Aquatic Macroinvertebrate Communities in the Dark, Sandy and Pike Rivers

Aquatic Macroinvertebrate Communities
in the Dark, Sandy and Pike Rivers

Gary Montz
Jodene Hirsch

Aquatic Biology Laboratory


Division of Ecological Services
Minnesota Department of Natural Resources

October 2001


Executive Summary

Aquatic macroinvertebrates were collected over two years from the Dark, Sandy and Pike rivers. A proposal to discharge taconite tailings holding pond water prompted the study, which was designed to provide baseline information on the fauna and to assist resource managers in decision-making. The invertebrate communities were analyzed using widely accepted water quality metrics. The Dark River is unique in this part of the state, with groundwater inputs sustaining a trout fishery, as compared to more typical warmwater streams. The data showed that the invertebrate community in the Dark River is very diverse, contains many taxa which are intolerant of water quality changes and is susceptible to major impacts from water quality or quantity alterations. Recolonization of impacted areas may be slow or not occur at all, due to factors including: many surrounding waters are warmwater streams which lack taxa found in the Dark, discharges would occur in the headwaters region of the Dark, spreading the impacts throughout the length of the river, the Dark Lake presents a barrier to any upstream movement. Impacts to the invertebrate community in this river could be major, long-term and have serious impacts for the fisheries in the river.

Invertebrate communities in the Sandy and Pike Rivers were less diverse, and more typical of warmwater streams and rivers. They had less intolerant taxa, and less insects within the community. These taxa are more adapted to the warmer waters and less solid substrate within these rivers. While these communities may be more tolerant of any discharges, they may also be impacted by poorer water quality in any discharges.

The invertebrate communities should be monitored if any discharges are permitted in these systems. This monitoring can help determine any degradation that may occur as a result of any discharge to these systems.

Introduction:

Aquatic macroinvertebrates, particularly aquatic insects, are a key component of the fauna in streams and lakes. These organisms serve as the food base for fish populations, and are fed upon in the adult life stages by terrestrial animals, such as birds. Their importance in the ecosystem has been long known and accepted, and more attention is being given to the diversity and abundance in many areas (Healey 1984).

Additionally, macroinvertebrates have been widely used in assessing water quality. Many states and different countries have developed and used indices that examine diversity and density of aquatic macroinvertebrates to determine water quality, or to assess impacts from specific actions and perturbations (Davis and Simon 1995).

Samples of invertebrates can yield valuable information for resource managers working on streams and rivers throughout North America (Barbour et al. 1999).

Baseline data collected prior to any significant changes to the system (logging, housing development, changes in discharge, excessive sedimentation) provides not only a picture of the water quality and invertebrate community conditions prior to the change, but can also be compared to post-event samples to document if any changes have occurred. Pre-event sampling can also point out if intolerant taxa sensitive to even minor changes are present, and can help managers make recommendations to avoid negative impacts to such intolerant fauna.

The Dark River in St. Louis County, Minnesota differs from typical streams in this area of the state. While many of the waters in this area are slower flowing, warm-water fisheries, the Dark River receives groundwater input in the lower reaches. This alters the temperature enough to allow a trout fishery in the stream. With a proposed potential significant discharge to the Dark River from holding ponds for a taconite processing plant, a study was developed to assess the invertebrate community in the river prior to any permit modifications. Knowledge of the fauna, in particular with its implications as food resource for the trout populations, can help resource managers determine how proposed changes could impact the biological resources in the stream.

The objectives for the invertebrate study were to survey, document and assess the aquatic macroinvertebrate fauna of the Dark, Sandy and Pike Rivers. These data would establish baseline information on this fauna to assist management decisions and allow future monitoring of the water quality and macroinvertebrate community in these waters. The Sandy and Pike Rivers were included as the Sandy River, which flows into the Pike River, was proposed as a possible alternative to receive the taconite holding ponds discharge.

Methods:

Study Area

The Dark River flows through St. Louis County in northern Minnesota. The river flows through wetlands and low riparian areas prior to entering Dark Lake. Further downstream from the outlet from Dark Lake, groundwater input moderates water temperature enough to support a trout fishery for a part of the river. The riparian area is intact in much of the river. Suspended sediment in the river is low, and water clarity is good with slight tannic staining.

