Macroinvertebrate Sensitivity Index Analysis: 2024 vs. 2025

As part of Project CREEKWISE (Community Research Efforts Evaluating Key Water Indicators and Stream Ecology), students collected and analyzed macroinvertebrate samples from multiple stream sites across Ionia County, Michigan. Each sample was identified and assigned a sensitivity score based on pollution tolerance. These scores allowed us to calculate an average sensitivity value for each site and year.

It’s important to note that we are using a modified version of the Hilsenhoff Biotic Index (HBI), where:

• Low sensitivity values (closer to 0) indicate an abundance of pollution-sensitive organisms (e.g., stoneflies, caddisflies, and mayflies), and therefore better water quality.
  
  

• High sensitivity values (closer to 10) indicate dominance by pollution-tolerant species (e.g., aquatic worms, leeches, and true fly larvae like mosquitoes), suggesting poorer water quality.
  
  

✅ Reliability of the Sensitivity Rating Method

The sensitivity index is widely used in stream bioassessment and is a reliable bioindicator of long-term water quality trends. Unlike water chemistry, which can fluctuate rapidly, the macroinvertebrate community structure reflects cumulative impacts over time. However, its reliability hinges on:

• Accurate species identification

• Adequate sample size

• Consistent collection methods

• Multiple habitat types (e.g., riffles and pools)
  
  

We followed standard protocols, though future sampling should aim for greater replication and standardization to increase statistical power and reduce variability.

📊 Summary of Sensitivity Score Trends

Site 2024 Avg. Sensitivity 2025 Avg. Sensitivity Change Interpretation

Grand River 5.19 5.06 ↓ Slight Slight improvement

Lake Creek 4.17 6.86 ↑ Major Significant decline

Bellamy Creek 3.75 5.65 ↑ Moderate Moderate decline

Sessions Creek 4.02 5.24 ↑ Moderate Decline in quality

Tibbetts Creek 3.63 3.95 ↑ Slight Slight decline

🌊 Discussion: Diversity & Community Shifts

1. Diversity vs. Sensitivity

While average sensitivity values provide a useful water quality indicator, they don’t fully capture biodiversity. A site with a mid-range score may host a wide variety of species from all tolerance levels—or be dominated by just a few moderately tolerant ones.

Adding metrics such as:

• Taxa richness (number of different taxa)

• EPT Index (number of Ephemeroptera, Plecoptera, Trichoptera taxa)

• Shannon Diversity Index

...would give a fuller picture of the ecological health and stability of each site.

2. Lake Creek – A Curious Case

The sensitivity score increased from 4.17 to 6.86, indicating a decline in water quality. This aligns with our observation that almost no caddisfly larvae were found in 2025—a major red flag, since caddisflies are among the most sensitive taxa.

Possible reasons for this shift:

• Habitat alteration (e.g., increased sedimentation smothering cobble substrates used by caddisflies). This was specifically noted by students sampling macroinvertebrates below the rails to trails bridge. There is a substantial sediment load accounting for the majority of the stream bed all the way down to the confluance with the Grand River. This sediment could create an inhospitable environment for macroinvertebrate communities. 

• Water temperature increase (possibly due to loss of riparian vegetation). As vegitation is removed along the stream solar energy has the potential of being absorbed by the waterway. Elevated temperatures could account for some species being absent from the environment. It could also lead to lower levels of dissolved oxygen which would contribute to a deline in those species requiring a greater level of oxygen. 

• Localized pollution events (agricultural runoff or nutrient spikes from failing septic systems). Lake Creek has had notable accidental fertilizer applications . Similar events could acount for water body specific eutrophication that created a negative effect on macroinvertebrate populations. 

• Sampling timing (caddisflies may have already emerged or were missed due to life cycle timing). Though this seems less the probable given sampling occuring in the same timeframe as last year.
  
  

This kind of taxonomic shift—loss of sensitive species and replacement by more tolerant ones—highlights how important it is to consider species composition, not just the total index value.

📌 Next Steps

To strengthen future analyses, we recommend:

• Including replicates at each site

• Adding a diversity or richness index

• Tracking presence/absence of key indicator species (e.g., Hydropsychidae, Baetidae, Chironomidae)

• Documenting site-level changes (shade cover, sediment, flow rate)