Toxicity to freshwater organisms from oils and oil spill chemical treatments in laboratory microcosms

Toxicity and temporal changes in toxicity of freshwater-marsh-microcosms containing South Louisiana Crude (SLC) or diesel fuel and treated with a cleaner or dispersant, were investigated using Chironomus tentans, Daphnia pulex, and Oryzias latipes. Bioassays used microcosm water (for D. pulex and O. latipes) or soil slurry (for C. tentans) taken 1,7, 31, and 186 days after treatment. SLC was less toxic than diesel, chemical additives enhanced oil toxicity, the dispersant was more toxic than the cleaner, and toxicities were greatly reduced by day 186. Toxicities were higher in the bioassay with the benthic species than in those with the two water-column species. A separate experiment showed that C. tentans' sensitivity was intermediate to that of Tubifex tubifex and Hyallela azteca. Freshwater organisms, especially benthic invertebrates, thus appear seriously effected by oil under the worst-case-scenario of our microcosms. Moreover, the cleaner and dispersant tested were poor response options under those conditions.     

Fate and effects of heavy metals in salt marsh sediments

The fate and effects of selected heavy metals were examined in sediment from a restored salt marsh. Sediment cores densely covered with Spartina patens were collected and kept either un-amended or artificially amended with nickel (Ni) under standardized greenhouse conditions. Ni-amendment had no significant effect on the fate of other metals in sediments, however, it increased root uptake of the metals. Metal translocation into the shoots was small for all metals. Higher Ni concentrations in plants from amended cores were accompanied by seasonal reductions in plant biomass, photosynthetic capacity and transfer efficiency of open photosystem II reaction centers; these effects, however, were no longer significant at the end of the growing season. Root colonization by arbuscular mycorrhizal fungi (AMF) resembled that of natural salt marshes with up to 20% root length colonized. Although Ni-amendment increased AMF colonization, especially during vegetative growth, in general AMF were largely unaffected.     

Cleaning oiled shores: laboratory experiments testing the potential use of vegetable oil biodiesels

A series of laboratory experiments were carried out to test the potential of vegetable oil biodiesel for the cleaning of oiled shorelines. In batch experiments, biodiesel was shown to have a considerable capacity to dissolve crude oil, which appears to be dependent on the type of biodiesel used. Pure vegetable oil biodiesels (rapeseed and soybean) were significantly more effective in the cleanup of oiled sands (up to 96%) than recycled waste cooking oil biodiesel (70%).In microcosm and mesocosm experiments, oiled sediments were sprayed with biodiesel and subjected to simulated tides. Microcosm experiments revealed that, of those tested, the highest ratio of biodiesel to crude oil, had the highest effectiveness for cleaning fine sands, with ratios of 2:1 (biodiesel:crude oil) giving the best results. In the mesocosm experiments a ratio 1:1 of soybean biodiesel to crude oil removed 80% of the oil in cobbles and fine sands, 50% in coarse sand and 30% in gravel. Most of the oil was removed with the surface water, with only a small amount being flushed through the sediments. Particle size and pore size were important determinants in the cleanup and mobility of crude oil in the sediments in these static systems. It is expected that the biodiesel effectiveness should improve in the natural environment particularly in exposed beaches with strong wave action. However, more laboratory and field trials are required to confirm the operational use of biodiesel as a shoreline cleaner.     

Review article: effects of oil pollution, chemically treated oil, and cleaning on thermal balance of birds

The acute effect of oil pollution on birds is on their thermal balance. Oil adheres to the plumage and causes a reduction in water repellant properties of the plumage, causing water to penetrate into the plumage to displace the insulating layer of air. The effect of oil on the plumage insulation is dose-dependent. The effect of oiling is greatly enhanced when the oil is spread in the plumage due to preening. In water, plumage oiling may cause the heat loss to exceed the bird's heat production capacity, resulting in hypothermia. If the oiled bird is ashore, with a dry plumage, it may have a normal thermal insulation. Bird species dependent upon feeding in water (such as diving birds) are therefore much more susceptible to the harmful effects of oil pollution than are semi-aquatic species that can feed ashore. It is possible to restore the water-repelling and insulative properties of the plumage by the process of cleaning if all the oil and soap is removed, and if the plumage is completely dry. Chemical treatment of oil has been suggested as a way to reduce the impact of oil spills on avian life. However, very few reports seem to have addressed the effects of chemically treated oil on the thermal balance of birds, and the results from one study actually indicate that oil treated with dispersants may be more harmful to birds than oil. The urgent need for more information about the effects of chemically treated oil on aquatic birds is therefore stressed.     

