Dissertation Blogging, Part 3: Community Structure at Lau Back-Arc Basin Vents

In preparation for my comprehensive examination next week 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
Part 2: Seep Primary Production of POM

___________________________________________________________________

Community Structure Associated With Chemoautotrophic Foundation Species at the Eastern-Lau Spreading Center in Relation to Stress and Substrate

Only one paper has addressed the ecology here, published by Desbruyères et al. (1994) it was based on targeted collections and video observations of the North Fiji and Lau back-arc basins. More recently, Henry et al. (submitted) found significant differences between the physiological tolerances of the ELSC chemoautotrophs in laboratory experiments while Podowski et al. (submitted) have found significant differences in the in situ thermo-chemical environment associated between various ELSC community types including the three foundation species’ communities. Communities associated with A. hessleri are exposed to high H2S, low O2 and high temperatures, while communities associated with B. brevior are exposed to low H2S, higher O2 and much lower temperatures. The thermo-chemical tolerances of I. nautilei overlap these two extremes. This gradient is apparent in their stratification, often resembling a "bull's eye" pattern around a vent opening with A. hessleri at the center surrounded by I. nautilei and then B. brevior.

My work seeks to understand the structure of chemoautotrophy-based communities at the Eastern-Lau Spreading Center (ELSC) in greater detail through quantitative sampling of foundation species. Two main questions are guiding this research: 1) What is the role of substrate in community structure? and 2) Are communities associated with chemoautotrophic foundation species structured along stress gradients? The data I have collected to address these questions include biometric data and abundance, in addition to mass and shell volume of a subset of the three chemoautotrophic foundation fauna from each collection. All associated fauna from quantitative collections of each community type were identified, enumerated and weighed for biomass.

The first question is derived from the observation that there appear to be wider diffuse venting communities on andesitic substrate, whereas on basaltic substrates venting appeared more localized. These two lava types are closely related but differ in concentrations of metals (i.e. iron, magnesium, silica) and physical properties (i.e. permeability, fragility). Additionally, polymetalic sulfides created as a result of hydrothermal venting provide a third substrate, likely the most extreme with reference to temperature and exposure to metal, hydrogen sulfide and oxygen concentrations. Two of our four study sites are basaltic while the other two are andesitic, but sulfides occur at all four sites in the way “chimneys” (see picture below) though the ones I was able to obtain collections from are only from the two basaltic sites.

Hydrothermal chimney built up from deposition of polymetalic sulfides. Encrusted with vent fauna, dark snail appears to be all I. nautilei and mussel is B. brevior. Photo copyright C.R. Fisher/Ridge2000.

• Null hypothesis 1: Mussels are in the same physiological condition regardless of substrate
o Response: condition index (g AFDW/internal shell volume)
o Predictor: substrate type
[figure and part of discussion removed]
The data suggests that mussels from andesitic communities are in better physiological condition over basalt and sulfide communities.

• Null hypothesis 2: There is no difference in biomass and abundance of chemoautotrophic fauna between substrates
o Response: biomass (kg/m2) and abundance (No. of individuals)
o Predictor: substrate type
[figure and part of discussion removed]
Biomass was not significantly different between substrate and between site. Abundance is significant only between andesitic and sulfide substrates and weakly significant between Kilo Moana and Tu’i Malila. The null hypothesis is only weakly rejected in certain cases, perhaps due to uneven sampling or high variability within a substrate type or site.

• Null hypothesis 3: There is no difference in the diversity (excluding chemoautotrophic fauna) between substrates
o Responses: species richness, evenness, Shannon index, Fisher’s alpha
o Predictor: substrate type
[figure and part of discussion removed]
The use of the various community indices suggest that andesitic communities are higher in diversity and lower in dominance.

