1 

 Quantification of Injury to Benthic Resources from the Chalk Point Oil Spill on the 

Patuxent River 

by 

Charles H. Peterson, PhD 

University of North Carolina at Chapel Hill 

Institute of Marine Sciences 

Morehead City, NC 28557 

cpeters@email.unc.edu

5 March 2002 

Introduction 

This report presents the quantification of benthic resource injury in the subtidal 

soft-bottom habitat following the Chalk Point oil spill of 7 April 2000. About 126,000 

gallons of No. 2 and No. 6 fuel oil were released into upper Swanson Creek, which then 

spread into lower Swanson Creek and the Patuxent River. Quantification estimates are 

made possible by availability of results from (1) quantitative sampling in June 2000 and 

again in September 2000 in a Versar report by Llanso and Volstad (2001); (2) 

quantitative sampling in September 2000 in an Academy of Natural Sciences report by 

Osman (2001); (3) long-term multi-year sampling data from the Maryland Chesapeake 

Bay Long-term Benthic Monitoring Program (LTB); and (4) a growing body of scientific 

understanding of the effects of oil on marine benthic invertebrates. This published 

literature on impacts of petroleum hydrocarbons on benthic invertebrates was used to 

interpret the empirical field data and develop the conceptual and mechanistic basis for the 

injury quantification. By evaluating injury on a species-by-species basis and matching 

empirically demonstrated patterns of change temporally and spatially related to the spill 

to mechanistic understanding of known sensitivity to contaminants in those species, this 

quantification procedure yields the most scientifically defensible identification and 

quantification of injuries (Peterson 2001). Any alternative based on risk assessment 

modeling is characterized by much higher uncertainty and is biased towards 

underestimating injury for many reasons (Peterson 2001). 

Methods 

 The approach taken to developing the injury quantification for subtidal benthic 

resources was to review the data contrasts and statistical analyses conducted in Llanso 

and Volstad (2001) and Osman (2001). Specifically the evidence for and against spill 

impacts to the soft-bottom benthos was assembled and organized by geographic area and 

time frame. The results of these statistical analyses, along with tables of data on average 

densities, were then used to identify those species or higher taxonomic groups that 

demonstrated responses, positive or negative, to the spill and the geographic extent and 

temporal duration of the responses. The biomass contrasts for each of those affected 

species or taxa were then used to estimate the magnitude of the lost production per unit 
 

2 

area (m-2), the area of the geographic region of each impact was then calculated by GIS 

based on the shoreline in the SCAT report (Entrix 2000), and the product of these latter 

two factors was computed to estimate the total biomass change induced by the oil spill at 

that sampling date for each affected taxon. Finally, an estimate of the biomass 

production lost in 2000 from the spill was made based on inferences from the density and 

biomass changes over the season and literature knowledge of the life histories of the 

affected species to integrate the production losses over the entire warm season of 2000. 

Projections of recovery and duration of injury in subsequent years (2001 and 2002) were 

made by examining the degree of convergence of oiled and unoiled areas in 2000 and by 

using the scientific literature on recovery rate of analogous taxonomic groups from past 

oil spills. 

Results of Injury Demonstration 

Patuxent River 

 There is little evidence that benthic impacts from the Chalk Point oil spill 

extended into the Patuxent River. The most rigorous and probably the statistically most 

powerful analysis on which this conclusion rests is the failure of a BACI (Before After 

Control Impact: Stewart-Oaten et al. 1986) analysis on total abundance of benthic 

organisms to reveal a statistically significant response to the spill or the data even to 

suggest a possible decline following the spill (Llanso and Volstad 2001: Fig. 8). This 

BACI analysis utilized an impact station in the Patuxent River near Chalk Point and 

compared it to an environmentally analogous control river site in the Chester River for 

which biannual sampling data extended back for 10 years prior to the spill (until 1990). 

The failure of this analysis to detect a spill response is not a problem of insufficient 

power but rather reflects a true lack of response in total benthic abundance. 

 The computations of the Benthic Index of Biotic Integrity (B-IBI) as a measure of 

the biotic health of the benthos in the Patuxent River also failed to exhibit any evidence 

of the Chalk Point oil spill extending into the River itself. The LTB monitoring stations 

sampled in the Patuxent River in September 2000 exhibited a pattern of benthic 

community degradation only well downstream of Chalk Point, typical of impacts of 

oxygen stress where waters are deeper and more strongly stratified, not evidence of an oil 

spill response (Llanso and Volstad 2001: Fig. 3). 

 The only suggestions of a potential impact of the oil spill on benthos in the 

Patuxent River come from examination and analysis of the long-term monitoring data in 

the Patuxent mainstem. A contrast of total abundance and biomass both show 

significantly lower values in June samples from 2000 after the spill than in the earlier 

1990-1993 period (Llanso and Volstad 2001: Fig. 4). The bivalve Rangia cuneata

appeared to explain most of the difference in biomass between June 2000 and earlier 

years. Shannon diversity and numbers of species failed to indicate any significant 

difference between the June 2000 and 1990-1993 years (Llanso and Volstad 2001: Fig. 

4). Such differences among years cannot be unambiguously interpreted and cannot be 

used in isolation to infer impacts of the oil spill, when the BACI test that controls for 
 

3 

inter-annual differences failed to reveal even a suggestion of an impact. Rangia cuneata

populations are known to fluctuate greatly among years in response to its sensitivity to 

low temperatures and its reliance on higher salinity for successful recruitment (Hopkins 

1973). 

 Sampling of the mainstem of the Patuxent River in September 2000 produced 

estimates of the Shannon species diversity, number of species, total benthic abundance, 

and biomass that did not differ significantly from the 1995-1999 summer samples (Llanso 

and Volstad 2001: Fig. 5). Consequently, this subsequent more intensive sampling of the 

river does not suggest any oil spill impact that is detectable in September 2000. Given 

that the impact to Rangia cuneata in upper Swanson Creek did appear to persist until at 

least the September sampling there (see below), this result further supports the 

interpretation that no oil spill impact appeared in the Patuxent River mainstem, even to 

Rangia cuneata. Nevertheless, it is possible that the oil spill effect on Rangia cuneata

extended out of Swanson Creek and into the Patuxent River. The failure of the BACI test 

to exhibit an impact of the oil spill on total abundance of benthic invertebrates does not 

preclude the possibility of one of the component species showing an effect. Uncertainty 

exists about the spatial scope of the reduction in Rangia cuneata abundance following the 

Chalk Point oil spill. 

Swanson Creek 

 There is strong evidence that the Chalk Point oil spill induced impacts to the 

subtidal benthic community and several large changes in several component taxa of the 

soft-bottom benthic community within Swanson Creek. Most of the responses were 

apparently limited to the upper Swanson Creek, but the most sensitive species among the 

dominant members of the community exhibited injury in lower Swanson Creek also. The 

injuries were evidenced in the June 2000 samples and recovery had only begun to occur 

by the September 2000 samplings. The empirical sampling evidence of injury is derived 

by first comparing upper Swanson Creek and lower Swanson Creek to Hunting Creek as 

a control. Where lower Swanson failed to demonstrate evidence of a response to the oil 

spill for a given taxon, then for that species or taxon, lower Swanson Creek and Hunting 

Creek were both used as control sites against which to judge the magnitude of the spillrelated difference. The average biomass values were weighted equally in making such 

contrasts that used two control sites. 

The Hunting Creek control is suitably located in geographic proximity about the 

same distance along the Patuxent and of similar size and area (Llanos and Volstad 2001: 

Fig. 3). Furthermore, sampling stations were positioned in similar habitat, allowing for 

justifiable contrasts of benthic populations and communities. Habitat information 

collected during sampling (Llanso and Volstad 2001: Appendices) shows similar mean 

station depths (1.0 m for upper Swanson, 1.1 m for lower Swanson, and 1.2 m for 

Hunting Creeks), silt/clay contents of sediments (84.9 % for upper Swanson, 90.5 % for 

lower Swanson, and 84.8 % for Hunting Creeks), and salinities (4.3 ppt for upper 

Swanson, 5.5 ppt for lower Swanson, and 6.0 ppt for Hunting Creeks). These similarities 

in salinity and sedimentology represent compelling grounds to consider the three 
 

4 

geographic areas comparable for the purposes of inferring impacts of the oil spill. 

Because other factors also contribute to differences in benthic community composition 

and abundances of component taxa, however, some level of uncertainty persists in even 

the most rigorous method of inference of impact from spatial contrasts in the absence of 

information from before the spill. However, evidence of partial to complete convergence 

over time in the impacted resources at the three sites, as exists for this spill in the benthic 

data in the Llanso and Volstad (2001) report (see below), provides confirmation that the 

other unmeasured environmental variables do not seriously confound the ability to use 

spatial contrasts to infer injury. 