CH4 and N2O emissions from Spartina alterniflora and Phragmites australis in experimental mesocosms

Spartina alterniflora, a perennial grass with C(4)-photosynthesis, shows great invading potential in the coastal ecosystems in the east of China. We compared trace gas emissions from S. alterniflora with those from a native C(3) plant, Phragmites australis, by establishing brackish marsh mesocosms to experimentally assess the effects of plant species (S. alterniflora vs. P. australis), flooding status (submerged vs. non-submerged), and clipping (plants clipped or not) on trace gas emissions. The results show that trace gas emission rates were higher in S. alterniflora than P. australis mesocosms due to the higher biomass and density of the former, which could fix more available substrates to the soil and potentially emit more trace gases. Meanwhile, trace gas emission rates were higher in non-submerged than submerged soils, suggesting that water might act as a diffusion barrier in the brackish marsh mesocosms. Interestingly, methane (CH(4)) emission rates were lower in clipped non-submerged mesocosms than in non-clipped submerged mesocosms, but nitrous oxide (N(2)O) emissions were enhanced. CH(4) emissions were significantly correlated with the plant biomass and stem density (R(2)>0.48, P<0.05) for both species, suggesting that both the two species might play important roles in CH(4) production and transport and also act as suppliers of easily available substrates for the methanogenic bacteria in wetland ecosystems. N(2)O emissions, however, were not significantly correlated with plant biomass and density (P>0.05).     

Field investigation on the toxicity of Alaska North Slope crude oil (ANSC) and dispersed ANSC crude to Gulf killifish, Eastern oyster and white shrimp

A field investigation was conducted on a Louisiana Spartina alterniflora shoreline to evaluate the toxic effects of crude oil (Alaska North Slope crude oil, ANSC) and dispersed oil (ANSC + dispersant Corexit 9,500) on three aquatic species indigenous to the Gulf of Mexico: Fundulus grandis (Gulf killifish), Crassostrea virginica (Eastern oyster), and Litopenaeus setiferus (white shrimp). Results indicated that total hydrocarbons concentration value in oiled treatments decreased rapidly in 3h and were below 1 ppm at 24h after initial treatment. Corexit 9,500 facilitated more ANSC fractions to dissolve and disperse into the water column. L. setiferus showed short-term sensitivity to the ANSC and ANSC + 9,500 at 30 ppm. However, most test organisms (>83%) of each species survived well after 24h exposure to the treatments. Laboratory tests conducted concurrent with the field investigation indicated that concentrations of crude oil higher than 30 ppm were required for any significant toxic effect on the juvenile organisms tested.     

Total mercury and methylmercury in freshwater and salt marsh soils of the Mississippi river deltaic plain

Mercury entering wetland environments can be microbially methylated to methylmercury. The purpose of this study was to investigate the historical rate of mercury accumulation and distribution of total and methylmercury in soil profile of Louisiana coastal marshes. Two sediment cores each were taken from Louisiana freshwater marsh and salt marsh. Vertical accretion was determined using the 137Cs dating technique. Total and methylmercury were determined with depth in the soil profiles. The fresh marsh soil on a dry weight basis contained more total and methylmercury than the salt marsh. Average vertical accretion rates in freshwater marsh and salt marsh were 0.90 and 0.75 cm year(-1), respectively. Average total and methylmercury content (to a depth of 30 cm) was 140 and 4.19 microg kg(-1) and 80 and 1.34 microg kg(-1) for the fresh and salt marsh, respectively. Due to greater sediment input resulting in a higher bulk density the salt marsh contained more total mercury per m2 (to 30 cm depth) than the fresh water marsh (5340 microg m(-2) as compared to 2929 microg m(-2)). The amount of methylmercury per m2 to depth of 30 cm was approximately the same for each marsh.     