Null hypothesis 4: Community similarity is the same across substrate type
o Responses: presence/absence, abundance, biomass, trophic guild biomass
o Predictors: substrate type, community type
o Method: Bray-Curtis similarity cluster analysis, multi-dimensional scaling
[figure and part of discussion removed]
Andesitic sites cluster together with varying chemoautotrophic foundation species biomass, suggesting that andesitic communities of different heterogeneities tend to be assembled similarly. All sulfide communities analyzed are from hydrothermal vents at basaltic sites, which they tended to cluster together with. ABE has a lot of overlap with the basaltic/sulfide sites. Tu’i Malila has little no overlap with basaltic sites and little overlap with ABE, the other andesitic site. Kilo Moana, TowCam and ABE are all relatively closer together while Tu’i Malila is farther south and shallower. These plots suggest that distance may be more of a factor in determining community structure than substrate type.

The second question makes the assumption that each foundation species occupies a certain niche on a stress spectrum. As previously described, the work of Henry et al. (submitted) and Podowski et al. (submitted) has shown a well-defined gradient of thermo-chemical tolerance both in lab experiments and in situ thermo-chemical sensing of the environments occupied by the three foundation species. I will test whether their respective communities are structured according to foundation species thermo-chemical tolerances as a proxy for environmental stress. To do this, I assign the highest stress to communities associated with A. hessleri, intermediate stress to communities associated with I. nautilei and the lowest stress to communities associated with B. brevior. Several samples were either chosen as communities of mixed foundation species or discovered upon retrieval to be mixed communities. These communities are analyzed separately and considered as an intermediate between A. hessleri/I. nautilei and I. nautilei/B. brevior communities.

"Bulls eye" pattern of chemoautotrophic foundation species around a fissure with diffusive venting. Photo copyright C.R. Fisher.

• Null hypothesis 1: There is no difference in diversity metrics (excluding chemoautotrophic fauna) between chemoautotrophic community type
o Responses: species richness, evenness, Shannon index, Fisher’s alpha
o Predictor: foundation species type

• Null hypothesis 2: There is no relationship between diversity and biological characteristics of foundation species.
o Responses: species richness, evenness, Shannon index, Fisher’s alpha
o Predictors: shell length, shell volume, biomass, percentage of most dominant fauna

• Null hypothesis 3: There is no difference in the diversity, abundance and biomass of mussel communities between different physiological condition indices.
o Response: species richness, evenness, Shannon index, Fisher’s alpha
o Predictors: condition index (g AFDW/internal shell volume)

• Null hypothesis 4: There is no community structure across foundation species type.
o Response: presence/absence, abundance, biomass of non-chemoautotrophic foundation species
o Predictors: foundation species type

• Null hypothesis 5: Within mussel bed communities, there is no relationship between diversity, biomass and abundance of the associated community to the mussel’s physiological condition.
o Response: species richness, evenness, Shannon index, Fisher’s alpha
o Predictors: physiological condition indices

• Other objectives:
o Plot shell length frequencies to test if there are patterns to recruitment within and across site and substrate type
o Report sampling effort using rarefaction
o Test whether taxonomic diversity is significantly different across site, substrate and foundation species type
o Explore the use of additional multivariate techniques
o Compare results and species composition to other western Pacific back-arc basins to discern any patterns, as well as to other hydrothermal vent sites.

Literature Cited

Bergquist DC, Fleckenstein C, Szalai EB, Knisel J, Fisher CR (2004) Environment drives physiological variability in the cold seep mussel Bathymodiolus childressi. Limnology and Oceanography 49:706-715

Desbruyères D, Alayse-Danet A-M, Ohta S, Scientific Parties of BIOLAU and STARMER cruises (1994) Deep-sea hydrothermal communities in Southwestern Pacific back-arc basins (the North Fiji and Lau Basins): composition, microdistribution and food web. Marine Geology 116:227-242

Fisher CR, Childress JJ, Arp AJ, Brooks JM, Distel DL, Favuzzi JA, Felbeck H, Hessler RR, Johnson KS, Kennicutt II MC, Macko SA, Newton A, Powell MA, Somero GN, Soto T (1988) Microhabitat variation in the hydrothermal vent mussel, Bathymodiolus thermophilus, at the Rose Garden vent on the Galapagos Rift. Deep-Sea Research 35:1769-1791

Smith KL (1985) Deep-sea hydrothermal vent mussels: nutritional state and distribution at the Galapagos Rift. Ecology 66:1067-1080