Journal of Applied Phycology 11: 551–558, 1999. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 551 Effect of total dissolved solids and irradiance on growth and toxin production by Nodularia spumigena P. Hobson 1 , M. Burch 2 & H.J. Fallowfield 1,∗ 1 Department of Environmental Health, School of Medicine, Flinders Medical Centre, Flinders University of South Australia, Bedford Park, South Australia, 5042 2 Australian Water Quality Centre, Private Mail Bag, Salisbury, South Australia ( ∗ Author for correspondence) Received 12 May 1999; revised 6 October 1999; accepted 9 October 1999 Key words: Nodularia, growth, hepatotoxin production, nodularin, salinity, irradiance Abstract The objective of this work was to determine the influence of total dissolved solids/salinity (TDS mg L −1 ) on growth and biomass specific rates of nodularin (hepatotoxin) production by Nodularia spumigena 001E isolated from Lake Alexandrina, South Australia. Maximum biomass yield (dry matter, chlorophyll a and particulate organic carbon /POC) at 80 μmol photon m −2 s −1 was recorded at 3300 mg TDS L −1 and decreased at salinities above or below this value (p< 0.05). The maximum biomass yield (dry matter and chlorophyll a) at 30 μmol m −2 s −1 occurred at a higher salinity of 9900 mg TDS L −1 . Cultures grown at 80 μmol m −2 s −1 , at a TDS > 6600 mg L −1 , had significantly (p< 0.05) lower nodularin content (ml −1 medium) than cultures grown at the same salinities at 30 μmol m −2 s −1 . The maximum total toxin concentration (mL −1 medium) occurred at 9900 and 3300 mg TDS L −1 at 30 μmol m −2 s −1 and 80 μmol m −2 s −1 respectively. Toxin per unit biomass, expressed as dry matter, chlorophyll a and POC was similar for cultures grown at 30 μmol m −2 s −1 or 80 μmol m −2 s −1 at salinities < 6600 mg TDS L −1 . At salinities > 9900 mg TDS L −1 the toxin content per unit biomass decreased at both irradiances, however, cultures grown at 30 μmol m −2 s −1 had a higher toxin content than those grown at 80 μmol m −2 s −1 . The results indicate that not only do changes in irradiance and salinity directly influence growth and toxin production but that changes in irradiance affected the influence of salinity. Abbreviations. DM, Dry matter; chl a, chlorophyll; POC, particulate organic carbon; TC, total carbon; IC, inorganic carbon; TDS, total dissolved solids Introduction Blooms of Nodularia spumigena occur worldwide mostly in salty or brackish waters – typically estu- aries and coastal lagoons (Jones et al., 1994). Lake Alexandrina in the south-east of South Australia has a history of toxic blooms of N. spumigena with the first well documented outbreak of toxic cyanobacteria in the world recorded at the lake by Francis in 1878. The occurrence of N. spumigena blooms in recent times has resulted in the water supply being switched to local reservoirs and the recreational use of the lake being restricted (Codd et al., 1994). N. spumigena produces the hepatotoxin nodularin which when consumed by animals may destroy liver structure, through inhibition of protein phosphatases (Carmichael & Fujiki, 1994), and in acute cases cause death of the animal through liver failure (Car- michael, 1994). Nodularin has also been shown to act directly as a liver carcinogen which is of partic- ular concern for people chronically exposed to toxin (Ohta et al., 1994). Humans may be exposed to cy- anobacterial toxins through drinking water supplies