Journal of Applied Phycology 8: 193-200, 1996. 193 © 1996 Kluwer Academic Publishers. Printed in Belgium. Water treatment of land-based fish farm effluents by outdoor culture of marine diatoms S. Lefebvre, J. Hussenot* & N. Brossard Centrede Recherche en EcologieMarine et Aquaculture, (CNRS-IFREMER), B.P 5, 17137 L'Houmeau, France (*Authorfor correspondence; phone: +33 4650 9440;fax: +3346500600; e-mail: jhusseno@ifremer.fr) Received 27 April 1996; revised 17 June 1996; accepted 18 June 1996 Key words: water treatment, mariculture, effluents, phytoplankton, diatoms, Skeletonema, Chaetoceros, mass culture Abstract The feasibility of using fish farm effluents was evaluated as a source of inorganic nutrients for mass production of marine diatoms. Batch cultures were conducted from May to July 1995 in 16-L outdoor rectangular tanks, homogenized by gentle aeration (0.2 Lair L -1 h-l). The effluents from the two fish farms studied were both characterized by high concentrations of inorganic materials (NH 4 -N, P0 4 -P, Si(OH) 4 -Si) and were shown to support production of marine diatoms. Moreover, periodic measurements of inorganic matter levels in the cultures showed that clearance was efficacious (90% in 3-5 days). Water purification efficiency and culture productivity were further increased through appropriate nutrient balancing. When effluents were limited in silicate, addition of Na 2 SiO 3 induced a significant increase in both diatom biomass and nutrient removal efficiency. In this case, up to 720000 cell mL -1 were produced dominated by Skeletonema costatum. By contrast, in effluents loaded with silicate, adjustment of the N:P:Si ratio by NH 4 -N and P0 4 -P supplementation then gave increased biomass production. In this case, the maximum cell density found was 450 000 cell mL- , dominated by Chaetoceros spp. Introduction Effluents from intensive fish farming facilities are char- acterized typically by high levels of dissolved matter (Handy & Poxton, 1993), mostly inorganic (Dosdat et al., 1996), making them a valuable source of nutri- ents for the mass production of microalgae (Vonshak, 1986; Gesamp, 1991). This is valuable in two ways. First, through assimilation, microalgal culture can help to reduce the load of nutrient components in the efflu- ents (Lincoln & Earle, 1990) and so decrease the envi- ronmental impact of concentrated NH 4 -N discharge (Videau & Merceron, 1992; Boyd, 1995). Second, such primary production could enhance the socio-economic value (Huntley, 1995) of integrated aquaculture sys- tems where finfish and shellfish (e.g. oyster) farming are conducted simultaneously (Shpigel et al., 1993). Coupling of these activities requires, however, that the microalgae produced be relatively large, such as diatoms (Enright, 1986). Moreover, such cells present two additional advantages: diatoms, and particular- ly Skeletonema costatum and Chaetoceros spp., are widespread in French atlantic coastal waters (Rince, 1979); they show high growth rate (Eppley, 1977; Suzuki & Takahoshi, 1995). Moreover, using the prop- er stimulation procedure, these species can easily be made dominant in otherwise natural microalgae pop- ulations (Dunstan & Tenore, 1974). Previous studies have shown that blooms of Skeletonema costatum (0.5- 1.106 cell mL- 1 ) can be obtained, without inoculation, from natural coastal seawater enriched with nutrients (N:P:Si atomic ratio 10:1:3-4; Roden & O'Mahony, 1984; Hussenot & Gautier 1994, 1995). Such blooms have also been generated in saline well water natural- ly high in silicate, although in this case, inoculation with Skeletonema costatum (100000 cell mL -1 ) was necessary (Baud & Bacher, 1990). In the present investigation, we tested the possi- bility of using fish farm effluents to induce diatom- dominated microalgal blooms in outdoor batch cul-