Chapter 15 – Benthic Macroinvertebrates
Release Date: April 2013
Benthic macroinvertebrates are organisms that live at the bottom of streams, lakes and wetlands, are visible to the naked eye and have no backbone. They include insect larvae (e.g. dragonflies; stoneflies) (Figure 1), annelids (e.g. leeches; worms), crustaceans (e.g. crayfish), mollusks (clams and mussels), and gastropods (snails).
Figure 1. Stonefly nymphs observed in the Credit River.
Similar to other organisms, benthic macroinvertebrates are influenced by their environment. Environmental factors that are of particular importance to macroinvertebrate diversity and abundance include: sediment composition and chemistry, land use, riparian vegetation cover, water chemistry and temperature, dissolved oxygen, and hydrological factors such as streamflow and water depth. Because benthic communities are structured by their environment, they serve as effective biological indicators of local environmental conditions. Benthic organisms also have low mobility compared to other animals such as fish and birds and are an indicator of local environmental conditions (e.g. particular riffles in streams). Monitoring benthic communities therefore provides a useful assessment of the spatial extent of potential environmental impairments.
Benthic Macroinvertebrate Monitoring
In 2011, benthic macroinvertebrates were monitored at 52 stations across the watershed (Figure 2). Benthic community characteristics such as abundance, composition and diversity were calculated to quantify the degree of environmental impairment and assess trends through time. This Watershed Health Report focuses on the Hilsenhoff Biotic Index (HBI; Hilsenhoff 1987), which is commonly used to summarize benthic communities in stream environments based on known tolerances of organisms to organic pollution. A benthic community classified as impaired (HBI score between 8 and 10) or possibly impaired (HBI score between 6 and 8) is one that is made up predominantly of organisms that are tolerant of organic pollution, while a community classified as unimpaired is dominated by organisms that are sensitive to pollution (HBI score between 1 and 6).
Figure 2. Location and status (2011) of benthic macroinvertebrate monitoring stations.
Benthic Macroinvertebrate Status (2011)
Overall, benthic macroinvertebrate status in 2011 is unimpaired (Figure 2). Many small tributaries in urbanized areas, however, are classified as possibly impaired. These small streams are often influenced by storm runoff containing excess sediments, chlorides (i.e. road salt) and other contaminants. In addition, land use change that narrows the riparian zone would further increase the rate of contaminant transfer to these streams and potentially reduce food supply, breeding habitat and shade. The environmental stress on benthic communities would be further compounded by reduced cool clean groundwater and changes to streamflow, which result from more impervious surfaces and fewer groundwater recharge areas.
Benthic communities in larger tributaries and in the Credit River have a status of unimpaired. The unimpaired status of these larger streams may be related to the River Continuum Concept (Vannote 1980), which suggests greater flow stability in larger streams that increases habitat complexity and productivity resulting in greater species diversity. Larger streams also contain more water allowing for greater dilution capacity of pollutants associated with urbanization, water pollution control plants, and agriculture.
The urban Lower Watershed has the most impaired benthic community with an average HBI of 6.24. In contrast, the more rural Middle and Upper Watershed have largely unimpaired benthic communities with average HBI scores of 4.71 and 5.00 respectively. The only impaired site (HBI 8.13) occurs in Fletchers Creek in the Lower Watershed. This site has poor water and habitat quality with no storm water controls which may be responsible for the impaired status of the benthic community.
Benthic Macroinvertebrate Trends
Trend analysis was completed using monitoring data collected at 35 stations over a 12-year period (2000 to 2011). Not all of the 52 monitoring stations sampled in 2011 have a continuous long-term record and therefore not all of the 52 stations could be included in the trend analysis.
A statistically significant linear trend was evident in the Upper Watershed suggesting improved water quality in this region (Figure 3) possibly in response to improvements in water pollution control plants. Although slight improvements were evident in the Lower and Middle watershed, only the Upper Watershed had a statistically significant trend. Given the fully urbanized status of the Lower Watershed, any substantial improvement to the benthic community may be difficult to achieve, however, improved storm water management and employing low impact development practices may help to further improve stream conditions. The Upper and Middle Watershed have generally had unimpaired conditions over the monitoring record, however, HBI scores in the Upper and Middle Watershed could further improve with more refined agricultural practices and advances to water pollution control plants.
Figure 3. Average Hilsenhoff Biotic Index (HBI) trends over time with standard error (error bars) for the Upper, Middle and Lower Watershed. The trend in the Upper Watershed is statistically significant (p<0.05) whereas the Middle and Lower Watershed showed no statistically significant change through time.
Examination of the benthic macroinvertebrate community at 52 monitoring stations in the Credit River Watershed indicates that overall the Credit River Watershed was largely unimpaired in 2011. The exceptions to this unimpaired status were tributaries located in the highly urbanized Lower Watershed which were primarily classified as possibly impaired. Long-term (12 years) trend analysis of the HBI indicated that benthic communities in the Upper Watershed suggest improved water quality over the monitoring period in this region. The Lower and Middle Watershed show fairly stable or slightly improving conditions in HBI over the monitoring period. Overall these are positive results and suggest improvements to water and/or habitat quality in the Credit River Watershed. It is important to remember, however, that human activities can decrease water and habitat quality and effective management, informed by monitoring data, is likely required to ensure these positive results continue.
In the next chapter we will go fishing. How healthy are fish communities in the Credit River Watershed and what does this tell us about the health of the Credit River and its tributaries?
Hilsenhoff, W.L. 1987. An improved biotic index of organic stream pollution. Great Lakes Entomol. 20:31-39.
Vannote R.L., G.W. Minshall, K.W. Cummins, J.R. Sedell, C.E. Cushing: “The River Continuum Concept”. Canadian Journal of Fisheries and Aquatic Sciences. 37.1980,1 Ottawa, 130-137.