O

34-1084 Polychaetes and copeopods were present Olu et al. 1997 g at similar densities; kinorhynchs M

present at one site H

Nematodes, oligochaetes. polychaetes. Jensen etal. 1992 Leptonemella aphanothecae with external symbiotic bacteria, penetrates to 22 cm seeps (16 m, Montagna et al. 1987), eastern Pacific methane seeps in Monterey Bay (906 m, mainly nematodes and ciliates, Buck & Barry 1998), at the Barbados Accretionary Prism (Olu et al. 1997) and in microbial mats in the Gulf of Mexico (2,230 m) and on Blake Ridge (2,150 m, Robinson et al. 2004). Most of these density enhancements are modest compared with the order-of-magnitude enhancement seen for megafauna relative to ambient sediments. However, Olu et al. (1997) documented one to two orders of magnitude greater meiofauna densities on mud volcanoes at 5000 m than expected for non-seep sediments at these depths. In contrast, little or no density difference from control sites was observed for meiofauna from hydrocarbon seeps off Santa Barbara, California (15 m water depth, Montagna & Spies 1985), the Hatsushima seep off Japan (1170 m, Shirayama & Ohta 1990) or brine seeps in the Gulf of Mexico (70 m, Powell & Bright 1981, Powell et al. 1983). Reduced meiofaunal densities occurred at shallow methane seeps in the North Sea (150 m, Dando et al. 1991) and off Denmark (10 m, Jensen et al. 1992). Often the density patterns are driven by nematodes. Variability of meiofaunal densities appears to be higher within than outside seep sediments due to increased habitat heterogeneity (Montagna & Spies 1985).

While counts or biovolume are the most common means of assessing meiofaunal abundance, Sommer et al. (2002) used DNA and ATP estimates of small-sized benthic biomass. At gas hydrate-fuelled seeps on the Oregon margin (790 m) they found DNA inventories 3.5-3.9 times higher in clam-bed and bacterial mat sediments than in background sediments. Total adenylates from seeps exceeded those from non-seep settings by 3.5 and 5.9 times in clam-bed and bacterial mat sediments, respectively.

Most seep studies record nematodes as the dominant taxon (Table 2), but this is typically true of ambient deep-sea sediments as well. Nematode:copepod ratios range from 4 to 10 at shallow seeps but can exceed 1000 in deep-seep sediments (Table 2). Nematodes exceed foraminiferans as the dominant biomass contributor in Monterey Bay seeps (Buck & Barry 1998) and in density at Hatsushima Cold Seep (Shirayama & Ohta 1990). At the Hatsushima seep, the fraction of nematodes dropped from 94% in the centre of a Calyptogena soyae bed to 55% near the edges and 64% in non-seep sediments; nematode:harpacticoid copepod ratios were 188, 4.2 and 6.8, respectively (Shirayama & Ohta 1990). Nematodes formed a higher percentage (88%) of the fauna at an active hydrocarbon seep off Santa Barbara than in low seepage conditions (76%) or non-seep sediments (78%) and the ratio of nematodes:harpacticoid copepods dropped from 40.1 to 9.7 with decreasing seepage (Montagna et al. 1987). Only four copepod species were present inside Beggiatoa mats at Santa Barbara seeps, compared with 34 species outside (Montagna & Spies 1985). In Beggiatoa mats in Alaminos Canyon, Gulf of Mexico, nematode representation (percentage of total) was equivalent to that in non-seep sediments (75%) (Robinson et al. 2004).

Nematodes are not always the dominant meiofaunal taxon at seeps. At a shallow brine seep in the Gulf of Mexico (72 m, East Flower Garden Bank) the meiofauna was dominated by gnathos-tomulids, with platyhelminths, aschelminths, nematodes and amphipods present (Powell & Bright 1981, Powell et al. 1983). The Flower Garden fauna is described as a thiobios that is dependent on continuous presence of hydrogen sulphide and has well-developed detoxification mechanisms. On the Blake Ridge, nematodes from Arcobacter mats and mussel beds formed only 36-56% of the metazoan meiofauna and harpacticoid copepods were surprisingly well represented (34-50%) in these settings (Table 2) (Robinson et al. 2004). Shirayama & Ohta (1990) noted the absence of kinorhynchs and ostracods at Japanese methane seeps, but Olu et al. (1997) reported kinorhynchs from Barbados mud volcanoes. Both groups are present in Alaminos Canyon in the Gulf of Mexico (Robinson et al. 2004). Because many of these studies are based on only two or three cores at each site, definitive statements about seep avoidance by specific taxa cannot be made.

