10 A
. Bennett et al. J. Exp. Mar. Biol. Ecol. 242 1999 1 –20 Table 1
Changes in carbon:nitrogen ratios over 60-day interval in Low-PAH and High-PAH microcosm sediments collected from Pass Fourchon, LA; error bars indicate one standard deviation
Day C:N in Low-PAH sediment
C:N in High-PAH sediment Without
With With
Without With
With snails
Low-Exposure High-Exposure
snails Low-Exposure
High-Exposure snails
snails snails
snails 60.8666.03
60.8666.03 60.8666.03
19.7961.71 19.7961.71
19.7961.71 28
18.6365.30 73.95656.72
17.4862.68 54.89632.62
49.73648.83 20.0062.98
60 19.2960.21
17.4160.58 15.9462.49
77.98663.20 19.0567.55
30.1361.66
experiment in the treatments with Low-PAH sediments without L . irrorata Fig. 7.
Fucoxanthin, zeaxanthin, and chlorophyll-a showed the same rapid decay in High-PAH sediments within the first ten days of the experiment Figs. 4, 7 and 8. Fucoxanthin
2
concentrations were significantly correlated with chlorophyll-a r 5 0.82 in all treatments throughout the duration of the microcosm experiment data not shown.
Zeaxanthin was rarely detected in Low-PAH sediments Fig. 8. Zeaxanthin con- centrations in High-PAH sediments did not change significantly from Day 28 to Day 60,
and did not show any significant differences due to the presence of L
. irrorata Fig. 8. In all four treatments containing snails, L
. irrorata experienced weight loss over the course of the 60 days of the microcosm experiment Fig. 9. High-Exposure snails
preexposed to High-PAH concentrations experienced more total wet weight loss over the 60 day period than did Low-Exposure snails collected from Low-PAH sediments.
In all treatments L . irrorata experienced some weight gain during the early stages of the
experiment Fig. 9. This weight gain occurred consistently earlier in the High-Exposure snails than in Low-Exposure snails. We were not able to detect significant differences in
PAH concentrations in whole animal tissues collected from Low-Exposure and High- Exposure snails at the beginning and end of the experiment due to the high variability
high standard deviations in replicate concentrations.
4. Discussion
There were significant decreases in the concentrations of total PAHs in all microcosm treatments. These decreases were likely due primarily to bacterial metabolism of PAHs
Cerniglia and Heitkamp, 1989 and secondarily to flushing of PAHs out of the system due to continuous water exchange throughout the experiment. However, the initial
concentrations were maintained up to Day 30, and the relative decreases were the same across all treatments. The relative abundances of HMW and alkylated PAHs in the
High-PAH treatments are indicative of petroleum hydrocarbon contamination at Pass Fourchon, LA Clark, 1989 Fig. 3. The relatively high abundance of HMW
compounds reflects the historical chronic input of PAH at the produced-water site. LMW compounds are relatively rapidly removed by bacterial metabolism and weathering while
A . Bennett et al. J. Exp. Mar. Biol. Ecol. 242 1999 1 –20
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21
Fig. 4. Changes in chlorophyll-a concentrations mg g dry sediment over a 60-day interval in Low-PAH
A and High-PAH B microcosm sediments collected from Pass Fourchon, LA. Error bars indicate one standard deviation.
HMW compounds are resistant to both weathering and metabolism, and accumulate over time Carman et al., 1996.
High-PAH sediments initially had a lower C:N ratio than did Low-PAH sediments due to the dominance of microphytobenthos at the High-PAH field site. These results are
consistent with the premise that a contaminant can eliminate benthic grazers and allow for microphytobenthos to flourish Carman et al., 1996, 1997. Although the standard
deviations of some of the C:N ratios were quite high, we speculate that the decrease in
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. Bennett et al. J. Exp. Mar. Biol. Ecol. 242 1999 1 –20
21
Fig. 5. Changes in phaeophytin-a concentrations mg g dry sediment over a 60-day interval in Low-PAH
A and High-PAH B microcosm sediments collected from Pass Fourchon, LA. Error bars indicate one standard deviation.
