Journal of Applied Phycology 2: 333-339, 1990. © 1990 Kluwer Academic Publishers. Printed in Belgium. 333 An algal medium based on fertilizers and its evaluation in mariculture M. Corsini' & M. Karydis 2 ,* 'Hydrobiological Station of Rhodes, 85 100 Rhodes, Greece; 2 Department of Environmental Studies, University of the Aegean, 811 00 Mytilini, Greece (*author for correspondence) Received 18 August 1990; revised 2 October 1990; accepted 3 October 1990 Key words: algal growth, Tetraselmis suecica, fertilizers, mass culture, Brachionus plicatilis Abstract Five agricultural fertilizers were tested as potential nutrient enrichments for the mass culture of Tetraselmis suecica. Maximum algal growth was observed for the trade fertilizer IgromurtonikR (Murphy Ltd, England) adjusted for a nitrate to phosphate ratio of 24: 1. The gross biochemical composition of T. suecica grown in the enriched fertilizer was compared to the composition of the alga grown in control medium. The nutritional value of the algal material was then tested on the rotifer Brachionus plicatilis. The medium based on the fertilizer is as an inexpensive substitute for mass algal culture of Tetraselmissuecica, a food source for the rotifer Brachionus plicatilis. Introduction Mass culture of microalgae has been used widely in aquaculture, both for bivalve rearing (Walne, 1981) and as a live food in intensive hatchery systems (Witt et al., 1981). However, the cost of algal mass-culture for larval rearing can be con- siderable and attempts have been made to reduce it by producing high density cultures (Spectorova etal., 1986) or by substituting standard algal media with inexpensive agricultural fertilizers (Gonzalez-Rodriguez & Maestrini, 1984; Gel- denhuys et al., 1987). There are many media that can support algal cultures at high densities (Shelef & Soeder, 1980) but the biochemical com- position of the algae differed (Frabegas et al., 1985) with different nutrient ratios. This can affect the nutritive value of an alga used as food for mass culture of zooplankton. It is, therefore, important that, in substituting a standard medium with agri- cultural fertilizers, both the biochemical com- position of the alga and its nutritional value to the predating zooplankton are assessed and com- pared to cultures grown in the standard medium. The present study reports the development of a medium based on a number of agricultural fertil- izers for the marine microalga Tetraselmis suecica, a species widely used in aquaculture (Walne, 1979). At the same time its gross biochemical composition was studied and its nutritional value was evaluated for the rotifer Brachionus plicatilis. Materials and methods Culture techniques The marine flagellate Tetraselmis suecica (Kylin) Butch (CCAP 66/4) was used for the experi- ments. The cultures were grown in a constant