Steven G. Ackleson

Size and refractive index of individual marine particulates: a flow cytometric approach

Flow cytometric measurements of light scatter, near-forward light scatter (theta = 1.5-19 degrees ) and side scatter (theta = 73-107 degrees ), from individual marine particles is modeled using Mie theory. Particles are assumed to be homogeneous and nearly spherical in shape. Uniform polystyrene microspheres and oil suspensions are used to estimate model accuracy. Within the particle diameter range of 1-32 microm, the mean error for near-forward scatter and side scatter is 16.9% and 30.1%. respectively. The model is used to estimate size and refractive index of several nannoplankton species and the results are compared with microscopic measurements of cell size and published values of phytoplankton refractive index. Within the refractive-index range of 1.01-1.1, the model may be inverted to yield refractive index with an absolute error of between 0.01 and 0.003. Measurements of particle size distributions in clear ocean water indicate this range accounts for 99% of all nannoplankton and 39% of all particles within the size range from 1 to 10 microm.

Blooms of the coccolithophore Emiliania huxleyi with respect to hydrography in the Gulf of Maine

Abstract We present results of oceanographic surveys of visually turbid blooms of the coccolithophore Emiliania huxleyi in the Gulf of Maine during the summers of 1988, 1989 and 1990. In each year, hydrographic stations within the blooms could be distinguished from non-bloom stations on a temperature-salinity diagram. In 1988 and 1989 the blooms were confined to the surface waters of the central western Gulf of Maine; T-S analyses showed they occurred in higher salinity surface waters at stations characterized by a well-defined upper mixed layer overriding a sharp pycnocline. Nutrients (not measured in 1988) were near depletion in the surface waters of both bloom and non-bloom stations in 1989, with surface phosphate being lower in the bloom waters (0.02–0.16 μM in the top 15 m) than in non-bloom waters (0.21–0.49 μM). Phosphate was not as low in the surface waters of the 1990 bloom. The bloom that year was much smaller in areal extent than in 1988 or 1989, and was limited to the northern part of the Great South Channel and western Georges Bank area of the Gulf of Maine. T-S analyses indicated significant mixing of different water masses in the area of the bloom in 1990, with the bloom being confined to those stations having less dense surface waters, of lower salinity, than the non-bloom stations. There also was evidence of a subsurface salinity minimum beneath the bloom waters in 1990. Blooms of E. huxleyi with surface expressions of visually turbid waters do not occur every year in the Gulf of Maine, and we discuss possible causative factors, specifically as related to the age or maturity of surface waters and macro- and micro-nutrient levels, that could facilitate bloom formation and which could vary between years.

Response of water-leaving radiance to particulate calcite and chlorophyll a concentrations: A model for Gulf of Maine coccolithophore blooms

A coupled atmosphere and ocean radiative transfer model, the Gulf of Maine (GOM) model, was developed to simulate water-leaving radiance from a vertically stratified ocean containing a bloom of the coccolithophore Emiliania huxleyi. The model is based largely on atmospheric and ocean data representing the Gulf of Maine. The atmospheric submodel simulates direct sunlight and diffuse skylight illuminating the sea surface and is adjusted to account for seasonal changes in atmospheric aerosols. The optical properties of E. huxleyi, required by the ocean submodel, are derived from measurements collected in Gulf of Maine coccolithophore blooms occurring in 1989 and 1990. The modeled response of volume reflectance to the combined effects of chlorophyll and particulate calcite compares favorably with field measurements of E. huxleyi cell abundance and coastal zone color scanner (CZCS)-derived volume reflectance representing a coccolithophore bloom in the northeast Atlantic Ocean. The GOM model was used to investigate the response of normalized water-leaving radiance, modeled for visible CZCS and sea viewing wide field of view sensor (SeaWiFS) bands, to particulate calcite and chlorophyll a. Ranges in the concentrations of particulate calcite, chlorophyll a, and colored dissolved organic material (CDOM) are selected to represent conditions reported for coccolithophore blooms. Water-leaving radiance increases with increasing particulate calcite concentration, primarily because of a disproportionately large amount of backscatter from detached coccoliths (about an order of magnitude larger than is predicted using Mie theory). As a result, CZCS plant pigment algorithms based upon radiance ratios may be corrupted more severely than previously estimated. As an alternative to radiance ratio-based algorithms, an iterative procedure (also referred to as optimization) is used to invert the GOM model in order to simultaneously retrieve particulate calcite and chlorophyll a concentrations. The approach uses normalized water-leaving radiance computed for all visible CZCS or SeaWiFS bands. Tests of the approach suggest that independent of errors associated with instrument calibration and atmospheric correction, errors in the retrieved concentrations are small, even when high concentrations of CDOM and vertical structure within the water column are neglected, i.e., with the assumptions that CDOM concentration is small and the water is vertically homogeneous. However, since there are no data sets of contemporaneous chlorophyll a concentration, particulate calcite concentration, and CZCS imagery, a rigorous test of the model and inversion technique must wait for the launch of new ocean color scanners such as the NASA SeaWiFS.