 The statistical tests conducted by Llanso and Volstad (2001) on various 

community parameters for the soft-bottom benthos provide the basis for a conclusion that 

the oil spill exerted an impact on upper Swanson Creek primarily and that the impact was 

reduced but still detectable in September 2000. In June 2000, upper Swanson Creek had 

significantly lower Shannon diversity and significantly higher species richness and total 

benthic abundance than lower Swanson Creek or the Hunting Creek control (Llanso and 

Volstad 2001: Fig. 9). These differences can be largely explained by the enhanced 

dominance by the spionid Streblospio benedicti in upper Swanson Creek and the 

enhanced numbers of polychaete species in that site. In September 2000, upper Swanson 

Creek exhibited significantly lower Shannon diversity and significantly lower total 

biomass than lower Swanson Creek or Hunting Creek (Llanso and Volstad 2001: Fig. 

11). Shifts in relative abundance of taxonomic contribution to the communities were also 

evident, with upper Swanson Creek possessing relatively more polychaetes and fewer 

bivalves and crustaceans than lower Swanson or Hunting Creeks (Llanso and Volstad 

2001: Figs. 12, 13). This pattern existed in both June 2000 and September 2000, 

although its intensity had decreased by September. Calculation of the index of 

community health (B-IBI) for the benthos from the September samples confirmed that 

upper Swanson Creek was still degraded at the end of the warm season relative to lower 

Swanson Creek and Hunting Creek. In upper Swanson Creek, only 2 of 10 sampling 

sites met benthic community restoration goals, while 8 were degraded or severely 

degraded (Llanso and Volstad 2001: Fig. 14). In contrast, 8 of the 10 sites in lower 

Swanson Creek met restoration goals and 6 of 9 (with the remaining one marginally 

degraded) sampling sites in Hunting Creek met restoration goals (Llanso and Volstad 

2001: Fig. 14). 

 The species of high biomass and production in Swanson Creek that exhibited 

detectable changes in abundance and biomass are two bivalve molluscs, Macoma 

balthica and Rangia cuneata, the amphipod Leptocheirus plumulosus, and a group of 

largely spionid polychaetes (Table 1). The oligochaete Tubificoides spp. also exhibited 

differences in abundances among creek sites in June 2000, but this species did not 

represent one of the biomass dominants in the system (Llanso and Volstad 2001). 

Macoma balthica exhibited a pattern of dramatically lower density in upper Swanson 

Creek than in the control areas of lower Swanson Creek and Hunting Creek during both 

the June and September 2000 samplings (Table 1). Macoma balthica is a species with 

known sensitivity to mortality from oil exposure (Shaw et al. 1976), so the experimental 

support exists in the literature for the conclusion from the Llanso and Vostad (2001) 
 

5 

sampling data that the oil spill decreased the abundance of this species in upper Swanson 

Creek through acute mortality. Rangia cuneata exhibited a similar but less strong pattern 

of lower density in upper Swanson Creek than in lower Swanson Creek or in Hunting 

Creek during the Llanso and Volstad (2001) June sampling (Table 1). By September, the 

Rangia abundance differences had narrowed considerably but only because of dramatic 

reductions in abundance in the control creeks, not from any increase or recovery in upper 

Swanson Creek (Table 1). Both Macoma balthica and Rangia cuneata are classified as 

pollution sensitive species in the Chesapeake Bay B-IBI (Weisburg et al. 1997). For both 

Macoma balthica and Rangia cuneata, the densities in lower Swanson Creek were 

actually intermediate between upper Swanson and Hunting Creek (Table 1), raising the 

possibility that the oil impact also affected these bivalves in lower Swanson also but to a 

lesser degree than in upper Swanson. One interpretation of the density patterns in Rangia 

cuneata after the oil spill is that effects did indeed extend not only to lower Swanson 

Creek but also into the Patuxent River, where analogous reductions in density were 

suggested by the sampling data in Llanso and Volstad (2001). This interpretation cannot 

be fully discounted, but the typically high variability of Rangia cuneata abundance 

(Hopkins 1973) renders the support for such a conclusion relatively weak. 

 The burrowing amphipod Leptocheirus plumulosus exhibited complete absence 

from upper Swanson Creek and an almost 98 % lower density in lower Swanson Creek 

than in the control Hunting Creek in June 2000 (Table 1) using data in Llanso and 

Volstad (2001). By September 2000, Leptocheirus had not returned to upper Swanson 

Creek, but the densities in lower Swanson Creek and the Hunting Creek control had 

converged through apparent increases in abundance in lower Swanson Creek and a 

dramatic decline in abundance in Hunting Creek (Table 1). Sampling in late September 

2000 of the heavily oiled site in Swanson Creek and in the control Hunting Creek by 

Osman (2001) confirmed the absence of Leptocheirus in Swanson and its presence in 

Hunting Creek. Amphipods are typically among the most sensitive taxa to oil pollution, 

with dramatic, often long-term (typically multi-year) declines documented in 

experimental studies and in impact assessments (Bonsdorff and Nelson 1981, Elmgren et 

al. 1983, Dauvin 1987, Kingston et al. 1995, Jewett et al. 1999). Because of their known 

sensitivity to contaminants, amphipods including Leptocheirus plumulosus are commonly 

used in laboratory tests to assess the toxicity of sediments (Schlekat et al. 1992, U.S. EPA 

1994). In addition, field and laboratory studies with Leptocheirus plumulosus indicate 

that it is indeed a good indicator of sediment contamination (McGee et al. 1999). 

Consequently, there is strong mechanistic support in the literature for the conclusions that 

the abundant amphipod Leptocheirus plumulosus suffered mortality from the Chalk Point 

oil spill, that this impact extended over both upper and lower Swanson Creek in June 

2000, but that only upper Swanson was still affected in September 2000 (Table 1). No 

field sampling was conducted in Swanson Creek after September 2000, so the duration of 

injury to amphipods can only be inferred from the literature of past impacts to sensitive 

amphipods from oil spills. Dauvin (1987) demonstrated impacts of the Amoco Cadiz oil 

spill to peracarid amphipods that lasted for nearly a decade, while Jewett et al. (1999) 

showed that isaeid and phoxocephalid amphipods had not fully recovered 6 years after 

the Exxon Valdez oil spill. Consequently, impacts on Leptocheirus plumulosus in upper 

Swanson Creek probably extended into subsequent years. In the absence of empirical 
 

6 

field data beyond September 2000 and given the impossibility of measuring future 

change before it has happened, a reasonable estimate for duration of impact to this highly 

sensitive species that had not initiated recovery in upper Swanson Creek by September 

2000 extends the impact through both generations in 2001 and the first brood of spring 

2002. This expectation represents a balance between assuming immediate recovery in 

upper Swanson Creek in 2001, for which there is no evidence, and assuming that impacts 

on amphipod production stretched out for 6 or more years, as evidenced by other oil spills 

(Dauvin 1987, Jewett et al. 1999). 

 In contrast to the two bivalve molluscs and the aorid amphipod, polychaetes, 

driven almost exclusively by a guild of spionids, exhibited much higher densities in the 

upper Swanson Creek than in lower Swanson Creek or in Hunting Creek (Table 1), using 

the June 2000 sampling results reported in Llanso and Volstad (2001). The species 

involved in this contrast were primarily the spionids Streblospio benedicti and Hobsonia 

florida, and a capitellid, Heteromastus filiformis (Llanso and Volstad 2001). By 

September 2000, dramatic reductions in abundance of polychaetes had occurred at all 

three creek areas, creating near or complete convergence between the most heavily oiled 

upper Swanson Creek and its controls of lower Swanson Creek and Hunting Creek (Table 

1). The enhancement of certain sedentary polychaetes like capitellids and spionids that 

can feed on surface organic particles is consistent with a high incidence of opportunism 

in that phylum, which typically exhibits positive responses to organic enrichment, even 

driven by petroleum hydrocarbons (Pearson and Rosenberg 1978, Gray 1982, Conan 

1982, Swartz et al. 1986, Warwick and Clarke 1993, Peterson et al. 1996, Jewett et al. 

