Saturday, January 31, 2009

Mesocosm Experiments

Mesocosm Experiments are the right way to test Nualgi.

Mesocosm Experiments to Investigate Phytoplankton Competition

Several years of mesocosm experiments in the Norwegian fjords near Bergen (where Ehux is a common part of the phytoplankton succession) looked at the effects of nutrient and other factors on phytoplankton species composition. In particular, nutrients were added in different amounts and different ratios to different bags, and the resulting phytoplankton species numbers were counted. The mesocosm bags experienced natural temperatures and irradiances (they were suspended in the fjord water). The polyethylene mesocosm bags (90% light transmittance) were 4 metres deep, 2 metres in diameter, and constantly stirred. Each bag was initially filled by pumping in unfiltered fjord water, and so natural communities of zooplankton, bacteria and viruses were introduced at the beginning of each experiment. See (Egge & Hemidal, 1994) for a full description of the experimental set-up.

Many phytoplankton species were present in the bags, but the abundances of just two have been plotted against various parameters, in the diagram shown above. Concentrating solely on Ehux cell numbers, the diagram shows no correlation with nitrate, greater Ehux numbers at low phosphate, and a tendency for blooms to occur at higher temperatures and irradiances. Irradiances are averaged over the previous 5 days. Temperature and light will be highly correlated in the shallow mesocosm bags, and it is thought likely that the correlation with high temperature is probably because Ehux does better at high light, rather than because it competes better at high temperatures.

Ehux home page

Ocean acidification – why were we experimenting in Norway?

During the Bergen experiment, a blog of our daily activities was kept. This is now closed to further commenting, but a PDF archive of the entries can be downloaded here. The experiment was also mentioned on the CEH news site, and can be seen on both this page and, in more detail, on this one.

The effect of burning fossil fuels
It is well known that we are changing the climate by burning coal and oil. Everyone is aware of the greenhouse effect – increasing carbon dioxide (CO2) in the air is trapping heat in the atmosphere. With a warmer planet, we know that the polar ice caps will shrink, releasing huge amounts of freshwater into the ocean. We expect sea level to rise, causing flooding of coastal areas; we expect the warmer ocean to increase the intensity of storms and hurricanes; we think it is possible (but we are not certain about this) that Northern Europe could get colder in an overall warmer world, because the major oceanic current – the Gulf stream – that brings heat to us, may became weaker or even switch off. There is much to be concerned about as we enter a high CO2 world.

One other feature of high CO2 in the atmosphere is that it makes the oceans more acidic. When CO2 dissolves in seawater, it forms carbonic acid. This is a basic consequence of chemistry – there may be sceptics who doubt that the greenhouse effect will change our climate – but they cannot argue with chemistry. The amounts of CO2 that we are putting into the atmosphere WILL change the oceans and WILL make them more acidic.

What do we mean by ‘more acidic’? Well, it will not mean that ships will dissolve in sea water, nor will it fizz like a cola drink. In fact, there will be a relatively small change. We measure acidity or alkalinity with something called pH, which is a measure of hydrogen ion concentration. The pH scale goes from 0 (very, very acid) to 14 (very, very alkaline). The oceans are currently slightly alkaline, with an average pH of about 8.2. How much will the pH change in the future? Within a century, we expect the pH of seawater to be about 7.8 – doesn’t sound much but is actually a huge change. pH is a logarithmic scale; since the beginning of the industrial revolution 200 years ago, CO2 entering the atmosphere and dissolving in the ocean has changed the pH by 0.1 units – but this small number hides a massive increase of about 30% in the hydrogen ion concentration. The important point is that apparently small changes in pH are caused by very large changes in hydrogen ion concentration.

Why does this matter?
The oceans have been slightly alkaline for a very long time. In the geological past, there have been times when oceans have been much more acidic than we predict for the next century but ocean pH has actually been very constant for a very long time – probably for as much as 20 or 25 million years. More important, never in the history of the planet has there been as rapid a change in pH as we are now seeing. We do not know how life in the oceans will adjust to this rapid change.

We already know that some marine creatures may disappear. Corals will not form so readily in a high CO2 world – their hard structures are made from calcium carbonate that is only laid down at current pH; it will dissolve at more acidic pH. Other organisms that are under threat are minute plants called coccolithophores which also have a calcium carbonate shell. These are the organisms that formed chalk (e.g. the white cliffs of Dover) in previous phases of the history of the planet – and are still important today forming vast blooms in the ocean that can be seen from satellites in space. In a high CO2 world, they too will have difficulty in making the calcium carbonate structures that they need.

These are obvious potential problems in a more acidic ocean but we know almost nothing about how the rest of the ocean will respond. We – humans – have only known a stable world. In the million or so years of our existence, mankind has always relied on a productive ocean. We get food from the sea but there are also other essential services that we take for granted. For example, about half of the oxygen that is produced by plants each year comes from the sea. In fact, it was the activity of minute plants in the ocean that first produced oxygen – about 3 billion years ago – that led to favourable conditions for animals to evolve. Today, the oceans are very important in regulating the whole planet – the oceans interact with the atmosphere, the atmosphere with the land to form an interdependency that maintains the whole planet – the Earth system. One of the most significant, yet invisible players in this control mechanism are microbes such as bacteria and phytoplankton.

Life in the ocean
Seawater does not look as though it contains much life. Mostly the water looks clear and blue and empty. There are some obvious plants, such as seaweeds, but they are restricted to a very narrow band around the coasts. However, in the vast majority of the oceans, the plants that produce so much of the oxygen that we need are tiny, microscopic algae called phytoplankton.

Other microbes, such as bacteria are also very abundant. In any litre of seawater, there will an average of one thousand million bacteria. They have a crucial role in maintaining the health of the Earth system. They recycle nutrients that are needed by plants; they breakdown waste organic matter; they degrade harmful compounds. In short, they maintain the planet. How will marine microbes respond to a more acidic ocean? We do not know – which why we are doing this experiment.

What is a mesocosm experiment?
A mesocosm has been defined as “an experimental system that simulates real-life conditions as closely as possible, whilst allowing the manipulation of environmental factors”.Basically, this means “a large enclosure that we can manipulate”.

The mesocosms that we are using in this experiment each contain about 12000 litres of seawater. This is a large enough volume to contain most of the bacteria, plants and animals (except fish) that interact together to form a community. They are also a manageable size that can be manipulated. By bubbling with air enriched with high CO2 (at 750 parts per million – the concentration that will be in the atmosphere in the year 2100 at the current rate of coal and oil burning), we have reduced the pH to 7.8 in just 3 days.

We can now begin to investigate how ocean acidification will affect microbial life in the seas. Will everything be the same as at current pH levels? In which case, we have little to worry about. Will undesirable phytoplankton and bacteria find the new conditions to their liking? In which case, how will that affect the future of the oceans? We do not have answers to these questions. But this experiment will begin to give us some clues about the future of the oceans in a high CO2 world.

Nualgi can reduce ocean acidification.

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