Experiments by Invited Researchers


Flow of material into food webs versus the biological pump: variation with phytoplankton and zooplankton communities and mineral ballast

Project acronym: HyIV-NTNU-01
Name of Group Leader: Professor Christina De La Rocha
User-Project Title: Flow of material into food webs versus the biological pump: variation with phytoplankton and zooplankton communities and mineral ballast
Facility: Sletvik
Data Management Report: Report

User-Project Objectives

The goal of this work has been to examine the influence of upper ocean food web structure and functioning on both the natural and artificially enhanced sequestration of carbon within the ocean. Data obtained in the primary mesocosm experiment run in the Bay of Hopavågen in August 2012 will be used to assess the extent to which organic matter produced within four different food webs is retained in the upper ocean food web versus remineralized back to carbon dioxide and inorganic nutrients (ammonium, dissolved silicon, phosphate) versus exported from the system in the form of rapidly sinking particles. Data from a second, related experiment run concurrently will be used to assess the extent to which mesozooplankton-containing food webs enable the dissolution of calcium carbonate particles in the surface ocean, increasing upper ocean alkalinity and the ocean’s ability to serve as a sink for carbon dioxide. It will also be used to discern the impact of calcium carbonate addition on food web structure and functioning and on fluxes and sinking velocities of particulate organic matter.

Short description of the work carried out

The project was largely carried out as planned, with four different food webs successfully established in the main mesocosm experiment, and clear differences observed in the use of nutrients, standing stocks of biomass, and amount and character of sinking fluxes in the different treatments. In addition, the side experiments on aggregation and zooplankton vital rates were completed successfully as well. In the secondary set of mesocosms, the ones containing varying levels of calcium carbonate particles, clear differences were also seen in sinking fluxes and other parameters, indicating that the particles had a significant influence on processes occurring within those mesocosms. In terms of difficulties, the main one was the amount of work needed to be done to run the experiment. It was too much for the number of people present at the experiment. We also had a problem with the mesocosm raft dragging its anchors out of position during especially strong tides associated with the full moon towards the end of the experiment.

Highlights of important research results

Although we are still analyzing samples and examining the resulting data, several general but key results are clear. The first was that higher standing stocks of biomass and higher rates of nutrient use were associated with the food webs based on diatoms rather than on non-siliceous phytoplankton (probably picoplankton, although this will not be certain until the pigment data are in). Interestingly, highest standing stocks of biomass occurred in the diatom mesocosms that also contained a population of copepods. Secondly, sinking fluxes of material (and therefore the operation of the biological carbon pump) was greatest in the mesocosms with the diatom-based food webs. Likewise, the fluxes appear to be higher (and more dominated by fecal pellets than phytoplankton aggregates) in the diatom mesocosms containing copepods. Thus it appears that food web structure plays a key role in both the primary fixation of CO2 into organic matter and its export out of the surface ocean as particulate organic carbon.

De La Rocha C., Gallinari M., Moriceau, B., Iversen M. H., Evertsen A. J., Giering S., Le Goff M., Masson A., Lampitt R. Highest sinking flux in an outdoor mesocosm experiment associated with diatom-based food web. , 2013
De La Rocha C., Gallinari M., Moriceau B., Coffineau N., Evertsen A.-J., Giering S., Iversen M., Le Goff M., Masson A., Donval A., Lampitt R., Boutorh J., Koski M.Sinking fluxes of organic matter and biogenic silica were well correlated in a non-turbulent outdoor mesocosm experiment. (in prep) , 0
Moriceau B., Gallinari M., De La Rocha C., Iversen M. Impact of plankton community changes on aggregates formation and sinking (in prep). , 0