Organic and inorganic nutrient effects on growth rate–irradiance relationships in the Texas brown‐tide alga Aureoumbra lagunensis (Pelagophyceae)1

Pelagophyte species in the genera Aureococcus and Auroumbra form brown tides in coastal bays that cause food‐web disruption and extensive shading of benthic primary producers. Organic nutrients have been suggested as key factors in the origination and persistence of the East Coast (USA) brown‐tide alga Aureococcus anophagefferens Hargraves et Sieburth. To evaluate this finding for the Texas brown‐tide alga Aureoumbra lagunensis D. A. Stockw., DeYoe, Hargraves et P. W. Johnson , we grew strain TBA‐2 with dissolved inorganic nitrogen (DIN; or ) or dissolved organic nitrogen (DON; urea or glutamate) as the nitrogen (N) source under eight light intensities. Maximum growth rates decreased with N source from (1.0 div · d −1 ) to (0.48 div · d −1 ). Neither growth rate efficiency (α) nor I k varied significantly between N treatments. Both inorganic phosphorus (P) and β‐glycerophosphate supported growth. Aureoumbra lagunensis can utilize at least some forms of organic N and P and can use them to persist or grow when inorganic forms become limiting. We found no evidence to support the hypothesis that organic utilization enhances or supplements growth at low light levels.