V. Measures of Population Density and Biodiversity

 

Sampling Methods

 

The macrobenthos of Foundry Cove were surveyed to determine whether the metals in Foundry Cove had changed the taxonomic composition of the benthic macrofauna. South Cove, located 1.5 miles downstream from Foundry Cove, served as a control site with respect to metal pollution. Gradients of sediment cadmium concentration from a few ppm to 10,000 ppm occured over only a few hundred meters in Foundry Cove; this provided an obvious gradient over which to search for faunal change.

One station in South Cove and 3 stations in Foundry Cove differing in metal concentration (500, 7000, and 52000 µg Cd per g dry sediment) were sampled. The highest concentration, 52000 µg Cd per g sediment, was found near the factory outfall pipe. Sampling was done at 2 month intervals; initially 4 replicate cores were taken at each station. Due to low animal abundances, replication was increased to 15 cores per station. A corer of 35 cm2 surface area was pushed 5 cm into the sediment and organisms retained on 500 µm mesh were preserved and identified.

 

Dominant Groups

 

The most abundant taxa were oligochaetes and larval chironomids. Densities of macrobenthic organisms (per m2) did not differ among stations within Foundry Cove or among Foundry Cove and South Cove. There is no evidence that the high metal levels in Foundry Cove reduced the number of individuals present. When one compares the number of species among coves, the station with the highest metal levels (outfall pipe station) showed a reduced taxonomic richness compared to all other stations (Klerks 1987, Klerks & Levinton 1992). Certain taxa, e.g. the oligochaete, Limnodrilus hoffmeisteri, and the chironomid, Tanypus neopunctipennis, as well as several nematode species, were more abundant at the outfall pipe station. This increase in these particular species may relate to an expansion in the absence of some other species that are eliminated at very high cadmium concentrations.

 

Although an obvious gradient of Cd concentration exists in Foundry Cove and independent evidence demonstrates that Cd is toxic, this does not correspond to an obvious gradient in faunal density or diversity, except at the highest concentration site near the outfall pipe, which is site FC3.

Figure 1. Number of soft bottom benthic taxa in South Cove (FC1); a Foundry Cove site with modest metal concentration (FC1), concentrations of tens of thousands of ppm (FC2); and the outfall site, where sediment concentrations may reach 25 percent cadmium! Date are from Klerks and Levinton 1989a.

Figure 2. Total counts of oligochaetes and chironomids in the same sampling sites. Date are from Klerks and Levinton 1989a.

 

The drop in species richness near the outfall may be due to ecological factors other than sediment Cd concentrations; the outfall pipe is located in a small cul de sac and ecological factors such as ice formation in winter months may be responsible for lowering diversity. To test this possibility, a site of comparable ecological characteristics was chosen and sampled in a manner identical to the outfall site. This control site had a Cd concentration of 10,200 µg/g compared to the outfall site concentration of 52,000 µg/g. The results matched those reported above. Density did not differ among these two sites, but species richness was lower in the outfall site (Klerks & Levinton 1992).

Figure 3. Comparison of population density (numbers per square meter) in the outfall site FC-3 and another site that is similar in area (FC -4).

Figure 4. Taxon richness for the same samples.

There was surprisingly no difference between diversity in a control area versus sites in the main part of Foundry Cove with Cd concentrations as high as 7000 µg/g. Some of the spatial homogeneity in density may have been built up over time owing to the evolution of resistance in certain taxa. In conclusion, general trends of diversity and abundance may not be sensitive indicators of pollution effects; other measures of pollution effects are discussed in Chapter VI.

 

After the Cleanup

What happened after the cleanup? We sampled Foundry Cove and estimated the total number of bottom-dwelling taxa from extensive sampling and we also took a look at the density of bottom organisms. We also sampled a cove north of Cold Spring, which we called North Cove. This was an attempt to have two coves that had not been polluted with cadmium for comparison with Foundry Cove. So here are the results for overall biodiversity:

We hope you can see that there are no obvious differences in overall species numbers before (1984) and after the cleanup. That is encouraging. Overall biodiversity has not been affected by the extensive dredging. What about abundance? We can show you some counts for oligochaete worms and nematodes:

You can see that we have a very different story here. Oligochaete population densities have dropped substantially since the cleanup. Also, larger nematodes have increased substantially in South Cove and North Cove, but remained low in Foundry Cove. We do not know why they increased so much in the two nonpolluted coves.

So, overall, the picture since the cleanup is complex. Overall biodiversity was not apparently altered by the major disruption that must have been caused by the dredging. But population abundances have changed both at the polluted site and in the non-polluted sites. This makes it difficult to judge whether the drop of oligochaete worm abundance in Foundry Cove was caused by the dredging or just resulted from other causes.