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Aquatic Life

Urban Riparian Wetland Benthic Invertebrate Monitoring

2002-2004

Assessments of the benthic macro-invertebrate community are commonly used to evaluate stream health. Benthic macro-invertebrates are useful as habitat and water quality indicators because they are affected by multiple and synergistic effects of different pollutants, they are sensitive to both chemical and physical impacts to habitat, and they are less mobile than fish, and thus cannot avoid impacts. The benthic species composition and abundance provides an indication of water quality, because certain species are sensitive to pollutants, while others thrive under altered conditions. For more information on benthic invertebrates visit the Aquatic Life page.

Between 2002 and 2004, eight sites along the Bronx River were sampled by NRG. Three samples were taken at each site using a Hess sampler in each of the three years. All individuals were identified to the order level, almost all were identified to the family level, and many were identified to the genus and species level. The abundance of each taxon in each sample was determined and formed the raw data for our analysis.

A total of 82 distinct taxa were found (9935 individuals) over the three years of the NRG study. The most common taxonomic groups in the river as a whole include oligochaeta (38.4% of individuals), polychaeta (21.6%), chironomidae (14.9%), and amphipodes (14.1%) – groups that are all moderately to very tolerant of pollution. Amongst individuals for which feeding data was available (9428 individuals) the vast majority were found to be collector gatherers (79.6%) – a signal that there is abundant organic material within the river system.

The data was analyzed to determine the values of four commonly used water quality indices (Hilsenhoff biotic index, EPT richness, species richness, percent model affinity) and to determine percentage of individuals in various feeding guilds and in major taxonomic groups.

Methods

Criteria for calculating and calibrating water quality indices

The New York State Department of the Environmental Conservation (NYS DEC) has developed methods for standardizing the values of the above four indices onto a scale of 0-10 where 0-2.5 indicates a severely impacted waterway, 2.5-5 indicates a moderately impacted waterway, 5-7.5 indicates a slightly impacted waterway and 7.5-10 indicates a non-impacted waterway.

Although NYSDEC has developed ways of adapting index values calculated in multiple habitat types (riffle, etc.) and using different methods (Ponar, slide, kick-sampling, etc.) There are, as of yet, no NYSDEC standards available for Hess samplers used in mostly riffle habitats (the cases at all the non-estuarine sites). Comparing results from Hess and kick sampled data is extremely difficult because Hess samplers sample a small area intensively, while kick samples come from a larger area with more edge and thus are likely to sample across a great variety of microhabitats and thus to pick up more diversity. As a result we might expect kick samples to have greater diversity than Hess samples, even if more individuals are identified from the Hess sample. Fortunately NYS DEC had conducted two surveys (in 1998 & 2003) in the Woodlawn portion of the Bronx River so we were able to compare their results (based on a 100 sub sample of individuals from a 5m long transect) to our results (based on all the individuals in a Hess sampler). Because we had 2-3 for each year from Woodlawn, we were able to determine yearly values for all the indices based on summing all the samples and averaging all the samples. In addition, we determined the 3 – year average for all the indices. The following table shows the results of these analyses.

Agency & Year Number of Individuals Species Richness HBI EPT Richness Model Affinity
NYS Dec 1998 100
sub-sample
16 5.88 2 41
NYS Dec 2003 100
sub-sample
16 6.2 2 39
NRG 2002 (Avg.) 234 13 7.12 1 37
NRG 2002 (Sum) 468 16 7.11 1 38
NRG 2003 (Avg.) 23 6 6.49 0 39
NRG 2003 (Sum) 70 10 6.32 0 35
NRG 2004 (Avg.) 131 11.3 6.35 1.3 37.7
NRG 2004 (Sum) 392 17 6.43 3 36
3-Year Avg. 116 9.9 6.66 0.78 38

The first thing to note is that the samples from 2003 contained far fewer individuals than those from 2002 and 2004 and this has a huge impact on both the 2002 values and the 3-year average values for Woodlawn. Secondly, this table suggests that summing all the samples from 2002 and 2004 gives values of Species Richness and EPT Richness that are comparable to NYSDEC’s values; however both HBI and Model Affinity are reasonably similar irrespective of whether the yearly samples are averaged or summed. Based on these comparisons we decided to use the yearly sums from the years where we had sufficient sample size to determine values of EPT and Species Richness for each site and we decided to use the 3-year average to determine HBI and Model Affinity. These values were then compared to NYSDEC standards to determine the corresponding water quality index.

Although we are confident that these indices are useful in comparing sites and tracking changes over time, we stress that the comparison to state standards is somewhat problematic and future research using both methods at a few sites could prove fruitful. In addition EPT and PMA were not calculated for the estuary section of the river as they are not appropriate for this type of environment.

