Additional Nutrients Intensify Dead Zones In Oceans

As more and more nutrients from land and air enter the world’s oceans, the dead zones without oxygen in the water will increase in size and intensity. That is the warning that PhD student Zoë van Kemenade, an organic geochemist at NIOZ, draws from her analysis of drill cores from the ocean floor off the coast of California. The results of this research were published in the journal Biogeosciences.

Natural dead zone
Van Kemenade and colleagues looked at drill cores that were taken from the Pacific Ocean floor off the coast of California as early as 1997. "There is a very interesting, natural 'dead zone' there, that is relatively low in oxygen due to specific ocean currents from the poles and from the Equator," Van Kemenade says. "By looking at how that lack of oxygen over the past 2.5 million years has correlated with warmer and colder periods, we wanted to learn how that lack of oxygen might continue to evolve in the future under the influence of changing climate."

Warmer or nutrient richer
Oxygen depletion can occur as water gets warmer. This is because warmer water can hold less dissolved gases. Because of the current warming of the oceans, they already contain 2 percent less oxygen than they did half a century ago. But lack of oxygen can also be caused by extra supply of nutrients, Van Kemenade explains. "Extra nitrogen causes extra growth of algae. Initially all those little plants in the upper layer of the water produce oxygen, but when they die and are 'eaten' by bacteria at the bottom of the ocean, the scale tips and more oxygen is consumed than produced."

Fossil molecules
For her reconstruction of oxygen levels, Van Kemenade used a very specific molecule from the drill core. "A distinct group of bacteria can break down nitrogen compounds under oxygen-free conditions. These so-called anammox bacteria produce very special molecules in the process, ladderanes. These should protect the cell from the extremely reactive metabolites in that chemical process. An additional advantage for us as researchers is that we can find those characteristic ladderanes literally hundreds of thousands of years later, as traces of these bacterial processes under oxygen-free conditions.”

Mainly nutrients
Whereas Van Kemenade had expected to see the conditions with low oxygen in the ‘history book’ of the ocean floor going up and down with warmer and colder periods in the past, she saw that ladderanes were quite common during both the ice ages and interglacials. "So, for this particular spot, we have to conclude that oxygen depletion was related not only to the fluctuations in temperature, but more importantly to the amount of nutrients in the water," Van Kemenade said.

First experience
This study was one of the first to use information from ladderans on this scale. "We should therefore not immediately translate the results to all seas where oxygen-free conditions can occur," Van Kemenade warns. "At the same time, it does serve as a warning that extra nutrients in the water can cause major problems for ocean biodiversity. Oxygen-less conditions can have major consequences for the ecosystem, but also for fisheries, for example."

Extra nutrients intensify dead zones in oceans
As more and more nutrients from land and air enter the oceans, this will increase the size and intensity of 'dead zones' in the world's oceans. That is the warning that PhD student Zoë van Kemenade, organic geochemist at NIOZ, draws from her analysis of drill cores from the ocean floor off the coast of California. The results of this research have been published in the journal Biogeosciences.

Natural dead zone
Van Kemenade and colleagues looked at drill cores that were taken from the bottom of the Pacific Ocean off the coast of California as early as 1997. “There is a very interesting, natural 'dead zone' there, which is relatively oxygen-free due to specific ocean currents from the poles and the Equator,” says Van Kemenade. “By looking at how this oxygen deficiency was associated with warmer and colder periods over the past 2.5 million years, we wanted to learn how this oxygen deficiency could also develop in the future under the influence of the changing climate.”

Warmer or more nutritious
Oxygen deficiency can occur because the water becomes warmer. After all, warmer water can retain fewer dissolved gases. Due to the current warming of the oceans, they already contain 2 percent less oxygen than half a century ago. But lack of oxygen can also be caused by the supply of nutrients, Van Kemenade explains. “Extra nitrogen ensures extra growth of algae. Initially, all those plants in the top layer of the water produce oxygen, but when they die and are 'eaten' by bacteria at the bottom of the ocean, on balance more oxygen is consumed than produced.”

Fossil molecules
For her reconstruction of the oxygen levels, Van Kemenade used a very specific molecule from the drill core. “A separate group of bacteria can break down nitrogen compounds under oxygen-free conditions. These so-called anammox bacteria produce very special molecules, ladderanes. These must protect the cell against the extremely reactive intermediate products of that chemical process. An additional advantage for us as researchers is that we can still find those characteristic ladderanes literally hundreds of thousands of years later, as traces of these bacterial processes under anoxic conditions.

Especially nutrients
While Van Kemenade had expected that she might see anoxic conditions in the history book of the ocean floor go up and down with warmer and colder periods of the past, she saw that ladderanes were quite common during both the ice ages and the interglacials. “For this specific place we must therefore conclude that anoxia was not only related to fluctuations in temperature, but especially to the amount of nutrients in the water,” says Van Kemenade.

First experience
This study was one of the first to use information from ladderanes on this scale. “We should certainly not immediately translate the results to all seas where anoxic conditions can arise,” Van Kemenade warns. “At the same time, it is a warning that extra nutrients in the water can cause major problems for biodiversity in the oceans. Oxygen-free conditions can have major consequences for the ecosystem, but also for fishing, for example.”