The diurnal variability ofAureoumbra lagunensis,dissolved metals (Cd, Pb, Mn, Cr, Cu, Zn, Ni, Se, and Fe), and environmental parameters (salinity, temperature, turbidity, pH, and dissolved oxygen) were studied to understand the role of metals in the occurrence of the Texas “brown tide.” TheA. lagunensisdensities were high in the afternoon; suggesting the influence of light on the growth ofA. lagunensis. The results indicate that metals, Cd, Pb, Mn, Cr, and Se may not be influencing the growth ofA. lagunensis. The dissolved Cu, Zn, and Ni concentrations were inversely related withA. lagunensisdensities while Fe had positive relationship withA. lagunensis. The spectroscopic characterization of the surfaces ofA. lagunensissuggests that these metals might be playing a role in the occurrence of Texas “brown tide” through involvement of ‐OH and ‐CH functional groups on the surfaces ofA. lagunensis.