VERTIMAR-2005
Symposium on Marine Accidental Oil Spills
Decrease and rebound effect of some indicator taxa after the
Prestige oil spill in the Galician Atlantic shelf
Alberto SERRANO1, Francisco SÁNCHEZ1, Izaskun PRECIADO1, Santiago PARRA2
and Inmaculada FRUTOS2
1 Instituto Español de Oceanografía, Santander, Spain
2 Instituto Español de Oceanografía, A Coruña, Spain
ABSTRACT. Two years after the Prestige oil spill (POS) an assessment of the effects on benthic fauna was carried
out using the data obtained in 5 multidisciplinary surveys. Otter trawl was used to study demersal, and megaepibenthic
communities and a beam trawl was used to study macro- and megaepibenthic fauna along eight transects perpendicular
to the coastline. Beam trawl was also employed to quantify the amount of tar aggregates on the continental shelf. No
correlation between tar concentration and indicator densities was detected. Nevertheless, a decrease in the densities of
several megafaunal indicators was detected the first year after spill, followed by a noteworthy recovery in 2004. Nonmacroscopic toxicity and some oceanographic agents are suggested as possible causes of these shifts.
1. INTRODUCTION
The Prestige oil spill (POS) resulted in the release of over 50 000 tons of heavy oil (type M100) 250 miles from the Galician coastline in oceanic waters (Northwest Iberian Peninsula) in
November 2002. Heavier fractions of oil reached the bottom by dropping from the water column
as tar aggregates with low bioavailability or in the form of small toxic particles in sea snow.
Therefore, the taxa initially affected by those sedimented oil components are assumed to be
secondary producers, suspension feeders and detritivorous organisms, followed by
planktophagous and benthophagous organisms in the trophic web. These possible shifts in the
abundances of lower trophic levels would unleash cascading bottom-up type ecosystem effects
(Peterson et al., 2003). On the other hand, top-down effects may also be relevant due to spatiotemporal prohibitions on trawling following the POS, which reduced fishing mortality and led to
an enhancement in the biomass of top predators that affected lower trophic levels. These
cascading changes may affect benthic taxa in different ways. Several sensitive or opportunistic
taxa can be used as indicators. Megabenthic species of slow growth and with slow recovery
capability, mainly crustaceans and echinoderms, show a high sensitivity to oil exposure. Several
studies have shown a fall in populations of sensitive species, such as megafaunal crabs,
gastropoda and echinoderms (Elmgren et al., 1983; Feder & Blanchard, 1998). Furthermore,
initial mortalities may be followed by extreme fluctuations of stress-tolerant or opportunistic
species (Suchanek, 1993). The main goal of the present work was to study variations in
indicator taxa following the POS and to analyse the relevance of tar aggregates on the
abundance and composition of taxa distribution.
2. RESULTS AND DISCUSSION
The results obtained in the present work suggest that tar aggregates found on the bottoms
of the Galician continental shelf after the POS did not affect the distribution of demersal and
benthic indicators. Tar aggregate abundance did not show any significant correlation with
indicator densities. The low bioavailability of tar aggregates probably provides the explanation
for these results. Nevertheless, negative correlations between depth and Triglidae, Catshark
and Paguridae densities were found showing that variable as the main factor in indicator
distribution.
The lack of correlations between the distribution of macroscopic tar aggregates and the
distribution of all benthic compartments does not imply an absence of effects of the POS in shelf
communities. Soluble components and toxic sea snow (microparticled oil, oiled plankton) are
not estimated by our methodology, and these sources of toxicity may affect the whole area.
