Prosimulium and Simulium in Pennsylvania
Matt Shank
2022-05-12
Background
When macroinvertebrate samples are dominated by Prosimulium and/or Simulium larvae, metric and IBI scores will be depressed at sites with little stress. Due to the low tolerance value of Prosimulium, there is a possibility that IBI scores could increase as this organism proliferates. These influences on IBI scores confound assessment determinations and may require biologists to resample the site after the seasonal larval blooms, or the biologist may have to rely on a more qualitative analysis of metric scores, sample composition, and site conditions to arrive at an assessment decision (Shull 2017).
The objective of this document is to provide guidance on spatial and temporal dynamics of Prosimulium and Simulium. Avoiding the ‘blooms’ of these taxa is desirable to prevent additional effort required to make an appropriate assessment. Additionally, we aim to provide guidance on the relative abundance of each taxa in samples from sites across a gradient of stress that would result in tenuous assessments.
Methodology
Data were queried from the SLIMS database for all samples that used Freestone/SWMMI (6-Dframe Composite, 200 subsample), Limestone (2-Dframe Composite, 300 subsample), and Multihabitat (10 Jabs, 200 subsample) methods. Samples with >= 50 organisms were retained to include heavily impacted sites (e.g. AMD) in the dataset. This resulted in 13,929 samples collected between 1999 - 2022. Samples were collected throughout all months, with most samples collected during March-May and November-December (Figure 1).
Figure 1. Temporal distribution of statewide samples, by method. 
We will look at the occurrence, abundance, and spatio-temporal dynamics of Prosimulium and Simulium to provide insight into proliferation of these taxa throughout Pennsylvania as a whole, and within each DEP region.
We will also look at the influence of increasing Prosimulium and Simulium relative abundance on metric and IBI scores. We selected macroinvertebrate samples that represented different levels of stress to examine the influence of Simulidae across a stressor gradient. All collection protocols were 6D 200 subsample that can be assessed using the freestone wadeable method.
Low stress: Mouth of Slate Run, a tributary to Pine Creek in Lycoming County. Slate Run at this location is a 4th order stream with a drainage area of 45 square miles, which is best represented by the large freestone IBI (Shull 2017). This sample (20130422-1300-smeans) was subsampled to 184 organisms, with no Prosimulium or Simulium present. The sample received a large freestone IBI score of 98.5.
Medium Stress: Deer Creek near Dutton Road just north (upstream) of the Maryland border in York County. Deer Creek at this location is a 3rd order stream with a drainage area of 11.8 square miles, which is best represented by the small freestone IBI (Shull 2017). This sample (20201118-1015-ablascovic) was subsampled to 202 organisms, with no Prosimulium or Simulium present. The sample received a small freestone IBI score of 49.5.
High Stress: Little Kishacoquillas Creek just upstream of the confluence with Kishacoquillas Creek in Mifflin County. L. Kish Creek at this location is a 4th order stream with a drainage area of 12.2 square miles, which is best represented by the small freestone IBI (Shull 2017). This sample (20201118-1015-ablascovic) was subsampled to 216 organisms, with 3 Simulium present. The sample received a small freestone IBI score of 31.2.
The stress classifications were based on the freestone IBI scores (Table 1), with supporting rationale derived from additional land use/land cover (Table 2), physical habitat (Table 3), and water chemistry data (Table 4).
Table 1. Freestone IBI and Subsample Size.| STR_STATION_ID | Stream | Stress_Category | Subsample Size | Freestone IBI Score |
|---|---|---|---|---|
| 20130422-1300-smeans | Slate Run | Low | 184 | 98.5 |
| 20201118-1015-ablascovic | Deer Creek | Mid | 202 | 49.5 |
| 20200512-1200-MifflinCCD | Little Kish Creek | High | 213 | 31.2 |
| STR_STATION_ID | Stream | Stress_Category | Cultivated | Developed | Forest | Drainage Area |
|---|---|---|---|---|---|---|
| 20130422-1300-smeans | Slate Run | Low | 0.36 % | 3.21 % | 93.43 % | 45.06 sq. mi. |
| 20201118-1015-ablascovic | Deer Creek | Mid | 47.79 % | 25.96 % | 25.05 % | 12.34 sq. mi. |
| 20200512-1200-MifflinCCD | Little Kish Creek | High | 50.61 % | 12.78 % | 33.44 % | 13.2 sq. mi. |
| STR_STATION_ID | Stream | Stress_Category | Embeddedness | Sediment Deposition | Condition of Banks | Bank Vegetative Protection | Total Score |
|---|---|---|---|---|---|---|---|
| 20130422-1300-smeans | Slate Run | Low | 20 | 17 | 18 | 17 | 221 |
| 20201118-1015-ablascovic | Deer Creek | Mid | 11 | 9 | 10 | 15 | 167 |
| 20200512-1200-MifflinCCD | Little Kish Creek | High | 11 | 11 | 15 | 16 | 159 |
| STR_STATION_ID | Stream | Stress_Category | ALKALINITY | CHLORIDE | Nitrate-N | PHOSPHORUS T | SPC @ 25.0 C |
|---|---|---|---|---|---|---|---|
| 20130422-1300-smeans | Slate Run | Low | 9.8 mg/l | 1.4 mg/l | < .04 mg/l | < .01 mg/l | 40.50 umhos/cm |
| 20201118-1015-ablascovic | Deer Creek | Mid | 43.2 mg/l | 44.19 mg/l | 4.63 mg/l | < .01 mg/l | 286.00 umhos/cm |
| 20200512-1200-MifflinCCD | Little Kish Creek | High | 125.8 mg/l | 9.0 mg/l | 4.03 mg/l | 0.058 mg/l | 322.00 umhos/cm |
For each sample, we rarefied the sample 10 times for each increment of 10 individuals, from 10 less than the subsample size down to < 10 individuals. For example, Slate Run had a subsample size of 184, we rarefied the sample in increments of 10, from 174 to 4, and replaced the remaining taxa list with either Prosimulium and Simulium. The objective of this part of the analysis was to determine the relative abundance of Simulidae taxa where the IBI score was affected to an extent equal to the temporal precision of 10 points for sites experiencing various levels of stress. We also examined the effect on each of the six individual metrics that comprise the freestone IBI score.
