The purpose of this post is to give a brief overview of how the activity of radionuclides correspond to the concentration of radionuclides measured in environmental samples. There appears to be some confusion in the public and within the scientific community as to how units are used and the degree of their interchangeability. This post is somewhat technical but falls into the category of “In Case You’re Interested” (ICYI), an acronym I am shamelessly borrowing from a fine book (Everything and More: A Compact History of Infinity) by one of my favorite writers David F. Wallace.
The purpose of this post is to address what impact the Fukushima Dai-ichi disaster has had and is having on the growth of photosynthetic algae or phytoplankton in the North Pacific Ocean ecosystem. There is some concern among the public that the radioactivity released from Fukushima represents a potentially acute and chronic risk to algae or phytoplankton that represent the base of the marine food web. A simple internet search will raise stories which speculatively describe the North Pacific Ocean as a “dead-zone” suggesting that activities of radionuclides from Fukushima are killing phytoplankton and leading to biological desert-like conditions in this important ecosystem. Microbes, algae included, are some of the most radiation resistant organisms on the planet that can survive acute and chronic doses of radiation that would kill multi-cellular organisms like ourselves. Satellite data can estimate phytoplankton biomass from space and this post uses NASA’s MODIS satellite data accessed through their really useful GIOVANNI data portal.
Satellite measurements of ocean temperature and the abundance of marine algae going back to 1997 suggest that Fukushima has had little if any impact on phytoplankton in the coastal waters of Japan and offshore waters of the North Pacific to this point. This diary is not meant to be an exhaustive survey of the state of the North Pacific ecosystem but is aimed at using remote sensing to address whether or not widespread collapse of phytoplankton populations occurred in the Pacific following the Fukushima disaster. As stated above all the animations and data in this diary can be accessed using NASA’s fantastic online portal called Giovanni.
What are algae and why do we care?
Algae, also called phytoplankton, are autotrophic organisms that can use sunlight as an energy source to produce glucose. They are the base of the food web and ultimately the yield of the major world fisheries depends on how productive these microscopic plants are year to year in the ocean. On long timescales the carbon dioxide they transform into organic matter at the ocean surface helps to determine how much carbon dioxide is the in atmosphere which helps to control Earth’s climate. Given the importance of phytoplankton to the health of the marine ecosystem, fisheries productivity and the capacity of the oceans to absorb fossil fuel derived carbon dioxide from the atmosphere there is much interest in the oceanographic research community to determine what controls the growth of algae in the ocean.
What controls how much and when phytoplankton grow?
To grow algae need light (which decreases exponentially with depth in the ocean) and nutrients like nitrogen and phosphorus (like fertilizer for your garden) which are dissolved in seawater. As algae grow they remove dissolved nutrients, including carbon dioxide, from the surrounding seawater and make new microscopic copies of themselves. The cells that grow eventually die and sink out of the surface of the ocean to the dark ocean interior where they rot, decompose, and return the nutrients again to dissolve in the subsurface ocean water. The growth of phytoplankton thus leaves the surface ocean with very low nutrient concentrations and when they run out of nutrients their growth stops. So, there is lots of light at the surface but very little nutrients and lots of nutrients but very little light in the deeper parts of the ocean. It can be tough being marine algae, or in other words, it ain’t easy being green.
Marine algae and radiation tolerance
Significant amounts (as reported in Povinec et al. 2013; open-access) of radioactive isotopes were released directly and indirectly via the atmosphere to the Pacific Ocean following the Fukushima Dai-ichi nuclear power plant disaster in March 2011. Releases to the coastal ocean continue to this day though at levels that are much diminished compared to release rates in March and April 2011 with maximum activities occurring off the coast and in Spring 2011 and 10,000-100,000 fold lower activities that are about 100-1000 fold higher than pre-Fukushima background ~1 km from the plant site. How the activities of these radionuclides will impact marine organisms is an ongoing concern and the focus of much research.
