Parameterisation of Microprotozooplankton Grazing and Growth: From data analysis to simulations in ecosystem model coupled to general circulation-biogeochemical model.

Abstract

In the context of global warming and climate change Biogeochemical Ocean General Circulation Models (BOGCM) are hard pressed to provide clear and realistic answers as to how ecosystems and the carbon cycle are affected. Ecosystem models have developed from the NPZ type (Nutrient-Plankton-Zooplankton) towards the use of several plankton functional types (PFTs) to enhance the prediction of ecosystem feedback on climate change. PFTs are selected on their impact on biogeochemical cycles. Zooplankton PFTs, for example, are mostly defined by size. Microzooplankton, one of these size classes, is a group of interest due to a high biomass and growth rates which allow these organisms to follow fluctuations in prey concentration. Furthermore, they are known to graze ~40-75% of particulate primary production in the surface ocean, against ~10-15% for the mesozooplankton. As a size class, microzooplankton includes several organisms pelagic ciliates, heterotrophic dinoflagellates, foraminifera, metazoans larva and copepods nauplii with different feeding modes, food preferences, grazing and growth rates. Ciliates and heterotrophic dinoflagellates are the main microzooplankton organisms. Although they are both protozoans, their feeding behaviour and preferred prey size have a substantial difference. In order to assess their differences, results from laboratory experiments were compiled from the literature a total of 342 for ciliates and 161 for dinoflagellates. It emerged that both organisms have a growth and a grazing threshold, also both grazing and growth rates depends on organism size and the size ratio with their prey (size expressed as diameter, volume or carbon content). Ciliates exhibit an increase of their maximal grazing rate past the optimal prey:ciliate ratio of 1:10. Dinoflagellates have a maximal grazing rate which increases to a prey:dinoflagellate ratio of 2:1, then continues to increase past this value, with a marked preference for diatoms over other possible prey types. As both ciliates and heterotrophic dinoflagellates have different size ratios with their prey, they will target different prey types. Moreover, both have different functional responses to fluctuations in prey concentration. Ciliates, with a higher threshold concentration and lower half-saturation concentration, will commence grazing later than dinoflagellates, but reach their maximal rates faster. They differ further from dinoflagellates with a higher maximal grazing rate and a lower metabolism. Parameterisation for a microzooplankton, ciliates and heterotrophic dinoflagellates PFTs were obtained from the data and used in a BOGCM. The 12 PFTs have a different impact on the ecosystem and biogeochemical cycles. The dinoflagellate PFT reduces the export and alters the distribution area of high primary production. The ciliate PFT has a similar impact to that of microzooplankton. It is doubtful that the microzooplankton PFT in itself correctly represents ciliates and heterotrophic dinoflagellates. Consequently a separation of both organisms in future models is recommanded to provide a better representation of the ecosystem and its response to climate change.