Aquatic Plants: an Opportunity Feedstock in the Age of Bioenergy

For reprint orders, please contact [email protected] REVIEW Aquatic plants: an opportunity feedstock in the age of bioenergy Biofuels (2010) 1(2), 311–321 Ann C Wilkie† & Jason M Evans There is a growing impetus to identify and develop bioenergy feedstocks that can be harnessed in ways that do not require major land-use intensification or use of food crops. Although invasive aquatic plants have long been regarded as an intriguing potential feedstock because of their high growth rate in natural water bodies, most contemporary management is based on plant control rather than utilization. This review pres- ents a comparative life cycle overview of modern aquatic plant control and alternative bioenergy utilization programs, highlighting costs and benefits associated with both approaches. Given recent advances in har- vester and bioenergy conversion technologies, it may be cost effective to incorporate utilization techniques in many water bodies, particularly if ancillary benefitsassociated with nutrient removal and greenhouse-gas reductions are given monetary credit. Pilot projects and site-specific evaluations are, however, needed to determine the ultimate scale in which bioenergy production from aquatic plants will be feasible. Critical problems such as anthropogenic climate change, be cultivated with extremely high areal biomass yield dwindling oil supplies and rural under development have rates [12,13]. However, major challenges and limitations sparked global interest in harnessing renewable energy identified for large-scale utilization of perennial grasses sources. While biofuels, such as ethanol, and other forms [14,15], residual biomass [16–18] and algal culture [19] make of biomass-based energy (i.e., bioenergy) are widely it clear that there is no ‘silver bullet’ solution to sustain- regarded as being among the most promising renewable- able bioenergy production likely. Instead, there will have energy pathways [1,2], there is significant concern that to be many solutions that are designed to take advantage land-use changes implied by large-scale bioenergy prod- of the specific resources available in different local and uction could pose substantial risks to both the natural regional contexts. environment [3–5] and human well-being [6]. This review makes the case that invasive aquatic plant Accordingly, there is a growing impetus to iden- biomass represents an untapped potential source of tify and develop bioenergy feedstocks that can be bioenergy that, while still having major challenges and harnessed in ways that do not require major land-use limitations, is intriguing in the sense that a whole suite intens ification or use of food crops. Much attention, of socioenvironmental benefits can be seen to accrue in for example, has been given to potential environmental conjunction with increased utilization. Since invasive benefits associated with cultivation of perennial grasses aquatic plants produce enormous amounts of biomass for bioenergy production on marginal and/or degraded and adversely affect natural environments (i.e., areas crop land [7–9]. Other pathways that are being exten- without direct human impact), sustained removal of sively explored for their potential to reduce bioenergy’s this biomass will generally have benefits for the nutrient land footprint range from increased utilization of resi- balance and native ecology of affected aquatic eco- dues from existing crop and forestry lands [10,11], to the systems. Moreover, beneficial bioenergy utilization biotechnological development of algal species that can of what is essentially a nuisance waste can be seen as †Author for correspondence Soil and Water Science Department, University of Florida-IFAS, PO Box 110960, Gainesville, FL 32611-0960, USA Tel.: +1 352 392 8699; Fax: +1 352 392 7008; E-mail: [email protected] future science group 10.4155/BFS.10.2 © 2010 Future Science Ltd ISSN 1759-7269 311 Review Wilkie & Evans Key terms having the effect of correspondingly native species, with prominent examples including cat- [22] Aquatic plants: Refers to plants that are lessening the need for land, water, tails (Typha spp.) in the Florida Everglades and duck- biologically adapted to grow in fertilizer and pesticide inputs assoc- weed (Lemna spp.) in Lake Maracaibo, Venezuela [23]. wetlands, lakes, rivers and other iated with the production of other Since such native species overgrowth is typically man- water bodies bioenergy feedstocks. aged in a similar way as invasive overgrowth by non- Eutrophication: Process of lake aging This review begins with a general native species, we suggest that the lines of reasoning caused by accumulation of sediments and organic matter. Often exacerbated discussion of invasive aquatic plants developed in this review