The Macroinvertebrate Communities of
the Great Swamp Watershed:
2000 and Subsequent
A Report to the
Ten Towns Great Swamp Management Committee
|Leland W. Pollock, Ph.D.
Department of Biology
The types and numbers of macroinvertebrates (mostly insect larvae/nymphs) that form the biological community at a particular stream location are influenced by the composite environmental conditions flowing by the site during the recent past. As the drainage focus for the broader upstream landscape, in-stream conditions are a direct reflection of the degree of environmental stress in the surrounding area. A strategically placed collection of macroinvertebrate samples can provide a method for evaluating water quality of the entire watershed and for pin-pointing specific problem areas within it.
Macroinvertebrates are particularly attractive water quality study subjects, with advantages over other community members. While the flying adult stages of many insects have sufficient mobility to permit them to reach anywhere in the entire watershed, they are only able to survive as aquatic larvae in those stream locations sustaining tolerable environmental conditions. Macroinvertebrate communities respond predictably to human induced stress. Because species differ in their tolerance to pollutants, particular species make useful "indicators" of conditions. They are large enough to be seen with the unaided eye, making them relatively easy and inexpensive to collect, yet they are far less mobile than fish, making them dependable markers for particular site observed. Because they are relatively abundant, there is little danger of depleting sparse populations through sampling. With some practice and modest equipment, they are relatively easy to identify.
Physical/chemical conditions within a stream can be monitored directly, although this tells you only about conditions "at the moment". As long-term inhabitants of streams, the presence of macroinvertebrates reflects stream conditions over the preceding days, weeks, or months. The presence of the biological community or of particular "indicator" species found at a given location depends on the availability of a range of required conditions during the past several weeks or months. Therefore, studies of macroinvertebrate communities provide valuable historical perspective missing in direct physical/chemical studies.
From 1992 through 1999, the National Resources and Conservation Service (NRCS) of the Morris County Soil Conservation Service conducted an annual, early-summer survey of macroinvertebrate communities within the Great Swamp Watershed. Following the expiration of NJ DOT funding support for this project, the NRCS decided to terminate their studies with their 1999 survey. Since June, 2000, I have been engaged by the Ten Towns Great Swamp Management Committee to continue this valuable data series.
The current study includes 17 sampling locations (most of which are identical to sites used in the 1992-1999 surveys) placed among the five major streams traversing the Great Swamp watershed, including, from east to west, 2 sites on Black Brook (BB1,2), 4 sites on Loantaka Brook (LB1-4), 4 sites on Great Brook (GB2-5), 3 sites on Primrose Brook (PB1-3), 4 sites on the upper Passaic River (PR1-3) and 1 site on a tributary of the Passaic River, Indian Grave Brook (IG1). Site numbers range from low numbers closest to their entry into the Great Swamp Wildlife Refuge and higher numbers located farther upstream. Beginning in June, 2000, one of the original (1992-1999) sites, GB1, was abandoned as representing a substrate type so unlike all the remaining sites as to make comparative interpretations difficult. In its place, a new site, IG1, was added to serve as a "reference site" with minimally disturbed conditions, thus hosting a regional macroinvertebrate community reflecting the least degree of human influence.
shows the approximate location of these sites. Table 1 includes brief descriptions of the sites and links to upstream and downstream photos of each study site.
Standard field and laboratory methods, adapted from the Environmental Protection Agency's Rapid Bioassessment Protocols (Barbour et al., 1999), were applied to the sample locations. Riffle habitats (i.e., turbulent water flow over stretches of shallow, cobble substrate) were used for this survey because such habitats are generally acknowledged to support the greatest community richness. At each field site, using a Surber sampler (a 500 μm mesh bag connected to a 1 foot square sampling frame), my assistant and I collected and preserved a composite of three independent macroinvertebrate samples representative of the range of water flow conditions present within the riffle habitat. In the field, we isolated and preserved, in 70% ethanol, all macroinvertebrates present in this composite sample.
In the laboratory, a random-subsampling technique was used to isolate from the original composite from each site a subsample of at least 200 individuals. These animals were sorted, enumerated, and identified. Animals remaining in the composite sample were surveyed, and single individuals representing species not already included in the 200+ individual-subsample were added to it. This step permitted us to note the presence of potentially important indicator species in the sample that otherwise would have been omitted. Many stream macroinvertebrates are small in size. Often closely related genera and species are difficult to distinguish in the field. Consequently, specimens were identified in the laboratory to the lowest taxonomic level possible, usually to the genus or species level, using dissecting and compound microscopy.
The widely used Strahler (1964) method of categorizing streams assigns a numerical "order" to specific stream reaches according to the degree to which tributaries enter upstream. Small brooks that lack tributaries are "first order" streams. The stream reaches that are joined by first order streams are termed "second order" streams. Reaches increase in order number when tributaries of equal order join (e.g., two second order streams fuse to form a third order reach). When streams of unequal order join, the subsequent reach carries the higher ordinal number of its sources.
