Cross-shelf transport of sub-thermocline nitrate by the internal tide and rapid (3–6 h) incorporation by an inshore macroalga

Abstract

During summer in shallow waters off Baja California, Mexico, the internal tide is a dominant thermal feature of the water column. However, its importance for sub-thermocline nutrient provision to benthic macroalgae is unknown. In order to determine if internal motions provide nutrients to macroalgae in summer, Ulva lactuca was outplanted at inshore stations for short (3 and 6h) intervals, at the surface, 5 and 10m depth, and tissue nitrogen content was measured before and after each deployment. Concurrently temperature, currents, and nutrients were measured using moored thermistors, current profilers, CTDs, Niskin bottles, and an in-situ UV absorbance nitrate sensor (ISUS). Discrete pulses of cool, nutrient-rich water were horizontally displaced at least 4km on the shelf and shoaled more than 20m depth at the semidiurnal frequency, resulting in more than a 10-fold change in the concentration of nitrate. Inshore, tissue nitrogen of Ulva outplants increased significantly during longer exposures to this cool water. At this site, the semidiurnal signal dominates water column temperature fluctuations from April to November, with summer showing the greatest cooling (up to 5°C) in a one-hour period. We estimated that 11% of the days of a year show internal waves that would cause a significant change in nutrient availability to macroalgae at 5m depth. This study supports the hypothesis that nitrate can reach and be rapidly incorporated by inshore macroalgae such as Ulva through transport forced by the internal tide, and that even very short (<1h) nutrient pulses in nature are reflected in macroalgal tissue. We propose that at this site, the internal tide provides a significant, yet understudied, high frequency nutrient source to inshore primary producers, particularly in summer.