ACCESS NUMBER: M10AC2008

STUDY TITLE: Biomass and mass-balance isotope content of seep populations on the 

upper slope Gulf of Mexico, determined from archived samples

REPORT TITLE: Biomass and mass-balance isotope content of seep populations on 

the upper slope Gulf of Mexico, determined from archived samples

CONTRACT NUMBER: M10AC20008

SPONSORING OCS REGION: Gulf of Mexico

APPLICABLE PLANNING AREA(S): Central Planning Area

FISCAL YEAR(S) OF PROJECT FUNDING: 2010–2014

COMPLETION DATE OF REPORT: TBD

COSTS: BOEM $ 88,550. LSU $88,550

PROJECT MANAGER(S): Robert S. Carney

AFFILIATION: Louisiana State University

ADDRESS: Energy Coast and Environment Bldg, LSU, Baton Rouge, La 70803

PRINCIPAL INVESTIGATOR(S)

*

: Robert S. Carney

KEY WORDS: Seep Community, Chemosynthesis, Mixotrophic, Trophic Ecology

BACKGROUND: Benthic communities based on foundation chemosynthetic organisms 

are an important part of the larger deep Gulf of Mexico ecosystem. Associated with 

hydrocarbon seepage, these systems may be put at risk by oil and gas development. As 

such it is important to know natural conditions and be able to distinguish them from 

conditions that might reflect impacts. This project seeks to determine from a large 

number of samples what the trophic conditions of two natural populations are. In 

addition it is being determined if mussel shells alone may provide similar information. 

Archived specimens were used.

OBJECTIVES: There were four objectives: 

1. to determine if the mass percent of carbon, nitrogen, and sulfur in tissue of the 

seep mussel Bathymodiolus childressi provide a useful tool for assessing the 

overall condition of populations; 

2. to determine if the stable isotope composition of carbon, nitrogen and sulfur in 

mussel tissue provides a useful indication of trophic condition of a population 

capable of both chemosynthesis and heterotrophic feeding; 
ACCESS NUMBER: M10AC2008

3. to determine if when trophic condition of a foundation species is determined is 

that condition also reflected in a consumer organism such as the snail 

Bathynerita naticoidea in the same system; 

4. to determine if analysis of mussel shells have the potential of providing 

information on the trophic condition that might be used to assess past seepage 

condition.

DESCRIPTION: Archived formalin-preserved specimens of the mussel Bathymodiolus 

childressi and the snail Bathynerites naticoidea from Bush Hill and Brine Pool NR-1 

were selected for analysis. Mass and stable isotope composition of carbon, nitrogen,

and sulfur were determined for tissue samples. On a subset of organisms, shell carbon 

stable isotopes were also determined. Isotope values were used to estimate 

proportional consumption of four trophic sources. Analyses of variance were used to 

test significances between seeps, at different positions within seeps and over time.

SIGNIFICANT CONCLUSIONS: The dominant upper-slope chemosynthetic mussel 

Bathymodiolus childressi is mixotrophic rather than strictly dependent on methanebased symbiosis as previously assumed. Four food sources were identified; two make 

use of methane and two make use of detritus consumed by filter feeding on algae and 

sulfur-using microbes living in the seep environment. The degree of use varies between 

places and through time. Shell retains a record of methane consumption and could be 

used to determine cause of mortality in death assemblages.

STUDY RESULTS: Stable isotope and elemental mass was determined for mussels 

and snails from archives of the Bush Hill (GC-185) and Brine Pool NR-1 (GC-233) sites. 

Statistical analysis of carbon data confirmed that the former receives thermogeneic 

methane and the latter biogenic. Analyses of nitrogen and sulfur indicated that filter 

feeding on detritus was also an important trophic pathway. Such feeding was a greater 

part of the diet at Bush Hill. These results showed that seep communities live within a 

more complex set of resources than previously thought. Seeping resources are 

important and to a lesser degree phytoplankton detritus. It was unexpected that filter 

feeding also results in the consumption of a seep-generated detritus as well based on 

sulfide-dependent chemosynthesis. Use of detritus adds resiliency to seep systems 

during periods of declining seepage. The mixed diets of the mussels are also found in 

the heterotrophic snail associates. Stable carbon from shells was found to reflect 

methane availability and can be used in the analysis of death assemblages.

STUDY PRODUCT(S): Carney, S.R. 2015. Biomass and mass-balance isotope content 

of seep populations on the upper slope Gulf of Mexico, Determined from Archived 

Samples. U.S. Dept. of the Interior, Bureau of Ocean Energy Management, Gulf of 

Mexico OCS Region, New Orleans, Louisiana. OCS Study BOEM 2015-051. 65 p.

 *

P.I.’s affiliation may be different than that listed for Project Manager(s).