Alkane biodegradation and dynamics of phylogenetic subgroups of sulfate-reducing bacteria in an anoxic coastal marine sediment artificially contaminated with oil

For 503 days, unoiled control and artificially oiled sediments were incubated in situ at 20m water depth in a Mediterranean coastal area. Degradation of the aliphatic fraction of the oil added was followed by GC-MS. At the same time, terminal restriction fragment length polymorphism (T-RFLP) of 16S rRNA encoding genes was used to detect dynamics in the sulfate-reducing bacteria (SRB) community in response to the oil contamination. Specific polymerase chain reaction (PCR) primer sets for five generic or suprageneric groups of SRB were used for PCR amplification of DNA extracted from sediments. During the experiment, hydrocarbons from C(17) to C(30) were significantly degraded even in strictly anoxic sediment layers. Of the five SRB groups, only two groups were detected in the sediments (control and oiled), namely the Desulfococcus-Desulfonema-Desulfosarcina-like group and the Desulfovibrio-Desulfomicrobium-like group. Statistical analysis of community patterns revealed dynamic changes over time within these two groups following the contamination. Significant differences in community patterns were recorded in artificially oiled compared with control sediments. Cloning and sequencing of 16S rRNA encoding genes performed after 503 days showed that many of the most abundant sequences were closely related to hydrocarbonoclastic SRB which could have played an active role in the observed biodegradation of aliphatic hydrocarbons. Results from the present study provide useful information on the dynamics of dominant SRB in heavily oil-contaminated sediments and their potential for anaerobic biodegradation for the treatment of spilled oil in anoxic marine environments.     

Mercury cycling and sequestration in salt marshes sediments: an ecosystem service provided by Juncus maritimus and Scirpus maritimus

In this study two time scales were looked at: a yearlong study was completed, and a 180-day decay experiment was done. Juncus maritimus and Scirpus maritimus have different life cycles, and this seems to have implications in the Hg-contaminated salt marsh sediment chemical environment, namely Eh and pH. In addition, the belowground biomass decomposition rates were faster for J. maritimus, as well as the biomass turnover rates. Results show that all these species-specific factors have implications in the mercury dynamics and sequestration. Meaning that J. maritimus belowground biomass has a sequestration capacity for mercury per square metre approximately 4-5 times higher than S. maritimus, i.e., in S. maritimus colonized areas Hg is more extensively exchange between belowground biomass and the rhizosediment. In conclusion, J. maritimus seems to provide a comparatively higher ecosystem service through phytostabilization (Hg complexation in the rhizosediment) and through phytoaccumulation (Hg sequestration in the belowground biomass).     

Response of methane emission to invasion of Spartina alterniflora and exogenous N deposition in the coastal salt marsh

Spartina alterniflora exhibits great invading potential in the coastal marsh ecosystems. Also, nitrogen (N) deposition shows an apparent increase in the east of China. To evaluate CH₄ emissions in the coastal marsh as affected by the invasion of S. alterniflora and N deposition, we measured CH₄ emission from brackish marsh mesocosms vegetated with S. alterniflora and a native plant, Suaeda salsa, and fertilized with exogenous N at the rates of 0 and 2.7 g N m^?2, respectively. Dissolved porewater CH₄ concentration and redox potentials in soils as well as aboveground biomass and stem density of plants were also monitored. The averaged rate of CH₄ emission during the growing season in the S. alterniflora and S. salsa mesocosms without N application was 0.88 and 0.54 mg CH₄ m^?2 h^?1, respectively, suggesting that S. alterniflora plants significantly increased CH₄ emission mainly because of higher plant biomass rather than stem density compared to S. salsa, which delivered more substrates to the soil for methanogenesis. Exogenous N input dramatically stimulated CH₄ emission by 71.7% in the S. alterniflora mesocosm. This increase was attributable to enhancement in biomass and particularly stem density of S. alterniflora driven by N application, which transported greater photosynthesis products than oxygen into soils for CH₄ production and provided more pathways for CH₄ emission. In contrast, there was no significant effect of N fertilization on CH₄ emission in the S. salsa mesocosm. Although N fertilization significantly stimulated CH₄ production by increasing S. salsa biomass, no significant increase in stem density was observed. This fact, along with the low gas transport capacity of S. salsa, failed to efficiently transport CH₄ from wetlands into the atmosphere. Thus we argue that the stimulatory or inhibitory effect of N fertilization on CH₄ emission from wetlands might depend on the gas transport capacity of plants and their relative contribution to substrates for CH₄ production and oxygen for CH₄ oxidation in soil.     

