Victor S. Kuwahara

Mesoscale Eddies Drive Increased Silica Export in the Subtropical Pacific Ocean

Mesoscale eddies may play a critical role in ocean biogeochemistry by increasing nutrient supply, primary production, and efficiency of the biological pump, that is, the ratio of carbon export to primary production in otherwise nutrient-deficient waters. We examined a diatom bloom within a cold-core cyclonic eddy off Hawai`i. Eddy primary production, community biomass, and size composition were markedly enhanced but had little effect on the carbon export ratio. Instead, the system functioned as a selective silica pump. Strong trophic coupling and inefficient organic export may be general characteristics of community perturbation responses in the warm waters of the Pacific Ocean.

Variability in the Relative Penetration of Ultraviolet Radiation to Photosynthetically Available Radiation in Temperate Coastal Waters, Japan

UVR and PAR wavelengths are attenuated to different extents within the water column, causing variations in spectral composition with depth. The present investigation (a) describes the variability of UVR and PAR penetration at a station in the temperate coastal waters of Sagami Bay and determines (b) the characteristics of relative UVR penetration to the euphotic zone. Examination of the seasonal irradiance profile measurements indicated eight measurements displaying two distinct attenuation coefficients (Kd) for specific UVR wavelengths and PAR. The two attenuation coefficients observed from specific wavelengths in the water column may be caused not only by chlorophyll pigments, but also by dissolved organic material in the upper layer. The 1% depth of surface UVR at 305, 320, 340, and 380 nm averaged 10.8 ± 5.7, 14.9 ± 9.5, 19.8 ± 12.1, and 30.4 ± 17.6 m, respectively. The depth of euphotic layer displayed less variability averaging 62 ± 15 m throughout the entire study. Relative UVR penetration within the euphotic zone averaged 17.8 ± 8.1, 22.9 ± 10, 30.5 ± 13.8, and 46 ± 46.9% for 305, 320, 340, and 380 nm, respectively. A large variation of the relative transmission of UVR within the euphotic zone was found although the spectral composition was relatively stable in the air throughout the study.

Variability of Bio-optical Factors Influencing the Seasonal Attenuation of Ultraviolet Radiation in Temperate Coastal Waters of Japan¶

Abstract The study identifies the relative contribution of various bio-optical factors to the total attenuation of ultraviolet radiation (UVR) wavelengths and photosynthetically active radiation (PAR) in temperate coastal waters of Japan by surveying the physical properties of the water column, UVR and PAR penetration, and the absorption characteristics of dissolved and particulate material. Spectral absorbance properties of pigment (aph), detritus (ad) and chromophoric dissolved organic material (aCDOM) displayed both seasonal and wavelength specific variability. On an annual basis, absorbance by aCDOM was the highest absorbing fraction (47–59%) for the UVR wavelengths measured (305, 320, 340 and 380 nm) but decreased (32%) at 450 nm. Contribution of pigments to total absorbance was highest (40–60%) during a spring bloom for both UVR and PAR. A large variability (C.V. > 42%) for annual average attenuation coefficients (Kd[λ]) at respective wavelengths observed suggests that the spectral composition of the water column changes throughout the year in this region. A significant relationship was observed between Kd(λ) and aCDOM at 305, 320, 340 and 380 nm only (P < 0.01) but not for 450 nm (PAR) indicating the role of CDOM in regulating variations in Kd(λ), particularly in the UVR range. The slope S, obtained from a natural-log plot of the absorption coefficient of CDOM against wavelength, ranged between 0.014 and 0.036 nm−1 annually (average = 0.020±0.007, C.V. = 35%) and suggests seasonal changes in the origin of CDOM between terrestrial (low S) and biogenous (high S) CDOM.

Seasonal variability in chromophoric dissolved organic matter in the Sakawa River and Sagami Bay, Japan

We investigated the chromophoric dissolved organic matter (CDOM) of river water in the Sakawa River and of surface water in the vicinity of the river’s mouth in Sagami Bay, Japan, during the period from July 2003 to July 2004. Absorption by CDOM was modeled as a logarithmic function. As a qualitative index of CDOM, the slope (S) of this function was estimated for a wavelength range from 280 to 500 nm. As a quantitative index of CDOM, the integration of absorption was determined between 280 and 500 nm (Σ280500aCDOM). High seasonal variability of S values was observed at the marine station. The S values at the upstream stations were related to chlorophyll a concentrations but not to bacterial abundance, whereas the integrated values at upstream stations were correlated with precipitation. Seasonal variability in the integrated values was low at the downstream stations, where the effect of effluent from nonpoint sources and sewage treatment plants was strong. Anthropogenic CDOM at the downstream stations appeared to be degraded by microbial utilization and photodegradation, whereas terrestrially derived CDOM at the upstream stations was less degraded. These qualitative differences in CDOM and the marked effect of dilution by seawater near the Sakawa River mouth suggest that the dynamics of CDOM in riverine and coastal environments should be studied with careful consideration of both spatial and temporal variations, particularly in small estuaries.

