In high-nitrate environments, seagrasses can be shaded by epiphytic algae and macroalgae (Drake et al., 2003) or sometimes by phytoplankton blooms (Nixon et al., 2001). Seagrasses affect the entire estuarine food web because they stabilize sediments; serve as habitats and temporary nurseries for fish and shellfish; are sources of food for fish, waterfowl, benthic invertebrates, or manatees; and provide refuges from predation. Eutrophication and other nutrient-related effects could be a concern for cultivation of microalgae or macroalgae in large suspended offshore enclosures (for example, Honkanen and Helminen, 2000).

Eutrophication also has implications for social acceptability (Codd, 2000), for example, because of eutrophication-related aesthetic concerns (Grant, 2010), and aesthetics can affect the recreational value of water bodies. It is unknown whether rare releases of culture water or the physical appearance of open ponds for algae cultivation could have negative effects on the social acceptability of algal biofuels. Opportunities for Mitigation

Quantifying water losses from raceways, ponds, or photobioreactors would indicate whether repairs of small leaks are necessary. These culture systems can be designed and tested to withstand natural disasters that are possible during the lifetime of the infrastructure. In coastal locations, for example, facility and infrastructure designs would need to consider the probabilities that hurricane winds and water surges could reach the algae cultivation site (Guikema, 2009). Mitigation plans for accidental releases would be desirable. Open-pond algae cultivation also can be sited in locations that are not prone to hurricanes or away from lakes and streams. With respect to harvest water, engineering solutions can maximize recycling.

5.1.3 Waterborne Toxicants Potential Environmental Effects

Some compounds present in algal ponds or photobioreactors could be toxic to humans or other organisms depending on exposure levels. Herbicides often are added to open systems to prevent growth of macrophytes and for selective control of algae (NALMS, 2004), but their application likely would be regulated as in the case of agriculture. If wastewater or oil well-produced water (Shpiner et al., 2009) is used as a water source for algae cultivation, heavy metals could be present. Wastewater could include industrial effluent (Chinnasamy et al., 2010) and municipal wastewater that has undergone various levels of treatment (Wang et al., 2010). The composition and amount of toxicants vary by the type of wastewater. Produced water (water contained in oil and gas reservoirs that is produced in conjunction with the fossil fuel) may contain high levels of organic compounds, oil and grease, boron, and ammonia (NH3) (Drewes et al., 2009). Many algal species including cyanobacteria, diatoms, and chlorophytes can bioconcentrate heavy metals (Watras and Bloom, 1992; Vymazal, 1995; Mathews and Fisher, 2008). Mercury could be introduced into feedstock production waters if unscrubbed flue gas from coal-fired power plants is used as a carbon dioxide (CO2) source (O’Dowd et al., 2006). Therefore, potential risks from using each type of produced water need to be identified so that adequate containment and mitigation measures can be implemented in cultivation and processing.

Waterborne toxicants (toxic substances made or introduced into the environment anthropogenically, not including algal toxins) potentially pose risk to humans or other

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