1996, Peterson 2001). Consequently, there is good empirical literature support and 

mechanistic understanding to reach a conclusion that this difference in polychaete 

abundance between upper Swanson Creek and the two controls, lower Swanson Creek 

and Hunting Creek, represents evidence of a density enhancement by the spill. This 

enhancement was evident in Llanso and Volstad’s (2001) June 2000 sampling data but 

had largely disappeared by their September 2000 sampling (Table 1). The mechanism of 

enhancement probably involves organic enrichment from the hydrocarbons and oildegrading microbes, but it may also reflect initiation of succession after removal of the 

bivalve biomass dominants by the oil toxicity. 

 

Results of Injury Quantification 

The only compelling evidence of injury to subtidal benthic resources comes from 

Swanson Creek. The injury is most appropriately quantified by using biomass rather than 

numerical density because biomass represents a better standard that can be used to 

estimate lost production. This creates a standard unit that is comparable across species 

and taxa, of special importance if restoration of damages cannot be done precisely in 

kind. In addition, biomass production better reflects the value of a taxon as energy for 

transfer to higher trophic levels and as a biogeochemical processor of materials within the 

ecosystem. These are the two ecosystem services of greatest concern in any 

compensation or restoration. 
 

7 

 The average biomass per m2 of each major taxon can be computed for upper 

Swanson Creek, lower Swanson Creek, and the Hunting Creek from sample data 

provided in Llanso and Volstad (2001: Appendices). Table 2 provides these means for 

the June 2000 and Table 3 for the September 2000 samplings in units of Ash Free Dry 

Weight (AFDW). Assuming that the bivalve injury was restricted to upper Swanson 

Creek and that the somewhat lower June 2000 densities of both Macoma balthica and 

Rangia cuneata in lower Swanson Creek than in Hunting Creek (Table 1) are not 

reflective of a significant decline in lower Swanson Creek, then the bivalve biomass lost 

from the spill is 1.14 g m-2 as estimated from June data or 2.73 g m-2 as estimated from 

the September data. There is no evidence in the numerical density data (Table 1) of 

intense recruitment of any of the injured bivalves during the 2000 summer period, so the 

increase in the magnitude of the biomass difference largely reflects growth and 

production foregone during summer. Thus, this higher (September 2000) estimate of 

biomass injury to bivalves is a more complete one. Because growth slows dramatically 

as water cools in the fall (Holland et al. 1987), and because Macoma balthica is largely 

an annual species with strong year classes living little more than a year (Holland et al. 

1987), the difference in biomass at the end of the warm season in September represents a 

reasonable estimate of total production lost from the oil spill during 2000. While Rangia

has the potential to live 5-6 years (Tenore et al. 1968), the Rangia abundance in Hunting 

Creek in September dropped to approximately 10% of the June numbers. Thus for the 

year 2000, treating Rangia as an annual represents a reasonable estimate of the injury. 

To calculate the injury, the loss of 2.73 g m-2 is then multiplied by the area affected 

(about 708,000 m2

 of upper Swanson Creek) to yield the total bivalve biomass production 

lost in 2000, 1,932.8 kg (Table 4). 

 The total biomass production lost by the amphipod Leptocheirus plumulosus

requires two separate calculations, one for June when injury extended from upper 

Swanson Creek through lower Swanson Creek, and a second for September, when only 

upper Swanson Creek remained impacted (Table 1). This species produces multiple 

broods per year and reproduction is continuous from May to November, with peaks of 

reproduction and population growth in spring and fall (Spencer and McGee 2001). 

Hence, an estimation of injury that sums the biomass differences documented in June and 

September represents the best estimate of Leptocheirus biomass production lost in a 

single year. In June 2000, the lost amphipod production was 0.1067 g m-2 in upper 

Swanson Creek and 0.1024 g m-2 in lower Swanson Creek (Table 4). Lower Swanson 

Creek has an area of about 1,320,000 m2. Consequently, the total biomass production 

lost from the spring population peak is the sum of the products of loss per unit area and 

total area for each of the two segments of the creek, or 75.5 kg for upper Swanson Creek 

and 135.2 kg for lower Swanson Creek (Table 4). The September 2000 injury, 

presumably to the second population peak, only appeared in upper Swanson Creek and 

amounted to 42.6 kg of biomass production lost (Table 4). Thus the total Leptocheirus

amphipod production lost from the oil spill in 2000 was 253.3 kg (Table 4). Although 

Leptocheirus plumulosus injury may have extended for many years, in the absence of 

multiple years of sampling, we assume that only three additional broods were affected, 

those of spring 2001, late summer 2001, and spring 2002. We also assume that only 

heavily oiled upper Swanson Creek continued to exhibit this injury. Thus we add another 
 

8 

75.5 + 42.6 + 75.5 kg of injury (totaling an additional 193.6 kg) corresponding to loss of 

the spring 2001, summer 2001, and spring 2002 broods from upper Swanson Creek, 

followed by complete recovery for all successive generations and years (Table 5). Thus, 

the total lost production estimated for Leptocheirus plumulosus is 446.9 kg. If more than 

two broods of Leptocheirus were affected annually or if impacts continued for longer 

than spring 2002, then this estimate of production lost is an underestimate. 

 The enhancement of production of polychaetes, largely spionids but also a 

capitellid, should be considered as partial mitigation for the loss of bivalve and amphipod 

production. These small polychaetes, like the injured bivalves Macoma balthica and 

Rangia cuneata and the injured amphipod Leptocheirus plumulosus, to some degree serve 

a role as prey for higher trophic level consumers in the system. In specific, hogchokers, 

mummichogs, and striped killifish are likely consumers of these small polychaetes 

(Homer and Boynton 1978). The bivalve molluscs of this system are prey for generally 

larger predators, importantly, blue crabs, white catfish, white perch, yellow perch, and 

overwintering ducks like canvasbacks (Lovvorn 1987, Hines et al. 1990). The 

amphipods are likely prey for all of the fishes that consume either polychaetes or bivalves 

(Hines et al. 1990). Because of their greater longevity and their greater capacity to filter 

water, the bivalves Rangia cuneata (a suspension feeder) and Macoma balthica (a 

facultative suspension/deposit feeder) probably serve a more important biogeochemical 

function in protecting water quality of the system than the polychaetes, implying that the 

biomass credit for enhanced polychaete production should not be credited against lost 

bivalve production on a one-to-one basis. Similarly, substantially more of the amphipods 

produced are expected to be preyed upon by higher trophic levels than the opportunistic 

polychaetes because amphipods and other small crustaceans are highly preferred fish 

foods and these sessile, opportunistic polychaetes typically die from food limitation and 

decompose in the sediments in large numbers (Marsh and Tenore (1990). Thus, 

production of opportunistic polychaetes also should not be credited against amphipod 

production on a one-to-one basis. The biomass enhancement of polychaetes was greatest 

in June 2000 (Table 1), when it represented 0.349 g m-2. Totaled over the affected area of 

upper Swanson Creek, the oil spill resulted in 247.1 kg of increased polychaete 

production (Table 4). The enhancement in abundance was no longer evident in 

September (Table 1), implying that only one brood was likely affected. The decline in 

abundance of polychaetes in upper Swanson Creek between the June and September 

samplings (Table 1) to the point of convergence with abundance in the control Hunting 

Creek probably represents evidence of the combined effects of starvation because of 

limited food resources (Holland et al. 1987, Marsh and Tenore 1990) and predation 

(Holland et al. 1980), implying a partial trophic transfer for this unnaturally high level of 

polychaete production. Holland et al. (1980) showed that predation during spring and 

summer on Streblospio benedicti and Macoma balthica explained much of the sharp 

seasonal decline of these two species in muddy-sand habitats of Chesapeake Bay. Other 

studies (Holland et al. 1987, Marsh and Tenore 1990) imply that food limitation is the 

cause of the summer crash in abundances of opportunistic polychaetes in estuarine 

systems, including the Chesapeake Bay. A full credit for the enhanced production of 

polychaetes of 247.1 kg is not warranted because of the high likelhood that a substantial 

fraction of this production of opportunists suffered food limitation, died, and 
 

9 

decomposed. A credit of 50% might account fairly for the existence of two credible and 

well-supported alternative fates for the opportunistic polychaetes. That would reduce the 

total AFDW injury of subtidal benthos by 123.6 kg (Table 5). 