Monitoring Site Map

Click the image to view the PDF (376KB)

Bronx River Benthic Monitoring Site Map

Results

Taxonomic Composition Along the Bronx River

Woodlawn Shoelace Bronx Forest New York Botanical Gardens
Number of Samples 10 9 8 9
Avg. Number of Individuals per Sample 92.8 43.4 87 337.8
River Mile 7 6 4.9 4.1
% Chironomidae 44.00 28.00 45.90 18.50
% Oligochaeta 16.80 9.70 9.70 52.90
% Trichoptera 1.60 1.90 4.00 1.00
% Coleoptera 2.20 0.00 0.60 0.00
% Polychaetes 0.00 0.00 0.00 0.00
% Leaches 1.90 5.70 14.40 15.90
% Mollusca 9.00 36.00 12.00 3.90
% Amphipodes 13.50 12.90 12.20 6.20

Taxonomic Composition Along the Bronx River (con't)

Bronx Zoo West Farms Cement Plant Lafayette
Number of Samples 8 9 7 7
Avg. Number of Individuals per Sample 61.3 164.6 204 131.6
River Mile 3.7 2.6 1.7 1.2
% Chironomidae 15.80 10.20 1.20 0.30
% Oligochaeta 19.70 22.20 34.60 0.00
% Trichoptera 0.00 1.10 0.00 0.00
% Coleoptera 0.00 0.00 0.00 0.00
% Polychaetes 0.00 0.00 45.90 96.20
% Leaches 13.20 0.60 0.00 0.00
% Mollusca 23.50 17.10 0.70 0.00
% Amphipodes 20.10 42.70 0.10 0.30

The charts summarize how the taxonomic composition changes as the river flows towards it mouth. In the upper portions of the river, Chironomidae, Amphipodes, Mollusca, and Oligochaeta dominate the community, with leaches increasing in the forested parts of the river and Oligochaeta increasing drastically in the New York Botanical Gardens section of the Bronx River. Notice that in the estuarine sections of the river Polychaetes become an increasingly dominant part of the benthic community.

Community Composition Along the Bronx River

Changes in Community Composition Riverwide

Given the shifts in the taxonomic groups as one travels downstream along the Bronx River, it is not surprising to note that the breakdown of feeding guilds also changes dramatically at locations closer to the mouth of the river. As this figure indicates the community is increasingly dominated by collector-gatherers at locations closer to the mouth of the Bronx River.

Water Quality, As Measured by Benthic Invertebrates, Along the Bronx River

Changes in Community Composition Riverwide

In the figure above the values of the four water quality indices (SPP-Species Richness; HBI-Hilsenhoff biotic index; EPT-EPT richness; PMA-percent model affinity) are plotted for each of the 8 sites along the river. The line connects the averages at each site. The average value of the water quality indices indicates that most of the Bronx River is moderately impacted, with the Botanical Gardens site classified as severely impacted.

Values of Water Quality Indicators at the Parkland Section Sampling Sites

Values of Water Quality Indicators at the Parkland Section Sampling Sites

Given the shifts in the taxonomic groups as one travels downstream along the Bronx River, it is not surprising to note that the breakdown of feeding guilds also changes dramatically at locations closer to the mouth of the river. As this figure indicates the community is increasingly dominated by collector-gatherers at locations closer to the mouth of the Bronx River.

Values of Water Quality Indicators at the Botanical Garden/Zoo Section Sampling Sites

Values of Water Quality Indicators at the Parkland Section Sampling Sites

Between 2002-2004 NRG monitored two sites within the NYBG/ZOO section of the river – one within the Bronx Zoo portion of the section, the other in the NYBG section of the river. The water quality values for Bronx Zoo are similar to values found elsewhere in the river, although there was a complete absence of caddisflies at this site (EPT = 0). The NYBG site, on the other hand, was found to have the lowest values of HBI amongst all 8 sites and the lowest PMA and species richness values of the 6 predominantly freshwater sites that were sampled. In addition this site was found to be dominated by oligochaeta (52.9% of individuals) suggesting that the site is probably impacted by severe pollution and/or very low oxygen levels.

Values of Water Quality Indicators at the West Farms Section Sampling Site

Values of Water Quality Indicators at the Parkland Section Sampling Sites

Samples taken from the West Farms section of the Bronx River in 2002 were highly dominated by polychaetes (97% of all individuals) from the family tubificidae (Non Cap. Setae) and contained no freshwater shrimps. In 2003 and 2004, however, the percentage of individuals found to be freshwater shrimp (Gammarus spp.) increased drastically to 44.3% and 70.2% respectively, while the no polychaetes were found in either year. These trends taken together with observed increases in species richness over the three years indicate a shift towards less tolerant species. These samples represented poor values and suggest that the community was either recovering from a disturbance or is highly unstable.

Values of Water Quality Indicators at the Estuary Section Sampling Sites

Values of Water Quality Indicators at the Parkland Section Sampling Sites

Two sites were sampled within the Estuary section of the Bronx River during 2002-2004. As with plants and fish, there are different communities of macroinvertebrates in salt and fresh water. Because of the differences in communities, two of the benthic invertebrate indices, PMA and EPT, are inappropriate for estuarine sections and were not applied to these sections. In addition, the standards for the other two indices should be adjusted for estuarine sections. Unfortunately, because no directly comparable standards are available for the sampling method used by NRG, data was compared to freshwater standards. Based on differences in standards for multiple plate (Ponar) sampling, which can be used in both estuarine and freshwater habitats, it appears that the water quality is probably slightly better at the two estuarine sections than is suggested in the below figure. Within the Estuarine section the vast majority of individuals (99.1%) were found to be collector-gatherers, and approximately 90% were found to be some form of annelid worm (oligochaeta or polychaetes).

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