The average elevational change per stream length, or gradient, was calculated for the stream or stream segment lying upstream from each sampling site, measured either to the next upstream station site or to the stream head as appropriate. Lengths were measured using quadrangle maps and the elevational change was estimated by counting the number of 10' map contour lines crossed by the stream section in question. The gradient was determined by dividing the vertical drop (# contours x 10') by the total linear length.
To add a physical-chemical-habitat context to these surveys, one day during each June sampling period is devoted to the collection of environmental data at each of the sampling sites. Because lotic (moving water) systems tend to show minimal short term (daily) variability, this single-day approach provides a better between-site comparison than would possible from environmental observations made at the times of sample collection scattered over a two week period of much more widely variable conditions. At each site, we record metered-readings of dissolved oxygen and temperature (YSI Model 85), pH (Hanna Instruments Model H9024C), total dissolved substances (Oakton Model WD-35661-57), turbidity (Hanna Instruments Model HI93703), current speed (Swoffer Model 2100) and water depth at 3 evenly spaced locations forming a transect across the stream. The latter two factors are combined to estimate the stream discharge rate at that location. We also fill out a standard EPA Habitat Assessment Field Data Sheet for High Gradient Streams (Barbour et al., 1999), which includes the coding for 10 habitat parameters, each on a scale of 0 (low) to 20 (high), guided by photographs and descriptions. These parameters include diversity of epifaunal substrate, available in-stream habitat cover, degree of embeddedness of stones, variety of velocity/depth regimes, extent of sediment deposition, status of channel flow, degree of channel alteration, frequency of riffles, stability of stream banks, extent of vegetation cover, and width of riparian zones. Experience working in the Great Swamp watershed has shown that some of these factors can be more objectively determined and more useful than are others. I have combined results of the most consistent among these characteristics and those showing the highest correlation to macroinvertebrate community quality (i.e., embeddedness, sedimentation, riffle frequency, bank stability and vegetational cover) to derive a "Habitat Value" for each site.
Metrics and Statistical Treatments
Metrics are quantitative representations of single (simple metric) or combined (multi-metric) characteristics of a sampled biological community. Using methods widely employed in the study of stream communities, two metrics were calculated for the macroinvertebrate community at each sampling station. The simple metric, Beck Index, was calculated along with the multi-metric Benthic Index of Biological Integrity (B-IBI). In time, viewing metrics from one collection period in the context of other comparable observations can help to distinguish "normal" variations and overall trends of change. Ultimately, we can use substantial changes in important biotic characteristics at a particular site to alert us to possible changes in stressful conditions there. It is important to keep in mind that such changes may be the result of site-specific environmental characteristics to which we may be able to respond, or they may reflect region-wide features (e.g., rainfall patterns) over which we have no control.
Beck Biotic Index
The Beck Biotic Index, is calculated to characterize the biotic community found at each site. Based on genus-level identifications, three categories of macroinvertebrate genera are tallied using criteria described in the SCS Water Quality Indicators Guide: Surface Waters. Class 1 organisms are "sensitive or intolerant of pollution"; Class 2 organisms are "facultative" and thus can tolerate varied conditions, perhaps including moderate levels of pollution; Class 3 organisms are "pollution tolerant". An undisturbed community should be comprised mostly of Class 1 and perhaps a few Class 2 or 3 species. Settings heavily impacted by organic pollution will be dominated by Class 3 animals. Locales in which Class 2 organisms are the most abundant reflect at least some significant source of stress, either natural or human caused.
The Beck Index for a particular site is calculated by doubling the number of Class 1 genera found there and adding to that the number of Class 2 genera. Class 3 genera are not included in the calculation. The formula for Beck Index is:
BI = 2n1 + n2
Where BI = Beck Index
n1 = number of Class 1 genera identified
n2 = number of Class 2 genera identified
Beck Index values calculated at the genus level correspond to the following water quality assessments.
||Water Quality Assessment
||Clean but limited habitat quality
|30 or higher
Although it has the advantage of being easy to calculate, Beck Index (sometimes referred to as the Florida Index) also has drawbacks. It was developed to help categorize macroinvertebrates in Florida streams, especially with reference to the impact of organic pollution on community composition. Because organic pollution does not appear to be a dominant source of stress in our area, there are other, widely employed metrics that may be even more useful for our local circumstances. Still, it may be useful to know local Beck Index scores to compare to those of other areas using this popular measure.
Benthic Index of Biological Integrity (B-IBI)
The B-IBI metric combines several distinctive, stress-influenced community characteristics into a single aggregate value that can be used to compare the level of stress evidenced by communities from different stream localities. For comparison, this Index is also applied to communities found at minimally disturbed, "reference" sites within the region. A B-IBI metric is tailored to a particular region by selecting for inclusion in the measure those community characteristics which correlate most closely with a sequence of sampling sites arrayed by personal observation along a known gradient from least to most disturbed (Karr & Chung, 1999). In this case, disturbance reflects regionally appropriate sources such as sedimentation, run-off from congested areas, flow interruption by impoundments, etc. Using the 17 sampling sites from the Great Swamp watershed, we have found that among an initial 37 community characteristics examined, . most closely follow such a human-related stress gradient.