 The net loss of production by subtidal benthic invertebrates from the April 200 

Chalk Point oil spill thus involves summing the losses to each taxon by year and then 

applying partial credit for the enhancement of opportunistic polychaetes (Table 5). In 

2000, lost bivalve production was 1,932.8 kg, and lost Leptocheirus plumulosus

production was 253.3 kg. The additional losses of Leptocheirus plumulosus production 

are projected to be another 118.1 kg in 2001 and 75.5 kg in 2002, totaling an additional 

193.6 kg. Thus the total injury to amphipods, not discounted by year of occurrence, was 

estimated to be 446.9 kg. Giving a 50% credit for enhancement of production by 

opportunistic polychaetes reduces overall injury by 123.6 kg. Consequently, the 

undiscounted sum of all injuries and credits to the subtidal benthos is 2,256.1 kg of 

AFDW. Because these injuries occur in different years, discounting is required to sum 

them. Assuming that restoration is initiated in 2002, we apply discounting accordingly to 

express these injuries to subtidal benthic invertebrates in 2002, using the standard 

discount rate of 3% annually. This requires application of two years of discounting 

(multiplication by 1.03) for injuries in 2000, one year of discounting for injuries in 2001, 

and no discounting for injuries expected in 2002. This set of calculations (Table 5) yields 

a net injury of 2,385.3 kg of AFDW discounted to year 2002. For every year that passes 

beyond 2002 before restoration is initiated, this number requires further adjustment by 

multiplication by the annual discount rate of 3%. 

The estimation of benthic injury identified in this report is a conservative 

estimate, for three reasons. First, the calculations assume that the lower densities of both 

Macoma balthica and Rangia cuneata in lower Swanson Creek compared to the control 

Hunting Creek observed in June 2000 (Table 1) do not indicate bivalve injury extending 

into lower Swanson Creek. If this does indicate a true bivalve injury in lower Swanson 

Creek, then the bivalve injury from the spill would be enhanced by 324.1 kg of additional 

production lost. This figure comes from the same method used in Table 4, that produces 

an estimated difference in biomass density between lower Swanson Creek and Hunting 

Creek in June 2000 of 0.246 g m-2 or 324.1 kg total summing over the whole area of 

lower Swanson Creek. Furthermore, the lower density of Rangia cuneata in the Patuxent 

River near the convergence with Swanson Creek is also assumed to be the consequence 

of natural variation and not an impact of the oil spill. If injury to Rangia cuneata actually 

extended into the Patuxent River, then our estimate fails to include it. Second, the 

calculations of benthic production lost from the oil spill in Table 4 make the assumption 

that only 2 cohorts in 2000, 2 cohorts in 2001, and 1 cohort in 2002 of Leptocheirus 

plumulosus were affected and one cohort in 2000 of Macoma balthica. Because 

Leptocheirus plumulosus achieves multiple broods during the summer May-to-September 

warm season with a peak in the October/November period (Spencer and McGee 2001), 

the estimates in Table 4 are conservative. Similarly, Macoma balthica produces a new 

cohort each year in the fall-to-spring time period (Holland et al. 1987), so its next cohort 

too may have suffered an impact if there was elevated residual contamination. Third, all 

the calculations of biomass production lost (or gained) as impacts of the Chalk Point oil 
 

10 

spill apply to the 2000 year, with effects through spring 2002 limited to amphipods. No 

fall 2000 or any 2001 sampling data exist to assess empirically whether impacts persisted 

so as to influence production in subsequent years. The most likely of the impacts to 

persist beyond spring 2002 would be those to the amphipod Leptocheirus plumulosus, 

arguing from its evident greater sensitivity (it alone unambiguously revealed an impact 

that extended throughout Swanson Creek: Table 1) and from the multi-year duration of 

impacts to other amphipods in other oil spills (e.g., Dauvin 1987, Jewett et al. 1999). 

Leptocheirus plumulosus had apparently recovered in lower Swanson Creek by 

September 2000, but when recovery in the more directly oiled upper Swanson Creek 

might be predicted to occur is uncertain. Absent sampling in summer 2001, assuming 

recovery of Leptocheirus plumulosus in upper Swanson Creek after the first brood of 

spring 2002 on the basis of the recovery dynamics from June to September in lower 

Swanson Creek and on the basis of prediction from multi-year recovery periods for other 

amphipods following oil spills is accompanied by some uncertainty. As species 

somewhat less sensitive to contaminants, the two bivalves Macoma balthica and Rangia 

cuneata can be predicted to recover quickly enough that injury will not also recur in 

2001. 

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des sables fins de la Pierre Noire (Baie de Morlaix, Manche occidentale) apres la 

catastrophy de l’Amoco Cadiz. Mar. Envir. Res. 21: 247-273.

Elmgren, R., S. Hansson, U. Larsson, B. Sundelin, and P.D. Boehm. 1983. The Tsesis oil 

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11 

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12 

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13 

Table 1. Average density m-2 of the species and higher taxa that showed the greatest 

responses to the Chalk Point oil spill of April 2000. 

June 2000 Sept 2000 

Species/Taxa Upp Sw Low Sw Hu Upp Sw Low Sw Hu 

Polychaetes (total) 10,633.7 622.4 1232.8 386.4 25.3 315.1

(mostly spionids) 

 

Bivalves (total) 46.0 144.6 294.4 26.8 186.3 193.2

Malcoma balthica 15.3 55.9 108.1 0 133.4 165.6

Macoma mitchelli 7.7 19.7 18.4 29.9 48.3 9.2

Rangia cuneata 23.0 69.0 167.9 6.9 4.6 18.4

 

Crustaceans (total) 214.7 105.2 1214.4 73.6 250.7 262.2

Cyathura polita 161.0 78.9 110.4 66.7 78.2 64.4

Leptocheirus 

 plumulosus 0 16.4 1044.2 0 172.5 170.2

Upp Sw = Upper Swanson Creek (spill source) 

Low Sw = Lower Swanson Creek 

Hu = Hunting Creek 
 

14 

Table 2. Average gm AFDW biomass m-2 of all higher taxa in June 2000 after the 

Chalk Point oil spill of April 2000. 

Taxon 

Upper 

Swanson 

(n = 3) 

Lower 

Swanson 

(n = 7) 

Hunting 

(n = 10) 

Polychaetes (mostly spionids) .6011 .2701 .2337

Bivalve molluscs .1273 1.1451 1.3906

Crustaceans .2860 .0872 .2375

Nemerteans .0828 .0118 .0235

Tubificids .0069 .0035 .0045

Gastropod molluscs .0004 .0022 .0078

Insect larvae .0008 .00003 .0022

Burrowing anemones .0529 0 .0007
 

15 

Table 3. Average gm AFDW biomass m-2 of all higher taxa in Sept. 2000 after the 

April 2000 Chalk Point oil spill 

Taxon 

Upper 

Swanson 

(n = 10) 

Lower 

Swanson 

(n = 10) 

Hunting 

(n = 10) 

Polychaetes (mostly spionids) .4945 .0252 .0515

Bivalve molluscs .2298 3.5031 2.4187

Crustaceans .2327 .3227 .2513

Nemerteans .0745 .0196 .0281

Tubificids .0030 .0031 .0045

Gastropod molluscs 0 0 0

Insect larvae .0029 .0283 .0119

Burrowing anemones .0004 0 0

Others Low 
 

16 

Table 4. Estimation of subtidal benthos injury during 2000 in units of AFDW biomass 

production lost for Chalk Point oil spill of April 2000. 

Injured or 

affected 

resource 

Date 

Biomass 

difference 

(ImpactControl) 

Affected 

area 

Total biomass 

change over 

affected area 

Biomass 

production 

losta

 in 2000b

Bivalve 

molluscs 

(mostly 

Macoma 

balthica, also 

Rangia 

cuneata) 

June 

Sept 

-1.14 g m-2

-2.73 g m-2

708,000 m2 

708,000 m2

-807.1 Kg 

-1,932.8 Kg -1,932.8 Kg

 

Polychaetes 

(mostly 

spionids, also 

capitellids) 

June +0.349 g m-2 708,000 m2 +247.1 Kg +247.1 Kg

 

Crustacean 

amphipod 

 

Leptocheirus 

 plumulosus 

June -0.1067 g m-2 in 

Upper Sw 708,000 m2 -75.5 Kg 

 

June -0.1024 g m-2 in 

Lower Sw 1,320,000 

m

2

-135.2 Kg 

 

Sept -0.0602 g m-2 in 

Upper Sw 708,000 m2 -42.6 Kg -253.3 Kg

a = negative number means a loss in production 

b = assumes 2000 impacts to a single cohort of bivalves, two cohorts of Leptocheirus; 

and only one cohort of polychaetes 
 

17 

Table 5. Summation across years and across taxa of injuries to subtidal benthos in units 

of biomass production lost (in kg of AFDW), with inclusion of discounting. 

