MEPS 235:29-42 (2002)  -  doi:10.3354/meps235029

Fluorescence patches in high-frequency internal waves

Cleridy E. Lennert-Cody1,*, Peter J. S. Franks2

1Inter-American Tropical Tuna Commission, 8604 La Jolla Shores Drive, La Jolla, California 92037-1508, USA
2Scripps Institution of Oceanography, University of California San Diego, La Jolla, California 92093-0218, USA

ABSTRACT: Phytoplankton patchiness associated with high-frequency internal waves was investigated by analyzing chlorophyll fluorescence along isotherms. In summer, patches of fluorescence were found over wave troughs through the upper part of the water column and occasionally under wave crests deeper down. A similar pattern was observed in spring; day-to-day variability in the vertical structure of fluorescence patchiness was however greater in spring than in summer. Maximum fluorescence within patches was typically less than 130% of the background fluorescence, ranging from 104 to 168% in spring and 102 to 153% in summer. The largest values were generally found along isotherms nearest the surface. The patterns of fluorescence and wave form, with additional data on beam attenuation coefficient, were used to test hypotheses of patch formation mechanisms, including non-photochemical quenching, swimming-gradient induced patchiness, and concentration-gradient induced patchiness. While non-photochemical quenching apparently created patches of fluorescence, the magnitude and pattern of the patches could not be completely accounted for by this dynamic. Much of the patchiness was consistent with increases in biomass driven by swimming of phytoplankton in the divergent wave circulation. The contribution of any mechanism to patch formation depends on the phytoplankton community structure: blooms dominated by dinoflagellates will show patches created by swimming and changes in fluorescence yield, while the non-swimming phytoplankton community will tend to show patches of enhanced fluorescence yield.

KEY WORDS: Patchiness · Phytoplankton · Fluorescence · Internal waves

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