Temporal variability of light penetration depth at Station ALOHA: Potential effect in productivity and phytoplankton community structure in the lower euphotic zone

R. M. Letelier¹, D. M. Karl², M. R. Abbott¹, R. R. Bidigare², J. S. Nahorniak¹

¹College of Oceanic and Atmospheric Sciences, Oregon State University 104 Ocean. Admin. Bldg., Corvallis, OR 97331-5503, US

²School of Ocean and Earth Science and Technology, University of Hawaii 1000 Pope Road, Honolulu, HI 96822, US

Abstract

Nearly monthly deployments of a Profiling Reflectance Radiometer (PRR) made since February 1998 at Station ALOHA (22 45 N 158 00 W) document 40m excursions of isolume surfaces between winter and summer months. While the 1% light level for summer months (~0.53 mol quanta m^-2day^-1) reaches on average 125 m, this photon flux shoales to 85 m during winter. The seasonal cycle of light depth penetration results from the combined effect of changes in sea surface solar irradiance and light attenuation (absorption and scattering) in the upper water column. Superimposed on this seasonal cycle, we observed short-term change in light attenuation in the mixed layer resulting from sub-surface diatom blooms during summer months of years 1998 and 2000. Colored Dissolved Organic Matter (CDOM) and chlorophyll (Chl) solar-induced fluorescence (683 nm) profiles derived from PRR data also display seasonal patterns in the lower euphoric zone. For a given isolume flux smaller than 0.53 mol quanta m^-2day^-1, the CDOM and Chl -specific fluorescence signals appear to increase between August-November and decrease between December-February. Since the photon flux is constant along a given isolume, this variability is attributed to physiological changes within the phytoplankton community in the lower euphotic zone. Because the temporal vertical displacement of the nutricline is forced by the displacement of isolumes we suggest that some of these fluorescence and CDOM trends result from successional patterns driven by the seasonal penetration and subsequent retreat of light into the nutricline. The difference in nitrate concentrations profiles in the upper water column between winter and summer months indicates that the lower euphotic zone contributes at least 30% of the annual new production in this area of the North Pacific subtropical gyre and the majority of the euphotic zone nitrate uptake.