Evaluation of alternative oil spill cleanup techniques in a Spartina alterniflora salt marsh

Three oil spill situations which cause long-term impact were simulated in 1 m(2) salt marsh plots to evaluate the effectiveness of alternative cleanup techniques at removing oil and reducing damage to Spartina alterniflora. Cleanup techniques, implemented 18-24 h after oiling, were not effective at removing oil after sediment penetration. When oil remained on the sediment surface, flushing techniques were most effective at removal, reducing levels of added oil by 73% to 83%. The addition of dispersant to the flushing stream only slightly enhanced oil removal. Clipping of vegetation followed by sorbent pad application to sediment was moderately effective, reducing added oil by 36% to 44%. In contrast to flushing and clipping, burning increased the amount of oil in sediment by 27% to 72%. Although flushing and clipping were effective at oil removal, neither technique reduced initial damage to plants or enhanced long-term recovery. While flushed plots sustained no additional plant damage due to cleanup, clipped and burned plots sustained additional initial plant damage. Based on these results, first considerations should be given to natural tidal flushing as the means to remove oil, especially in salt marshes subject to ample tidal inundation. Although our results do not support cleanup in salt marshes with ample tidal inundation, low pressure flushing may be warranted when fuel oils or large quantities of crude oil impact salt marshes subject to reduced tidal flushing. Flushing, when warranted, should be initiated prior to oil penetration into the substrate. Clipping may be considered as a cleanup response only when heavy oil cannot be effectively removed from vegetation by flushing. Burning is not recommended because it enhances oil penetration into sediment and causes substantial initial plant damage.     

Survival and activity of Streptococcus faecalis and Escherichia coli in petroleum-contaminated tropical marine waters

The in situ survival and activity of Streptococcus faecalis and Escherichia coli were studied using membrane diffusion chambers in tropical marine waters receiving oil refinery effluents. Protein synthesis, DNA synthesis, respiration or fermentation, INT reduced per cell, and ATP per cell were used to measure physiological activity. Cell densities decreased significantly over time at both sites for both S. faecalis and E. coli; however, no significant differences in survival pattern were observed between S. faecalis and E. coli. Differences in protein synthesis between the two were only observed at a study site which was not heavily oiled. E. coli was more active in protein synthesis and respiration than S. faecalis at both oiled and unoiled sites, and the percentage of the E. coli population that was respiring was significantly higher than S. faecalis fermenting cells at both sites. However, S. faecalis cells were more active in DNA synthesis and higher in ATP content than E. coli cells at both sites. Although fecal streptococci have been suggested as a better indicator of fecal contamination than fecal coliforms in marine waters, in this study both E. coli and S. faecalis survived and remained physiologically active for extended periods of time. These results suggest that the fecal streptococci group is not a better indicator of fecal contamination in tropical marine waters than the fecal coliform group, especially when that environment is high in long-chained hydrocarbons.     

EROD activity and stable isotopes in seabirds to disentangle marine food web contamination after the Prestige oil spill

In this study, we measured via surgical sampling hepatic EROD activity in yellow-legged gulls from oiled and unoiled colonies, 17 months after the Prestige oil spill. We also analyzed stable isotope composition in feathers of the biopsied gulls, in an attempt to monitor oil incorporation into marine food web. We found that yellow-legged gulls in oiled colonies were being exposed to remnant oil as shown by hepatic EROD activity levels. EROD activity was related to feeding habits of individual gulls with apparent consequences on delayed lethality. Capture-recapture analysis of biopsied gulls suggests that the surgery technique did not affect gull survival, giving support to this technique as a monitoring tool for oil exposure assessment. Our study highlights the combination of different veterinary, toxicological and ecological methodologies as a useful approach for the monitoring of exposure to remnant oil after a large oil spill.     

Effects of consolidated tailings water on red-osier dogwood (Cornus stolonifera Michx) seedlings

As part of their tailings management, the oil sand industries plan on producing consolidated (composite) tailings (CT), in which an inorganic coagulant aid (gypsum) is added to create a non-segregating deposit. The water associated with this treatment contains potentially phytotoxic levels of sodium, sulfate, chloride, boron, aluminum, fluoride and strontium. Since CT water is expected to saturate deposits in the reclamation areas, it may affect successful reclamation of these sites. Red-osier dogwood (Cornus stolonifera Michx) was demonstrated to be relatively salt resistant and to have high potential for the reclamation of mining areas. In the present study, we used red-osier dogwood to examine the effects of CT water on the accumulation of ions within plant tissue, growth, gas exchange, water potentials and chlorophyll concentration. CT water reduced shoot lengths and dry weights in treated plants. The roots of treated plants accumulated higher concentrations of sodium and chloride than did shoots. The accumulation of sodium and chloride was accompanied by an increase in magnesium and calcium and a decrease in potassium in the roots, while the levels of potassium increased in the leaves. CT water altered gas exchange and water potentials in seedlings, and resulted in a decrease in chlorophyll's a and b. The results suggest that the mechanisms of salt resistance in red-osier dogwood seedlings involve the restriction of sodium transport from roots to shoots.     

