ABSTRACT
Cyanobacteria blooms occur worldwide, their intensity depending on climatic and hydrochemical conditions (Codd 1995). Harmful algal blooms give the water an unpleasant smell (earthy, musty) and colour and may lead to the accumulation of various toxins (hepatotoxins, such as microcystins and nodularins, or neurotoxins, such as anatoxin-a and saxitoxins) in the water column (WHO 1999). Cyanobacterial toxins can bioaccumulate in aquatic organisms (fish, mussels, and zooplankton) and consequently, they can poison humans, livestock and pets (Kankaanpää et al. 2002, Ozawa et al. 2003, Saker et al. 2004). Effective management of algal blooms is therefore highly desirable to prevent or at least mitigate their potential health hazards. One of the oldest intervention techniques is the use of algicides (e.g. copper(II) sulphate, potassium permanganate, chlorine, sodium oxochlorate). Copper sulphate (CuSO4, the most commonly applied) is considered effective, easy to apply and economical (Cooke et al. 2005) but, unfortunately, it has widespread ecological impacts (McKnight et al. 1983). Application of CuSO4 causes some changes in species succession (Effler et al. 1980, Soldo et al. 2000, van Hullebusch et al. 2002), and copper, like other heavy metals, is not biodegradable and is likely to accumulate in bottom sediments (Sanchez et al. 1978, Hanson et al. 1984, Garcia-Villada et al. 2004). For these reasons, other methods are needed to inhibit or limit cyanobacterial bloom. The natural extracts of some aquatic plants (macrophytes) (Mulderij et al. 2005, Xian et al. 2006) or decomposing barley straw have algicidal effects (Barrett et al. 1992, Martin et al. 1999), and may
be an inexpensive, environmentally-friendly alternative to the use of algicides such as copper sulphate that have undesirable side effects. The growth of bluegreen algae (Anabaena sp., A. variabilis, Microcystis aeruginosa) has been shown to be strongly inhibited by freshwater macrophytes (Ceratophyllum demersum, Elodea nuttallii and Myriophyllum spicatum) in a number of independent studies (Saito et al. 1989, Gross et al. 1996, 2003; Nakai et al. 1999). In particular, Erhard and Gross (2006) found that the growth of Pseudanabaena cf. catenata Myr 980, Synechococcus sp. Cha 9817 and S. nidulans Pot 9801 was inhibited by an average of 56-92% by extracts of Elodea nuttallii or Elodea canadensis. However, more quantitative environmental studies are needed to investigate the potential impact of macrophyte extracts or decomposed barley straw on target organisms and the level of toxins released to the environment.
