By Jay T. Cullen

The purpose of this post is to address common questions related to Fukushima monitoring efforts being conducted by the Integrated Fukushima Ocean Radionuclide Monitoring (InFORM) network in the northeast Pacific Ocean and coastal waters of Canada. This diary continues a series aimed to report the results of scientific research into the impact of the Fukushima disaster on the environment. I am asked routinely why we do not measure contamination in marine microalgae, the base of the marine foodweb, given that they concentrate radionuclide contamination from Fukushima found in seawater into their cells as they grow. The extremely low levels of contamination found from Fukushima in the northeast Pacific Ocean combined with the very small amounts of microalgae present in oceanic waters make such monitoring logistically infeasible. Follow below the fold for the detailed answer.
In Masset, Haida Gwaii on June 2 during a public discussion about the InFORM project I was asked the following question, one that I have been asked on other occasions, by a local resident (paraphrased):

Q. Why don’t you measure marine phytoplankton, the microscopic algae that concentrate radioactive elements from seawater at the base of the marine foodweb? They concentrate isotopes by more than 100 times. Why focus on seawater and salmon when the algae matter most?

This question is sometimes posed online as well with the suggestion that by avoiding looking for radionuclide contamination in marine algae, and focusing on seawater measurements and/or organisms higher in the food chain, one can avoid determining the “real” impact of Fukushima on the environment.

A. In actuality it is logistically challenging and infeasbile to measure Fukushima contamination of ocean phytoplankton given the trace levels of radionuclide activities in the northeast Pacific Ocean.

Let’s walk through a few simple calculations looking at radiocesium (¹³⁷Cs half life ~30 years and ¹³⁴Cs half life ~2 years) activities to understand why this is so. Our monitoring efforts tend to focus on radiocesium for the following reasons:

1. ¹³⁴Cs has a half life that is short enough that all other human sources to the environment have decayed away making it an ideal tracer for Fukushima contamination
2. next to the short lived Iodine-131 (half life ~ 8 days), Cs isotopes were released in greatest activity to the environment from Fukushima and would be most likely to represent a radiological health risk given their chemistry and propensity to be taken up by the biota
3. other isotopes were released in much lower amounts from Fukushima relative to Cs (see other posts here and search for plutonium and strontium for example) and would therefore be much more difficult to detect
4. because they are gamma emitters (unlike Pu isotopes and ⁹⁰Sr which emit alpha and beta radiation respectively) they are relatively easy and resource efficient to detect

How Much Radiocesium Can We Detect in Practice?

Our detection limit using sensitive gamma spectrometers like the one described by our partner organization Our Radioactive Ocean is about 0.2 Bq m^-3 or 2 x 10^-4 Bq L^-1 of seawater. We process 20 liters of seawater to achieve this detection limit so we are able to calculate the absolute activity of ¹³⁷Cs that goes into the instrument:

Detection Limit (Bq L^-1) x Liters Processed (L) = Absolute Activity (Bq)

2 x 10^-4 x 20 = 4 x 10^-3 Bq or 4 milliBq

So in order to have a detectable signal there must be 4 x 10^-3 Bq ¹³⁷Cs in the detector. How many liters of seawater do we have to filter to get enough phytoplankton so that we can have measurable amounts of ¹³⁷Cs?

Marine Microalgae Bioconcentrate Radiocesium From Seawater

The first consideration is that marine phytoplankton do concentrate radioactive cesium from the seawater in which they grow. For a primer on how organisms concentrate various natural and artificial radionuclides from seawater please consult this diary. Marine phytoplankton tend to concentrate radiocesium about 100-fold relative to seawater activities in which they grow (see Table VIII in the IAEA report here) meaning they have a concentration factor (CF) of 100. This means that if the activity of ¹³⁷Cs in seawater was 1 Bq L^-1 the marine algae would have about 100 Bq kg^-1 in their cells.

Thus far the activity of ¹³⁷Cs has been below 10 milliBq L^-1 or 10 x 10^-3 Bq L^-1 but lets assume conservatively that ¹³⁷Cs reaches 10 x 10^-3 Bq L^-1 sometime this year or early next. This is in keeping with our measurements offshore and model predictions of the timing of the contamination in our waters. That would mean we would expect the amount of contamination in phytoplankton to be:

Algae ¹³⁷Cs (Bq kg^-1) = Seawater ¹³⁷Cs (Bq L^-1) x Concentration Factor (L kg^-1)

1 Bq kg^-1 = 10 x 10^-3 Bq L^-1 100 (L kg^-1)

To calculate how many liters of seawater we have to filter to get enough phytoplankton radiocesium to detect we have to know how much algae there is (the mass of microalgae) in a liter of ocean seawater.

How Much Algae is there is a Liter of Ocean Seawater?

The short answer is “not very much at all”. Phytoplankton growth follows a seasonal cycle in the northern hemisphere where their numbers and mass increase dramatically in the spring during what is called a bloom. This is when the mass of algae in a liter of water is at a maximum. We will be conservative here and assume that we will go looking for phytoplankton during this peak bloom where there is on the order of 2 x 10^-3 grams (2 milligrams) of algae per liter (~1 kg) of seawater. So the number of kilograms of phytoplankton in each liter is 2 x 10^-6 kg. There is roughly 1,000,000 times more water by mass than there is algae.

So How Much Seawater Must Be Filtered to Detect the ¹³⁷Cs in the Algae?

We established about that we need about 4 x 10^-3 Bq in our detector to get a useful signal to quantify. We calculated above that with 10 x 10^-3 Bq L^{-1 137}Cs in seawater the algae will have about 1 Bq kg^-1 meaning that there is about 2 x 10^-6 Bq (2 microBq) per liter of seawater owing to the presence of contamination in the algae present. We can calculate the number of liters of seawater that must be filtered to obtain one sample with enough activity to quantify as follows:

Minimum Filtration Volume (L) = Minimum Detectable Activity (Bq)/Algae Activity per Liter of Seawater (Bq L^-1)

2000 L = 4 x 10^-3 Bq/2 x 10^-6 Bq L^-1

So assuming a relatively high level of contamination in northeast Pacific seawater and high levels of phytoplankton biomass we need to pass 2000 liters of seawater through a filter with poresizes small enough to retain micron (10^-6 meter) sized organisms. No small feat I can assure you. During most of the year phytoplankton mass would be 10-fold lower meaning one would need to filter 20,000 liters to get a signal.

We are not avoiding sampling marine algae because we are afraid of what we might find. Indeed, we are likely to find absolutely nothing at all unless we exert what amounts to a herculean effort. Collecting algal samples with the resolution we are achieving for seawater monitoring in not feasible. With quality seawater measurements we can make reliable estimates of what levels of contamination we can expect in marine algae and organisms higher in the food web. In addition, for those interested coastal macroalgae, the logistics of sampling of which are much simplified, are being monitored for Fukushima derived contamination by Kelp Watch. These organisms are useful sentinels for coastal contamination as they grow rapidly, stay in place and concentrate radionuclides from the seawater in which they grow. It is much easier to get the required mass of macroalgae compared to their microscopic cousins as the photo below attests.