The Ultimate Limiting Nutrient for Oceanic Primary Production

Programme website: http://inspire-dtp.ac.uk

Project Rationale:
In order to understand how Earth’s climate regulation system works, we need to understand the controls on oceanic primary production, which largely determines the rate of carbon uptake from the atmosphere to the ocean and thence the strength of Earth’s greenhouse effect. It also controls the rate at which oxygen is added to the ocean-atmosphere system and the food supply for the rest of the food web. Ecosystem productivity affects how quickly biogenic rocks such as chalks are formed at the bottom of the ocean. A full understanding of Earth history is only possible with knowledge of past ocean primary production levels.

It has long been argued, from first principles, that the ultimate limiting nutrient (ULN, the one whose supply dictates productivity over long timescales) for ocean primary production is phosphorus. One reason for this is the ability of some organisms, the nitrogen-fixers, to utilize dinitrogen as their nitrogen source and flourish if nitrate runs out. Many modelling studies (e.g. [1]) have concluded that the ULN is phosphorus. However, other studies (e.g. [2]) suggest that iron, the proximate limiting nutrient for primary production across ~30% of the ocean surface and whose scarcity often restricts nitrogen fixation [3], is instead the ULN.

Methodology:
This project will involve running ocean biogeochemical models that, unlike [2], are capable of running routinely for longer than the residence times of both phosphorus and iron in the oceans (order 20,000 and 1,000 years respectively). An existing box model of N and P cycling [1] will be expanded to add spatial complexity and include Fe. The main model used will however, be the intermediate complexity cGENIE model, which you will adapt to fit the purposes of the project.

You will make use of new information, for instance from studies of the persistence of iron in hydrothermal vent plumes, to parameterize a key aspect of the model: the rate of scavenging of iron from seawater. More generally, the research will involve use of up-to-date representations of iron cycling and iron-phytoplankton interactions, taking advantage of new datasets and understanding coming out of the ongoing international GEOTRACES programme, of which Prof. Lohan is a leading participant. The modelling will also be informed by results of previous bioassay studies measuring whether iron, nitrogen or phosphorus is the proximate limiting nutrient in different locations [3]. You will assess the ULN by inspection of model primary production results when P, N and Fe inputs are altered.

Training:
The INSPIRE DTP programme provides comprehensive personal and professional development training alongside extensive opportunities for students to expand their multi-disciplinary outlook through interactions with a wide network of academic, research and industrial/policy partners. The student will be registered at the University of Southampton and hosted at the School of Ocean and Earth Science. Specific training will include:

You will be trained in various aspects of biogeochemical and ecosystem modelling and computer programming, depending on prior experience, such as: debugging, distributed version control, dimensional checking of equations, mass balance checks, sensitivity analyses, MATLAB. You will attend masters-level courses as appropriate, such as Computational Data Analysis, and an international course on cGENIE. You will benefit greatly from interactions with the large number of PhD students, PDRAs and academic staff involved in marine biogeochemistry and biogeochemical modelling in OES and the National Oceanography Centre Southampton.