All megafaunal indicator taxa well-sampled in the otter trawls (rays and urchins did not
present significant yearly variations), except pouting, monkfish and starfishes showed a
significant decrease in density in 2003 with respect to pre-POS years (Fig. 1). Indicators
VERTIMAR-2005
Symposium on Marine Accidental Oil Spills
showing a clearer decrease in 2003 were benthic species with a higher muddy-affinity, and
benthophagous fishes. All taxa except pouting and Paguridae presented a significant recovery
the following year with respect to 2003 (Fig. 1). Figures 2-5 show the decrease in 2003 and the
recovery of 2004, and how these shifts in indicators densities affected the whole area, not only
the maximum impact zone. SIMPER multivariate analysis on the megafaunal indicators in the
maximum impact zone indicated that the same taxa that typified the post-POS decrease typified
the clear recovery in 2004. Plesionika heterocarpus, Munida sp. and flatfishes were the three
species that most percentage of similarity explained in the perturbation dichotomy (2002-03),
and also in the recovery dichotomy (2003-04).
This rapid recovery of indicator taxa in autumn 2004 is remarkable. Several studies reported
benthic long-term recovery over 3-15 years following oil spills (e.g. Feder & Blanchard, 1998). In
contrast, there are scarce reports on the quick rebound of benthic fauna after post-spill
decreases (Elmgren et al., 1983; Dean et al., 1996) that is explained as a consequence of the
recovery from unoiled adjacent sites. Nevertheless, the absence of unoiled areas in the shelf
after the POS and the evidence of density decreases throughout the Galician shelf may indicate
that the origin of recovery is to be found in causes other than emigration. Enhanced nutrient
availability and changes in interspecific relationships, such as a decrease in predation or
competition pressure, may be causes of the rebound effect detected.
The fall in the indicator densities in 2003 may reflect bottom-up changes derived from the
POS, although these changes may also be related to other factors not taken into account in this
work. Factors such as upwelling and outwelling processes or the strength of the Navidad” flow
are of great importance to the benthic regime of the Galician shelf, determining primary and
secondary production changes or larval advection. Nevertheless, a historical perspective is
required to corroborate the megafaunal decrease/rebound shifts of 2002-2003 or the absence of
infaunal indicator changes, since important interannual fluctuations in benthic fauna may be
characteristic in the Galician continental shelf.
REFERENCES
Dean, T.A., Jewett, S.C., Laur, D.R., Smith, R.O., 1996. Injury to epibenthic invertebrates
resulting from the Exxon Valdez oil spill. In R.B. Spies, D.A. Wolfe, B.A. Wright,
Proceedings of the Exxon Valdez Symposium, pp. 424-439. American Fisheries Society
Symposium
Elmgren, R., Hansson, S., Larsson, U., Sundelin, B., Boehm, P.D., 1983. The "Tsesis" oil spill:
acute and long-term impact on benthos. Mar. Biol. 73, 51-65.
Feder, H.M., Blanchard, A., 1998. The deep benthos of Prince William Sound, Alaska, 16
months after Exxon Valdez oil spill. Mar. Poll. Bull. 36, 118-130.
Peterson, C.H.; Rice, S.D.; Short, J.W.; Esler, D.; Bodkin,J.L.; Ballachey, B.E. & Irons, D.B.
2003. Long-term ecosystem response to the Exxon Valdez oil spill. Science, 302, 20822086
Suchanek, T.H., 1993. Oil impacts on marine invertebrate populations and communities. Amer.
Zool. 33, 510-523.
VERTIMAR-2005
Symposium on Marine Accidental Oil Spills
Figure 1. Densities (number / 30 min otter trawl ± SE) of indicator species or groups of species
in the pre-POS and post-POS periods.
350
30
250
number / 30 min traw l
number / 30 min t raw l
300
200
150
100
20
10
50
0
0
Four-spot megrim
Other flatfis hes
Forkbeard
Spotted
dragonet
Triglidae
15
6000
number / 30 min traw l
number / 30 min t rawl
5000
10
5
4000
3000
2000
1000
0
0
Pouting
Catshark
Monkfish
Plesionika heterocarpus
Munida sp
15
50
number / 30 min traw l
number / 30 min traw l
40
30
20
10
5
10
0
0
Crabs
2001
Nephrops
Paguridae
2002
--- POS ---
Starfishes
2003
2004
VERTIMAR-2005
Symposium on Marine Accidental Oil Spills
Figure 2. Spatial distribution of flatfishes (L. boscii excluded) densities in the study area in the
pre-POS and post-POS periods (historical series of surveys 2001-04).
