Eddies and algal stoichiometry: Physical and biological influences on the organic carbon pump

Omta, A. W. 2009. Eddies and algal stoichiometry: Physical and biological influences on the organic carbon pump.
PhD-thesis, Vrije Universiteit, Amsterdam

Abstract Nederlandse versie

The partitioning of carbon between the atmosphere and the (deep) ocean is determined by a number of processes, amongst which the organic carbon pump is an important one. This process involves the growth, sinking and remineralisation of plankton and it leads to an increase of the carbon concentration with depth in the ocean. Various physical and biological phenomena may have an impact on the organic carbon pump; the main objective of this dissertation was to understand some of these impacts. Variations in the elemental stoichiometry (or more precisely: the carbon:nutrient ratio) of the plankton are crucial, because the strength of the organic carbon pump increases with increasing carbon:nutrient ratio of the sinking organic material, both under steady-state conditions and in situations far from steady state. In most of the simulations that are presented in this dissertation, the novel Phytoplankton Internal Nitrogen & Carbon (PINC) model was therefore used. This model is able to make predictions about the elemental stoichiometry of phytoplankton. This PINC model was based on the Dynamic Energy Budget (DEB) theory, a modelling framework for organisms and their stoichiometries based on physico-chemical principles.

In Chapter 3, the PINC model coupled to a one-dimensional water-column model was used to investigate the e ects of the mixed-layer depth and water temperature on the strength of the organic carbon pump. According to the model, such environmental conditions have a strong impact on the elemental stoichiometry of organic matter and therefore on the strength of the organic carbon pump. More speci cally, in the light-limited regime, the algal carbon:nutrient ratio turned out to decrease with increasing mixed-layer depth and temperature. This led to more carbon in the deep ocean and lower atmospheric carbon concentrations, if the water temperature was lower. Thus, the model suggested the existence of a positive feedback between temperature and atmospheric CO2 content which may have contributed to the glacial/interglacial cycles in the atmospheric CO2 concentration.

In Chapter 4, the PINC model was coupled to a three-dimensional hydrodynamic model to study the impact of a submesoscale ocean eddy on the organic carbon pump. The eddy turned out to have little e ect on the strength of the organic carbon pump, because the ow brought carbon in inorganic form from the deep ocean to the surface and this carbon was subsequently returned to depth through sinking of organic matter. The overall e ect was a slight weakening of the organic carbon pump, mainly because of a decrease of the carbon:nutrient ratio of the sinking organic matter.

The impact of a submesoscale eddy on plankton distributions was studied in Chapter 5. Under high-irradiance conditions, the highest plankton concentrations emerged inside the regions with a large vertical exchange: the eddy centre and surrounding lobes. However, if the light intensity was lower, then the highest plankton concentrations were seen in the regions around the eddy centre and the lobes. These ndings could be explained using the concepts of critical light and critical turbulence and considering the e ects of light and nutrients on phytoplankton: if the plankton is nutrient-limited, then vertical exchange promotes plankton growth by transporting nutrients to the ocean surface, whereas vertical exchange leads to a net decline of the plankton by transporting it into the deep ocean, in the case that the plankton is light-limited.

In Chapter 6, the organic carbon pump was studied at the basin scale by means of one-dimensional simulations of the PINC plankton model at di erent locations at the ocean surface. In the tropical and temperate regions, there was a strong increase of the carbon:nutrient ratio (and hence of the strength of the organic carbon pump) with latitude. This was caused by the prediction of the PINC model that the algal carbon:nutrient ratio decreases with increasing water temperature.

The interpretation of satellite chlorophyll observations was the subject of Chapter 7. Various coupled hydrodynamic-biological models were used to reproduce the seasonal variation pattern of chlorophyll in the Mozambique Channel. Unexpectedly, the simulated seasonal cycle of the algal chlorophyll:nitrogen ratio turned out to give a much better correspondence with the observations than the simulated seasonal cycle of phytoplankton biomass. This suggests that the seasonal cycle of chlorophyll in the Mozambique Channel reflects a variation in the algal cellular chlorophyll content rather than a variation in algal biomass.

In this dissertation, the impact of ow conditions, light and nutrients on the organic carbon pump has been investigated. According to the models, these ow conditions, light and nutrients mainly in uence the strength of the organic carbon pump through their impact on the algal carbon:nutrient ratio. Furthermore, ow and light conditions appear to have a strong impact on the spatial distributions as well as on the chlorophyll:carbon ratio of phytoplankton. Variations in this ratio may even be the primary source of seasonal cycles of the chlorophyll concentration at the ocean surface in (sub)tropical regions such as the Mozambique Channel. The issue of the origin of (seasonal) chlorophyll variations in di erent parts of the World Ocean could therefore provide a stimulating starting point for further research.

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