence of certain families (i.e. Hydropsychidae, Gammaridae
and Baetidae), giving lower scores to heavily polluted streams and higher scores to
pristine streams (Marchal, 2005). Nonetheless, it is important to clearly understand
the effectiveness of metrics at this level of taxonomic resolution because of the
prevalence of family and order level macroinvertebrate indices in volunteer stream
monitoring, and initial assessments.
In conclusion, most streams in the MMBD portions of Jersey County were
shown to have similar benthic invertebrate communities despite differing LULC
distributions. In spite of these similarities, changes in agricultural land use were
shown to have an impact on these streams with an increase in % Hydropsychidae
being the best indicator of community response to agriculture. H’, richness,
evenness, and Hilsenhoff’s FBI showed no correlation with the LULC data suggesting
that these bioassessment metrics may not be effective in the region’s low-diversity,
medium impact agricultural streams.
22. 22
Acknowledgements
We thank M. Rhaesa (aquatic ecology consultant) and A. Ringhausen (Great
Rivers Land Trust) for their support of this research. Additionally, we are grateful to
Principia College students L. Pyne, C. Chichester, and K. Krishniswami for helping
with the fieldwork. Finally, we appreciate the support of the entire Principia College
Biology and Natural Resources Department.
23. 23
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