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Ocean urea fertilization for carbon credits poses high ecological risks

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Abstract

The proposed plan for enrichment of the Sulu Sea, Philippines, a region of rich marine biodiversity, with thousands of tonnes of urea in order to stimulate algal blooms and sequester carbon is flawed for multiple reasons. Urea is preferentially used as a nitrogen source by some cyanobacteria and dinoflagellates, many of which are neutrally or positively buoyant. Biological pumps to the deep sea are classically leaky, and the inefficient burial of new biomass makes the estimation of a net loss of carbon from the atmosphere questionable at best. The potential for growth of toxic dinoflagellates is also high, as many grow well on urea and some even increase their toxicity when grown on urea. Many toxic dinoflagellates form cysts which can settle to the sediment and germinate in subsequent years, forming new blooms even without further fertilization. If large-scale blooms do occur, it is likely that they will contribute to hypoxia in the bottom waters upon decomposition. Lastly, urea production requires fossil fuel usage, further limiting the potential for net carbon sequestration. The environmental and economic impacts are potentially great and need to be rigorously assessed.

Introduction

Large-scale ocean enrichment experiments have been conducted over the past two decades to understand the response of the oceans to limiting nutrients. Most of these experiments have involved iron additions to the equatorial North Pacific, the subarctic Pacific, and the Southern Ocean, which are known as high-nutrient, low-chlorophyll (HNLC) areas where there is apparent ample macro-nutrient availability (nitrogen, phosphorus), but limited micro-nutrients (iron), and thus limited phytoplankton accumulation (de Baar et al., 2005, Boyd et al., 2007, Buesseler et al., 2008). Application of such research has led some to suggest that with such ocean enrichment, carbon sequestration can be enhanced, and this may serve as one approach to reduce the build-up of greenhouse gases in the atmosphere.

Iron experiments, now totaling more than a dozen (de Baar et al., 2005, Boyd et al., 2007, Buesseler et al., 2008), have shown that phytoplankton blooms can be successfully manufactured, and this has caught the attention of the business community as a means to promote engineered solutions to climate change. The carbon-offsets market is rapidly expanding, and new enterprises are seeking methods to sequester atmospheric carbon as part of cap-and-trade programs. If phytoplankton blooms can lock away carbon through sinking to the deep sea, the market for these carbon offsets could be very large, particularly if an international quota system for carbon trading is agreed upon. However, large-scale manipulation of iron in regions of the ocean where this element is limiting is economically challenging. Thus, entrepreneurs are turning to regions where the limiting nutrient is nitrogen, not iron, with the hopes of enriching waters with this element.

A current plan, as announced by the Ocean Nourishment Corporation of Sydney, Australia, calls for the dispersement of 1000 tonnes of urea in the Sulu Sea, off the coast of the Philippines beginning in 2008 (Young, 2007), although preliminary trials apparently have already begun (Aning, 2007). Broader oceanic applications are also projected beyond the Sulu Sea in the future (http://www.oceannourishment.com/technology.asp). The goal is not only to remove carbon from the atmosphere by increasing algal biomass production and sequestration through sinking, but also to enhance primary production that leads to enhanced local fish production.

The effects of nutrient enrichment on an ecosystem must be considered from multiple perspectives, including physical, biological, and socioeconomic (e.g., Nixon, 1995, Howarth et al., 2000, Cloern, 2001). Urea ((NH2)2CO) is a nitrogen fertilizer and feed additive, the global use of which has increased 100-fold in the past 4 decades (Glibert et al., 2006), and there is a large and growing body of literature on nitrogen cycling and urea metabolism by phytoplankton. These data permit us to propose several specific predictions about the Sulu Sea in particular and the fate of urea pumped into tropical or subtropical seas in general. Here we argue that this plan will likely not lead to enhanced fisheries or carbon sequestration. Instead, there is a real possibility that fisheries and the regional aquatic ecosystem could be significantly damaged for years to come, yielding environmental damage that could far outweigh the gains of carbon offsets.

