Benthic foraminifera living in Gulf of Mexico bathyal and abyssal sediments: Community analysis and comparison to metazoan meiofaunal biomass and density

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Abstract

Benthic foraminiferal biomass, density, and species composition were determined at 10 sites in the Gulf of Mexico. During June 2001 and 2002, sediment samples were collected with a GoMex box corer. A 7.5-cm diameter subcore was taken from a box core collected at each site and sliced into 1-cm or 2-cm sections to a depth of 2 or 3 cm; the >63-μm fraction was examined shipboard for benthic foraminifera. Individual foraminifers were extracted for adenosine triphosphate (ATP) using a luciferin–luciferase assay, which indicated the total ATP content per specimen; that data was converted to organic carbon. Foraminiferal biomass and density varied substantially (∼2–53 mg C m−2; ∼3600–44,500 individuals m−2, respectively) and inconsistently with water depth: although two ∼1000-m deep sites were geographically separated by only ∼75 km, the foraminiferal biomass at one site was relatively low (∼9 mg C m−2) while the other site had the highest foraminiferal biomass (∼53 mg C m−2). Although most samples from Sigsbee Plain (>3000 m) had low biomass, one Sigsbee site had >20 mg foraminiferal C m−2. The foraminiferal community from all sites (i.e. bathyal and abyssal locales) was dominated by agglutinated, rather than calcareous or tectinous, species. Foraminiferal density never exceeded that of metazoan meiofauna at any site. Foraminiferal biomass, however, exceeded metazoan meiofaunal biomass at 5 of the 10 sites, indicating that foraminifera constitute a major component of the Gulf's deep-water meiofaunal biomass.

Introduction

Foraminifera are commonly occurring constituents of deep-sea benthic communities (e.g., Gooday, 1986; Gooday et al., 2000). In some cases, benthic foraminifera are known to be very abundant in the bathyal to abyssal zones (e.g., Sen Gupta et al., 1981; Bernhard et al., 2000; Gooday, 2002). Numerous publications exist regarding the benthic foraminiferal fauna of the Gulf of Mexico slope, but many reports are on total assemblages in which live and dead specimens were not distinguished from each other. Publications with information on living (or recently living) populations (utilizing the rose Bengal or copper sulfate stains) include Phleger and Parker (1951), Parker (1954), Reynolds (1982), Sen Gupta and Aharon (1994), Sen Gupta et al. (1997), and Robinson et al. (2004). Of these studies, the first two were regional in scope, covering much of the northern Gulf, but mainly targeting the total assemblage. The others, although focused on cytoplasm-containing populations, covered only small portions of the northwestern Gulf, including cold seeps. Here we present a report on unequivocally live benthic foraminifera, including agglutinated and tectinous species, from the Gulf of Mexico slope and abyss.

Few studies have compared foraminiferal biomass and density to those of metazoan meiobenthos in the deep sea. In those that directly compare foraminifera and metazoan meiofauna, results commonly show (1) dominance of foraminifera in terms of biomass in bathyal and abyssal areas (e.g., Coull et al., 1977) and (2) foraminiferal densities exceeding those of other meiofauna and macrofauna (e.g., Snider et al., 1984; Gooday, 1986; Smith, 1992; Gooday et al., 2000; Cornelius and Gooday, 2004). In addition, deep-water areas with depleted oxygen concentrations or sulfidic conditions are also known to have communities where foraminiferal biovolume exceeds that of metazoan meiofauna (Buck and Barry, 1998; Bernhard et al., 2000). With regard to the Gulf of Mexico, however, the only published comparison of foraminiferal and metazoan meiofaunal densities and biomass is just for a few hydrocarbon seeps (Robinson et al., 2004). Here, we present another comparison, but for a much larger area.

Section snippets

Methods

Foraminifera were obtained from 10 sites using the RV Gyre in June 2001 and 2002 (Fig. 1). Site locations included a variety of water depths (range ∼550–3700 m) and physiographic settings as well as considerable coverage in latitude and longitude across the Gulf (Table 1). Sites included two from the Mississippi Trough (MT3, MT6), two from the salt-diapir basinal complex (Bush Hill, C7), two from the Florida Escarpment region (S36, S42), and four from the Sigsbee Abyssal Plain (JSSD1, JSSD2,

Density and biomass

In the surface centimeter, benthic foraminiferal densities ranged over an order of magnitude (∼3600–45,500 individuals m−2) and foraminiferal biomass ranged from ∼2 to 53 mg C m−2 (Table 1). When data from the top 3 cm were integrated, foraminiferal densities ranged from ∼9100 to 82,000 m−2 and biomass ranged from 3.4 to 98.1 mg C m−2 (Table 1). In six cases, the biomass and density in the subsurface interval (1–2 or 1–3 cm) equaled or exceeded that of the surface cm (Fig. 2). Integrated foraminiferal

Foraminiferal density and biomass

The foraminiferal densities obtained from the Gulf of Mexico slope and abyss were lower than those determined in a previous study (using ATP analysis) across a similar water-depth range in the eastern Pacific (624–3728 m, ∼3000–408,000 foraminifers m−2; Bernhard, 1992). In general, Gulf of Mexico foraminiferal densities determined in this study were comparable to the density of foraminifers estimated using rose Bengal at one site in the Arabian Sea (3350 m, ∼58,000 foraminifers m−2; Gooday et al.,

Conclusions

The Gulf of Mexico bathyal and abyssal foraminiferal fauna is dominated by agglutinated forms. Foraminiferal abundance and diversity do not simply vary with water depth in the Gulf of Mexico, but there appears to be a weak relationship of biomass with depth. Additional phenomena responsible for the observed patterns could be associations with geographic features, each with particular forces that affect the faunal distributions (e.g., proximity to seeps, proximity to the Mississippi River Delta,

Acknowledgments

We thank the RV Gyre captain and crew for shipboard support, all scientific party participants on DGoMB 6/01 and 6/02 cruises for sampling assistance; Sophie De Beukelaer (TAMU) for photography of “tree foraminifers”; Christie Robinson and Jessica Blanks (USC) for laboratory assistance; and three anonymous reviewers and Andrew J. Gooday for their useful comments on an earlier version of this manuscript. Funded by Minerals Management Service Contract 1435-01-99-CT-30991 to G.T. Rowe (Texas A&M

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