Injured or affected 

resource 

Biomass production lost 

(-) or gained (+) in Year: 

2000 2001 2002

Production discounted 

to 2002 basis 

Bivalve molluscs -1,932.8 0 0 -2,050.5 

Leptocheirus plumulosus

amphipod -253.3 -118.1 -75.5 -465.9 

Opportunistica polychaetes +123.6 0 0 

 

 +131.1 

 Total -2,062.5 -118.1 -75.5 

 

-2,385.3 

a = the credit applied here is 50% of the enhancement in production (see text for 

explanation) 
1

Comments on Draft Quantification of Injury to Benthic Resources from the Chalk 

Point Oil Spill on the Patuxent River 

Prepared by: 

A. Fred Holland, 

Chairman of the Science Board, 

Hollings Marine Laboratory 

& 

S.C. Marine Resources Research Institute 

P.O. Box 12559 

Charleston, SC 29422 

I reviewed the document entitled "Draft Quantification of Injury to Benthic 

Resources from the Chalk Point Oil Spill on the Patuxent River" and the 

supporting information provided in the associated technical reports (Llanso and 

Volstad, 2001, Osman 2001). My review of this work is summarized below. 

 Overall Assessment 

The data that were obtained for this study are of a high quality (e.g., few 

missing data, taxonomically complete, well summarized and presented) and were 

applied in a reasonable and through manner to develop a synthesis and 

conclusions about the impacts of the Chalk Point April 2000 Oil Spill on benthic 

resources in the Patuxent Estuary. The synthesis that was produced and the 

conclusions reached are reasonable and defensible. The assessment is 

consistent with the technical data provided. The conclusion that most of the 

damage from the Chalk Point Oil Spill was limited to Upper Swanson Creek is 

reasonable and valid. Upper Swanson Creek is probably the area where 

restoration efforts should be focused. If the benthos of upper Swanson Creek 

can be demonstrated to have recovered from the damage associated with the 

Chalk Point Oil Spill then it is reasonable to assume the rest of the system has 

recovered. 

The use of a biomass standard to estimate ecological damage was an 

ecologically meaningful and prudent choice. As the assessment indicates, 

biomass has great ecological relevance (e.g., fish eat mass not numbers, energy 

flow is based on mass not numbers, the nutritional value of benthic biomass of 

the dominant biota at Chalk Point is relatively similar), lower variability, and can 

be used to evaluate the degree of restoration in a relatively unambiguous 

manner. Had the assessment been based on a biodiversity or abundance 

conclusions would have had greater uncertainty and less ecological relevance. 

This does not suggest that the biodiversity and abundance data should not be 

used in the assessment process. These parameters reinforce the validity of the 
2

biomass information and address damage to other ecological attributes of the 

ecosystem. 

The assumptions made for the analyses used to quantify the ecological 

damage resulted in a conservative (i.e., minimal) estimate of the impact on 

benthic biomass. As the author indicates, less conservative assumptions and 

interpretations could have been made resulting in greater estimates of damage 

(e.g., “One interpretation of the density patterns in Rangia cuneata after the oil 

spill is that the effects did indeed extend not only into lower Swanson Creek but 

also into the Patuxent River”) Therefore, both the extent and duration of the 

impact may have been underestimated. For example, rather than assume 

proportional damage (e.g., 25% of lower Swanson Creek was damaged), no 

impact was assumed for this stratum. It appears, however, from the data in 

Llanso and Volstad (2001) that some damage (albeit small) may have occurred 

in the upper portions of the lower stratum. In addition, impact on Rangia biomass 

was assessed as if this bivalve was an annual species; Rangia, however, is a 

relatively long-live benthic organism (>5 years) with highest production in later 

years. As a result, the damage to the productivity and filtering capacity of this 

clam is likely underestimated. Finally, the ecological value of the observed 

increases in polychaete biomass was treated as if they were equivalent (1:1) with 

the observed declines in bivalve biomass. The importance of bivalves as 

ecological filters is generally considered to be greater that of an equivalent mass 

of spionid polychaete. The assessment recognized this deficiency but did not 

adjust for it (e.g., weight the biomass data for the spionid polychaetes differently 

from the data for the bivalves) in any manner. 

It is unfortunate that the sediment chemistry data that were collected were 

not processed and used to quantify the spatial extent of the exposure to spilled 

oil. The sediment chemistry information would have greatly reduced the 

uncertainty in the estimates of the spatial extent of the oiled sediments to which 

benthic organisms were exposed (i.e., the strata used to estimate damage would 

have been based on exposure data and geographical boundaries defined by less 

objective techniques). In addition, a single reference site (Hunting Creek) was 

the basis of most of the comparisons/contrasts used to quantify damage. 

Addition reference stations were sampled but the data collected for them were 

not processed. Failure to process the samples from addition reference sites 

increased the uncertainty associated with estimating the spatial extent of the 

damage from spilled oil. 

Year-to-year, seasonal, and spatial variation in natural environmental 

conditions such as salinity, temperature, freshwater inflow, depth, sediment 

characteristics including total carbon, and other environmental parameters are 

known to affect the abundance and biomass of benthic stocks. The text of the 

document(s) reviewed, however, suggests that “Habitat information collected 

during sampling (Llanso and Volstad 2001: Appendices) shows similar mean 

station depths (1.0 m for upper Swanson, 1.1 m for lower Swanson, and 1.2 m 
3

for Hunting Creeks), silt/clay content of sediments (84.9% for upper Swanson, 

90.5% for lower Swanson, and 84.8% for Hunting Creeks), and salinities (4.3 ppt 

for upper Swanson, 5.5 ppt for lower Swanson, and 6.0 ppt for Hunting Creeks)” 

implying these factors did not greatly influence benthic distributions in space and 

time. No analyses (e.g., regressions of environmental factors vs benthic biomass 

or abundance), however, were provided to support this statement, and no 

adjustments were made to the biomass/abundance data collected to account for 

differences among years or space due to “natural” environmental parameters. 

These data are available (e.g., Llanso and Volstad 2001: Appendices) and it is 

possible that these environmental parameters varied among stations and years 

in a manner that was associated with the distributions of benthic abundance and 

biomass. Preliminary correlations of just the Swanson Creek data suggest a 

relatively strong associations between Strebliospio benedicti and Rangia cuneata

abundances and total carbon, silt/clay and even the small salinity gradient that 

occurred. Such adjustments (e.g., Analysis of Covariance using silt/clay content 

of sediments as a covariant) may have increased the sensitivity and power of the 

statistical contrasts evaluated (especially the contrasts across years). I 

recommend that preliminary analyses be conducted to validate that the 

environmental gradients among the sample sites were not a significant 

contributing factor to the observed distributional patterns. This analysis could be 

done in less than a manday using the data in Llanso and Volstad (2001). 

Correlations and regressions that account for less that 25% of the variance 

should probably be considered as unimportant and would not likely alter findings. 

Correlations that account for more than 25% of the variance may alter findings 

(either increasing or decreasing the estimated damage). 

Assumptions 

In order to conduct an assessment of the damage resulting from the Chalk Point 

April 2000 spill on the natural resources of the Patuxent Estuary it appears that 

the assessment made the following assumptions. I listed the assumptions to 

assist me in evaluating the reliability of the data and conclusions. These 

assumptions are reasonable and are likely result in a conservative estimate of 

ecological damage. 

• Damage to intertidal habitats (mud flats) was assumed to be equivalent to 

that which occurred in subtidal areas (i.e., the aerial estimates for intertidal 

plus subtidal habitats were used as the geographical basis for the 

assessment and the subtidal benthic data were used as the biological 

basis for the damage assessment). 

• Gradients in benthic distributions observed in Swanson were the result of 

the oil spill incident not due to changes in environmental conditions (e.g., 

salinity, depth, sediment characteristics, and the amount of emergent 

marsh vegetation). 
4

• Watershed properties (size, land cover) in the drainage basins of the 

Swanson and Hunting Creeks were assumed to have similar effects upon 

benthic distributions. 

• Rangia was assumed to be an annual species and Leptocherius was 

assumed to have an average of two broods over the summer period. 

These assumptions may have resulted in an underestimate of damage to 

Rangia and Letocherius biomass. 

• The ecological value of a unit of benthic biomass is equivalent across the 

dominant species occurring at Chalk Point. This is probably true for the 

nutritional value of the dominant benthic biota at Chalk Point, but may not 

be true for other ecological attributes. For example, the filtering value of 

bivalves may be superior to that of polychaetes. 

• Chronic impacts from the oil spill were minimal and the majority of the 

damage occurred between April 2000 and September 2000. This may not 

have been true and would have resulted in an underestimate of damage. 

