1J9_ 



A more effective method for evaluating diversity, especially for comparison between 

 different sample sizes, is the rarefaction curve approach (Sanders 1968). Rarefaction 

 curves plot the expected species for different population sample sizes. While these curves 

 cannot extrapolate beyond existing abundance data, they can interpolate species numbers 

 for smaller sample sizes and facilitate comparison of samples. These curves can also be 

 used to rank the relative ecological impact of dredged material deposition (Figure 3-22). If 

 disturbance is scaled to diversity, calculated by the rarefaction method, we would rank the 

 stations from high to low disturbance as follows: Center > 75E >150N >150W > 300WSW 

 >300SE. 



Similarly, the REMOTS® Organism-Sediment Index (OSI) can also be used to rank 

 stations. Unfortunately, the REMOTS® data from the stations sampled with the grabs was 

 limited by a variety of confounding factors. Many of the stations had indeterminate OSI 

 values, or the value was based on one replicate. Successional stage could not be determined 

 adequately at the center station (CTR) due to the presence of plastic glacial clay. The OSI 

 evaluation, however, does provide some insight into the relationship of disturbance to 

 benthic community structure across the mound. The lower OSI values (particularly <+6) 

 indicate greater impacts. Based on the OSI, the stations are ranked in the order of highest to 

 lowest impact: (No data on Station Center or 150 N) 150W > 300WSW > 300SE > 75E. 



The major difference in these three rankings is the relative position of Station 75E. 

 Based on the Shannon-Wiener H' statistic, and rarefaction curves, Station 75E was 

 comparable in disturbance to the CTR station. The OSI plot indicates that, while showing an 

 impact, Station 75E had a relatively high OSI (5.5). This result is attributed to one of the 

 station replicates showing the presence of Stage HI feeding voids at depth. Other than this 

 discrepancy, the overall station ranking was comparable between the rarefaction curves and 

 the OSI. 



Spatial trends in faunal composition (numbers of species, individual abundances, and 

 biomass) are related to organic enrichment gradients (in both space and time). These 

 qualitative relationships are shown in Figure 4-1. Similarly, organism-sediment 

 relationships, as measured by the REMOTS® OSI, also tend to change across zones of 

 organic/physical impacts (Figure 4-1). Based on faunal trends in declining species richness 

 (S), declining abundance (A) (Figure 4-2), OSI values between +4 and +5.5, and relatively 

 high H' values, the Seawolf Mound at the time of the 1997 survey appeared to fit into the 

 ecotonal transition in the SAB/OSI diagram (Figure 4-1). 



Station abundance ranged from a minimum of 1250 individuals per square meter at 

 Station CTR to a peak of ca. 28,000 per square meter at Station 300SE (Figure 4-2). These 

 abundances are far less (by one to two orders-of-magnitude) than is typically observed at the 



Monitoring Cruise at the New London Disposal Site, Seawolf Mound 1995 - 1998 



