A Natural History of Millbrook Marsh,
A Wetland In An Urbanizing Setting

Invertebrates are a key part to the wetland ecosystem as an invaluable part of the food web as well as their newer but more commonly known use as indicators of the health of aquatic systems. Much is known about the invertebrate species that are water quality indicators, but not nearly as much is known of others. It is probable, though, that small invertebrate consumers are even more important with regard to detritus decomposition in wetlands than are the large invertebrates, unlike the situation in terrestrial ecosystems (Bursey 1989). At the bottom of the food chain in the wetland, plant litter is colonized by microorganisms and this detritus is used by the first invertebrates, shredders or grazers, for example, amphipods and snails. The litter particles are reduced in size by their feeding activity and the smaller particles are then used by either collectors or filter feeders (chironomids for example), depending on their size. Invertebrates also feed on algae, usually abundant in wetland ecosystems, though algae is sometimes not as plentiful in vegetated systems (Cole, pers. comm.). They are one of the critical links in the food chain, prey for fish, waterfowl and other birds, amphibians, reptiles and mammals. They are a part of the nutrient cycling processes both directly and indirectly (Murkin and Wrubleski 1988).

Stream Macroinvertebrates

The macroinvertebrate communities in the stream reaches of Thompson Run and Slab Cabin Run were surveyed in July and October of 1997 (Brooks et al. 1998). Stations 1, 5, and 6 were located in Thompson Run and Stations 2, 3, and 4, in Slab Cabin Run. They revealed moderately depauperate macroinvertebrate fauna in both streams (Table 6-1). In Slab Cabin Run, 24 species were found in July and 31 species in October. The most common taxa were chironomids and hydropsychid caddisflies, and the average species richness was 17.2. In Thompson Run, 19 species were found in July and 15 in October. Midge larvae and blackfly larvae were most common and the average species richness was 11.0. The least disturbance in the macroinvertebrate community shown by RBP III was at the "Slightly Impaired" Station 2, compared to all other stations which were classified as "Moderately Impaired". The number of pollution intolerant taxa present in both streams, the mayflies, stoneflies, and caddisflies, was low at all stations, and the Hilsenhoff Biotic Index suggested that Station 2 of Slab Cabin Run is the only one not affected by organic pollution as are the other stations in Slab Cabin and Thompson Runs. Additionally, there were low values for scrapers/filterers at all but Station 4, indicating a high value for filtering collectors. This suggests a disproportionate amount of fine particulate organic matter. There was also an overabundance of pollution tolerant chironomids, shown by the EPT/Chironomidae Index. A large portion of the total macroinvertebrate community was made up by the most dominant taxon, which, in a healthy community, should contribute less than 20% (Brooks et al. 1998). All of this suggests a generally unbalanced community.

Table 6-1
1997 Millbrook Marsh Benthic Macroinvertebrates (Brooks et al. 1998)

Table 6-1 continued
1997 Millbrook Marsh Benthic Macroinvertebrates (Brooks et al. 1998)

Table 6-1 continued
1997 Millbrook Marsh Benthic Macroinvertebrates (Brooks et al. 1998)

The earliest of the historical data found for benthic macroinvertebrate communities was a survey conducted in 1978 (Miller 1979). Station 2 was on Thompson Run just below East College Avenue. Station 2 had a community characteristic of organic pollution. There were only 8 taxa, with high numbers of midges and snails and no stoneflies, mayflies or caddisflies. Station 1, also Thompson Run, was located upstream of Millbrook Marsh and had similar taxa to Station 2. This station was upstream of the sewage treatment plant discharge, and for this reason, the structure of the macroinvertebrate community, at Station 1 at least, can most likely be attributed to stormwater runoff. Station 4 was on Slab Cabin Run just above the Thompson Run confluence. There was good species diversity and a high number of pollution sensitive species. There were 19 taxa collected from Slab Cabin Run. Table 6-2 shows the benthic macroinvertebrates collected on July 11, 13 and 19, 1978 from Stations 1 and 2 of Thompson Run and Station 4 of Slab Cabin Run.

Table 6-2
1978 Millbrook Marsh Qualitative Benthic Macroinvertebrates
(Miller 1979)

Table 6-2 continued
1978 Millbrook Marsh Qualitative Benthic Macroinvertebrates
(Miller 1979)

P = present C = common A = abundant

The PennDOT macroinvertebrate surveys took place in May 1980. Some of the historical data collected for the Mt. Nittany Expressway Environmental Impact Statement included inventories by D. Reinhold of the Pennsylvania Fish Commission in 1959, and G. Miller of the Pennsylvania Department of Environmental Resources in 1978 (PennDOT 1981). Both collection sites, Reinhold’s 1959 Station 12, and Miller’s 1978 Station 27, were in the same general vicinity on Slab Cabin Run as PennDOT’s 1980 Station 3. In 1959, the invertebrates included 3 genera of stoneflies, mayflies of the genus Stenonema, and caddisflies, all pollution intolerant species (Reinhold 1959). The 1978 data included 19 species that were indicative of good stream conditions, though again, as with the fish inventory, they were warm water species rather than cold, as the stream is classified. As in 1959, there were pollution sensitive taxa present (Miller 1979). The May 1980 invertebrate collection also included 4 pollution sensitive taxa. The sample from station 3 with the highest species count and the highest diversity index of 2.63 had 22 species and was taken from a riffle with rock substrate.

Station 27 of 1978 was near the 1980 Station 4 and included 4 pollution sensitive taxa. The sample from Station 4 at the riffle had the highest diversity index of 3.89 (PennDOT 1981). Table 6-3 shows the taxa collected on May 14, 1980 from riffles and pools in Slab Cabin Run upstream of the Thompson Run confluence (PennDOT 1981).

Table 6-3
1980 Millbrook Marsh Benthic Macroinvertebrates (PennDOT 1981)

Table 6-3 continued
1980 Millbrook Marsh Benthic Macroinvertebrates (PennDOT 1981)