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Saturday, November 17, 2007

Dissertation Blogging, Part 2: Seep Primary Production of POM

In preparation for my comprehensive examination in less than 2 weeks and because its International Dissertation Writing Month, I will be posting my thesis proposal as I madly try to finish it all in time over the next few days. Feel free to question, correct, nitpick, criticize (constructively, I'm in a fragile state right now!), comment, praise me and make suggestions for improvement. And yes, I'm freakin' out!!!!

Part 1: Introduction


This chapter is completed and under internal review right now and will be submitted soon. Hence, I will not go into much detail about it right now, you'll have to wait till it gets published! Below is the abstract for the paper:
A Stable Isotope Bioassay for Seep Primary Production of Particulate Organic Matter

Hydrocarbon seeps provide a rich source of chemical energy in the aphotic, nutrient-poor deep-sea. Yet it is unclear how much chemosynthetic primary production enters the pool of particulate organic matter (POM) that can be utilized by suspension-feeding taxa. To address these questions, tissue stable carbon, nitrogen and sulfur isotope content was analyzed from suspension-feeding taxa at a hydrocarbon seep community dominated by the methanotrophic mussel, Bathymodiolus childressi, in the northern Gulf of Mexico. Two end-member stable isotope mixing models were used to determine the relative input of seep-derived POM to photosynthetic-derived POM that is assimilated into the tissues of the suspension-feeding community. Our results indicate that seeps play an active role in the production of POM, which is assimilated by suspension-feeders. Additionally, we constrain possible sources of nutrients to the POM pool.
OK, a little of the basics. A methane seep is an area where natural gas and oil diffuse from reservoirs below the sediment naturally. The seep I studied is called a brine pool. All you need to know if that a brine pool is weird, like an undersea lake of brine (see picture below). Salt concentrations ~120psu or about 4 times the salinity of seawater. The geologic history is complex, but has to do with a salt diapir from the depths of the sediment that recharges the brine pool. Particulate organic matter (POM) cold be anything with an organic origin but not currently alive; bacteria, dead piece of meat floating down from the surface, fish poop, mussel pseudofaeces, etc. This is filter-feeder food. Though passive predators, such as anemones, can phagocytose (engulf) these particles and take up dissolved inorganic carbon (DIC), this is a minor part of their diet. They are after all predators. I hypothesize that they are eating zooplankton hanging around the seeps. Unfortunately, no one has yet to study the zooplankton at the brine pool, so I do not know they exist there, but past experience shows me that where there is energy there is zooplankton! The zooplankton do feed from the POM pool. Hence passive predators are consumers of the POM indirectly. This is becuase stable isotope trophic fractionation for carbon, sulfur and nitrogen behave in a predictable manner (with some caveats). Meaning, "you are what you eat".

Brine Pool NR-1 "shoreline" composed of the methanotrophic mussel Bathymodiolus childressi with a syntactic foam marker visible. Density of mussels is ridiculously high (rope on marker is ~50cm). Photo copyright Ian MacDonald (TAMU).

I basically picked off filter-feeders (barnacles, sabellids, serpulids, sponge) and passive predators (anemones and hydroids) from foam markers at the brine pool, a few hundred km south of Louisiana. These markers had been down there since the early to late-mid 90s and recovered in 2004. Plenty of time to get fouled up! After torturing them a bit, I ground them up and sent them to get their isotopes analyzed. Previous authors have already determined the surface phytoplankton carbon and nitrogen stable isotope values right over our study site and the sulfur isotopes are known to generally be pretty similar from ocean basin to ocean basin (unlike carbon and nitrogen). These values became the photosynthetic end-member into the model. Another previous study by a member of our lab had determined the tissue stable isotope values of all 3 elements for B. childressi at the same location. These values became the methanotrophic end-member. Tissue stable isotopes from chemosynthetic (sulfide-oxidizing) clams and tubeworms were used as an input for thiotrophy. Thiotrophic input was deemed neglible because there were no standing stocks of thiotrophic organisms, though sulfide concentrations were elevated in the outer section of the gigantonormous bed of mussels and mats of sulfide oxidizer bacteria. I included it in my model for good measure and the model agrees with me.

If you plot the stable isotope values of carbon against nitrogen, a nice nearly-linear continuum between methanotrophy and photosynthesis is visible, suggesting mixing of sources in the POM pool. To make a long story short, the model results suggest that seeps play an active role in contributing to the pool of available POM, something close to a quarter. More later when its published!

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