Estimating the Diurnal Cycle and Daily Insolation of Ultraviolet and Photosynthetically Active Radiation at the Sea Surface

Accurate determination of the diurnal variability and daily insolation of surface (0+) and subsurface (0−) irradiance are essential to estimate several physical, chemical and biological processes occurring at the surface layer of marine environments. Natural downwelling PAR and spectral UVR were examined on eight occasions at 0+ and 0− to refine empirical models, particularly in the UVR spectrum. The diurnal variability in UVR and PAR were wavelength dependent and were modeled by a sinusoidal equation. The best fit for PAR at 0+ and 0− was the sinusoid power of n = 2 and n = 2.5, respectively. In the UVR spectrum, sinusoids increased as wavelengths decreased ranging from n = 2–5. Higher n values in the UV‐B spectrum suggest sharper increase/decrease near sunrise and sunset hours, ultimately reducing the final value of daily insolation at specified wavelengths. Calculated daily insolation of UV‐B/(UV‐A + PAR) ratio suggests that photoinhibition from exposure to UV‐B occurs within a shorter biologically effective day length than PAR, and is high during summer and low during winter. These results suggest that biogeochemical calculations based on diurnal models of irradiance measurements would benefit from accurate solar noon references and wavelength specificity, particularly in the UVR spectrum.

Bacterial growth rate and the relative abundance of bacteria to heterotrophic nanoflagellates in the euphotic and disphotic layers in temperate coastal waters of Sagami Bay, Japan

This study aimed to clarify the vertical differences in bacterial growth and grazing pressure on bacteria by heterotrophic nanoflagellates (HNF) and to identify the controlling factors of bacterial growth in temperate coastal waters of Sagami Bay, Japan. In addition to environmental factors, the annual monthly variations in bacterial growth rate (BGR) and the relative abundance of bacteria to HNF (BA/HNFA) were investigated in the euphotic and disphotic layers between May 2012 and May 2013. Significant vertical differences in BGR and BA/HNFA were evident between the two layers during the thermal stratification times of May to October 2012 and April to May 2013. BGR indicated significantly stronger limitation of bacterial growth in the euphotic layer compared to the disphotic layer. In contrast, significantly lower BA/HNFA was observed in the euphotic layer, suggesting significantly higher grazing pressure on bacteria by HNF. However, significant differences in BGR and BA/HNFA were not observed between the two layers from November 2012 to Match 2013, when the water column was well-mixed vertically due to the cooling and wind-induced mixing of surface water. This study indicates that bacteria in the euphotic layer grow less actively and are more vulnerable to predatory grazing by HNF relative to the disphotic layer during the stratification period. Further, multiple regression analyses indicate that bacterial growth was most controlled by the concentrations of chlorophyll a and dissolved organic carbon in the euphotic and disphotic layers, respectively.

Inovations in ocean optics for coastal and open ocean mooring applications

The subdiscipline field of ocean optics has advanced dramatically in the last 2 decade due to a culmination of advances. These advances include large strides in our quantitative understanding of theoretical ocean optics, technological innovations in instrumentation and progress in utilizing interdisciplinary observational platforms. Theoretical understandings of ocean optical processes have evolved from the ability to quantify phytoplankton biomass from fluorescence to the quantification of phytoplankton classes and the development of proxies for quantification of biogeochemical parameters. Technological innovations have evolved from one-cast broadband scalar photosynthetically active radiation (PAR) sensors to multifunctional hyperspectral sensors capable of long-term deployments on moored buoy platforms. The facilitation of interdisciplinary observational platforms such as ocean color remote sensing satellites, moored buoys, drifters, autonomous underwater vehicles (AUVs) and gliders has also advanced our understanding of spatial and temporal variability of various ocean optics properties. The objectives of this study are to evaluate the progression of our understanding of theoretical ocean optics due to recent advances in optical sensors and their applicability to advanced ocean observational platforms, such as moored buoy observatories. Specifically, we will present recent advances in measurement techniques for the inherent and apparent optical properties of seawater obtained from time-series measurements from moored buoy platforms from both complex coastal and relatively stable open ocean environments. The in situ time-series measurements of inherent and apparent optical properties, namely the optical absorption and backscattering coefficients, and upwelling and downwelling attenuation coefficients, as potential proxies for biogeochemical parameters will be discussed. We will also suggest potential quality control and data analysis protocols necessary for quality time-series optical measurements from moored buoy platforms. Special consideration will be given to the current and future direction of ocean optics towards deriving the concentration, composition and size of biological and geochemical properties while assessing innovations in technology pertaining to ocean optics.