Section snippets

The Sulu Sea

The Sulu Sea, a deep oceanic basin, is isolated from the surrounding ocean by a chain of islands, making it a region of restricted water exchange (Jones, 2002). It is connected to the South China Sea in the south through the Balabac Strait, and in the north via the Palawan shelf and Mindoro Strait (Jones, 2002, Campos and Villanoy, 2007). These connections are believed to be pathways for water exchange and, more importantly, for plankton and larval exchange between adjacent basins, the

Elemental stoichiometry: limits to carbon biomass from nitrogen enrichment

Nitrogen fertilization stimulates the production of a higher phytoplankton biomass, but only to the extent that other nutrients are not also limiting. The chemical composition, by atoms, of a typical algal cell is 106 carbon: 16 nitrogen: 1 phosphorus: 0.0001 iron. Therefore, for each added unit of iron, about 1,000,000 units of carbon biomass can be produced, assuming all other elements are sufficiently available. However, for each unit of nitrogen that is added to a nitrogen-limited region,

Urea fertilization likely to have eutrophication impacts

In addition to the specific regional concerns about the potential loss in biodiversity in the Sulu Sea, were urea fertilization to be undertaken on a large scale, there are other significant concerns which may apply to other tropical or subtropical regions.

While stoichiometry argues against efficient conversion of added urea to algal biomass, any increased algal biomass is of concern. The goal of the proposed plan is to enhance phytoplankton biomass to a concentration of 200 μg L−1 of chlorophyll

Urea fertilization may alter phytoplankton species composition and change carbon sequestration efficiency

The efficiency of the urea enrichment program is dependent on the efficiency of carbon burial to the deep sea, but burial efficiency will depend on the species composition of the stimulated blooms. Urea enrichment is likely to cause alterations in algal species composition and a loss of phytoplankton biodiversity. Based on our understanding of phytoplankton species dynamics, it is suggested that urea enrichment would preferentially lead to the enhanced production of cyanobacteria,

Urea fertilization may not benefit fisheries

In support of urea fertilization, it has been argued that, “For every tonne of reactive nitrogen added to the ocean in the form of urea, …5.7 tonnes of phytoplankton will be produced, ultimately leading to roughly an extra tonne of fish” (Young, 2007). While trophic transfers to fish are known to be inefficient, and to decrease as primary production increases (e.g., Ryther, 1969), the inefficiencies are particularly large when cyanobacteria and dinoflagellates dominate the phytoplankton

Urea fertilization not likely to sequester carbon to the deep ocean

None of the major ocean fertilization experiments involving iron enrichment was able to confirm a significant enhancement of carbon transport to the deeper ocean (e.g., de Baar et al., 2005, Lutz et al., 2007, Buesseler et al., 2008). Carbon production and transport following iron enrichment were found to be dependent on light, temperature, depth of the mixed layer, and grazing (de Baar et al., 2005). While massive sedimentation following some blooms has been documented, for example following a

Ocean fertilization programs require well designed verification and monitoring

The need to establish global agreements on carbon offsets that are based on sound science is underscored by this poorly conceived urea fertilization program. It is not sufficient to verify that a bloom develops, for example, using satellite imagery as a measure of the sequestration of carbon, which is the proposed method of choice of the commercial enterprise. Such information does not provide a quantification of the export out of the surface waters, and does not enable quantification of the

Carbon offsets may be overestimated

The Ocean Nourishment Corporation has claimed that in the long run, beyond Sulu Sea trials, “One Ocean Nourishment plant will remove approximately 5-8 million tonnes of CO2 from the atmosphere for each year of operation, equivalent to offsetting annual emissions from a typical 1200 MW coal-fired power station or the short-term sequestration from one million hectares of new growth forest” (www.oceannourishment.com/technology.asp). However, urea production is itself a high-energy consuming

Conclusions

In sum, the proposed plan for urea fertilization in the oceans to enhance carbon sequestration and fish production is a plan that will fail. The economics, efficacy and safety of large-scale iron fertilization in order to enhance carbon sequestration have been questioned and repeatedly debated (e.g., Buesseler et al., 2008). The concerns for nitrogen enhancement in the form of urea thus appear to be even greater. Urea enrichment is not likely to produce the desired phytoplankton assemblage

Acknowledgements

This paper was developed under the Global Ecology and Oceanography of Harmful Algal Blooms (GEOHAB) core research project on HABs and Eutrophication and the GEOHAB regional focus on HABs in Asia. GEOHAB is supported by the International Oceanographic Commission (IOC) of UNESCO and by the Scientific Committee on Oceanic Research (SCOR), which are, in turn, supported by multiple agencies, including NSF and NOAA of the USA. This paper represents the views of the authors and is not made on behalf

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