Impact of crude oil exposure on nitrogen cycling in a previously impacted Juncus roemerianus salt marsh in the northern Gulf of Mexico

This study investigated potential nitrogen fixation, net nitrification, and denitrification responses to short-term crude oil exposure that simulated oil exposure in Juncus roemerianus salt marsh sediments previously impacted following the Deepwater Horizon accident. Temperature as well as crude oil amount and type affected the nitrogen cycling rates. Total nitrogen fixation rates increased 44 and 194 % at 30 °C in 4,000 mg kg^?1 tar ball and 10,000 mg kg^?1 moderately weathered crude oil treatments, respectively; however, there was no difference from the controls at 10 and 20 °C. Net nitrification rates showed production at 20 °C and consumption at 10 and 30 °C in all oil treatments and controls. Potential denitrification rates were higher than controls in the 10 and 30?ºC treatments but responded differently to the oil type and amount. The highest rates of potential denitrification (12.7?±?1.0 nmol N g^?1 wet h^?1) were observed in the highly weathered 4,000 mg kg^?1 oil treatment at 30 °C, suggesting increased rates of denitrification during the warmer summer months. These results indicate that the impacts on nitrogen cycling from a recurring oil spill could depend on the time of the year as well as the amount and type of oil contaminating the marsh. The study provides evidence for impact on nitrogen cycling in coastal marshes that are vulnerable to repeated hydrocarbon exposure.     

Copper and lead concentrations in salt marsh plants on the Suir Estuary,Ireland

Concentrations of Cu and Pb were determined in the roots and shoots of six salt marsh plant species, and in sediment taken from between the roots of the plants, sampled from the lower salt marsh zone at four sites along the Suir Estuary in autumn 1997. Cu was mainly accumulated in the roots of monocotyledonous and dicotyledonous species. Pb was mainly accumulated in the roots of monocotyledons, while dicotyledons tended to accumulate Pb in the shoots. In the case of Aster tripolium there was a clear differentiation in the partitioning of Pb within the plant, between low and high salinity sites. At the low salinity sites, Pb accumulated only in the roots while at the high salinity sites there was a marked translocation to the shoots. The increase in Pb concentrations in roots and shoots of A. tripolium was accompanied by a concomitant decrease in sediment concentrations of Pb. This inverse correlation between sediment and plant concentrations of Pb was also recorded for Spartina spp. and Schoenoplectus tabernaemontani but in the case of these species the roots contained higher concentrations of Pb regardless of salinity levels. These differences in accumulation of Cu and Pb in various salt marsh species, and the influence of salinity on the translocation of Pb in A. tripolium in particular, should be taken into account when using these plants for biomonitoring purposes.     

The effects of oil spill and clean-up on dominant US Gulf coast marsh macrophytes: a review

The objective of this review was to synthesize existing information regarding the effects of petroleum hydrocarbons on marsh macrophytes in a manner that will help guide research and improve spill-response efficiency. Petroleum hydrocarbons affect plants chemically and physically. Although plants sometime survive fouling by producing new leaves, even relatively non-toxic oils can stress or kill plants if oil physically prevents plant gas-exchange. Plant sensitivity to fouling varies among species and among populations within a species, age of the plant, and season of spill. Physical disturbance and compaction of vegetation and soil associated with clean-up activities following an oil spill appear to have detrimental effects on the US Gulf coast marshes. Other techniques, including the use of chemicals such as cleaners or bioremediation, may be necessary to address the problem. Clean-up may also be beneficial when timely removal prevents oil from migrating to more sensitive habitats.     

