 |
 |
SITEMAP | SEARCH | CONTACT US |  |
|
 |
  The Role of Appendicularians in Southern Ocean Carbon Flow and TrophodynamicsSupervisors Graham Hosie (AAD), Simon Wright (AAD), Tom Trull (UTas) (PhD only) Project Outline The project will test the hypothesis that appendicularians are a keystone species regulating carbon flow in low productivity waters on the Southern Ocean and in the high Antarctic waters. The Southern Ocean Continuous Plankton Recorder survey (PN 472) has consistently recorded very high abundances of zooplankton in surface waters of the permanent open ocean zone (POOZ). Abundances increase south of the sub-Antarctic Front (SAF) and decline dramatically in the sea ice zone (SIZ), notably in the vicinity of the sub-Antarctic Circumpolar Current Front (SACCF). Yet, the POOZ is generally considered oligotrophic, with some elevated primary production around the PFZ, but not as high as in the SIZ (Banse, 1996; Atkinson, 1998; Fiala et al., 1998). Dominant zooplankton in the region are small cyclopoid copepods, mainly Oithona similis, small calanoid copepods, small euphausiids and appendicularians. Sampling by CPR and Norpac nets have recorded mean abundances of appendicularians exceeding 50 ind. m-3 and representing a large fraction of total zooplankton abundance. They are typical of oceanic oligotrophic conditions (Bone, 1998; Deibel, 1998), but also occur in large numbers in the SIZ, coastal and fjordic waters. They form gelatinous houses with a delicate filtering mesh that can concentrate pico-, nano-plankton, and particulate matter to approximately 100 to 1000 times the ambient concentration (Davoll & Silver, 1986; Bedo et al., 1993). Their houses need to be discarded as often as several times per day, as the mesh becomes clogged with particulate matter plus various protozoans, auto- and heterotrophs, and small metazoans. The discarded houses are one of the major sources of marine snow (Davoll & Silver, 1986; Alldredge & Silver, 1988), which probably supports the small copepods and euphausiids in the region. Both the discarded houses and the appendicularians themselves are consumed by carnivorous zooplankton and adult and larval fish (Gorsky & Fenaux, 1998). Appendicularians also produce a high number of small faecal pellets per day (Vargas et al., 2002), an ideal food source for the coprophagous Oithona. Appendicularians play an important role in accumulating material that would be otherwise too small for direct consumption by filter feeders and carnivores. Marine snow is not only a food source for pelagic grazers, it probably also plays a significant role in the vertical flux of carbon from surface waters. Vargas et al. (2002) has suggested that due to the high efficiency by which appendicularians ingest nano- and picoplankton, their houses and faecal pellets represent a sink for small particles that would not normally fall from the euphotic zone. This may have significant affects on the functioning of biological carbon pump and particularly the role of the Southern Ocean as a major sink for atmospheric carbon. Most studies have focussed on tropical and northern hemisphere temperate species. However, we know almost nothing of the biology of Southern Ocean appendicularians. We lack basic knowledge of the filtration rate and number of house produced per day, necessary to begin defining the trophic importance of this group. A sub-tropical Japanese species can produce up to 16 houses per day and discarded houses can represent 490-1100% of appendicularian biomass per day (Sato et al., 2001). House production rate is expected to be considerably lower in the Southern Ocean, but this will be compensated by the much larger size of Southern Ocean species, producing substantially larger houses. Given that appendicularians are widely distributed through the Southern Ocean and Antarctic coastal waters, their relatively high abundance, and their capacity to ingest large quantities of nano- and picoplankton, detailed studies of this group are warranted to define their trophic importance in the region and their role in carbon transfer. The project will mainly involve experimental studies using live appendicularians, conducted on board ship and at Kingston, to address the following objectives. 1. Determine the generation time of appendicularians. 2. Determine feeding and daily production rates. 3. Determine the composition of the houses and protistan communities typically associated with the houses. 