Investigating drivers of phytoplankton blooms in the North Atlantic Ocean using high-resolution in situ glider data

Abstract

Autonomous buoyancy-driven underwater gliders represent a powerful tool for studying marine phytoplankton dynamics due to their ability to obtain frequent depth-resolved profiles of bio-optical and physical properties over inter-seasonal time scales, even under challenging weather conditions and low light. This thesis is based on a unique year-long deployment of pairs of gliders at the Porcupine Abyssal Plain Sustained Observatory located in the Northeast Atlantic Ocean, complemented by remotely sensed chlorophyll and photosynthetically active radiation (Aqua MODIS products), surface net heat (NCEP/NOAA reanalysis), surface wind stress (ASCAT products) and in situ measurements of nutrients, chlorophyll, microscale turbulence and meteorological parameters. The data were used to study drivers of autumn and spring phytoplankton blooms.

In the beginning of the deployment, the gliders captured the upper ocean dynamics during an autumnal storm. The onset of an autumn phytoplankton bloom due to nutrient intrusion was detected. Additional data collected during a simultaneous sampling campaign allowed quantification of the nutrient supply by two physical mechanisms associated with a storm event: entrainment of nutrients during a period of high wind forcing and subsequent shear-spiking at the pycnocline due to interactions of storm generated inertial currents with wind. The importance of the two mechanisms is discussed, and I conclude that storms play an important role in fuelling ocean primary production during periods of nutrient depletion.

The glider data from winter and spring captured the onset and development of the phytoplankton spring bloom. Mechanisms controlling the bloom onset were studied in light of the main competing hypotheses: the critical depth, the critical turbulence, and the dilution-recoupling hypotheses. The bloom onset was consistent with the critical depth hypothesis, if the decoupling between the actively mixing layer and the mixed layer is considered. However, the observed bloom developed slowly and was relatively low in magnitude. The frequent passage of storms and periods of convective mixing can significantly decrease mean growth rate for phytoplankton populations affecting the rate of bloom development.

Finally, the impact of biotic factors, such as zooplankton grazing, on spring bloom dynamics is discussed. In order to address potential zooplankton variability that underlies the observations, the glider data was coupled with a simple phytoplankton-zooplankton model. The model was forced with the phytoplankton growth rate evaluated based on the observational data. It is shown that gradual phytoplankton growth in winter results in tight coupling between phytoplankton and zooplankton that can hamper the formation of high-magnitude spring blooms in the North Atlantic Ocean.

More information

Identifiers

Catalogue record

Export record