    Does Toxic Microcystis aeruginosa Affect Aquatic Plant Establishment? Michelle T. Casanova, 1 Michael D. Burch, 2 Margaret A. Brock, 1 Peter M. Bond 2 1 Division of Botany, School of Rural Science and Natural Resources, University of New England, Armidale, N.S.W. 2351, Australia 2 CRC for Water Quality and Treatment, Private Mail Bag 3, Salisbury, S.A. 5108, Australia Received 2 December 1997; revised 8 March 1998; accepted 30 March 1998 ABSTRACT: Germination and establishment of aquatic plants in response to the addition of toxic and nontoxic cultures of Microcystis aeruginosa cells, the toxin, microcystin, or both nontoxic cells and dissolved microcystin was investigated in two glasshouse experiments and an in vitro germination experiment. The presence of high densities of cyanobacterial cells had a negative effect on aquatic plant establishment when cell densities were greater than 10 9 cells mL 1 . Plants that were shaded with 20% shade cloth also had reduced establishment compared to the control. The most sensitive plants were ( species of Chara and Nitella. There was no evidence that dissolved microcystin as cyanobacterial ) cell-free extracts alone had a deleterious effect on aquatic plant germination and establishment. Dense cyanobacterial blooms may, therefore, have a role in preventing the establishment of aquatic plants through shading.  1999 by John Wiley & Sons, Inc. Environ Toxicol 14: 97109, 1999 Keywords: cyanobacteria; blue – green algae; macrophytes; germination; seed bank; toxin; microcystin INTRODUCTION Ž The reported incidence of cyanobacterial blue  green . algal blooms in Australian water bodies has increased Ž . over recent years Verhoeven et al., 1992 . There is evidence that the abundance of submerged aquatic plants has coincidentally decreased where algal blooms Ž . occur Abe et al., 1996 . Results of a survey of farm dams in New South Wales indicate that aquatic plant communities are less diverse and abundant where Ž . cyanobacterial blooms occur Casanova et al., 1997 . This phenomenon is well supported by Northern Hemi- sphere studies in which the decline in submerged plant abundance has been attributed to shading of sub- Ž merged plants by algae in the water column Phillips et . al., 1978; Moss, 1991; Moss et al., 1996 . The situation Correspondence to: Michelle T. Casanova in Australia may be different because Australian inland water bodies are characteristically turbid, generally from suspended clay particles rather than algal biomass Ž . Bayly and Williams, 1973 . Many submerged and emer- gent aquatic plants germinate readily on damp soil at Ž . the edges of water bodies Britton and Brock, 1994 , so the edges of water bodies are a ‘‘nursery’’ for plant establishment. Because cyanobacterial blooms charac- teristically form scums at the edge of water bodies, it may be that the physical presence of dense scums inhibits the successful establishment of plants in these areas. Cyanobacteria could affect aquatic plant establish- Ž. ment and growth by 1 changing the physical environ- Ž. ment of germinating and established plants and 2 the production of toxins which could affect plant metabolic processes. Scums of Microcystis aeruginosa have been found to attenuate incident light, reduce free gas ex-  1999 by John Wiley & Sons, Inc. CCC 1520-4081 / 99 / 010097-13 97