Horizontal internal‐tide fluxes support elevated phytoplankton productivity over the inner continental shelf

Abstract

The small, free‐living, photosynthetic organisms collectively called phytoplankton are the base of the oceanic food web. The phytoplankton use the energy from sunlight to convert carbon dioxide into organic carbon. Besides the necessity for sufficient light, phytoplankton also require nutrients to photosynthesize and grow. They acquire these nutrients from the dissolved pool of nutrients in the surrounding ocean. In the surface waters, where there is sufficient light for phytoplankton growth, nutrients are often in short supply and act to limit the growth of the phytoplankton. The renewal of the nutrients in the surface ocean is controlled by the physical dynamics of the upper ocean, including waves and tides, and the influence of the wind. Since the phytoplankton are limited by the availability of nutrients, the rate at which physical dynamics supply nutrients is a fundamental control on the rate of phytoplankton productivity and thus on the entire oceanic food web. In the Southern California Bight (SCB), the nutrient fluxes that regulate the productivity of the phytoplankton are caused by internal waves. Internal waves are waves that propagate through the body of the ocean much in the same way that waves propagate on the surface of the ocean. In the SCB, the most energetic internal waves are internal waves of tidal frequency, called the internal tide . The internal tide causes transport and mixing of nutrient‐rich waters from deep and offshore to shallow coastal waters. This means that coastal waters are much more productive than the surface waters offshore (these productive coastal phytoplankton populations can even be observed from space). In this article, we demonstrate quantitatively that the horizontal flux of nutrients due to the internal tide controls the productivity of the coastal ocean in Southern California. The narrow continental shelf of the Southern California Bight (SCB) is characterized by elevated primary productivity relative to the adjacent open ocean. This persistent gradient is maintained by the nitrate fluxes associated with internal waves of tidal frequency (the internal tide). Here we provide the first estimates of the internal‐tide–driven horizontal fluxes of nitrate, heat, energy, and salinity, calculated from high‐resolution, full water‐column data gathered by an autonomous wave‐powered profiler and a bottom‐mounted current meter. The vertically integrated nitrate, heat, and energy fluxes were onshore over the 3‐week period of the field experiment. The inner‐shelf area‐ and time‐averaged dissipation rate due to the onshore horizontal energy flux, 2.25 × 10  − 7  W kg  − 1 , was elevated relative to open ocean values. The magnitude of the vertically integrated horizontal nitrate flux (136.4 g N m  − 1  d 1 ) was similar to phytoplanktonic nitrate uptake rates over the inner‐shelf. This nitrate flux was variable in time, capable of supporting 0–2800 mg C m  − 2  d  − 1 (mean approx. 774 mg C m  − 2  d  − 1 ) of “new” primary productivity, depending on the energetics of the internal tide and the cross‐shore distribution of nitrate. We postulate that the horizontal, internal‐tide–driven nitrate flux is the primary cause of the persistently elevated phytoplankton biomass and productivity over the narrow SCB inner shelf. Furthermore, these results suggest that horizontal fluxes of nutrients driven by internal waves may contribute significantly to primary productivity along the boundaries of aquatic environments.