The worldwide increasing recourse to chemical dispersants to deal with oil spills in marine coastal ecosystems is a controversial issue. Yet, there exists no adequate methodology that can provide reliable predictions of how oil and dispersant-treated oil can affect relevant organism or population-level performance. The primary objective of the present study was to examine and compare the effects of exposure to untreated oil (weathered Arabian light crude oil), chemically dispersed oil (Finasol, TOTAL-Fluides) or dispersant alone, upon the ability of fish for environmental adaptation. To reach that goal, we implemented high-throughput, non-lethal challenge tests to estimate individual hypoxia and heat tolerance as surrogate measures of their capacity to face natural contingencies. Experimental populations were then transferred into semi-natural tidal ponds and correlates of individuals' fitness (growth and survival) were monitored over a period of 6 months. In accordance with our stated objectives, the contamination conditions tested corresponded to those observed under an oil slick drifting in shallow waters. Our results revealed that the response of control fish to both challenges was variable among individuals and temporally stable (repeatable) over a 2-month period. Exposure to chemical dispersant did not affect the repeatability of fish performance. However, exposure to oil or to a mixture of oil plus dispersant affected the repeatability of individuals' responses to the experimental challenge tests. At population level, no difference between contamination treatments was observed in the distribution of individual responses to the hypoxia and temperature challenge tests. Moreover, no correlation between hypoxia tolerance and heat tolerance was noticed. During the field experiment, hypoxia tolerance and heat tolerance were found to be determinants of survivorship. Moreover, experimental groups exposed to oil or to dispersant-treated oil displayed significantly lower survival than control or dispersant-exposed groups. Finally, from the four experimental populations tested, the one exposed to chemically dispersed oil presented the lowest growth rate.

Laboratory experiments performed in a recirculating flume were designed to quantify the bioturbation influence of the mud snail Hydrobia ulvae, one of the most abundant deposit feeders on European intertidal mudflats. Variations in sediment moisture content that occur between bedforms in shorenormal, ridge and runnel systems of intertidal mudflats were added to the model definition. Sediment erosion thresholds, erosion rates and the microalgal pigment composition of resuspended material were quantified for different H. ulvae densities (0, 1000, 5000, 10 000 and 50 000 snails m− 2) and applied bed shear stresses. Two different sediment moisture contents were tested. In the absence of macrofauna, recorded turbidities increased up to the maximum applied value of 1.6 Pa and the addition of snails increased the resuspended mass at all bed shear stresses tested. The amount of resuspended mass depended on snail density; the amount of resuspended material was highest in sediment test beds that had the highest moisture contents (ridge-type sediments) and snail density. Resuspended and bioturbated sediment was characterised by an enrichment in phaeopigments compared to the underlying sediment. A nine-parameter model, which included sediment moisture content as a new variable, gave a reasonably good estimate of the resuspension of ridge-and runneltype sediments for the range of snail densities. An eight-parameter version of the model was adequate for quantifying erosion rates on intertidal mudflats not characterised by a ridge/runnel geomorphology.