University of Otago, New Zealand


Te Tari Hua-Ruanuku

Dr Rob Middag

contact photo
Dr Rob Middag
Tel: (03) 479 7907
Location: Science II, 3n13

Trace metals in the marine environment.

Our research focusses on the biogeochemistry of trace metals in sea water. This research involves large oceanic expeditions as well as local field work to determine the sources, sinks and cycling of metals in the world’s oceans and seas.

Analysis of dissolved aluminium and manganese with flow injection systems at sea

Biogeochemistry of Trace Metals

Many trace elements are critical for marine life and therefore influence the functioning of ocean ecosystems. For example, iron, zinc and cadmium play a crucial role in the growth of phytoplankton, the microscopic unicellular plants that form the basis of the ocean food chain (primary production). Metals like lead on the other hand, are toxic contaminants, negatively affecting primary production. Through their effect on primary production, trace metals affect the amount of atmospheric CO2 sequestered in the deep ocean via the biological pump, thereby affecting global climate. Other trace metals, together with a diverse array of isotopes, are used to assess modern-ocean processes and the role of the ocean in past climate change. For example, manganese can be used as a tracer of input from hydrothermal vents and the relationships between the metals cadmium, zinc and nickel with the major nutrients nitrate, phosphate and silicate are used to estimate ocean nutrient concentrations in the past. In 2014 we took part in an expedition to the Bellingshausen Sea, west of the Antarctic Peninsula to collect samples for 11 different metals, iron and uranium isotopes as well is iron binding ligands that keep iron in solution. Our most recent cruise was just north of New Zealand and this transect will be repeated in 2016.

Sampling for trace metals in the Titan Ultra-Clean sampling van.
You can see part of the Titan sampling system (white PVDF water samplers and the titanium frame)
and some of the sample bottles to be filled with a sub-sample of water for a variety of parameters.


We have several flow injection systems that can be used to determine concentrations of iron, manganese and aluminium while at sea. Moreover, we have a SeaFAST system, an ultra clean, low pressure ion chromatography system for seawater extraction and a High Resolution Sector Field Inductively Coupled Mass Spectrometer (Nu Attom). The SeaFAST is an automated system that buffers a small volume (typically 10-20 mL) of seawater to a pH of 6 before it is loaded onto a column containing a chelating resin. The resin binds the trace metals in the sample, but not the sea salts that cause interfere with the measurements on the mass spec. Additionally, the sample gets pre-concentrated as volume of the sample decreases by 5-50 fold, depending on the volume of seawater passed over the resin and the volume of acid used to elute the metals of the resin. This way we can accurately and precisely determine the low levels of metals that occur in the open ocean to study their distribution in influence on the marine ecosystem.

PhD Projects available!

Please contact me if you are interested in conducting a PhD in this research area. Scholarships are available for international and domestic students. The next research cruise will probably be north of New Zealand.


Mari112: Global Marine Systems

CHEM 201: Physical and Environmental Chemistry

CHEM 204: Environmental Chemistry 

CHEM 304: Environmental Chemistry (Coordinator)

CHEM 400 (Honours Module): Chemical Oceanography


Professional Background

Dec 2012-present Lecturer Chemical Oceanography at the University of Otago
Oct 2010-Nov 2012 Joint post doc with prof de Baar (Royal Netherlands Institute for Sea research; NIOZ) and prof Bruland (University of California Santa Cruz) stationed in Santa Cruz (USA)
Oct 2006-Sept 2010 PhD student with prof de Baar (NIOZ, the Netherlands)
Aug 2010-Sept 2006 Undergraduate and Msc in Marine Biology at the University of Groningen (the Netherlands)

Recent Publications

See for full list of publications

Peer reviewed papers

Book chapter:

The all-titanium ‘Titan’ Ultra-Clean sampling system for trace metals coming on deck after
a deep deployment in the Atlantic Ocean aboard RV Pelagia.
In this picture: Patrick Laan (yellow helmet, Royal Netherlands Institute for Sea Research),
Rob Middag (red jacket), Jeroen de Jong (bottom of the picture, Université Libre de Bruxelles)
and Cor Stevens (Pelagia technician)