Why Kelp?

Kelp Watch 2014 is an initiative to use our kelp forest species (Macrocystis pyrifera and Nereocystis luetkeana) as coastal detectors of Fukushima released radioisotopes (primarily 134Cs and 137Cs) predicted to arrive to the west coast of North America via the ocean currents by mid-2014. Kelps are a special group (Order Laminariales) of brown seaweeds. They are found in temperate and sub polar waters exclusively; they cannot tolerate warm water for long periods of time. Although they have a plant-like appearance, they are not plants. Their plant-like appearance is because they have holdfasts, blades and stipes. The holdfast anchors the kelp body; it does not specialize in nutrient absorption. The stipe connects blades to the holdfast, and also is a conduit for transferring nutrients and photosynthetic products throughout the kelp body. Although the entire kelp body can photosynthesize and absorb nutrients most of that occurs via the leaf-like blades which are broad and thin comprising most of the surface area of the kelp which is important when capturing photons and nutrient molecules. Macrocystis (Giant Kelp) and Nereocystis (Bull Kelp) are two surface canopy forming kelps that form extensive forests along the entire California coast and beyond. These kelps are an ideal organism to use as natural detectors of the presence of radioisotopes for several reasons.

1. Location

Kelp forests line the coastline in deep water wherever there is hard substrate. Thus, they are well positioned to come in contact with and be bathed by the seawater containing the Fukushima contaminants. The kelp species of choice is Macrocystis because it is found in cold water pockets off Baja Mexico and line nearly the entire coastline from California to Kodiak Islands, Alaska. The conspicuous sporophyte phase, which produces the highly visible forest, is perennial. In response to shorter winter days in northern latitudes, the sporophyte can be found only in shallow more protected waters to increase its capture of solar radiation. Above Ft. Ross CA (38.5°N) the deep water form is completely absent. North of Pt. Conception another canopy forming species Nereocystis can be found in deep and shallow waters up to Unalaska Island, Alaska. Although it overlaps in distribution with Macrocystis they usually are not found in the same exact habitats. Nereocystis solves the problem of short day lengths in northern latitudes by being an annual; the sporophyte disappears during the winter. The microscopic gametophyte remains, however, and will create a new sporophyte come spring. Nereocystis is included in this study because there are some significant stretches of the coastline that lacks Macrocystis or where it is difficult to collect.

2. Canopy Formation

These two kelp species produce surface canopies which are designed to harvest sunlight and dissolved nutrients. Approximately 98% of all photosynthesis of the Giant Kelp occurs in the surface canopy. Because these canopies are primarily made up of blades, most of the kelp sporophyte surface area is found on or near the surface. Being in contact with surface water as it flows by, contributes to kelp’s ability to efficiently extract substances dissolved in seawater, including the radioisotopes in question for this study. The long fronds that make up the canopy of the Giant Kelp can exist at least 6 months for southern California populations disappearing as a result of normal senescence. They are, however, continually replaced and material from senescent fronds can be translocated to younger growing fronds. Fronds from central California northward usually break off due to disturbance but can be much older. Macrocystis canopies, therefore, are exposed to seawater for long periods of time. Blade tissue of Bull Kelp is not usually present during the winter months; however, it is being sampled where needed and when available. New sporophytes begin growing in the spring rapidly forming canopies. This characteristic may be useful in resolving exact durations and amounts of exposure to radioactive seawater.

3. Efficient Uptake and Concentration of Anticipated Radioisotopes

Marine brown seaweeds are known to concentrate Cesium (Cs), Strontium (Sr) and Iodine (I) into their tissues among many other elements. This is especially true for kelps. Macrocystis tissue Cs levels are 20x that of its concentration in seawater (Manley unpubl), Sr levels are concentrated 8x (Rosell & Strivastava 1984) and I can be concentrated greater than 2,000x. Nereocystis is physiologically similar to Macrocystis and has been shown to concentrate Sr and I in similar fashion (Whyte et al. 1981; Rosell & Strivastava 1984). It is anticipated that Cs is similarly concentrated in Bull Kelp tissue. This ability to concentrate iodine was utilized to detect 131I in canopy blades from California populations of Macrocystis which absorbed this radioisotope after it was released into the atmosphere from the damaged Fukushima plant and deposited via rainfall onto coastal waters (Manley & Lowe 2012). Radioactive 137Cs from nuclear weapons testing during the 1950s and Chinese nuclear tests in the late 1970s has been detected many years after in Asian kelp species (Momoshima & Takashima 1985). Dr. Vetter’s group recently detected residual 137Cs in kelp growing on the outer breakwall of Long Beach / LA Harbor as part of preliminary Kelp Watch 2014 sampling. With an MDL<0.06 Bq/kg for both radioisotopes of Cs, we are convinced that if taken up by kelp we will detect it. Potassium (K) is another element that is concentrated in kelp tissue over its concentration in seawater. Potassium comprises greater than 8% of the dry weight of kelp tissue. A naturally occurring radioisotope, K-40, is also concentrated in kelp tissue and is the most abundant radioisotope found in kelp (~4,000 Bq per kg dry weight). We expect that the levels of Cs found in kelp to be much lower than the levels of K.

4. Ease of Collecting and Processing for Analysis

Because canopy tissue is being collected, it can easily be harvested by boat, SCUBA or snorkeling. Because this effort depends on many volunteers, the collection and processing has intentionally been simplified without compromising the integrity of the study. Simply stated, kelp blade tissue is dried, and milled to a uniform particle size and sent to LBNL for analysis.

5. Importance of the Kelp Forest Ecosystem

Kelp forests are one of the most productive and complex ecosystems on earth. They are as productive as terrestrial tropical rain forests. Frond growth of the Giant Kelp can exceed 45 cm (~ 18 inches) per day. There are over 500 species that are directly associated with the kelp forest and some estimates are much higher. There are 50 known organisms that directly graze on kelp tissue or its detached tissue. It is logical to assume that radioisotopes taken up by kelp and concentrated in its tissues will be taken up by the organisms that feed upon it, and distributed throughout the kelp bed community as a whole. One motivation for conducting this study is to actually determine the amount radioactive Cs accumulated in kelp, even though we anticipate that the amount that we find in kelp will be very small. Because of our concern for this important coastal resource, we feel it is important to be vigilant.

Contributed by Dr. Steven L. Manley; 2/13/14