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

ORIGINAL ARTICLE Horizontal internal-tide fluxes support elevated phytoplankton productivity over the inner continental shelf Andrew J. Lucas 1 , Peter J. S. Franks 1 , and Christopher L. Dupont 1,2 Abstract 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 27 W kg 21 , was elevated relative to open ocean values. The magnitude of the vertically integrated horizontal nitrate flux (136.4 g N m 21 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 22 d 21 (mean approx. 774 mg C m 22 d 21 ) 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. Keywords: internal waves, mixing, new productivity, nitrate flux, nutrient dynamics, Reynolds fluxes Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093, USA Present address: Microbial and Environmental Genomics, J. Craig Venter Institute, San Diego, California 92121, USA Correspondence to Andrew J. Lucas, ajlucas@ucsd.edu Introduction [1] In nutrient-limited oceanic environ- ments, the rate of nutrient supply to the euphotic zone—typically mediated by physical dynamics—controls the rate of phytoplankton productivity and the character of the phyto- plankton community and, at steady-state, sets the proportion of that productivity which can be exported to higher trophic levels and out of the euphotic zone (“new” productivity, sensu Dugdale and Goering 1967). Quantifying the physical supply of nutrients to the sunlit surface ocean is therefore of fundamental importance in understanding oceanic ecosystem function. [2] The elevated primary productivity of the coast ocean relative to adjacent offshore waters is due to the operation of physical dynamics that act to inject nutrients into the Limnology and Oceanography: Fluids and Environments † 1 (2011): 56–74 q 2011 by the American Society of Limnology and Oceanography, Inc. DOI 10.1215/21573698-1258185 Downloaded at UNIV CA- SAN DIEGO on May 26, 2011