Substrate in the river varies from sand, sand/gravel, cobble/rock to softer accumulations of silt. Instream vegetation can be common in the river above Dark Lake, while it is uncommon throughout much of the length below the lake. Woody debris is often present and can be abundant in some reaches.

The Sandy River is more typical of streams in this area. The river flows through many wetlands and there is much less coarse sediments, with bottom substrate dominated by softer organic sediments. Overhanging vegetation and wetland shrubs dominate in the riparian area. The water is very stained, and carries a higher sediment load. Temperatures are warmer, and occasional problems with dissolved oxygen reductions have occurred due from inflow from surrounding wetlands of organic sediments during extreme rainfall. The Pike River is similar to the Sandy River as a warmwater stream with predominantly sandy-silty substrate interspersed with some rock and instream vegetation.

Invertebrate collection

Aquatic macroinvertebrates were collected over two years (1999 - 2000) from sites in the three rivers during spring (June) and fall (September). These two periods were chosen to represent highest diversity in the invertebrate community. The sites on the Dark River and one site on the Sandy were sampled all four periods. A second site on the Sandy and one on the Pike (below where the Sandy River enters the Pike) were sampled only during 2000.

Invertebrate samples were collected from all sites with kick-nets. Samples were collected in two major habitats: rock type (rock/cobble/gravel), or multi-habitat (rock, woody debris, vegetation). The choice of habitat to be collected was determined by looking at the sampling area to determine what types of substrate were common. Kick net samples collected in rock type substrate were a composite of two kick net samples, each disturbing an area approximately equal to the width of the kick net from the front of the net. Multi-habitat samples were not quantified, but proportional amounts of the different substrates were collected. For example, if the dominant substrate was woody debris and small amounts of instream vegetation, most of the collection came from the woody debris with a small amount from the vegetated area. Adult dragonflies were collected, as well as any emerging exuvia, to try and document additional species not collected in larval sampling. All samples were preserved in ethanol in the field, placed in jars, labeled and brought back to the laboratory. There the samples were sorted with a dissecting microscope and macroinvertebrates were counted and identified to the lowest practical taxonomic level. The Chironomidae were identified only to the family level, due to the time needed for mounting and identifying this group.

Six sites were sampled on the Dark River (Figure ). One of these sites was located upstream of Dark Lake (DRup), one at the river’s outlet from Dark Lake (DRout), and four sites downstream of the lake corresponding to reaches sampled in an earlier fisheries survey (DR1-4). All sites were sampled for two years.

Two sites were sampled on the Sandy River (Figure ). One site was north of the city of Virginia, downstream of Sandy Lake (SR1). This site was sampled for two years. The other site (SR2) was northwest of Virginia, upstream of the confluence of the Sandy and Pike Rivers. This site was sampled for one year (2000).
One site (PR1) was sampled for one year (2000) on the Pike River (Figure ). This site was northwest of Virginia, downstream of the confluence of the Sandy and Pike Rivers.

The data for sites and dates were used to calculate various water quality metrics that are commonly used by many agencies and states throughout North American (Barbour, et al. 1999). The results from the metrics were compared to help assess water quality and invertebrate communities.

Results

Taxa totals

Total number of taxa collected from all sites ranged from a low of 86 (June 2000) to a high of 124 (June 1999). Insect taxa made up the majority of the taxa collected, comprising 90% of the invertebrate taxa over all sample periods (Appendices A - D). These totals would be higher if the Chironomidae were identified to genus level.

With insect taxa dominating the fauna identified, the total number and mean number of insect taxa were examined for each site (Figure ). Most sites in the Dark River had higher numbers of insect taxa compared to sites in the Sandy and Pike Rivers.

Mean numbers of insect taxa in the Dark River were lowest at the outlet site. This is typical of lake outlet areas, which can often be dominated by a few taxa adapted to efficiently use the food source from the lake. While mean numbers of insect taxa were comparable between the Dark River outlet site and site 1 on the Sandy River, both site 2 of the Sandy and the Pike River site had low numbers of insect taxa collected in their samples.