4. Determine the rate of degradation of the houses into marine snow and fate/consumption of the material. Workplan Experimental work will be conducted on board ship, and with specimens returned to the aquarium facilities at Kingston, to determine the feeding and house production rates for each of the abundant species of the two main genera found in the region Oikopleura and Fritillaria. These genera are known to have different feeding and house production strategies in other waters. House production is a function of temperature, and is also regulated by rate of clogging of the filters by larger diatoms and particulate matter. Temperature varies considerably through the region, latitudinally and seasonally. Attention will be paid to feeding and house production rates in the different zones of the Southern Ocean, i.e. the sub-Antarctic, Polar Frontal, Antarctic, and sea ice zones, as well as coastal waters. Seasonal variation in appendicularian activity will also be studied, especially in relation to variation in auto- and heterotrophic protistan production. Specimens will also be collected from each zone and seasonally to determine chemical composition, notably carbon and DNA signatures (see below). Studies on protistans trapped on the filters and the formation of marine snow-protistan communities will be conducted in association with the Marine Microbial Ecology Research Group (MME, Project 40), especially in the use of that project's minicosm chambers to study marine snow formation and grazing at a community level. MME has successfully used these minicosms to study marine snow created by self-aggregating particles, which are one source of marine snow, together with mucilage from algae and zooplankton such as appendicularians. These studies would involve determining the rates of grazing on phytoplankton, bacteria and detritus as well as feeding preferences and the fate of the marine snow particles produced. In addition, the fate of phytoplankton pigment markers will be determined, looking in particular at production and stability of degradation products to aid interpretation of field data. Ultimately the project will produce a model for appendicularian based trophodynamics that will account for regional and seasonal variation and contribute to biogeochemical processes studies in the Ocean Control of CO2 studies of ACE. Time line
Appendicularians have been identified in the Biology programme as a key taxon warranting further investigation. The project already has approval under PN 472 "Southern Ocean Continuous Plankton Recorder". It addresses a number of Biology KSOs of the Strategic Plan, notably KSO 1 and 3 of Goal 2 and KSO 1 of Goal 3 relating to effects of climate change on biogeochemical cycles and vertical flux of carbon. Only cursory studies have been conducted so far, mainly distribution and abundance data analysis, to funding and staffing limitations at the AAD. This project is well suited for a PhD study. The project will benefit both the Carbon Flux and Southern Ocean Ecosystem programs by determining the importance of this numerically abundant but little-studied group as grazers and their role in repackaging carbon for export to the deep ocean. References Alldredge, A.L. and Silver, M.W. (1988) Characteristics, dynamics and significance of marine snow. Progress in Oceanography, 20, 41-82 Atkinson, A. (1998). Life cycle strategies of epipelagic copepods in the Southern Ocean. Journal of Marine Systems, 15, 289-311. Banse, K. (1996) Low seasonality of low concnentrations of surface chlorophyll in the subantarctic water ring: underwater irradiance, iron, or grazing? Progress in Oceanography, 37, 241-291 Bedo, A.W., Acuña, J.L., Robins, D. & Harris, R.P. (1993). Grazing in the micronic and submicronic particle size range: the case of Oikopleura dioica (Appendicularia). Bulletin of Marine Science, 53, 2-14. Bone, Q. ed (1998) The biology of pelagic tunicates. Oxford University Press Davoll, P.J. & Silver, M.W. (1986). Marine snow aggregates: Life history sequence and microbial community of abandoned larvacean houses from Monterey Bay, California. Marine Ecology Progress Series, 33, 111-120. Deibel, D. (1998). Feeding and metabolism of appendicularians. In Q. Bone, The Biology of Pelagic Tunicates, (pp 139-149). Oxford: Oxford University Press. Fiala, M., Semeneh, M. & Oriol, L. (1998). Size-fractionated phytoplankton biomass and species composition in the Indian Ocean Sector of the Southern Ocean during the austral summer. Journal of Marine Systems, 17, 179-194. Gorsky, G. & Fenaux, R. (1998). The role of appendicularians in marine food webs. In Q. Bone, The Biology of Pelagic Tunicates, (pp 161-169). Oxford: Oxford University Press. Sato, R., Tanaka, Y. and Ishimaru, T. (2001) House Production by Oikopleura dioica (Tunicata, Appendicularia) Under Laboratory Conditions. Journal of Plankton Research, 23, 415-423 Vargas, C.A., Tönnesson, K., Sell, A., Maar, M., Møller, E.F., Zervoudaki, T., Giannakourou, A., Christou, E., Satapoomin, S., Petersen, J.K., Nielsen, T.G., and Tiselius, P. (2002) Importance of copepods versus appendicularians in vertical carbon fluxes in a Swedish fjord. Marine Ecology Progress Series, 241, 125-138 Contact A/Prof Tom Trull or tel +61 3 6226 2988 |
|
|
|
||