44.0°
500
44.0°
0
20
0
10
43.5°
43.5°
43.0°
43.0°
FLATFISHES
Density (nº ∙ haul)
Autumn 2001
FLATFISHES
Density (nº ∙ haul)
Autumn 2002
0
0
125
42.5°
125
42.5°
250
10.0°
9.5°
9.0°
8.5°
250
500
500
1000
1000
8.0°
10.0°
44.0°
44.0°
43.5°
43.5°
43.0°
9.5°
9.0°
8.5°
43.0°
FLATFISHES
Density (nº ∙ haul)
Autumn 2003
8.0°
FLATFISHES
Density (nº ∙ haul)
Autumn 2004
0
0
125
42.5°
125
42.5°
250
10.0°
9.5°
9.0°
8.5°
250
500
500
1000
1000
8.0°
10.0°
9.5°
9.0°
8.5°
8.0°
Figure 3. Spatial distribution of forkbeard (Phycis blennoides) densities in the study area in the
pre-POS and post-POS periods (historical series of surveys 200144.0°
500
44.0°
0
20
0
10
43.5°
43.5°
43.0°
FORKBEARD
Density (nº ∙ haul)
Autumn 2001
43.0°
FORKBEARD
Density (nº ∙ haul)
Autumn 2002
0
10
42.5°
0
10
42.5°
20
20
40
40
80
10.0°
9.5°
9.0°
8.5°
8.0°
80
10.0°
44.0°
44.0°
43.5°
43.5°
43.0°
FORKBEARD
Density (nº ∙ haul)
Autumn 2003
9.5°
9.0°
43.0°
8.5°
FORKBEARD
Density (nº ∙ haul)
Autumn 2004
0
10
42.5°
0
10
42.5°
20
20
40
40
80
04).
10.0°
9.5°
9.0°
8.5°
8.0°
8.0°
80
10.0°
9.5°
9.0°
8.5°
8.0°
VERTIMAR-2005
Symposium on Marine Accidental Oil Spills
Figure 4. Spatial distribution of spotted dragonet (Callyonimus maculatus) densities in the study
area in the pre-POS and post-POS periods (historical series of surveys 2001-04).
44.0°
500
44.0°
0
20
0
10
43.5°
43.5°
43.0°
SPOTTED
DRAGONET
Density (nº ∙ haul)
Autumn 2001
43.0°
SPOTTED
DRAGONET
Density (nº ∙ haul)
Autumn 2002
0
10
42.5°
0
10
42.5°
20
20
40
40
80
10.0°
9.5°
9.0°
8.5°
8.0°
80
10.0°
44.0°
44.0°
43.5°
43.5°
43.0°
SPOTTED
DRAGONET
Density (nº ∙ haul)
Autumn 2003
9.5°
9.0°
43.0°
8.5°
SPOTTED
DRAGONET
Density (nº ∙ haul)
Autumn 2004
0
10
42.5°
0
10
42.5°
20
20
40
40
80
10.0°
9.5°
9.0°
8.5°
8.0°
8.0°
80
10.0°
9.5°
9.0°
8.5°
8.0°
Figure 5. Spatial distribution of crab densities in the study area in the pre-POS and post-POS
periods (historical series of surveys 2001-04).
44.0°
500
44.0°
0
20
0
10
43.5°
43.5°
43.0°
CRABS
Density (nº ∙ haul)
Autumn 2001
43.0°
CRABS
Density (nº ∙ haul)
Autumn 2002
0
50
42.5°
0
50
42.5°
100
100
200
200
400
10.0°
9.5°
9.0°
8.5°
8.0°
400
10.0°
44.0°
44.0°
43.5°
43.5°
43.0°
CRABS
Density (nº ∙ haul)
Autumn 2003
9.5°
9.0°
43.0°
8.5°
CRABS
Density (nº ∙ haul)
Autumn 2004
0
50
42.5°
0
50
42.5°
100
100
200
200
400
10.0°
9.5°
9.0°
8.5°
8.0°
8.0°
400
10.0°
9.5°
9.0°
8.5°
8.0°