Invertebrate tolerances at varying levels of urban disturbance on Burnaby Mountain, BC

Hanna Jackson

Department of Biological Sciences, Burnaby, BC, Canada

*This report is a class project NOT peer-reviewed science


Question

Are stream invertebrates being affected by construction and urbanization on Burnaby Mountain?


Hypothesis

Reaches that are closer to the construction zones will consist mostly of invertebrates that have a higher tolerance and a higher HBI score and reaches that are further from the construction zones will consist of invertebrates with a variety of tolerances and an overall low HBI score. We also expect that the reaches with higher HBI scores will have finer sediments than those without disturbances.


Results

We sampled invertebrates and recorded their abundances and tolerance scores at multiple reaches of streams on Burnaby Mountain (Supplemental Material, Figure 4). Our disturbed reaches had an average tolerance of 5.48 and our reference, undisturbed sites had an average tolerance of 5.45 (Table 1), indicating no significant difference (ANOVA, F=1.709, p=0.149) between the tolerances of invertebrates at these two different levels of disturbances. This is also seen when looking at the distribution of tolerance scores among reaches (Supplemental Material, Figure 2), with all reaches having similar compositions, with the exception of reach E, where the median tolerance score was 2, much lower than the others despite similar variation present.

We also sampled each reach for average sediment size. All reaches showed no difference in sediment sizes, with the exception of reach E, which had a significantly larger average sediment size (Tukey HSD Test) than all others (Figure 1).

Table 1. We sampled stream invertebrates in reaches of streams that were close to construction (A and B) and that were further away from construction (C,D and E). We then calculated the HBI score for each reach, a measure of the tolerance of invertebrates present scaled by each of their abundances.




Figure 1. Sediment sizes at each sampling site within each reach were measured and averaged.


Discussion

Urbanization, such as that at the top of Burnaby Mountain, has been shown to substantially decrease stream quality (Stepenuck et al. 2002). It is negatively associated with the presence of intolerant macroinvertebrate species and genetic richness, and thus it degrades macroinvertebrate communities (Stepenuck et al. 2002). It has previously been shown that pollutant runoff from urbanization is a major contributing factor in this degradation of invertebrate communities (Hassett et al. 2018). The Hilsenhoff Biotic Index (HBI) is a fast and efficient method for quantifying a stream’s exposure to organic pollutants (Hilsenhoff 1988).


Contrary to our hypothesis, we found that both our disturbed streams and reference streams received, on average, a HBI score of approximately 5.6, indicating that both disturbed and reference streams have fairly substantial pollution (Table 1). However, the designation of reference and disturbed sites may require re-evaluation and it seems likely that the urbanization of Burnaby Mountain is affecting reaches further down than initially expected. Reaches A, C and D were shown to have poor, very poor and fair water quality, reach B was shown to have good water quality, and only the furthest reach, reach E was shown to be in excellent standing (Table 1 and Supplementary Material, Table 2). Further analysis of streams would be required to determine the extent, sources of, and biological effects of the pollution; however current data analysis points to the disruption of invertebrate communities being more widespread, and more variable, than initially expected.


This decrease in HBI index in reaches closer to the urbanization also was found to be associated with the habitat characteristics such as sediment size (Figure 2) and channel thalweg depth (Supplemental Material, Figure 3) Reaches A-D showed similar values for both variables, and reach E, the least impacted according to HBI score, showed a significantly higher average sediment size and average channel thalweg depth. While further analysis will be required to determine the cause of these results, this analysis tentatively supports our hypothesis and points to the fact that patterns in HBI scores may also be associated with abiotic habitat characteristics, rather than just organic pollutant content, whether that is alongside or independent of organic pollution cannot yet be known for this study system.

Supplemental Material

Figure 2. We sampled stream invertebrates in reaches of streams that were close to construction (reaches A and B) and that were further away from construction (reaches C, D and E). We then identified all the invertebrates and assigned them a tolerance score based on their ability to withstand organic polutant stressors.


Table 2. Evaluation of water quality using the Hilsenhoff Biotic Index. From Hilsenhoff, 1988.



Figure 3. Channel thalweg depth at each sampling site within each reach were measured and averaged.



Figure 4. Location of reaches sampled on Burnaby Mountain, BC, Canada



Literature Cited

Hassett, B. A., E. B. Sudduth, K. A. Somers, D. L. Urban, C. R. Violin, S. Y. Wang, J. P. Wright, R. M. Cory, and E. S. Bernhardt. 2018. Pulling apart the urbanization axis: patterns of physiochemical degradation and biological response across stream ecosystems. Freshwater Science 37:653–672.


Hilsenhoff, W. L. 1988. Rapid Field Assessment of Organic Pollution with a Family-Level Biotic Index Author ( s ): William L . Hilsenhoff Source : Journal of the North American Benthological Society , Vol . 7 , No . 1 ( Mar ., 1988 ), Published by : The University of Chicago Pre. Journal of the North American Benthological Society 7:65–68.


Stepenuck, K. F., R. L. Crunkilton, and L. Wang. 2002. Impacts of urban landuse on macroinvertebrate communities in southeastern Wisconsin streams. Journal of the American Water Resources Association 38:1041–1051.