Modeling biological-physical feedback mechanisms in marine systems

Abstract

The marine biosphere is an active and important component of the Earth system. Biologically induced changes in physical oceanic properties through phytoplankton cause potential positive and negative feedbacks. In particular, surface floating cyanobacteria can increase light absorption and the albedo at the ocean surface and decrease momentum input by wind. In this thesis I study the feedbacks mediated by marine cyanobacteria on the physics of the upper ocean. Using the water column model GOTM, I set up a coupled biological-physical model to investigate local effects of the feedbacks on the mixed layer dynamics. Extending these one-dimensional studies, I use the general circulation model MITgcm and set up a three-dimensional coupled biological-physical model to study also non-local effects on the ocean circulation on a basin-wide scale. I show that the absorption feedback by phytoplankton leads to a surface warming and a subsurface cooling. The temperature differences caused by cyanobacteria are more pronounced than those caused by other phytoplankton. The positive absorption and wind feedbacks mediated by cyanobacteria are stronger than the negative albedo feedback. Cyanobacteria mediate a local shallowing of up to 30% of the surface mixed layer due to the absorption feedback and of around 10% due to the wind feedback. By warming the ocean surface and shallowing the mixed layer cyanobacteria locally lead to environmental conditions promoting their own growth. Due to the circulation, colder subsurface waters can be transported and thus also lead to a surface cooling at other locations. Increased absorption by phytoplankton and cyanobacteria affects the meridional overturning circulation. Reduced surface wind stress mediated by cyanobacteria leads to a distortion of the subtropical gyre and to reduced subtropical downwelling and equatorial upwelling. In a warmer environment the local effects of the absorption and the wind feedback are stronger than today. With increasing temperatures cyanobacteria shift northwards and lead to stronger effects of the biological-physical feedbacks in some regions, but weaker effects in other regions. Increasing temperatures might lead to a spread of cyanobacteria, if they are able to adapt to temperatures higher than 30 degrees C, and thus a larger ocean region would be affected by the induced feedbacks. Yet, the model studies do not indicate a substantial increase in the area covered by cyanobacteria. This thesis provides the first quantitative estimate of how surface floating cyanobacteria feed back on their physical environment. Overall, the results suggest that surface floating cyanobacteria and their feedbacks on light absorption and wind stress need to be taken into account in ocean models used for climate scenarios in order to capture changes in the dynamics of the upper ocean.