can be directly applied to by anthropogenic loading of nutrients, as a problem that has emerged problematic native aquatic plants. such as phosphorus and nitrogen, in within the context of the past cen- The reach of invasive aquatic plants is truly which case it is referred to as tury’s rapid increase in both global global, stretching from the tropics (e.g., water hya- cultural eutrophication commerce and anthropogenic cinth and water lettuce), through all temperate zones Eichhornia crassipes Water hyacinth ( ): eutrophication. Strategies employed (e.g., common coontail and hydrilla) and even into A floating aquatic plant that has become naturalized throughout the to combat invasive aquatic plants are sub-arctic regions (e.g., Eurasian water milfoil). tropics and is commonly regarded as then explored, with consequences of In all areas, overgrowth associated with aquatic one of the world’s most problematic the most common strategies exam- plants often has quite detrimental socioecological con- invasive species. Owing to its prolific ined in some detail through the use sequences. If left unmanaged, the rapid growth and growth and ability to sequester many water-borne contaminants, water of a diagrammatic life cycle. Next, natural senescence of invasive aquatic plants can hasten hyacinth has also been the subject of a comparative review and life cycle the build-up of sediment nutrients and organic matter much research into biomass utilization diagram is developed for manage- associated with lake eutrophication. In the most severe and aquatic remediation ment strategies based upon the har- cases, waterways can become choked with vegetation vest and utilization of extant inva- mats to such an extent that navigation becomes impos- sive aquatic plant biomass for bioenergy production. sible and underlying waters become anaerobic, thereby The review concludes by suggesting public policy and destroying valuable fisheries [24]. Development of research frameworks that could facilitate development anaerobic conditions in lakes with major aquatic plant of integrated bioenergy and utilization programs for problems is further associated with large increases in the sustainable management of invasive aquatic plants the emission of methane [25], a gas that has a global in appropriate rivers, lakes and reservoirs. warming potential 21 times that of carbon dioxide. To be clear, the context in which we propose to Some invasive aquatic plant populations also provide consider bioenergy production from invasive aquatic habitat for vector organisms that spread serious diseases plants is in those water bodies in which such plants among local human [26] and/or wildlife [27] populations. are already widely established and there is no feasible Annual economic costs from the control of invasive means for eradication. Since the changes to native ecol- aquatic plants in the USA alone have been estimated ogy associated with invasive aquatic plants and their at over US$1 billion [28]. subsequent management are generally quite dramatic, we do not suggest that invasive aquatic plants should Underlying causes be introduced into natural systems where they are not Invasive aquatic plants emerged as a major problem over currently found. Instead, we suggest that bioenergy util- the past century, owing to exponential increases in two ization is a potential management alternative for what anthropogenic forces: global trade and commerce, and is, unfortunately, a ubiquitous environmental problem. nutrient enrichment of receiving waters, particularly with phosphorus (P) and nitrogen (N). Invasive aquatic plants: a global problem The impact of trade is straightforward, as plant spe- Invasive species are defined as non-native species that cies have been transported across great distances and cause or could potentially cause significant economic introduced into new areas at rates far exceeding those and/or environmental harm in areas where they are that would have occurred without human assistance. introduced [20]. Commonly listed among the world’s While some invasive aquatic plant populations may be most damaging invasive species are freshwater aquatic the result of intentional introductions for ornamental plants, such as water hyacinth (Eichhornia crassipes), water and/or agricultural use [29], unintentional spread has lettuce (Pistia stratiotes), hydrilla (Hydrilla verticillata), also occurred at a global scale through the careless Eurasian water milfoil (Myriophyllum spicatum), com- disposal of imported aquarium plants or discharge of mon coontail (Ceratophyllum demersum), giant water ship ballast contaminated with plant fragments [30]. fern (Salvinia molesta), alligatorweed (Alternanthera Local and regional spread of nascent invasive plants philoxeroides) and

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