Hilsenhoff Biotic Index

The Hilsenhoff Biotic Index (HBI) is a widely used and accepted method for assessing water quality through the use of aquatic macroinvertebrates (Hilsenhoff 1987). This index measures organic enrichment and the impacts of dissolved oxygen on the community. HBI values were calculated for each site and sample period and overall mean HBI values were calculated for each site (Figure ).

Invertebrate samples from the Dark River suggested two slightly different community zones. The upstream site through site 1 had mid-range HBI values, suggesting water quality of fair to good. The upstream site water quality could be influenced by the flow through numerous wetland areas, contributing organic matter and affecting the community. Additionally, habitat was primarily sand with some instream vegetation and woody debris, with very little rock or cobble. This habitat is similar to that found at site 1. The invertebrate community at the outlet site is likely influenced to a significant degree by the water from Dark Lake, which is carrying phytoplankton and higher nutrient levels from the lake. Invertebrate communities in lake outflow areas tend to be dominated by specific groups which are adapted to use these resources (Ward, 1992). Filter-feeding taxa such as hydropsychid caddisflies and black flies dominate the community in lake outflows until the planktonic food source is reduced and normal lotic process regain control of food sources. This is evident in the community in the outlet area of the Dark River.

The second “zone” suggested by the invertebrate communities extends from site 2 - 4. HBI values suggest good to very good water quality. These sites support higher populations of trout, and habitat tends to be more dominated by rock, cobble and consolidated substrate, with some woody debris. Even the multihabitat samples in site 4 indicate very good water quality.

The two sites in the Sandy River showed different HBI scores. The first site had the poorest HBI scores, with an average water quality value bordering fair-fairly poor. The second site had better HBI scores (based on one year of sampling) indicating good water quality. Habitat at the first site was soft sediments, with some instream vegetation, woody debris and slow current. The riparian zone was predominantly wetland habitat. The second site had sand with rock and cobble, some instream vegetation, and faster current. The riparian zone upstream was less dominated by wetland habitat, although downstream of this site the Sandy again entered into a large wetland area.

The single site at the Pike River, sampled for one year, had HBI scores falling between good and very good. The habitat was sand, rock and cobble, and some instream vegetation.

Number of Intolerant Taxa

The taxa in the HBI were examined, with those having tolerance values from 0 - 3 counted as “intolerant” (based on HBI scores of 0.00 - 3.50 as excellent, with no organic pollution). The numbers of intolerant taxa for each site during each sample period and the overall mean are shown in Figure 3.

A similar pattern to the HBI scores for the Dark River were seen in the intolerant taxa numbers. The mean number of intolerant taxa for upstream to Site 1 were in the 10 - 14 range. These means rose from 16 - 23 for sites 2 - 4.

The number of intolerant taxa were consistently lower for all sites in the Sandy and Pike Rivers. While HBI values were different, mean numbers of intolerant taxa ranged from 3 - 6 for all three sites.
% EPT Taxa to Total Taxa

The EPT taxa (Ephemeroptera, Plecoptera, Trichoptera) are generally indicators of better water quality, with many of the taxa in these groups sensitive to poorer water quality (EPA 2000, Resh and Jackson 1993, Stewart and Loar 1993). This metric is also widespread in use for assessing water quality. The % EPT results show slight differences from the HBI scores (Figure 4). While Sites 2 - 4 are overall consistently high, mean values for the upstream sites are also high (above 50% of the total taxa) with the exception of the multihabitat samples from the outlet site. However, the samples in the riffle area at the outlet site are equivalent to many of the samples in sites 2 - 4. While site 1 in the Sandy River indicated that less than a third of the community was the EPT taxa, site 2 and the Pike River had EPT taxa percent similar to many sites in the Dark River.

Examination of the actual number of EPT taxa collected helps clarify this metric (Table ). While the percentage of the EPT in the Sandy and Pike rivers may be near that of many sites in the Dark, the actual number of different taxa collected were substantially lower than nearly every site in the Dark, with the exception of the multihabitat samples from the outlet. These samples had a mean number of EPT similar to the Pike River. Again, the community at the outlet site is driven by the influence of the water from Dark Lake, and may not be representative of the community and water quality in the Dark downstream, when this lentic influence has been dissipated. Thus, while the percent of the EPT taxa may indicate that the Sandy and Pike sites are similar to the Dark sites, the actual numbers suggest that this fauna is much lower than most sites in the Dark River.