- Degree of Dominance (DOM): As diversity declines, a few taxa come to dominate the community assemblage. A few opportunistic species that can tolerate modified conditions replace more specialized types. Calculate what proportion of the total individuals present fall into the two most abundant taxa. Increases as stress builds.
- Total taxa richness (TAXA): Biodiversity of stream declines as flow regimes are altered, habitat is lost, chemicals are introduced, energy cycles are disrupted, and alien taxa invade. Decreases as stress builds.
- Proportional Contribution by Predators (PPRED): Predators can only thrive in the presence of a rich diversity of prey types and in habitat circumstances that permit them to locate prey (i.e., unhindered by strong chemicals, low turbidity, etc.) and to pursue them successfully (i.e., sediment-free pore spaces within substrata, good supply of dissolved oxygen, etc.). The contribution of predators to the community is greater under conditions of minimal stress. Decreases as stress builds.
- Presence of Stress-intolerant Indicator Species (IndINTOL). Animals most sensitive to degradation are among the first to be lost as disturbance pressure builds. Conversely, their presence indicates low stress conditions. Included among the stress intolerant species in Great Swamp watershed streams (and thus, presumably in Rockaway watershed streams as well) are species in the ephemeropteran (mayfly) families, Caenidae, Ephemerellidae, and Heptogeniidae, the plecopteran (stonefly) family, Perlidae, the dipteran (fly) family, Tipulidae, and the trichopteran (caddisfly) family, Philopotamidae. Decreases as stress builds.
- Number of Species of the Order Ephemeroptera (#EPH): Diversity declines in response to most types of human influence. Many ephermerotperans or mayflies graze on algae and are especially sensitive to chemical pollution that interferes with them or with their food sources. Stoneflies and caddisflies are less affected by heavy metals and other chemicals. In nutrient-poor streams, livestock feces and fertilizers from agriculture or domestic runoff can increase numbers and types of mayflies. If mayflies are up but stoneflies and caddisflies are low, enrichment may be the cause. Decreases as stress builds.
- Number of Species of the Order Plecoptera (#PLEC): Stoneflies, in the order Plecoptera, are most sensitive to human disturbance. Many are predators that stalk prey in nooks and crannies among substrate. Sedimentation fills in such spaces and preclude such species. Other stoneflies are shredders - relying on leaf litter from overhanging canopy. Most stoneflies require cool water temperatures and high oxygen to complete their life cycles. Decreases as stress builds.
- Number of Species of the Order Trichoptera (#TRI): Wide diversity of trophic styles from net-building filter feeders to rock-scraping herbivores to invertebrate predators characterize the Trichoptera or caddisflies. Disturbance-caused habitat loss within streams leads to declines in caddisfly diversity. Decreases as stress builds.
- Presence of Stress-tolerant Indicator Species (IndTOL). Animals least sensitive to degradation tend to thrive competitively as disturbance pressure builds. Conversely, their expanding presence indicates increasing stress conditions. Stress-tolerant types included here are species with the class Hirudinea (leeches), the order Isopoda (sow bug crustaceans), and the families Gammaridae (amphipod crustaceans or "scud", Physidae (pulmonate pond snails), Enchytraidae (small, aquatic, oligochaete annelid worms), Planariidae (free-living turbellarian flatworms), and members of the chironomid midge genera, Dicrotendipes and Tanytarsus. Increases as stress builds.
The range of numbers that might be observed for each of these characteristics is divided into 3 sub-ranges representing values expected from least stressed ("reference" sites), intermediate, and most stressed communities. Then, depending on the range into which a specific characteristic at a particular site falls, it is assigned a score of 5, 3, or 1 respectively. The B-IBI value is the sum of these character scores, generating a maximal (least stressed) score of 40 (8 characters each with a maximal score of 5) and a minimal value (most stressed) of 8 x 1 = 8. B-IBI values were calculated in this way for each site in this study.
B-IBI values calculated at the genus level correspond to the following water quality assessments:
||Water Quality Characterization
Combining Habitat Assessment and Biotic Metrics
To gain a broader perspective on the ecology of watershed streams, it is useful to view the distributions of environmental variables and of biological communities together. While it is difficult to determine precise cause and effect relationships between specific conditions present and the collection of organisms they support, correlation analysis can help us view these relationships more closely. Correlation coefficients vary from -1.00 (= a perfect negative correlation), if increases in the values for a variable increases tend to be matched by decreases in the corresponding community metric, to +1.00 (= a perfect positive correlation), if increases in habitat values tend to be matched by increases in community metrics. It is important to stress that correlation shows only that the two entities being compared co-vary. It does not necessarily demonstrate a causal relationship between the two.