Sulphidic seep sediments might be expected to reduce diversity and elevate dominance, as has been found in hydrothermal vent meiofauna (Vanreusel et al. 1997). Shirayama and Ohta (1990) noted reduced H' among meiofauna at seeps but recorded higher dominance in non-seep sediments.

In a North Sea pockmark, the edges exhibited greater nematode species richness per core (69 and 75 species) than the more active base (29 and 37 species) (Dando et al. 1991).

A detailed comparison of dominant nematode families and genera at the Hatsushima seep, the East Flower Garden Cold Seep, and East Pacific Rise by Shirayama & Ohta (1990) reveals some overlap in families (Xyalidae, Linhomoeidae, Chromadoridae, Cyatholaimidae were at two or three of these), but remarkably little overlap at the genus level. In contrast, nearby control and seep meiofauna had more genera in common. This difference led the authors to suggest that meiofauna may evolve adaptations to seep conditions locally. The species list of nematodes at North Sea pockmarks provided by Dando et al. (1991) also indicates that Linhomoeidae and Chromadoridae are abundant seep families, with large numbers of Comesomatidae, Leptolaimidae and Siphanolaim-idae also present in pockmarks.

Relation to sediment conditions

Strong gradients in sulphide and oxygen could be expected to regulate the biology and distribution patterns of metazoan meiofauna. Measurements of porewater solute concentrations made on the same scale as the meiofauna body size (mm) (sensu Meyers et al. 1988) could reveal much about the tolerances and preferences of taxa but such measurements have not been made for seep meiofauna. However, there are instances of careful documentation of vertical distribution patterns, symbioses and body morphology in relation to seep conditions that provide insight about how meiofauna interact with their sedimentary environment.

A deeper vertical distribution of seep meiofauna (compared with non-seep assemblages) has been observed for deep-water Japan cold seeps (Shirayama & Ohta 1990). In contrast, at an active shallow hydrocarbon seep the nematodes were concentrated in the upper 2 cm, with reduced density at 6-8 cm relative to control sediments (Montagna et al. 1989). Most other meiofaunal taxa were largely restricted to surface sediments in Montagna's study and thus showed no distinct vertical pattern. Powell et al. (1983) and Jensen (1986) propose that hydrogen sulphide is the primary control on gnathostomulid, nematode and other meiofaunal distributions and diversity in Gollum's Canyon, East Flower Garden in the Gulf of Mexico. None of these taxa, however, had symbionts. At methane-seep pockmarks in the North Sea, the symbiont-bearing nematode Astomonema sp. exhibited a density maximum at 5-8 cm, corresponding to the peak of elemental sulphur content (presumably a product of sulphide oxidation) occurring just above the zone of maximum sulphate reduction and sulphide concentration (Dando et al. 1991). The tight link between these properties suggests control of nematode vertical distribution by sediment geochemistry.

Jensen (1986) reported body elongation in thiobiotic nematodes from the Flower Garden brine seeps (Gulf of Mexico). In Monterey Bay (906 m) nematodes with the largest body diameter were from methane seeps (compared with control sediments) but these exhibited no difference in length:diameter relationships (Buck & Barry 1998).

A high incidence of bacterial symbioses has been reported for euglenoid and ciliate meiofauna from Monterey Bay seeps (Buck et al. 2000), which is similar to that observed for meiofauna in the low-oxygen Santa Barbara Basin (Bernhard et al. 2001). Symbiont-bearing nematodes have been reported from several shallow seeps. Leptonemella aphanothecae occurs in sandy seep sediments of the Kattegat, Denmark, to depths of 22 cm (Jensen et al. 1992) and Astomonema sp. was dominant in pockmark sediments from the North Sea (Dando et al. 1991). It is unknown whether the symbionts in these two species contribute to sulphide detoxification, nutrition or other functions.

Additional remaining questions include (a) the extent to which seep meiofauna show specialized adaptations to distinct microhabitats (e.g., clam beds, mussel beds, bacterial mats), (b) the modes of nutrition and importance of chemosynthetically fixed carbon sources, (c) successional sequences or relation to seepage intensity and (d) the evolution of specific groups in reducing conditions associated with vents and seeps.

