Growth and grazing on the 'Texas brown tide' alga Aureoumbra lagunensis bythe tintinnid Amphorides quadrilineata

Growth and ingestion by the loricate ciliate Amphorides quadrilineata expos ed to increasing dietary doses of the Texas brown tide alga Aureoumbra lagu nensis were investigated. The ciliate grew at a maximum rate of 0.38 d(-1), ingesting 0.032 ppm (similar to6.4 x 10(2) cells) prey d(-1) on a diet con sisting only of Isochrysis galbana. When A. quadrilineata was offered a mix ed diet of I. galbana and A. lagunensis, the growth rate decreased when A. lagunensis made up more than ca 40 % of the diet. Ingestion rate by A. quad rilineata remained constant with increasing A. lagunensis proportion in the diet, which indicates that A, quadrilineata did not avoid feeding on A. la gunensis. The behaviour of A. quadrilineata was largely unchanged when cili ates were fed A. lagunensis. However A. quadrilineata did perform a signifi cant transient response approx. 60 min after A. quadrilineata was exposed t o A. lagunensis by increasing the turning rate per unit time. This study su ggests that efficient top-down control of A. lagunensis by heterotrophic pr otozoans such as the studied ciliate may not happen as long as phytoplankto n organisms other than A. lagunensis make up a minor part of the standing p hytoplankton stock.