C:N ratios of Low-PAH sediments over time was due to an increase in the abundance of diatoms as indicated by increased fucoxanthin concentrations and possibly bacteria in
those treatments Table 1 and Fig. 7. Sediment C:N ratios at Day 0 were significantly lower P , 0.0001 in High-PAH than in Low-PAH sediments, indicating a higher
fraction of carbon sources from microalgae at the High-PAH site at the onset of the experiment. The C:N ratio signature at the Low-PAH site was predominantly vascular at
the beginning of the experiment ca. 60 and shifted to a more mixed microbial and
A . Bennett et al. J. Exp. Mar. Biol. Ecol. 242 1999 1 –20
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21
Fig. 6. Changes in total phaeophorbide concentrations mg g dry sediment over a 60-day interval in
Low-PAH A and High-PAH B microcosm sediments collected from Pass Fourchon, LA. Error bars indicate one standard deviation. BLD 5 below limits of detection.
vascular signature by Day 60 of the experiment ca. 20; over time, carbon resources increased in quality for microbial uptake, as shown by the shift in sedimentary C:N
ratios Table 1. Increases in chlorophyll-a and fucoxanthin concentrations after Day 40 indicated that increased diatom abundance was largely responsible for decreased
C:N ratios in Low-PAH sediments. Zeaxanthin concentrations never recovered after the initial decline, indicating that cyanobacteria did not contribute to observed changes in
C:N ratios.
Meiobenthos may have been more important than L . irrorata in grazing-down
microphytobenthos in microcosm sediments, as decreases in the abundance of micro- phytobenthos and changes in phaeopigment concentrations did not differ significantly
between treatments with and without snails Figs. 4–8. In another study, a significant increase in the abundance of microphytobenthos was observed after the removal of
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. Bennett et al. J. Exp. Mar. Biol. Ecol. 242 1999 1 –20
21
Fig. 7. Changes in fucoxanthin concentrations mg g dry sediment over a 60-day interval in Low-PAH A
and High-PAH B microcosm sediments collected from Pass Fourchon, LA. Error bars indicate one standard deviation.
meiobenthos harpacticoid copepods in experimental sediments collected from Cocod- rie, LA Carman et al., 1997. Our results support previous findings that meiobenthos
can significantly ‘‘graze-down’’ microphytobenthos in Louisiana salt marsh sediments Carman et al., 1997.
Other possible factors that may have contributed to the decrease of microphytobenthic abundance in all microcosm treatments, particularly over the initial 12 days of the
experiment, could have been reduced light and or nutrient availability as compared to
A . Bennett et al. J. Exp. Mar. Biol. Ecol. 242 1999 1 –20
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21
Fig. 8. Changes in zeaxanthin concentrations mg g dry sediment over a 60-day interval in Low-PAH A
and High-PAH B microcosm sediments collected from Pass Fourchon, LA. Error bars indicate one standard deviation. BLD 5 below limits of detection.
field conditions. Although nutrients were not measured and could have been lower than field concentrations, microcosm nutrient concentrations were not likely to be limiting to
microphytobenthos because of the continuous flow of filtered ambient salt marsh water used in this experiment. Although we have observed significantly higher concentrations
of dissolved inorganic nitrogen and phosphorus at Station 1 than at Station 3 Bennett et al., 1999, it is unlikely that any differences in the relative contribution of sedimentary
nutrients to microphytobenthos from the two sediment types was maintained due to the
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. Bennett et al. J. Exp. Mar. Biol. Ecol. 242 1999 1 –20
Fig. 9. Average wet-weight changes g of L . irrorata per sampling period over the 60-day duration of the
microcosm experiment in Low-PAH A and High-PAH B sediments collected from Pass Fourchon, LA. Error bars indicate one standard deviation.
high flushing rate of water through the experimental tank. Light levels in the microcosm were well within the known range in which diatoms can flourish Admiraal, 1984.
Although light levels were well below the observed light irradiance at field sites when
A . Bennett et al. J. Exp. Mar. Biol. Ecol. 242 1999 1 –20
17
sediments were exposed at low tide, the range of light levels observed in the field when
2 21
the sites were covered with water at slack and high tides 250–500 mmol m s
, Bennett et al., 1999 was similar to the light levels used in this experiment ca. 270
2 21
mmol m s . Diatoms were observed growing in thick mats on the surface of the
microcosm water table. The overall weight loss of L
. irrorata in all microcosm treatments may have been due to a limitation of microalgal resources because of meiobenthic grazing Fig. 9. Average
dry-weights of L . irrorata in populations collected from Grand Isle, LA, were shown to
have the least amount of weight change 175 to 201 mg dry weight during the months May to Sept. with the highest temperatures 23 to 278C Shirley et al., 1978. This
period of minimal weight change was considered to be a thermal-neutral zone where respiration and thermal stresses were in ‘‘balance’’. Weight losses of snails collected
from the High-PAH station at Pass Fourchon were greater than in snails collected from the Low-PAH station. Despite the greater weight loss by the High-Exposure snails, these
snails were clearly more active and contributed more to microphytobenthic loss in the early stages of the experiment than did the Low-Exposure snails. Developed resistance
has been demonstrated in benthic invertebrates exposed to environmental contaminants over several generations Klerks and Levinton, 1989.
5. Conclusions
