Krill, Copepods and Microbes in the Antarctic Fall

The microbial food web plays a central role in most aquatic systems, with on average 40% of the carbon fixed through primary production flowing through bacteria in the photic zone. In the Antarctic, the importance of the microbial food web is greatest during in the fall and winter, when primary production is minimal and the food web is net heterotrophic. The central question of the proposal is, to what degree is the microbial food web in the Antarctic connected to the classic food web, and what is the role of biodiversity in modulating this connection? It is well established that predation can alter the diversity of prey communities, and this can reduce the impact of predators on total prey biomass: less predation-vulnerable species increase as the competitively dominant prey species are grazed down. Recent work has proposed that this interaction between predation and biodiversity of the prey is dependant on the productivity of the system. In less productive systems, moderate predation pressure will have less of a positive impact on prey biodiversity, while at very low productivity, the same predation pressure may actually reduce biodiversity. It is often difficult to show strong connections between the microbial and classic food webs, with changing biomass in upper trophic levels having little or no effect on the heterotrophic and autotrophic flagellates or bacteria of the microbial food web. Preliminary work by the applicant showed that in late fall, removing almost all ciliate biomass through predation by the furcilia larvae of krill had no measurable effect on the trophic levels beneath ciliates. A possible, but as of yet untested, explanation for this is, that there were changes in abundance of individual species, but not in the community as a whole. It is proposed to test these hypotheses during the late austral summer and early fall, as primary productivity is declining. The applicant will take part in the Polarstern ANT XXIII/4 cruise to the Amundsen and Bellingshausen Seas, from 10 February to 11 April 2006. Surface water samples will be amended with a range of abundances of either krill furcilia larvae, calanoid, or cyclopid copepods. This will allow the grazing rates of the added predators on the entire prey community to be measured. Moreover, this will allow predator effects on prey diversity to be measured over a broad range of predation pressures. Biodiversity will be measured with both morphological and molecular methods. The same experimental design will be repeated over the period mid-February to early April, as primary productivity declines. In this way, the interaction of predation and productivity on biodiversity within the microbial food web can be examined. The proposed experiments will give insight on how the microbial and classic food webs in the Antarctic are connected, on how krill larvae use hetrotrophic food sources at times of declining primary productivity, and on how changes in biodiversity affect ecosystem functioning.

The microbial food web plays a central role in most aquatic systems, with on average 40% of the carbon fixed through primary production flowing through bacteria in the photic zone. In the Antarctic, the importance of the microbial food web is greatest during in the fall and winter, when primary production is minimal and the food web is net heterotrophic. The central question of the proposal is, to what degree is the microbial food web in the Antarctic connected to the classic food web, and what is the role of biodiversity in modulating this connection? It is well established that predation can alter the diversity of prey communities, and this can reduce the impact of predators on total prey biomass: less predation-vulnerable species increase as the competitively dominant prey species are grazed down. Recent work has proposed that this interaction between predation and biodiversity of the prey is dependant on the productivity of the system. In less productive systems, moderate predation pressure will have less of a positive impact on prey biodiversity, while at very low productivity, the same predation pressure may actually reduce biodiversity. It is often difficult to show strong connections between the microbial and classic food webs, with changing biomass in upper trophic levels having little or no effect on the heterotrophic and autotrophic flagellates or bacteria of the microbial food web. Preliminary work by the applicant showed that in late fall, removing almost all ciliate biomass through predation by the furcilia larvae of krill had no measurable effect on the trophic levels beneath ciliates. A possible, but as of yet untested, explanation for this is, that there were changes in abundance of individual species, but not in the community as a whole. It is proposed to test these hypotheses during the late austral summer and early fall, as primary productivity is declining. The applicant will take part in the Polarstern ANT XXIII/4 cruise to the Amundsen and Bellingshausen Seas, from 10 February to 11 April 2006. Surface water samples will be amended with a range of abundances of either krill furcilia larvae, calanoid, or cyclopid copepods. This will allow the grazing rates of the added predators on the entire prey community to be measured. Moreover, this will allow predator effects on prey diversity to be measured over a broad range of predation pressures. Biodiversity will be measured with both morphological and molecular methods. The same experimental design will be repeated over the period mid-February to early April, as primary productivity declines. In this way, the interaction of predation and productivity on biodiversity within the microbial food web can be examined. The proposed experiments will give insight on how the microbial and classic food webs in the Antarctic are connected, on how krill larvae use hetrotrophic food sources at times of declining primary productivity, and on how changes in biodiversity affect ecosystem functioning.