• The threshold values for the B-IBI for the low mesohaline area are 

appropriate to apply to the Chalk Point region of the Patuxent River and 

are equally applicable in Swanson and Hunting Creeks. The Chalk Point 

regions is characterized by strong environmental gradients and given the 

state-of-science for benthic index development this is the best assumption 

that could have been made. 

Sample Design Concerns 

 Below several concerns that affect the reliability and uncertainty 

associated with the assessment of the damage resulting from the Chalk Point Oil 

Spill are identified. These concerns provide the Trustees a framework for 

deciding if additional analyses would be beneficial (i.e., reduce the uncertainty or 

better define the existing assessment). Most importantly, they provide 

information that may be beneficial to future damage assessments. The 

approaches that are identified for addressing these concerns are not the only 

approaches that could be used. These approaches should not be considered 

recommendations. They are suggestions for use in future assessments. 

The sample design of the study required the damage assessment to make 

assumptions about the spatial extent of exposure to the spilled oil. Sediment 

chemistry data were not processed and were not available to better define the 

degree of exposure to spilled oil. Had sediment chemistry data been available 

then it would have been possible to use these data as covariates and to define 

the extent and degree of oiled benthic habitat. Under the best of circumstances 

benthic assessments consist of a triad of: (1) exposure data, (2) sensitive 

species toxicity data, and (3) benthic community response data. The present 
5

sample design is missing the exposure leg of this three-legged stool. The toxicity 

data are available from the literature (i.e., #2/6 fuel oil exposures kill sensitive 

biota at ppb levels) and the community data are available in Llanso and Volstad 

(2001). This deficiency/concern is identified to emphasize the value of following 

an exposure, toxicity, and response indicator strategy when conducting damage 

assessments. 

 The vast majority of the estimates of the impact of the oil spill was based 

on differences between a single reference location (Hunting Creek) and/or the 

mainstem Patuxent River. These reference locations may not have been 

appropriate reference locations because of differences in depth, water quality, 

sedimentation rates, proximity to emergent vegetation, and exposure to gradients 

resulting from the Chalk Point power plant. A higher level of confidence could 

have been obtained had an average/mean value for several reference creeks 

(upstream and downstream) been used for contrasts. That is it may have been 

beneficial to sample multiple reference creeks but take fewer samples in each. 

This concern is identified to emphasize the value of multiple reference sites to 

the damage assessment process. 

 The BACI (Before-After-Control-Impact) portion of the study is also based 

on contrasts to a single control station (Chester River) that is located in a 

different water body with a different drainage basin and watershed 

characteristics. A higher level of confidence in conclusions could have been 

obtained if the BACI analyses were based upon the average value of multiple 

reference stations representing a range of similar environmental settings (e.g., 

Chester, Severn, Potomac). This concern is identified to emphasize the value of 

multiple reference sites to the damage assessment process. 

The BACI analyses appeared to be based on a single parameter 

(abundance). This parameter is a variable one and more interpretable results 

may have been obtained had a suite of benthic parameters been used (e.g., the 

metrics used for the B-IBI). No data for less variable parameters (e.g., biomass) 

were provided for the Chester River. Thus, an evaluation of these parameters 

and the appropriate BACI hypothesis tests could not be conducted. This concern 

is identified to emphasize the value of using multiple ecological attributes in 

damage assessments and analyses like BACI. 

Sampling was not conducted following the major recruitment period for 

most of the biota composing benthic communities at Chalk Point (i.e., spring 

2001). It is not know if the differences observed in upper Swanson Creek in 

Spring 2000 reappeared in Spring 2001 representing a chronic response to the 

oil spill. Differences between upper and lower Swanson Creek may have 

reappeared in Spring 2001 suggesting an impact on recruitment processes and 

sensitive early life stages. This concern is identified because it is considered to 

be the greatest weakness in the study design. Had samples been obtained in 

the following spring recruitment period that upper Swanson Creek had benthic 
6

communities that were similar in the kinds, abundances, and biomass as 

occurred in appropriate reference sites then recovery would have been 

considered to have occurred. 

 Data were not collected on the size of benthic organisms at each site 

(e.g., length of Macoma and Rangia). Size data would have provided for a less 

ambiguous assessment of damage (i.e., impacts on growth could have been 

separated from impact on recruitment). This concern is identified to highlight the 

value of obtaining size/age data in the interpretation of assessment data. Size 

data need only be collect for "key" species. 

 A stratification of Hunting Creek was not a part of the design and an 

evaluation was not conducted to show such stratification was not appropriate. A 

gradient from the upper to lower Hunting did not appear to exist; however, no 

analysis was presented to validate this. A demonstration of no gradient in 

Hunting Creek would have made the conclusions related to the gradient 

observed in Swanson Creek more creditable. 
Response to “Comments on Draft Quantification of Injury to Benthic Resources from the 

Chalk Point Oil Spill on the Patuxent River”.

Prepared by: Charles Peterson (UNC), Roberto Llanso (Versar), and Beth McGee 

(USFWS).

Comment 1: “The assumptions made for the analyses used to quantify the ecological damage 

resulted in a conservative (i.e., minimal) estimate of the impact on benthic biomass. As the 

author indicates, less conservative assumptions and interpretations could have been made 

resulting in greater estimates of damage (e.g., “One interpretation of the density patterns in 

Rangia cuneata after the oil spill is that the effects did indeed extend not only into lower 

Swanson Creek but also into the Patuxent River”) Therefore, both the extent and duration of the 

impact may have been underestimated. For example, rather than assume proportional damage 

(e.g., 25% of lower Swanson Creek was damaged), no impact was assumed for this stratum. It 

appears, however, from the data in Llanso and Volstad (2001) that some damage (albeit small) 

may have occurred in the upper portions of the lower stratum.” 

Response: Our conclusions were based on an analysis that we thought was scientifically 

rigorous and defensible, one that we believe could withstand scrutiny in a court of law. Whether 

or not these estimates are conservative, is arguable. We acknowledge in the injury 

quantification report that there is a suggestion in the survey data of Llanso and Volstad (2001) 

that the injury to Rangia may extend beyond upper Swanson Creek into lower Swanson in June 

2000. However, no augmentation of the spatial scope of injury is added for this possibility 

because the biomass and numerical data for Rangia are not fully consistent in their indication of 

Rangia injury in lower Swanson Creek. In addition, there is no rigorous information available 

on which to base a quantified estimate for how far into lower Swanson Creek the injury to 

Rangia may have extended. The reviewer’s comment on this point illustrates the difficulty in 

quantifying injury in its arbitrary suggestion of 25% as a possibility for the proportion of lower 

Swanson Creek that was injured, without a compelling rationale for that choice.

Comment 2: “In addition, impact on Rangia biomass was assessed as if this bivalve was an 

annual species; Rangia, however, is a relatively long-live benthic organism (>5 years) with 

highest production in later years. As a result, the damage to the productivity and filtering 

capacity of this clam is likely underestimated.” 

Response: While the injury report acknowledges that Rangia cuneata can live for several years, 

perhaps up to a decade, the injury calculations for this spill are based on the assumption that the 

Rangia that died in Swanson Creek during spring 2000 because of the oil spill would not have 

survived past September 2000 in the absence of the spill. This assumption is based upon the 

disappearance of Rangia from the reference site, Hunting Creek, between June and September 

2000 as documented in the Llanso and Volstad (2001) report. Consequently, the addition of a 

term for production foregone in later years is not supported by the data. The reason for the 

disappearance of Rangia at the control site may be mortality due to increased salinity from June 

to September. That is, Rangia, which prefers low oligohaline conditions, can recruit into low 

salinity areas during the spring, but cannot tolerate the salinity increase during the summer 

(Llanso, Versar Inc, personal communication).
Comment 3: “Finally, the ecological value of the observed increases in polychaete biomass was 

treated as if they were equivalent (1:1) with the observed declines in bivalve biomass. The 

importance of bivalves as ecological filters is generally considered to be greater that of an 

equivalent mass of spionid polychaete. The assessment recognized this deficiency but did not 

adjust for it (e.g., weight the biomass data for the spionid polychaetes differently from the data 

for the bivalves) in any manner.” 

Response: The calculations of injury in the report are done separately by taxon so that the 

various taxa can later be weighted differently in scaling restoration to compensate for the losses. 

 In that scaling process, the reviewer’s point that polychaete biomass gain should be weighted 

less heavily than bivalve loss is, in fact, adopted. Changes in bivalve biomass are given double 

the weighting given to changes in polychaete biomass. The injury document has been revised to 

reflect this change.