Marine algae are made up of both prokaryotic (e.g. cyanobacteria) and eukaryotic (e.g. diatoms and coccolithophores) organisms. Generally microorganisms tend to be more tolerant of of ionizing radiation than animals with members of marine photosynthetic algae being well represented (e.g. Singh and Gabani 2011 Journal of Applied Microbiology). Recently, as an extreme example, a eukaryotic algae (link, link) was isolated from the spent fuel pool of a research reactor in France that can resist a dose of 20,000 Gy or more than 2,000 times the lethal dose to human beings. Basically, these amazing organisms live in distilled water and withstand withering amounts of radioactivity. Given that maximum offshore activities of Fukushima derived isotopes like cesium (137-Cs half life ~30 yr and 134-Cs half life ~2 yr) are ~20 Bq/m3 in seawater (Kumamoto et al. 2014) dose rates experienced by microbes are not likely to approach levels known to induce phytoplankton mortality.
Seasonal cycle of phytoplankton growth in the North Pacific
In the North Pacific, where there is a strong seasonal cycle between colder, dark winters and warm light filled summers, there is also a strong seasonal cycle in phytoplankton growth. Over the winter the upper ocean is mixed with the deeper ocean because cooling of surface water makes it more dense and stronger winds from winter storms stirs the soup. This brings nutrient-rich water to the surface. But, the lights aren’t on yet and there is not enough sunlight to make the phytoplankton happy. When spring rolls around and the sun shines bright the phytoplankton have all they need to be happy (light and nutrients) and the grow quickly. When they grow very quickly their numbers increase exponentially and we call this a spring bloom. Nutrients are used up quickly and chlorophyll a, the pigment that helps the plants harvest the suns energy, increases in concentration at the oceans surface in parallel with the numbers of algae. This leads to a strong seasonal cycle where chlorophyll a concentrations are low in the winter and increase dramatically in spring.
Shown below is the seasonal cycle of sea surface temperature in the North Pacific.
Cold surface temperatures dominate in the norther hemisphere winter that warm as we head into spring. Warming is accompanied by increased hours and intensity of sunlight. Cooling conditions dominate in late fall as day length and incoming solar energy intensity diminish.
The biological response to this seasonal cycle is shown in the animation of chlorophyll a concentrations below.
Algal biomass is low in the winter as deep mixing and lack of light limits the growth of the phytoplankton. However, this mixing returns nutrients to the surface that were depleted during the previous seasons growth. As the spring comes on more light warms the surface and allows a bloom of phytoplankton to develop that propagates northward following the sun. Very low amounts of phytoplankton are observed in the south as the summer progresses given that nutrients have been used up and surface warming prevents mixing of the more dense subsurface, nutrient rich waters up into the sunlit upper ocean.
Long term trends in algae growth in the North Pacific: Does Fukushima have a negative effect?
The following figure shows how this seasonal cycle of spring bloom and winter crash has operated over the period covered by satellite observations between 1997 and the present.
Peak chlorophyll concentrations during the Spring bloom fall in a range between ~0.75 and 1.9 mg/m3 over the period with the minimum occurring in 2008 and the maximum in 2004. The spring bloom in 2011, coincident with the disaster, was an average year for phytoplankton growth followed by the second highest bloom in the satellite record in 2012. In 2013 chloropyll concentrations were lower than in either 2011 or 2012, similar to 2003 and greater than 2008, 2007 and 1999. Most recent data for 2014 indicate that the spring bloom is underway with more data coming in at each months end.
Long-term trends do not indicate that there has been a collapse of the phytoplankton population following the Fukushima disaster. We kept an eye on what the spring bloom peak looked like for 2014. The following figure updates the data presented above with the chlorophyll a concentrations averaged for the month of April 2014. Chlorophyll a concentrations are significantly greater than those in 2013 and similar to levels in 2011 and 2012 and record levels in 2004. Given that it August now the bloom has subsided with a modest increase in chlorophyll expected in the Fall.
More study on the impacts of the Fukushima disaster on populations of marine organisms are required given the persistence and potential to concentrate in the biota of certain of the radionuclides.
Personal Aside: I recall seeing some of the first SeaWiFS images as a graduate student and being amazed at the productivity of the oceans. The power of remote sensing is impressive and affords us a great tool for studying the oceans.