Discussion

Dark River

The invertebrate community from the Dark River is diverse, abundant and has high numbers of taxa which are intolerant to water quality degradation. Sites further downstream of Dark Lake support more intolerant taxa, and suggest that the water quality in these areas is minimally impacted. These results are also supported by Fisheries surveys on the Dark (Karl Koller, personal communication).

There has been suggestion that groundwater inflows become more substantial downstream of the lake, and this input could be influencing this community. However, even the invertebrate community at the site upstream of the lake suggests a diverse healthy invertebrate fauna, which is likely influenced by the riparian zone which the river flows through before entering the lake.

The large number of intolerant insect taxa, the abundance of the EPT taxa, the diversity and complexity of the invertebrate community all suggest that water quality or quantity changes could have significant impacts on this fauna. Increasing flow by the discharge of taconite tailing holding pond water could have significant impacts to the community. The increase in discharge could significantly increase the normal discharge of the river during summer conditions. This water would be warmer than the river, and could overwhelm the influence of the groundwater input, raising the stream temperature. Temperature plays a critical role in aquatic invertebrate biology and ecology and can restrict the distribution or occurrence of taxa (Ward 1992, Minshall 1984). Temperature controls growth of insects, and impacts the levels of dissolved oxygen in the water. Warming the temperature over the river length could alter insect growth, lower dissolved oxygen levels which could eliminate intolerant lotic taxa such as stoneflies, and impact the community composition and abundance by changing the life cycles in aquatic insects.

Increases in flow can also impact the invertebrate community through increases in sediment transport and deposition. Aquatic insects are tied to the substrate of the stream, and the substrate has both direct and indirect impacts on this fauna (Minshall 1984). Generally, increasing substrate complexity and heterogeneity results in greater diversity and abundance in the aquatic insect community.

An increase in transported sediment can impact the invertebrate community in different ways. The transported sediment can scour filter-feeding insects from the environment. Additionally, sediment can be deposited in heterogenous substrate, filling in interstitial spaces and reducing or eliminating habitat.

Increased sediments can also impact the abundance and distribution of algal growth, which can impact food availability to grazing aquatic insects. Changes in taxa composition or abundance of the grazers in the community can impact the predator populations in the invertebrate fauna.

Increases in flow can also result in more organic matter transported directly to the stream. Higher water levels can flush organic sediments from wetlands into the stream. Additionally, increased inflow into the lake could result in more phytoplankton and nutrients being transported out of the lake, which raises the organic load in the river. The HBI looks at the impacts that organic enrichment can have on the invertebrate community, with intolerant taxa dropping out as the organic levels rise. Thus, any increases of this type could stress or eliminate a large number of taxa currently in the river (Wiederholm 1984).
It could be argued that springtime snowmelt conditions bring large amounts of organic material and/or sediments into the stream system. However, these are temporal events which occur on a seasonal basis. Invertebrate communities, like many of the other aquatic organisms, have evolved to adapt to these seasonal “perturbations”. Increases of water flow, organic material or sediments which occur long term can alter the community which has not evolved under these conditions.

The abundance of EPT taxa in the system are also sensitive to perturbations which may occur from changes in water chemistry. Different taxa respond differently to water chemistry changes in a variety of parameters, including temperature, pH, or metals. Depending on the chemistry of any significant inputs to the river, these taxa could be mildly to seriously impacted. If the water chemistry from the holding ponds is different from the river water (altered pH, elevated metals) various taxa could be seriously impacted or eliminated from the system.

Invertebrate taxa are not independent organisms existing in the aquatic system. The organisms are interrelated, partitioning food and habitat resources both spatially and temporally. A change in grazer fauna has the potential to alter the predator invertebrate fauna, through a change in their prey population. It may also impact the amount and type of epiphytic algal growth, which can in turn have further impacts on the herbivore fauna. Changes in the kinds and numbers of the invertebrate fauna can impact the food base for the fish in the system.

