derived material originating in the plankton, on land or in the littoral zone. Most existing knowledge about infaunal nutrition at seeps is derived from analyses of 813C, 815N, and sometimes 834S signatures of animal tissues, which reflect both diet and trophic level (Conway et al. 1994). Scientists have assessed the relative importance of chemosynthetic vs. photosynthetic food sources, and in some cases evaluated the significance of methanotrophy vs. thiotrophy or the relative contribution of methane-derived carbon using stable isotopic analyses.
A range of microbial metabolic processes involving a diversity of electron donors and electron acceptors may generate chemosynthetic food sources. Under aerobic conditions these may include H oxidation, S oxidation, Fe oxidation and methane oxidation. Anaerobic processes include meth-anogenesis, S and sulphate reduction, H oxidation, denitrification and methane oxidation (Tunni-cliffe et al. 2003). The relative importance of these in overall seep ecosystem metabolism has not been established. Growing use of lipid biomarkers, particularly fatty acids (Fullarton et al. 1995, Pond et al. 1997, 1998) offers promise for higher resolution of dietary components but these techniques are just starting to be applied to seeps.
There are few studies of protozoan or metazoan meiofaunal diets at seeps largely because the effort required to obtain sufficient amounts of organic matter for isotopic analyses is prohibitory. Documentation of ectosymbioses in seep nematodes (Dando et al. 1991, Jensen et al. 1992) and in euglenoids and ciliates (Buck & Barry 1998) indicates a possible nutritional role for chemosynthetic bacteria in these taxa. Nematode isotope signatures presented by Van Dover et al. (2003) for Blake Ridge seeps (2,155 m) reveal very light 813C (-45 to -50%c) but relatively heavy 815N (9-11%c), suggesting utilization of chemosynthetically derived organic matter, but at a high trophic level, possibly as decomposers. Werne et al. (2002) present molecular isotopic evidence that bacterivorous ciliates are feeding primarily on methane-derived carbon from archaea and bacteria at the Kazam mud volcano in the Mediterranean Sea, based largely on the isotopic signatures of tetrahymenol, a lipid biomarker produced by marine ciliates when they use procaryotes as their sole food source.
Among the macrofauna, stable-isotope based nutritional investigations have now been carried out for a variety of shallow- and deep-water seeps in the Atlantic, Pacific and Gulf of Mexico. The absence of significant chemosynthetic contribution to the nutrition of shallow-water seep infauna appears to be a widespread phenomenon. 813C signatures for seep infauna typically ranged between -16 and -20 %o for methane seeps at 115 m in the North Sea (Dando et al. 1991) and at 35-55 m off northern California (Levin et al. 2000), indicating a largely marine plankton-derived diet. Similarly, at bubbling reefs in the Kattegat (Denmark), the rock invertebrates had signatures of -17 to -34% (Jensen et al. 1992). There are some exceptions, notably nematodes, pogonophorans and a thyasirid in the North Sea (Dando et al. 1991, 1994). Similar observations of limited chemosynthetic contributions to diet have been made for epifauna on seep carbonates at 10-12 m in the Kattegat (Jensen et al. 1992), and for fishes and crustaceans at a 132-m seep off Oregon (Juhl & Taghon 1993). High availability of photosynthetically produced food may be partly responsible for these observations.
In contrast to results from shallow water, macrofauna from seeps at bathyal and abyssal depths show a highly variable dependence on chemosynthetic food sources (Figure 10). This dependence has been examined for seeps in the northeast Pacific Ocean off Alaska (4,445 m), Oregon (590 m), California (500 m), in the Gulf of Mexico (2200-3300 m) and the Blake Ridge in the Atlantic Ocean (2155 m). Average infaunal 813C signatures are exceptionally light at methane seeps in the Gulf of Alaska and on the Oregon Margin (-41 to -46%e) (Levin & Michener 2002), in sul-phidic/microbial mat sediments of the Florida Escarpment in the Gulf of Mexico (average -51%e) (Levin et al. unpublished data), and on the Blake Ridge (-40 to -50%e) (Van Dover et al. 2003). At these sites the majority of infauna appear to be using local chemosynthetic production from the seep. Estimates of methane contribution to the carbon pool for seep infauna range from 32-51% for Gulf of Alaska pogonophoran fields, 20-44% for Oregon microbial mats (Levin & Michener
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