Comment 4: “It is unfortunate that the sediment chemistry data that were collected were not 

processed and used to quantify the spatial extent of the exposure to spilled oil. The sediment 

chemistry information would have greatly reduced the uncertainty in the estimates of the spatial 

extent of the oiled sediments to which benthic organisms were exposed (i.e., the strata used to 

estimate damage would have been based on exposure data and geographical boundaries defined 

by less objective techniques).” 

Response: Under the Oil Pollution Act of 1990, injury to natural resources must be 

“observable” and “measurable”. While we agree that analysis of sediment chemistry may have 

provided information regarding exposure of benthic organisms to petroleum compounds, it is 

also true that exposure does not equal effects due to the variety of factors that influence 

biovailability of sediment-associated contaminants. Hence, we are not convinced that the 

chemical data would have improved our ability to delineate the impacted area. In addition, one 

of our concerns was that toxicity in the water column during the time of the spill may have 

affected recruitment of benthic larvae to subtidal areas. This effect would not be detected 

through the analysis of sediment contaminants, but rather by looking directly at the benthic 

macroinvertebrate community. Nonetheless, we will take this recommendation under advisement 

in the event we are involved with future oil spill NRDAs.

Comment 5: “In addition, a single reference site (Hunting Creek) was the basis of most of the 

comparisons/contrasts used to quantify damage. Addition reference stations were sampled but 

the data collected for them were not processed. Failure to process the samples from addition 

reference sites increased the uncertainty associated with estimating the spatial extent of the 

damage from spilled oil.” 

Response: There must be some confusion, as we did not collect samples at additional reference 

sites in the Patuxent River. We did collect benthic samples in Indian Creek and Trent Hall 

Creek in September; however, these sites were potentially impacted by the spill. These samples 

were not analyzed because the evidence suggested that it would be highly unlikely to detect 

measurable effects in these samples. This assessment was based on the degree of oiling of these 

tributaries relative to Swanson Creek, and results of analysis of intertidal benthic samples 
collected in these areas that did not suggest evidence of injury.

Comment 6: “Year-to-year, seasonal, and spatial variation in natural environmental conditions 

such as salinity, temperature, freshwater inflow, depth, sediment characteristics including total 

carbon, and other environmental parameters are known to affect the abundance and biomass of 

benthic stocks. The text of the document(s) reviewed, however, suggests that “Habitat 

information collected during sampling (Llanso and Volstad 2001: Appendices) shows similar 

mean station depths (1.0 m for upper Swanson, 1.1 m for lower Swanson, and 1.2 m for Hunting 

Creeks), silt/clay content of sediments (84.9% for upper Swanson, 90.5% for lower Swanson, 

and 84.8% for Hunting Creeks), and salinities (4.3 ppt for upper Swanson, 5.5 ppt for lower 

Swanson, and 6.0 ppt for Hunting Creeks)” implying these factors did not greatly influence 

benthic distributions in space and time. No analyses (e.g., regressions of environmental factors 

vs benthic biomass or abundance), however, were provided to support this statement, and no 

adjustments were made to the biomass/abundance data collected to account for differences 

among years or space due to “natural” environmental parameters. These data are available (e.g., 

Llanso and Volstad 2001: Appendices) and it is possible that these environmental parameters 

varied among stations and years in a manner that was associated with the distributions of benthic 

abundance and biomass. Preliminary correlations of just the Swanson Creek data suggest a 

relatively strong associations between Streblospio benedicti and Rangia cuneata abundances and 

total carbon, silt/clay and even the small salinity gradient that occurred. Such adjustments (e.g., 

Analysis of Covariance using silt/clay content of sediments as a covariant) may have increased 

the sensitivity and power of the statistical contrasts evaluated (especially the contrasts across 

years). I recommend that preliminary analyses be conducted to validate that the environmental 

gradients among the sample sites were not a significant contributing factor to the observed 

distributional patterns. This analysis could be done in less than a manday using the data in 

Llanso and Volstad (2001). Correlations and regressions that account for less that 25% of the 

variance should probably be considered as unimportant and would not likely alter findings. 

Correlations that account for more than 25% of the variance may alter findings (either increasing 

or decreasing the estimated damage).” 

Response: As recommended, additional data analyses were conducted to validate the assumption 

that environmental gradients among sampling sites in Swanson Creek and Hunting Creek were 

not a significant contributing factor to the observed distribution patterns of species abundance 

and biomass, and hence, to differences between strata that may confound impacts from the oil 

spill (see attached document by Llanso and Volstad, 2002). The objective was to determine if the 

amount of variation due to spatial differences in salinity and silt-clay in Swanson and Hunting 

Creeks was significant and sufficiently large to justify incorporation of these variables as 

covariates to improve sensitivity of the impact study. In brief, data were analyzed by ANOVA 

using the General Linear Model (GLM) procedure in SAS, which is applicable to unbalanced 

designs (unequal number of observations per cell). Dependent variables in the analysis were 

total abundance, total biomass, total species richness, Shannon Diversity, the abundance of the 

top five numerically dominant species (Leptocheirus plumulosus, Streblospio benedicti, 

Tubificoides spp., Macoma balthica, and Rangia cuneata) and the biomass of Macoma balthica, 

Macoma mitchelli and Rangia cuneata. Results indicated some significant relationships among 

grain size and/or salinity and a few of the dependent variables; however, in all cases, the effects 

of salinity and percent silt-clay could be explained in terms of differences among seasons, sites, 
the putative effects of the oil spill or visual examination of the data indicated the significant 

relationship was likely driven by outliers. Hence, the conclusion was that bottom salinity and 

grain size were not considered to be major contributing factors to the observed distribution 

patterns of species abundance and biomass.

Assumptions

Comment 7: “In order to conduct an assessment of the damage resulting from the Chalk Point 

April 2000 spill on the natural resources of the Patuxent Estuary it appears that the assessment 

made the following assumptions. I listed the assumptions to assist me in evaluating the 

reliability of the data and conclusions. These assumptions are reasonable and [are] likely result 

in a conservative estimate of ecological damage.” 

Response: We disagree with the premise that all the stated assumptions likely resulted in a 

conservative estimate of benthic injury. In particular, we note the following: First, the 

assumption of injury to benthic communities to intertidal mudflats being equivalent to subtidal 

injury. The Academy of Natural Sciences conducted a survey in the intertidal areas in Hunting, 

Swanson and Trent Hall Creeks. These limited data did not indicate differences in benthic 

community structure that could be related to the oil spill. However, rather than assume no injury 

to intertidal habitat based on these data, we instead chose to combine it with the subtidal injury 

assessment. Second, the assumption that benthic distributions in Swanson Creek were the result 

of the oil spill and not due to changes in environmental conditions. Had we attributed some or 

all of the changes in benthic community composition in upper Swanson Creek to factors other 

than oil, then our injury estimate would have been substantially lower. Third, the assumption 

that watershed properties were similar in Hunting and Swanson Creeks. The effect of 

differences in the drainage basins may have increased or decreased our estimate of injury.

Comment 8: “Rangia was assumed to be an annual species and Leptocherius was assumed to 

have an average of two broods over the summer period. These assumptions may have resulted in 

an underestimate of damage to Rangia and Leptocherius biomass.” 

Response: As indicated above in our response to Comment 2, Rangia was assumed to be an 

annual species based on a comparison with the reference site, Hunting Creek, and the fact that 

they had all but disappeared from this area by the September sampling. We agree that 

Leptocheirus may have multiple broods from spring through fall; however, they also suffer very 

high natural mortality rates during the summer due to food limitation and predation. Typically, 

the result is extremely low population densities throughout the summer. Leptocheirus 

populations generally experience two seasonal peaks, in the fall and spring, and so we felt it was 

reasonable to estimate injury based on the loss of production of two generations per year.

Comment 9: “The ecological value of a unit of benthic biomass is equivalent across the 

dominant species occurring at Chalk Point. This is probably true for the nutritional value of the 

dominant benthic biota at Chalk Point, but may not be true for other ecological attributes. For 

example, the filtering value of bivalves may be superior to that of polychaetes.” 

Response: See response to Comment 3.
Comment 10: “Chronic impacts from the oil spill were minimal and the majority of the damage 

occurred between April 2000 and September 2000. This may not have been true and would have 

resulted in an underestimate of damage.” 

