University of Leeds,  School of Earth and Environment: Earth Sciences

Prof. Michael Krom

Environmental Geochemistry


Contact details.

M. Krom photograph

I am Professor (Emeritus) in the School of Earth and Environment, University of Leeds, Leeds UK. I am Professor of Marine Biogeochemistry in the Department of Marine Biology, Charney School of Marine Sciences, Haifa University. I am Scientist in Residence at the Sdot Yam Marine Biological Station, Haifa University. I am also adjunct Professor at the Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, Canada.

Research Interests:

Present and past oceanography of the Eastern Mediterranean particularly the importance of the P cycle.

Satellite image of the Eastern Mediterranean.
Satellite image of the Eastern Mediterranean showing the transition between winter ‘bloom’ conditions and summer nutrient starved conditions.

The Eastern Mediterranean is unusual in two particular ways; it is ultra-oligotrophic with very low primary productivity despite the input of significant amounts of nutrients from the adjacent land-masses particularly Europe. This is caused by its anti-estuarine circulation. It is also the largest body of water in the world which is unequivocally phosphorus limited with a nitrate:phosphate ratio in the deep water of 28:1 far in excess of the Redfield ratio of 16:1. It is P starved with high N:P ratios for DOM and POM.

We have recently been able to explain why the basin is P limited. There is an excess of N over P in the nutrient supply, particularly the atmospheric supply. However, unlike other bodies of water where the N:P input is greater than 16:1, in the Eastern Mediterranean there is very limited denitrification and thus no biological process available to reduce the N:P ratio to Redfieldian numbers. This is because of the low primary productivity in the system. Thus we have been able to show that the two unusual features of the Eastern Mediterranean are connected. This work has continued with a 1-D mass-budget model that is now able to explain quantitatively many of the unusual features of the Eastern Mediterranean and now also the Western Mediterranean. This work carried out by Helen Powley and supervised by Philippe van Cappellen (U of Waterloo) has explained quantitatively the reasons for the differences in oligotrophy, nutrient concentration and N:P ratios between eastern and Western Mediterranean and between both and other inland seas such as the Baltic. We show that the sea is net heterotrophic which is unusual for an inland sea. We are working on predictions of both the effect of climate change and what the Mediterranean was like before pollution.

I was coordinator of the EU-CYCLOPS project, which was the first Lagrangian nutrient addition experiment that did not involve iron. We showed that the Eastern Mediterranean is phosphorus starved with no nutrient reservoir (DOM, POM, surface or deep water) having adequate amounts of P. In winter the system is P limited in the conventional sense. By contrast in summer when the Lagrangian experiment was carried out, to our surprise when phosphate fertilizer was added the chlorophyll content in the surface water decreased. We explained this observation by noting that surface water was P limited for bacteria for the entire year because they had access to the large DON reservoir but N & P co-limited for phytoplankton in summer. After phosphate was added both bacteria and phytoplankton were grazed. We showed that there was also a considerable excess of grazing potential, which resulted in a new process called trophic tunneling which transferred primary productivity rapidly to zooplankton grazers.

My latest work is based on development of an automated nutrient analyser capable to determining nutrient concentrations in the photic zone of Israeli coastal waters. With these data we are examining the seasonal changes in nutrients and how they interface with the biological changes in the same waters.

Atmospheric supply of bioavailable iron, phosphorus and nitrogen to marine systems

SEM picture of Fe nanoparticles found in Western Mediterranean Saharan dust
SEM picture of Fe nanoparticles found in Western Mediterranean Saharan dust

I have been involved in long term sampling of atmospheric inputs into the eastern Mediterranean including dust and carried out studies to quantify it importance to biogeochemical processes in the basin.

Mineral dust and aerosols are important sources of iron and phosphorus particularly to the open ocean where they are frequently the major source. In the atmosphere such particles often form cloud condensation nuclei and sites where acid gases condense. As cloud water evaporates the pH of the water film decreases. As a result such particles can often be subject to highly acid conditions in the atmosphere especially in areas where there is atmospheric pollution.

We have carried out detailed studies which have shown that iron dissolves under such acid conditions and when the pH becomes neutralized in clouds, chemically reactive Fe nanoparticles form. The potential for such reactive Fe to form is controlled by the amount and nature of chemical weathering in the source area as well as acid processes in the atmosphere. Our studies have found that the rate at which Fe is produced can be described most accurately by a three phase model. Our data is being used to calibrate more accurately global climate and cloud models.

We are now showing that similar processes cause P to be converted from relatively insoluble apatite and Fe-bound P to labile P available directly for phytoplankton uptake. This process is particularly important in the Eastern Mediterranean where mineral dust from the Sahara meets polluted air-masses for southern Europe but also is important in the western Pacific and the North Atlantic.

The Sphinx, Egypt.
87Sr/86Sr isotopic ratios in sediment provide evidence for reduction of rainfall in the Nile catchment compatible with the theory that a prolonged drought caused the collapse of the Old Kingdom in Egypt.

Paleoenvironmental studies

We have carried out studies to look at climate change in Africa over the past 25,000 years. Using 87Sr/ 86Sr we have examined the paleoclimate record of the Nile flood and Saharan dust in two sediment locations in the eastern Mediterranean. We have found that between 12ka BP and 4.2ka BP, there was a systematic increase in Nile water flow at the same time as a systematic decrease in the supply of both Blue Nile sediment and Saharan dust to the Mediterranean. This was caused by a increase in the intensity of the African monsoon which caused a 'greening' of the Sahara desert. At the end of this period there was a rapid decrease in rainfall which reached its climax with a long period of drought which caused the end of the Old Kingdom in Egypt.

Sedimentary Geochemistry:

Loch Duich.
Loch Duich has been used as the field site to study sources of anthropogenic trace metals and other sedimentary geochemical processes.

We have a major interest in understanding natural processes in recent sediments and in using sediments as a historical record of past environmental change.

We have long known that anthropogenic trace metals have been deposited in the sediment of Loch Duich since the beginning of the Industrial revolution. , We have shown that almost all of the Copper and Zinc was transported with particles from the Irish Sea while 50% of the lead was derived from atmospheric input. We have developed a gel probe to sample sediment pore waters at high resolution. This technology has been used to study biogeochemical processes in a variety of different environments including determining the present active front in Mediterranean sapropels, nutrient recycling in estuarine sediments in the Humber estuary and Loch Duich, Scotland , and most recently examining P cycling in freshwater sediments including a lagoon in an artificial wetland at Slimbridge bird reserve.

Water Quality processes in Mariculture:

Mariculture is the last untapped food resource on earth. We have been involved in developing intensive mariculture systems that are both commercially viable and environmentally friendly. Recently a fishpond-seaweed-abalone system developed from our work was put into commercial operation in Israel . A new system which works entirely on bacterial biofilters has been developed. Fish excrete ammonia which is toxic to the fish. This ammonia is oxidised to nitrate in a nitrification filter and then converted to nitrogen gas in an anaerobic denitrifying filter. This system, has operated successfully for several years. It runs without any exchange of water thus enabling mariculture to be carried far from the sea if necessary. It is now in commercial operation in Hudson, New York. Future research work will involve examining the dynamics of P cycling within this system.

Major recent publications 

Past and Present Oceanography and P cycling in the Eastern Mediterranean :  

Atmospheric supply of bioavailable iron, phosphorus and nitrogen to marine systems: 

Paleoenvironmental studies: 

Sedimentary Geochemistry and Marine Pollution: 

Water Quality Processes in Mariculture: