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2007
Levin, LA, Mendoza GF.  2007.  Community structure and nutrition of deep methane-seep macrobenthos from the North Pacific (Aleutian) Margin and the Gulf of Mexico (Florida Escarpment). Marine Ecology-an Evolutionary Perspective. 28:131-151.   10.1111/j.1439-0485.2006.00131.x   AbstractWebsite

Methane seeps occur at depths extending to over 7000 m along the world's continental margins, but there is little information about the infaunal communities inhabiting sediments of seeps deeper than 3000 m. Biological sampling was carried out off Unimak Island (3200-3300 m) and Kodiak Island (4500 m) on the Aleutian margin, Pacific Ocean and along the Florida Escarpment (3300 m) in the Gulf of Mexico to investigate the community structure and nutrition of macrofauna at these sites. We addressed whether there are characteristic infaunal communities common to the deep-water seeps or to the specific habitats (clam beds, pogonophoran fields, and microbial mats) studied here, and ask how these differ from background communities or from shallow-seep settings sampled previously. We also investigated, using stable isotopic signatures, the utilization of chemosynthetically fixed and methane-derived organic matter by macrofauna from different regions and habitats. Within seep sites, macrofaunal densities were the greatest in the Florida microbial mats (20,961 +/- 11,618 ind(.)m(2)), the lowest in the Florida pogonophoran fields (926 +/- 132 ind(.)m(2)), and intermediate in the Unimak and Kodiak seep habitats. Seep macrofaunal densities differed from those in nearby non-seep sediments only in Florida mat habitats, where a single, abundant species of hesionid polychaete comprised 70% of the macrofauna. Annelids were the dominant taxon (> 60%) at all sites and habitats except in Florida background sediments (33%) and Unimak pogonophoran fields (27%). Macrofaunal diversity (H') was lower at the Florida than the Alaska seeps, with a trend toward reduced richness in clam bed relative to pogonophoran field or non-seep sediments. Community composition differences between seep and non-seep sediments were evident in each region except for the Unimak margin, but pogonophoran and clam bed macrofaunal communities did not differ from one another in Alaska. Seep VC and delta N-15 signatures were lighter for seep than non-seep macrofauna in all regions, indicating use of chemosynthetically derived carbon. The lightest delta C-13 values (average of species' means) were observed at the Florida escarpment (-42.87 parts per thousand). We estimated that on average animal tissues had up to 55% methane-derived carbon in Florida mats, 3144% in Florida clam beds and Kodiak clam beds and pogonophoran fields, and 9-23% in Unimak seep habitats. However, some taxa such as hesionid and capitellid polychaetes exhibited tremendous intraspecific 613C variation (> 307.0) between patch types. Overall we found few characteristic communities or features common to the three deep-water seeps (> 3000 m), but common properties across habitats (mat, clam bed, pogonophorans), independent of location or water depth. In general, macrofaunal densities were lower (except at Florida microbial mats), community structure was similar, and reliance on chemosynthesis was greater than observed in shallower seeps off California and Oregon.

Whitcraft, CR, Levin LA.  2007.  Regulation of benthic algal and animal communities by salt marsh plants: Impact of shading. Ecology. 88:904-917.   10.1890/05-2074   AbstractWebsite

Plant cover is a fundamental feature of many coastal marine and terrestrial systems and controls the structure of associated animal communities. Both natural and human-mediated changes in plant cover influence abiotic sediment properties and thus have cascading impacts on the biotic community. Using clipping ( structural) and light ( shading) manipulations in two salt marsh vegetation zones ( one dominated by Spartina foliosa and one by Salicornia virginica), we tested whether these plant species exert influence on abiotic environmental factors and examined the mechanisms by which these changes regulate the biotic community. In an unshaded ( plant and shade removal) treatment, marsh soils exhibited harsher physical properties, a microalgal community composition shift toward increased diatom dominance, and altered macrofaunal community composition with lower species richness, a larger proportion of insect larvae, and a smaller proportion of annelids, crustaceans, and oligochaetes compared to shaded ( plant removal, shade mimic) and control treatment plots. Overall, the shaded treatment plots were similar to the controls. Plant cover removal also resulted in parallel shifts in microalgal and macrofaunal isotopic signatures of the most dynamic species. This suggests that animal responses are seen mainly among microalgae grazers and may be mediated by plant modi. cation of microalgae. Results of these experiments demonstrate how light reduction by the vascular plant canopy can control salt marsh sediment communities in an arid climate. This research facilitates understanding of sequential consequences of changing salt marsh plant cover associated with climate or sea level change, habitat degradation, marsh restoration, or plant invasion.

2000
Levin, LA, James DW, Martin CM, Rathburn AE, Harris LH, Michener RH.  2000.  Do methane seeps support distinct macrofaunal assemblages? Observations on community structure and nutrition from the northern California slope and shelf Marine Ecology-Progress Series. 208:21-39.   10.3354/meps208021   AbstractWebsite

Although the conspicuous epifauna of reducing environments are known to exhibit strong morphological, physiological, and nutritional adaptations for life in these habitats, it is less clear whether infaunal organisms do so as well. We examined metazoan macrofauna from methane-seep sediments on the northern California slope (500 to 525 m depth) and from seep and non-seep sediments at 3 locations on the shelf (31 to 53 m depth) to determine whether the community structure and nutritional sources of seep infauna were distinct from those in non-seep, margin sediments. Seep macrofauna consisted mainly of normal slope and shelf species found in productive settings. Several macrofaunal taxa, such as Capitella sp., Diastylopsis dawsoni, and Synidotea angulata, exhibited a preference for seeps. Other taxa, such as the amphipods Rhepoxynius abronius and R, daboius, avoided seeps. Species richness of shelf macrofauna, evaluated by rarefaction and diversity indices (H' and J'), generally did not differ in seep and non-seep sediments. Similarly, stable isotopic composition (delta C-13, delta N-15) Of active seep and non-seep macrofauna did not differ at the 3 shelf sites. Stable isotopic analyses of calcareous material confirmed the presence of methane-influenced pore waters at the slope study site. At one slope clam bed, macrofaunal delta C-13 signatures were lower and delta N-15 values were higher than at another clam bed, inactive slope sediments and shelf sites. However, only 1 of 14 macrofaunal taxa (a dorvilleid polychaete) exhibited isotopic evidence of chemosynthetic nutritional sources. At these sites, seep influence on the ecology of continental margin infauna appears spatially limited and relatively subtle. At their current level of activity, the northern California slope and shelf seeps appear to function as ephemeral, small-scale disturbances that are not sufficiently persistent to allow chemosynthesis-based trophic specialization by most infauna. Rather, we suggest that many of the infauna inhabiting these seep sediments are shelf and slope species preadapted to organic-rich, reducing environments.