Response: We agree that the assessment was not focused on measuring chronic effects and, 

unfortunately, that reflects the state of the science. Natural variability of benthic populations 

will likely mask the ability for one to detect chronic, sublethal effects on benthic populations, if 

they exist. Presently, the injury quantification does assume that recovery of the benthic 

community in Swanson Creek had occurred by spring 2001. The possibility that the duration of 

injury was underestimated is acknowledged in the injury quantification report. Absent additional 

field sampling, there is insufficient information with which to test the assumptions about 

duration of injury. The assumption of rapid recovery is based upon the field evidence of Llanso 

and Volstad (2001) of successful recovery of Leptocheirus, in lower Swanson Creek by 

September 2000. This amphipod is likely the most sensitive of all the major taxa to 

contaminants, so its rapid recovery in a portion of the affected area provided support for 

assuming that recovery in upper Swanson Creek would occur by 2001. Nonetheless, we 

recognize the uncertainty associated with this assumption. Consequently, we have changed our 

estimate of biomass loss to reflect this uncertainty. As stated in the revised injury quantification 

report, the most likely injury to persist would be to Leptocheirus, due to its sensitivity to 

contaminants and from the multi-year duration of impacts to other amphipods in other oil spills. 

 Therefore, we have extended the recovery of Leptocheirus populations until September 2002. 

Assuming a similar lost production for this time period years as occurred in 2000 (75.5kg for 

June 2001+ 42.6 for September 2001 + 75.5 kg for June 2002), we estimate the additional loss 

of biomass as 193.6 kg. Bivalves are typically less sensitive to contaminants; hence, we have not 

altered the recovery estimates for Macoma and Rangia. 

Sample Design Concerns

Comment 11: “The approaches that are identified for addressing these concerns are not the 

only approaches that could be used. These approaches should not be considered 

recommendations. They are suggestions for use in future assessments. 

The sample design of the study required the damage assessment to make assumptions 

about the spatial extent of exposure to the spilled oil. Sediment chemistry data were not 

processed and were not available to better define the degree of exposure to spilled oil. Had 

sediment chemistry data been available then it would have been possible to use these data as 

covariates and to define the extent and degree of oiled benthic habitat. Under the best of 

circumstances benthic assessments consist of a triad of: (1) exposure data, (2) sensitive species 

toxicity data, and (3) benthic community response data. The present sample design is missing 

the exposure leg of this three-legged stool. The toxicity data are available from the literature 

(i.e., #2/6 fuel oil exposures kill sensitive biota at ppb levels) and the community data are 

available in Llanso and Volstad (2001). This deficiency/concern is identified to emphasize the 

value of following an exposure, toxicity, and response indicator strategy when conducting 

damage assessments.” 
Response: See response to Comment 4.

Comment 12: “The vast majority of the estimates of the impact of the oil spill was based on 

differences between a single reference location (Hunting Creek) and/or the mainstem Patuxent 

River. These reference locations may not have been appropriate reference locations because of 

differences in depth, water quality, sedimentation rates, proximity to emergent vegetation, and 

exposure to gradients resulting from the Chalk Point power plant. A higher level of confidence 

could have been obtained had an average/mean value for several reference creeks (upstream and 

downstream) been used for contrasts. That is it may have been beneficial to sample multiple 

reference creeks but take fewer samples in each. This concern is identified to emphasize the 

value of multiple reference sites to the damage assessment process.” 

Response: We do not believe there were other appropriate reference sites within the Patuxent 

River. Salinity regimes in Patuxent River tributaries that were not impacted by the spill (either 

upstream or toward the mouth of the Patuxent River) are much different than the area around 

Swanson Creek. Tributaries within the same salinity regime (with the exception of Hunting 

Creek) were thought to be impacted by the spill. In hindsight, reference areas with similar 

physico-chemical characteristics could have been sampled in tributaries outside the Patuxent 

River, but given the circumstances, we believe our approach was rationale and defensible.

Comment 13: “The BACI (Before-After-Control-Impact) portion of the study is also based on 

contrasts to a single control station (Chester River) that is located in a different water body with 

a different drainage basin and watershed characteristics. A higher level of confidence in 

conclusions could have been obtained if the BACI analyses were based upon the average value 

of multiple reference stations representing a range of similar environmental settings (e.g., 

Chester, Severn, Potomac). This concern is identified to emphasize the value of multiple 

reference sites to the damage assessment process.” 

Response: We agree that properly replicated control areas constitute an important element of 

BACI design, as is finding control sites with similar environmental characteristics. In the BACI 

assessment, Station 68 in the Chester River was chosen as the control site because 

characteristics of the station, the salinity regime and the location in a small tidal basin, were 

most similar to those of Station 74 in the Patuxent River. In addition, Station 68 exhibited good 

benthic condition, indicative of good water quality, an important criterion in the selection of a 

control site. Ideally, control sites should be located in proximity to the impact site, should be 

independent of one another and should be randomly chosen. This is not always possible. Longterm fixed monitoring sites in Chesapeake Bay, for example, were specifically selected to 

measure trends in sensitive areas targeted for pollution abatement. Therefore, selection of 

control sites for this study was problematic. Another limitation of applying the BACI design to 

the Chalk Point data is there is inadequate temporal replication. Ideally, sites before and after 

the perturbation should be sampled at small time intervals. Times between successive sampling 

events should be random. None of this was possible because this study used data from a 

monitoring program with a fundamentally different monitoring objective and sampling regime. 

Because of the limited data, natural temporal variability in abundances make short-term impacts 

difficult to detect. Adding more control sites or using a different suite of benthic parameters 

would not have alleviated this problem. In fact, biomass data from low mesohaline sites 
throughout the Chesapeake Bay are extremely variable because of the influence of bivalves, 

which are sampled with varying efficiencies. Finally we note that, theoretically, it is possible 

that the use of multiple control sites might have reduced our ability to detect differences between 

impact and control sites. Control sites representing different systems will be influenced by 

varying levels of perturbation, variation among the controls will not be consistent, and there will 

be temporal interactions among controls that could result in a reduction in the sensitivity of the 

test.

Comment 14: “The BACI analyses appeared to be based on a single parameter (abundance). 

This parameter is a variable one and more interpretable results may have been obtained had a 

suite of benthic parameters been used (e.g., the metrics used for the B-IBI). No data for less 

variable parameters (e.g., biomass) were provided for the Chester River. Thus, an evaluation of 

these parameters and the appropriate BACI hypothesis tests could not be conducted. This 

concern is identified to emphasize the value of using multiple ecological attributes in damage 

assessments and analyses like BACI.” 

Response: See response to Comment 13.

Comment 15: “Sampling was not conducted following the major recruitment period for most of 

the biota composing benthic communities at Chalk Point (i.e., spring 2001). It is not known if the 

differences observed in upper Swanson Creek in Spring 2000 reappeared in Spring 2001 

representing a chronic response to the oil spill. Differences between upper and lower Swanson 

Creek may have reappeared in Spring 2001 suggesting an impact on recruitment processes and 

sensitive early life stages. This concern is identified because it is considered to be the greatest 

weakness in the study design. Had samples been obtained in the following spring recruitment 

period that upper Swanson Creek had benthic communities that were similar in the kinds, 

abundances, and biomass as occurred in appropriate reference sites then recovery would have 

been considered to have occurred.” 

Response: We recognize this as an area of uncertainty (see response to Comment 10) and have 

increased our estimate of injury in an attempt to account for some of the uncertainty associated 

with recovery estimates.

Comment 16: “Data were not collected on the size of benthic organisms at each site (e.g., length 

of Macoma and Rangia). Size data would have provided for a less ambiguous assessment of 

damage (i.e., impacts on growth could have been separated from impact on recruitment). This 

concern is identified to highlight the value of obtaining size/age data in the interpretation of 

assessment data. Size data need only be collect for "key" species.” 

Response: We will take this recommendation under advisement in the event we are involved with 

future oil spill NRDAs.

Comment 17: “A stratification of Hunting Creek was not a part of the design and an evaluation 

was not conducted to show such stratification was not appropriate. A gradient from the upper to 

lower Hunting did not appear to exist; however, no analysis was presented to validate this. A 

demonstration of no gradient in Hunting Creek would have made the conclusions related to the 
gradient observed in Swanson Creek more [credible].” 

Response: Stratification of Swanson Creek was conducted as part of a “near field-far field” 

design to evaluate pollution induced impact. Variability due to gradients in environmental 

factors (e.g., salinity) were believed to be negligible. However, as noted in the response to 

Comment 6, additional statistical analyses were conducted to address the concern about effects 

of potential gradients in salinity and grain size on species’ distributions in Swanson and Hunting 

Creeks. The conclusion was that bottom salinity and grain size were not considered to be major 

contributing factors to the observed distribution patterns of species abundance and biomass; 

hence, there was no evidence for a gradient effect in Hunting Creek.