Summary 1.Remnants from oil spills can persist for decades within anoxic coastal sediments affecting local flora and fauna, but few studies have examined the long-term impacts of the residual petroleum on these valuable coastal ecosystems. 2. The Wild Harbor salt marsh (Buzzards Bay, Massachusetts, USA) still harbours residual petroleum from a 1969 oil spill, which released 700 000 L of No. 2 fuel oil. Previous effects have been noted in fiddler crabs and ribbed mussels inhabiting the oiled marsh. 3. Spartina alterniflora biomass was sampled above-and below-ground in elevational transects through areas with different total petroleum hydrocarbon (TPH) content. Further, elevational differences were mapped in oiled and non-oiled locations. 4. Stem density and above-and below-ground biomass decreased in oiled areas. The decreased vegetation biomass led to unconsolidated sediments, increased topographical variation and, ultimately, loss of salt marsh habitat. 5. Four decades after the Florida spill, with only 100 kg of the original 595 000 kg of spilt oil persisting in salt marsh sediment, the effects on large-scale ecosystem functions are still evident. 6. Synthesis and applications . Our results demonstrate specific impacts of residual petroleum on S. alterniflora biomass and coastal erosion. Further, we have defined TPH content thresholds at which to expect long-term ecosystem impacts. The recalcitrant nature of these contaminants and their chronic large-scale ecosystem effects leads to the potential demand for alternative compensation, such as preservation of coastal systems at risk elsewhere.

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In September 1969, the Florida barge spilled 700,000 L of No. 2 fuel oil into the salt marsh sediments of Wild Harbor, MA. Today a substantial amount, approximately 100 kg, of moderately degraded petroleum remains within the sediment and along eroding creek banks. The ribbed mussels, Geukensia demissa, which inhabit the salt 2 marsh creek bank, are exposed to the spilled oil. Examination of short-term exposure was done with transplantation of G. demissa from a control site, Great Sippewissett marsh, into Wild Harbor. We examined the effects of long-term exposure with transplantation of mussels from Wild Harbor into Great Sippewissett. Both the short-and long-term exposure transplants exhibited slower growth rates, shorter mean shell lengths, lower condition indices, and decreased filtration rates. Our results add new knowledge about long-term consequences of spilled oil, a dimension that should be included when assessing oil-impacted areas and developing management plans designed to restore, rehabilitate, or replace impacted areas. Capsule Ribbed mussels exposed to residual petroleum in salt marsh sediments, in short-term and long-term transplant experiments, exhibited significant effects that should be considered when assessing oil-impacted areas and developing management plans.