MEPS 138:71-82 (1996)  -  doi:10.3354/meps138071

Distribution and abundance of benthic and demersal macrofauna within a natural hydrocarbon seep

Steichen DJ Jr, Holbrook SJ, Osenberg CW

Natural oil seeps provide a heightened supply of organic carbon that can potentially result in higher numbers of infaunal organisms, yet toxic effects of oil or its by-products could have negative impacts. These contrasting influences make oil seeps an ideal setting to investigate the effects of a tradeoff between enrichment and toxicity. We explored small-scale (1 to 10 m) spatial patterns of distribution of infauna and physical and chemical characteristics of sediments within an oil seep area located near Santa Barbara, California, USA. Correlation and principal components analyses revealed that physicochemical parameters of the sediments (dissolved sulfide, volatile solids, organic carbon and nitrogen, sediment grain size) generally covaried with one another; oil-rich samples had large grain size and high concentrations of organic material. Densities of nematodes were positively correlated with amount of oil present, whereas all other major taxa (polychaetes, gammarids, oligochaetes, bivalves, copepods, ostracods, cumaceans) were negatively correlated. These results contrasted with patterns that we observed at a much larger spatial scale (up to 1 km) where the number of organisms colonizing trays of clean sediments was greater at 2 seepage sites compared to 2 sites that lacked seeps. A field experiment tested the effects of hydrocarbon seepage on the distribution and abundance of macrofauna. Colonization by infauna was assessed in trays of sediments that varied in organic enrichment (clean sediment, sediment with fresh oil, weathered oil, or sulfide). The results were consistent with small-scale (within-seep) patterns observed in the field survey; after 24 h, nematode densities were 3 times higher in the enriched trays, whereas all other taxa were most abundant in the clean trays. The results suggest that the biological effects of oil seepage are scale-dependent and that behavioral mechanisms involving microhabitat selection may contribute to the observed spatial patterns of distribution.

Macrobenthos . Hydrocarbon seeps . Organic enrichment

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