Statewide Temporal Dynamics
Percent Occurrence
The first measure of Prosimulium and Simulium proliferation was percent occurrence, meaning the number of samples where either taxon was present in a sample, divided by the total number of samples collected in that month.
Statewide, the patterns of percent occurrence varied for Prosimulium and Simulium. Prosimulium was most often present in samples collected between December and April. Prosimulium percent occurence was highest (>70%) in January - March. Alternatively, Simulium was most often present in samples collected between May - September, with percent occurrence peaking (>63%) in May and June (Figure 2).
Figure 2. Statewide Prosimulium and Simulium percent occurrence, by month. Grey bars in background indicate the number of samples collected per month. Blue bars in front represent the number of samples where either taxa was present. The labels above each blue bar indicates the percent occurrence for each month.
Relative Abundance
The second measure of Prosimulium and Simulium proliferation was relative abundance, which is the total number of either taxon in a sample, divided by the total subsample size, expressed as a percentage.
Statewide patterns of relative abundance were similar to patterns of percent occurrence, where relative abundance of Prosimulium was elevated in December - April, while Simulium was elevated in April - June (Figure 3).
Figure 3. Statewide Prosimulium and Simulium relative abundance, by month. Points in the background represent the relative abundance of each sample. Boxplots are superimposed to show median and upper/lower quartile values. Triangles represent the mean relative abundance for each month. 
The statewide trends are revealing, but overly broad to provide actionable insight into sampling timing considerations over the vast territory of Pennsylvania. The following section looks at temporal patterns by DEP region to speak to spatial considerations.
Regional Spatio-Temporal Dynamics
A simple proliferation index was created that was the product of percent occurrence and relative abundance data for Prosimulium and Simulium. This product was rescaled on a 0 - 100 scale, which results in a comparable measure of occurrence and abundance for both taxa across months and regions.
There are 6 DEP Regional Offices throughout the state. Each Regional Office represents corresponding land area, as seen in Figures 6 and 7. The abbreviations are as follows (from east to west):
- NERO - Northeast Regional Office
- SERO - Southeast Regional Office
- SCRO - Southcentral Regional Office
- NCRO - Northcentral Regional Office
- NWRO - Northwest Regional Office
- SWRO - Southwest Regional Office
Prosimulium
The Prosimulium proliferation index value was highest in February for NCRO and March for the remaining regions. The months of January - March were elevated compared to other months for all regions. Index values approached 0 for May - November for all regions (Figure 4).
Figure 4. Prosimulium mean relative abundance, percent occurrence, and proliferation index, by month and region 
Simulium
The Simulium proliferation index value was highest in June for SERO, SWRO, and NERO; May for NWRO and NCRO; and February for SCRO (Figure 5). Compared to Prosimulium, Index values were lower for all regions and months, meaning abundance and occurrence of Simulium is lower comparatively.
Figure 5. Simulium mean relative abundance, percent occurrence, and proliferation index, by month and region 
Mapping Regional Spatio-Temporal Dynamics
Maps were generated to demonstrate the spatial component that may not have been well described using the graphics above. The maps are animated and loop through each month in chronological order. County and regional boundaries are shown. The small grey points are samples where the taxa (either Prosimulium or Simulium) were absent. Points where either taxa were present are colored and sized according to the relative abundance of the taxa in the sample (legends are present on the plot to further describe coloration and size).
Prosimulium
Figure 6. Prosimulium occurrence and relative abundance shown in map format. Background layers include county and regional boundaries.
Simulium
Figure 7. Simulium occurrence and relative abundance shown in map format. Background layers include county and regional boundaries.