Macrofauna Abundance, biomass, composition and endemism

Density Despite highly sulphidic conditions present in seep sediments, these environments often support surprisingly high densities of macrofauna. Estimates of density vary with the mesh size employed but values of >10,000 ind m-2 are common and local patches of >40,000 ind m-2 can occur (Table 3).

Comparisons of macrofauna from seep and non-seep sediments reveal that the total macrofaunal densities at seeps may be impoverished (North Sea, Dando et al. 1991), enhanced (Santa Barbara, Davis & Spies 1980, Oregon, Sahling et al. 2002, Gulf of Mexico, Levin et al. unpublished data), or identical (Levin et al. 2003) to those in nearby non-seep sediments. Seep macrofauna appear more likely to exhibit higher densities relative to ambient (background) fauna at greater water depths (e.g., >3000 m) (Table 3), perhaps because food is more limiting and methane provides a valuable additional carbon source (Levin & Michener 2002). Variability in the relationship between seep and non-seep macrofaunal densities appears not to be directly related to the geochemistry of seep sediments. Sediments with concentrations of H2S up to 20 mM appear to support high densities (albeit low diversity) of infauna (Sahling et al. 2002, Levin et al. 2003).

Biomass Biomass is generally dominated by tubeworms and bivalves, with single site values of 1000-3000 kg m-2 (wet wt) common (Sibuet & Olu-LeRoy 2002). There is a strong positive relationship between bivalve biomass and fluid flow that transcends seep types (e.g., mud volcano sides, slide scarps) (Sibuet & Olu-LeRoy 2002). Among the smaller macrofauna, biomass is highly variable, ranging from 2-170 g m-2 (Table 3). Macro-infauna of Calyptogena beds at Hydrate Ridge exhibited an order of magnitude higher biomass (162 g m-2) than those in background sites (10 g m-2) but microbial mat and Acharax communities did not (Sahling et al. 2002). So few measurements have been made of infaunal biomass, however, that these values are unlikely to represent the full range present at seeps.

Endemism The extent to which macrofauna inhabiting seeps form a distinct assemblage different from non-seep habitats appears to be partially a function of depth. Methane seeps on the shelves off California, Oregon and in the North Sea had dense macrofaunal populations but few endemics (Dando et al. 1991, Levin et al. 2000). Species showing a strong preference for sulphidic seeps on the northern California shelf (35-55 m) were the amphipod Cheiremedeia zotea, the isopod Syni-dotea angulata, the cumacean Diastylopsis dawsoni and the polychaete Capitella sp. (Levin et al. 2000). North Sea pockmarks had high densities of Siboglinum and Thyasira. From sampling cold seep sites on the outer shelf (160-250 m), upper slope (250-450 m), intermediate slope (450-800 m) and deeper bathyal zones (1450-1600 m) in the Sea of Okhotsk, Sahling et al. (2003) concluded that seep endemic faunas were confined to depths below 370 m. They suggested that higher predation pressure at shallower depths was partly responsible for the absence of seep specialists in shallow water. In studies of seep and non-seep macrofauna on Hydrate Ridge, Sahling et al. (2002) found 25% of the 36 families identified to be present exclusively at seeps. These included Vesicomyidae, Solemyidae, Nuculanidae, Provannidae, Pyropeltidae, Hyalogyrinidae, Dorvilleidae and Polynoidae. Ampharetid polychaetes were also very abundant, although not limited to seeps. In this study the proportion of endemic, heterotrophic seep fauna was greatest in the most sulphidic sediments (Beggiatoa covered) and least in the Acharax community, whereas the proportion of heterotrophic colonists (non-seep fauna) exhibited the reverse pattern.

Table 3 Characteristics of seep macrofaunal communities

Location

Depth

(m) Habitat type

Methods (mesh size)

Density Biomass

Gulf of Alaska. 4.445 Pogonophoran Kodiak Seep field

Gulf of Alaska. 4.445 Calyptogena Kodiak Seep phaseoliformis bed

Oregon. 590 Calyptogena bed

Hydrate Ridge

Oregon. 590 Microbial mat

Hydrate Ridge

Submersible box and tube corers (0.3 mm) Submersible box and tube corers (0.3 mm) Submersible box and tube corers (0.3 mm) Submersible box and tube

Oregon. 770

Hydrate Ridge

Oregon. 770

Hydrate Ridge

Acharax bed

Calyptogena bed (C. kilmeri & C. pacifica)

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