Methane gas seeps along the oxic/anoxic gradient in the Black Sea: Manifestations, biogenic sediment compounds and preliminary results on benthic ecology

Sediment parameters (grain size, organic content, chloroplastic pigments, adenylates, potential hydrolytic activity and electron transport system activity) and benthic biota were studied in a methane seep area south-west of the Crimean Peninsula in the Black Sea over a depth range from 60 to 260 m. A control transect with similar depths was performed in an adjacent nonseep area. Methane seepage in this region occurs from 35 to 785 m depth, passing the oxic/anoxic interface zone at 130 to 180 m. The methane seep areas were characterized by abundant carbonate precipitates which occurred in various shapes from small, flat structures to tall chimneys with increasing anoxia. The carbonates were associated with distinct bacterial mats. Most of the measured biochemical parameters in the sediment were quite similar in the seep and nonseep areas. The content of organic matter was higher and grain size was more uniform in the nonseep areas. However, the seep areas were characterized by higher proportions of the larger fauna (size classes 0.5-1 mm and > 1 mm) as well as increased total numbers of benthic fauna in the suboxic and upper anoxic zone. The animals did not show any seep-specific adaptations. In addition to methane seepage, highly variable hydrochemical and sedimentary conditions on the lower shelf and upper slope may also play an important role in structuring the composition and distribution of the benthic fauna.