Hasong Pak

On the intermediate particle maxima associated with oxygen-poor water off western South America

Abstract The distribution of suspended particulate matter was measured during 21 May and 18 June 1977 between 4 and 23°S from the coast of South America to about 500 nautical miles offshore. A well-defined maximum was observed over the continental margins at depths of about 200 m between ∼9 and 23°S, At 4°S, the main particle maximum was at approximately 400 m, but in the nearshore zone the maximum extended upwards to ∼200 m. A comparison of the particle and chemical data shows that the particle maxima are usually at approximately the core depth of the oxygen minimum layer. A nitrite maximum and a nitrate minimum were also observed at or near the particle maximum core depth south of ∼9°S. Near 4°S, a weak nitrite maximum was observed within the oxygen minimum layer at some stations. The protein distribution near 15°S suggests that the material in the particle maximum contains significant amounts of organic matter. The distribution of the particle maximum layer between 9 and 23°S and its relations to the density field and the cross-shelf flow suggest that most of the particles could originate in the bottom waters over the outer continental shelf and be transported offshore in a quasi-horizontal path. Offshore particle transport near the equator is probably supported by a westward current off northern Peru between and under the eastward extension of the Equatorial Undercurrent and the Subsurface South Equatorial Countercurrent. However, the source of the particles in this ∼400-m maximum has not been determined.

Meridional variations in the concentration of chlorophyll and microparticles in the North Pacific Ocean

Abstract The vertical distributions of chlorophyll a and microparticle concentration were recorded along two meridional transects of the eastern Pacific Ocean. One transect, obtained during the winter along 50°W, covered 15°S to 15°N, and the other transect, obtained during the summer along 155°W, covered 23°N to 57°N. Both distributions were measured optically: Chl a concentration was determined from the in situ fluorescence of Chl a , and microparticle concentration was determined from the transmittance of a collimated beam of light at 665 m. Two patterns are apparent from the data. First, meridional changes in the concentration of Chl a are parallel by changes in particle concentration. Chlorophyll concentrations are high at the equator and at high latitudes where the concentrations of microparticles are also high. An examination of the vertical distributions of water density and nitrate concentration suggests that this pattern appears to be determined largely by the concentration or rate of supply of nitrate to the euphotic zone. Second, for a given latitude the vertical distribution of Chl a is predominantly caused by increases in the mean concentration of pigment within the microparticles. The ratio of Chl a to particle concentration is lowest in the surface layer. It increases rapidly below the surface layer, and it reaches a maximum value at or just below the depth of the chlorophyll maximum. Below the chlorophyll maximum, the ratio decreases slowly with depth. These changes appear to be a result of photoadaptation by phytoplankton and are consistent with recent mathematical descriptions of this process derived from studies of laboratory cultures.

Equatorial Front in the Eastern Pacific Ocean

Abstract The equatorial front in the eastern Pacific Ocean observed during 14 October–7 December, 1971, from R/V Yaquina of Oregon State University is discussed. The front is clearly defined at the sea surface by a large horizontal gradient of temperature, salinity, σt and nitrate. Equatorial upwelling, which is believed to be driven by the prevailing wind and the Equatorial Undercurrent, appears as the primary source of the cold water on the south side of the front in the region west of the Galapagos Islands. In vertical distributions of various properties of the water across the equator, equatorial upwelling is apparent over the equator in the region between the Galapagos Islands and the Ecuadorian coast. It is suggested that the equatorial front in this region is also associated with the equatorial upwelling.

Effect of particle size distribution and chlorophyll content on beam attenuation spectra.

The relationships between beam attenuation spectra, chlorophyll and pheophytin pigment concentrations, and particle size distributions are examined for a coastal region (Monterey Bay area) believed to have negligible concentrations of terrestrially derived dissolved organic compounds (during May 1977) but large quantities of phytoplankton and resuspended sediments. It was found that the slope of the beam attenuation spectra increases when the hyperbolic slope of the size distribution increases. The magnitude of this increase in slope was consistent with calculations based on a range of particle diameters from 0.5 to 30 microm, so that it would be possible to predict the slope of the particle size distribution if the slope of the beam attenuation spectra is known. The ratio of chlorophyll and pheophytin pigments to suspended volume concentrations affected the beam attenuation spectra to a lesser degree and in a more complex manner. Because of the strong effect of slope, it was concluded that the chlorophyll and pheophytin pigment content of suspended particles could not be efficiently predicted by means of beam attenuation measurements.