HCHs and DDTs in salt marsh plants (Scirpus) from the Yangtze estuary and nearby coastal areas, China

HCHs and DDTs in salt marsh plants taken from intertidal flats in the Yangtze estuary and coastal area in April and July 2002 were determined by GC-ECD. A significant seasonal effect was observed for HCHs and DDTs in sources and concentration levels in different sample types including above-ground tissues and roots as well as the whole plants and rhizospheric sediments. The results indicated that the concentration of t-HCH was higher in the above-ground tissues than in their roots in April; however, the partitioning of DDTs between contaminated sediments and the roots showed the higher concentrations of t-DDT in their roots. HCHs and DDTs concentration levels were higher in above-ground tissues than in roots in July. BCFs of HCHs and DDTs exhibited lower values with higher levels of contaminants in sediments, and higher values with lower levels in sediments.     

Removal of methyl chloroform in a coastal salt marsh of eastern China

The atmospheric burden of methyl chloroform (CH(3)CCl(3)) is still considerable due to its long atmospheric lifetime, although CH(3)CCl(3) emissions have declined considerably since it was included into the Montreal Protocol. Moreover, CH(3)CCl(3) emissions are used to estimate hydroxyl radical (OH) levels, trends, and hemispheric distributions, and thus the mass balance of the trace gas in the atmosphere is critical for characterizing OH concentrations. Salt marshes may be a potential sink for CH(3)CCl(3) due to its anoxic environment and abundant organic matter in sediments. In this study, seasonal dynamics of CH(3)CCl(3) fluxes were measured using static flux chambers from April 2004 to January 2005, along an elevational gradient of a coastal salt marsh in eastern China. To estimate the contribution of higher plants to the gas flux, plant aboveground biomass was experimentally harvested and the flux difference between the treatment and the intact was examined. In addition, the flux was analyzed in relation to soil and weather conditions. Along the elevational gradient, the salt marsh generally acted as a net sink of CH(3)CCl(3) in the growing season (from April to October). The flux of CH(3)CCl(3) ranged between -3.38 and -32.03 nmol m(-2)d(-1) (positive for emission and negative for consumption), and the maximum negative rate occurred at the cordgrass marsh. However, the measurements made during inundation indicated that the mudflat was a net source of CH(3)CCl(3). In the non-growing season (from November to March), the vegetated marsh was a minor source of CH(3)CCl(3) when soil was frozen, the emission rate ranging from 3.43 to 7.77 nmol m(-2)d(-1). However, the mudflat was a minor sink of CH(3)CCl(3) whether it was frozen or not in the non-growing season. Overall, the coastal salt marsh in eastern China was a large sink for the gas, because the magnitude of consumption rate was lager than that of emission, and because the duration of the growing season was longer than that of the non-growing season. Plant aboveground biomass had a great effect on the flux. Comparative analysis showed that the direction and magnitude of the effect of higher plants on the flux of CH(3)CCl(3) depended on timing of sampling vegetation type. In the growing season the plant biomass decreased the gas flux and acted as a large sink of the gas, whereas it presented as a minor source in the non-growing season. However, the mechanism underlying plant uptake process is not clear. The CH(3)CCl(3) flux was positively related to the dissolved salt concentration and organic matter content in soil, as well as light intensity, but it was negatively related to soil temperature, sulfate concentrations, and initial ambient atmospheric concentrations of CH(3)CCl(3). Our observations have important implications for estimation of the tropospheric lifetime of CH(3)CCl(3) and global OH concentration from the global budget concentration of CH(3)CCl(3).     

Relationship between hydrocarbon measurements and toxicity to a chironomid, fish larva and daphnid for oils and oil spill chemical treatments in laboratory freshwater marsh microcosms

This research investigated the extent to which various common hydrocarbon measures can be used to predict toxicity to freshwater aquatic organisms due to fouling by oil. Actual toxicity results, on laboratory freshwater marsh microcosms using two water-column species and a benthic species, were described earlier. The hydrocarbon measures used were TPH(g), TPH(FID), TPH(MS), TTAH (sum of 41 target aromatic hydrocarbons), principal components of 41 TAHs, and each individual TAH. In general, toxicity was more closely related to TPH(MS) levels than to TPH(FID) and (especially) TPH(g) levels. The strongest relationships were found for TTAH levels and for the principal components of the TAHs. Regressions of toxicity on many individual TAHs were also strong, with a single group of compounds explaining as much as 59% of the variation in survival. While the various regressions were highly significant statistically and at times able to accurately predict broad differences in toxicity, the high variation in survival at a specific hydrocarbon concentration indicates that these hydrocarbon measures can not substitute for actual toxicity determinations in accurately ranking the toxicity of samples from oiled freshwater marshes.