As the proposed discharge to the Dark would occur in the headwaters of the stream, impacts have the potential to occur throughout the entire river, from source to confluence with the Sturgeon River. Thus, there would not be an upstream “invertebrate pool” to recolonize impacted reaches of the Dark River. Other possible recolonization mechanisms include upstream movement by larval stages and migration by aerial adult forms (Ward 1992). However, upstream movement would not be a mechanism, as the river could be impacted throughout it’s length. The receiving waters (Sturgeon River) are warmer and would not likely support the same fauna. Nor is it likely that the length of the Dark River could be recolonized through upstream movement. Additionally, the Dark Lake is a barrier to any upstream movement past the lake. Adult colonization can only be effective if there is a source of organisms within adult flight distance. As noted earlier, many or most of the rivers and streams in this area are not similar to the Dark, but are more typically warmwater systems that would not have the same invertebrate faunal composition. Thus, any impacts could results in community changes that would take a long time to reset to the current baseline conditions, if it occurred at all. Impacts to the invertebrate fauna in the Dark from discharges to the headwaters have the potential to result in long-term or permanent community composition changes.

Sandy and Pike Rivers

Fewer sites were sampled on these rivers due to problems with accessibility. At the sites sampled, the habitat was different than found in the Dark River. The riparian zone has much more wetland, with grasses and shrubs for riparian vegetation. Bottom substrates were softer sediments, with little rock or gravel in the areas sampled. The water in both rivers was dark colored, and had higher turbidity than the Dark River.

Invertebrate communities in the Sandy and Pike Rivers were less diverse than most sites in the Dark River. An average of 15 - 25 taxa were collected from these two rivers on any sample period. Nearly completely absent was the order Plecoptera, which are intolerant of warmer waters and lower dissolved oxygen. Additionally, the sites on these rivers had fewer intolerant taxa. Less than 10 taxa were considered intolerant by HBI values from the samples on these two rivers, and the average number of intolerant taxa was about 5.

This may not necessarily indicate degradation in the water quality, but may reflect the nature of the rivers and the riparian zones. More of the riparian area of the Sandy River is wetlands, and the river substrate is more soft unconsolidated sediments. The Pike River is also larger, with more input from riparian wetland areas. Both of these factors could contribute to a less diverse invertebrate community as well as one that is more tolerant of increased nutrient or sediment loads. The waters in these rivers are also warmer than that of the Dark, and this can shape the communities. It is also important to note that the HBI scores for the most downstream site in the Sandy as well as the single site on the Pike indicated good water quality, suggesting some but not overly large amounts of organic enrichment. This supports the possibility that riparian zone and instream habitat have set the baseline for the invertebrate community. It also suggests that the communities in these rivers may not be as seriously impacted from additional discharge as those of the Dark River. These two rivers are larger, and the proposed discharge would not comprise as significant portion of the flow at any given time. Due to the physical nature of these two rivers, impacts may be less noticeable than in the Dark River.

Conclusions

The data collected in the Dark River provide a solid baseline for future monitoring use. The invertebrate community in the Dark River is diverse, abundant and has a large number of taxa intolerant of water quality changes. These data suggest that any inputs (such as the proposed tailings pond discharge) to the Dark River have significant implications for impacting the invertebrate community. Any discharges to this river or changes in water quality should take this community into account when determining the appropriateness of such an action. Changes to the Dark River invertebrate community may have significant implications to the fisheries that are currently in the river. Recolonization from any impacts has serious limitations, and may either be long-term or not occur. Perturbations such as increased flow, increased sedimentation, alteration of water chemistry, or increased temperature could all have significant impacts on the invertebrates, which serve as the base of the food chain in flowing waters. If any discharges or water quality changes are to occur, monitoring of the invertebrate community should continue in the same methodology as used in this study. The invertebrate community may serve as an indicator of impacts and changes to the river system. Alterations to this river system should be viewed with extreme caution.

The data collected on the Sandy and Pike Rivers provide the start of a baseline for invertebrate monitoring and survey. The fauna in these rivers appears to be more resilient to water quality changes. However, the community contains intolerant taxa which may serve as indicators of adverse changes for any proposed discharges or water quality alterations. Continued monitoring of these areas is also suggested for any actions which may impact the water quality and the invertebrate fauna of these rivers.


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