Nitrogen uptake and growth by the opportunistic macroalga Ulva lactuca (Linnaeus) during the internal tide

To determine if an opportunistic macroalga could increase its tissue nitrogen content in response to nutrient pulses forced by the internal tide, independent samples of the green alga Ulva lactuca (Linnaeus), previously cultured in nutrient-poor tanks, were outplanted in the nearshore ocean for 12 h deployment periods (8 daylight periods, 7 night periods), at three different depths (1, 5, and 10 m), during stratified water column conditions and spring tides. Algal tissue nitrogen and growth weight (before and after each deployment), water-column temperature (continuous), and inorganic water-column nutrient concentrations (taken on 3 days during the experiment) were measured. Abrupt drops in temperature (up to 3 °C in an hour) occurred twice daily, at 10 m and occasionally at 5 m depth, associated with the internal tide. Date and deployment depth had significant effects on U. lactuca tissue nitrogen, showing a significant increase for deeper deployments and a positive correlation with the duration of cool water (< 15.8 °C, > 1 μM NO3−) at the deployment depth. On days when temperature remained above 15.8 °C at all depths (15.8 °C was established as the temperature below which nitrate was above 1 μM), no significant increase in tissue nitrogen was detected at any depth. Growth and tissue nitrogen were significantly and inversely correlated for mid-water (5 m) and bottom (10 m) deployments, but not at the surface. The data support the hypothesis that U. lactuca can increase tissue nitrogen in response to the cold phase of the semidiurnal internal tide.

► Ulva tissue N increased (up to 94%) concurrently with semidiurnal cold-water pulses.
► The significant increase correlated to the depth to which cold water shoaled.
► Ulva can rapidly increase tissue nitrogen in response to semidiurnal internal tides.
► Algae can respond to internal-tide induced upwelling at a scale of hours.