In situ optical characterization of summer blooms in the North Pacific Subtropical Gyre

J. Nahorniak¹, R. M. Letelier¹, A. Ashe¹, D. M. Karl², M. J. Church², L. A. Fujieki²

¹College of Oceanic and Atmospheric Sciences, Oregon State University, 104 COAS Admin. Bldg., Corvalis, OR 97331-5503

²Department of Oceanography, University of Hawaii, 1000 Pope Road, Honolulu, HI 96822

Abstract

The North Pacific subtropical gyre has been characterized historically as an oligotrophic pelagic ecosystem with low carrying capacity. However, over the past 15 years, the Hawaii Ocean Time-series (HOT) program has recorded several summer phytoplankton blooms where the dominant photoautotrophic taxa can be filamentous cyanobacteria or diatoms. These events are ecologically and biogeochemically important because they can trigger large exports of organic matter into the benthic environment. Because the depth distribution of these blooms is characterized by a surface or subsurface biomass maximum located at the base of the mixed-layer, making it difficult to quantify their magnitude from above water remote sensing observations. As part of the HOT program, nearly monthly deployments of a Profiling Reflectance Radiometer (PRR) in the upper water column (0-200m) have been made since February 1998. In addition, in October 2002, an optical package including a 9 wavelength absorption and attenuation meter (AC-9), a fast repetition rate fluorometer (FRRf, and a CTD was added to our monitoring program in order to assess the vertical and temporal variability of inherent optical properties and phytoplankton photosynthetic parameters in the euphotic zone. The PRR records display a distinct signature for summer blooms during the years 1998, 2000, and 2005. This signature consists of a significant increase in the attenuation coefficient at the base of the mixed-layer, a concomitant decrease in the PRR derived fluorescence quantum yield in this layer, and a shoaling of isolumes below it. In addition, although no significant blooms were observed in the PRR data between 2002 and 2004, the AC-9 measurements in the upper euphotic zone recorded a significant increase in scattering and changes in the particulate absorption spectrum that are consistent with the presence of filamentous cyanobacteria during autumn 2002 and 2003. This change in the absorption spectrum was not observed in a diatom dominated bloom during summer 2005. Our results suggest that it is possible to characterize the nature and magnitude of summer blooms and to quantify seasonal and interannual changes in the abundance of filamentous cyanobacteria in the North Pacific Subtropical gyre using vertical profiling moorings equipped with optical instruments similar to those used in this study.