Volume scattering function of suspended particulate matter at near-forward angles: a comparison of experimental and theoretical values

Narrow angle light scattering measurements were made for various sizes of spherical particles suspended in water. These were compared to calculated theoretical scattering values as derived from the theory of Mie (1908). Through measurements with different particle concentrations at angles between 0.2 degrees and 0.7 degrees the effect of the unscattered main beam light was removed. Results agreed well with Mie theory for these angles.

The vertical structure and size distributions of suspended particles off Oregon during the upwelling season

Abstract A simple numerical model of the vertical distribution of two size classes of particles is developed for situations common during the coastal upwelling season off Oregon. The total particle concentration is assumed to be proportional to the phytoplankton population for the surface layer. The two size classes of particles are thus distinguished by their maximum specific growth rates, their half saturation constants for nitrate uptake, and their settling rates. The resulting vertical distributions and size distributions were similar in shape to the average profiles for regions inshore and offshore of a particle front during August 1974 but overestimated particle concentration by 40%. This was attributed to ignoring grazing by zooplankton. Sensitivity analyses showed the size preference was most responsive to the maximum specific growth rates and nutrient half-saturation constants. The vertical structure was highly dependent on the eddy diffusivity followed closely by the growth terms.

Intermediate Nepheloid Layers Observed Over The Continental Margins Off Oregon

Intermediate nepheloid layers were observed by a beam transmissometer operating at 650 nm during 3-8 November 1976 over the continental shelf and slope off Oregon. Two well defined intermediate nepheloid layers were observed. One was located at about 150 m depth and extended westward from a point 10 NM offshore. The second was at about 375 m depth and went westward from a point 25 NM offshore. They were at least 15 NM wide (east--west dimension), and 50-150 m thick. They closely followed isotherms in the zonal section. The overall structure of the nepheloid layers remained unchanged during the period of the cruise. The intermediate nepheloid layers appear to have been generated by lateral diffusion and advection of turbid water from bottom nepheloid layers over the continental shelf and slope toward the deep water. Possible processes of diffusion and advection of bottom nepheloid layers to generate intermediate nepheloid layers are discussed.

Some observations on the effect of ozone treatment on suspended particulate matter in seawater

Effects of ozone treatment on suspended particles in seawater have been observed. The observations consist of measuring the particle size distribution using an electronic particle counter before and after treating the sample with ozone. Three kinds of response have been observed in the laboratory and in the field: (i) the total elimination of a peak in the size spectra; (ii) a shift of a peak to a smaller size with correspondingly less volume; and (iii) a mild decrease in the total particle concentration. Our interpretation of these responses as observed in the field are respectively: (i) phytoplankton with a very small amount of refractory structural materials (e.g. naked flagellates); (ii) particles that individually contain both refractory and non-refractory components (e.g. diatoms); and (iii) primarily non-organic and refractory organic materials.

Observations of natural fluorescence with an underwater radiometer

Measurements of subsurface irradiance spectra in the ocean consistently indicate intensities of long-wavelength visible light (λ≧589 nm) greater than could be derived from the penetration of sunlight. This can be attributed to natural fluorescence and/or to spectral crosstalk due to light leakage through the blocking filters of each discrete detector. A comparison of observed profiles with modelled contributions from these two possible sources indicates that both factors are contributing. At 671 and 694 nm wavelength, the excess signal appears to be predominantly fluorescence while that at 589 nm is mostly crosstalk. Both effects appear to be important at 625 nm although the amount of excess light is small compared to the other wavelengths.The above observations and interpretations are consistent with the optical design of the instrument used and the shape of natural irradiance spectra. Fluorescence efficiencies derived from the irradiance measurements correlate well with measuredin situ fluorescence.