Phytoplankton optical properties: flow cytometric examinations of dilution-induced effects.

Flow cytometry is used to measure dilution-induced changes in the optical properties of Dunaliella tertiolecta-light scatter, cell fluorescence, and refractive index. Observed changes in cell optical properties are compared to simultaneous measures of cell volume and count, extracted chlorophyll a concentration, and the (14)C uptake rate. Flow cytometric measurements reveal short-term dilution effects (within 1 h of dilution) that, are not evident in other morphological or physiological measurements such as cell volume, extracted chlorophyll a concentration, and (14)C uptake rate. Data are presented which suggest that these short-term changes in cell optical properties are the result of changes in the real component of refractive index, possibly due to a rapid and temporary rearrangement of the internal cellular structure. Long-term changes are observed in time series measurements of cell volume and count, extracted chlorophyll a concentration, and (14)C uptake rate.

Astronaut observations of the Persian (Arabian) Gulf during STS-45

Abstract As a result of the 1991 Persian Gulf war, between mid‐January and June 1991, the Persian Gulf was contaminated with an estimated 4 to 6 million barrels of crude oil, released directly into the Gulf from refinement facilities, transhipment terminals, and moored tankers along the coast of Kuwait, and precipitated from oil fire smoke plumes. To assess the environmental impact of the oil, an international team of marine scientists representing 14 nations was assembled under the auspices of the United Nations International Oceanic Commission and the Regional Organization for Protection of the Marine Environment to conduct detailed surveys of the Persian Gulf, the Strait of Hormuz, and the Gulf of Oman, including hydrographic, chemical, and biological measurements. To supplement the field surveys and to serve as an aid in data interpretation, astronauts aboard the Space Shuttle Atlantis photographed water features and coastal habitats in the Persian Gulf during mission STS‐45 (24 March to 02 April 1992...

Earth observations during space shuttle mission STS‐45 mission to planet earth March 24—April 2, 1992

A description is presented of the activities and results of the Space Shuttle mission STS-45, known as the Mission to Planet Earth. Observations of Mount St. Helens, Manila Bay and Mt. Pinatubo, the Great Salt Lake, the Aral Sea, and the Siberian cities of Troitsk and Kuybyshev are examined. The geological features and effects of human activity seen in photographs of these areas are pointed out.

Diffuse attenuation in optically-shallow water: effects of bottom reflectance

It is well-known that in the ocean, where the depth of the ocean floor is large compared with the attenuation length of irradiance, the diffuse attenuation coefficients for vector and scalar irradiance (K-functions) are not affected by the optical properties or proximity of the ocean floor. This is the case of an optically deep ocean where the attenuation coefficients are determined solely by the inherent optical properties of the water and the distribution of radiance. Since, within optical-deep water, variability in the K- functions due to radiance distribution are small relative to the effects of the inherent optical properties of water, K- functions have been treated as quasi-inherent optical properties. Furthermore, when the depth of the ocean floor is shallow enough so that it becomes illuminated by down- welling irradiance, i.e. when the ocean is optically shallow, the in-water light field is modified by the optical properties of the ocean floor. The effect decreases with increasing depth and distance from the ocean floor. It is not generally appreciated, however, that the associated K- functions will also be affected by both the optical properties and the proximity of the ocean floor and, therefore, cannot be treated as quasi-inherent optical properties. If these effects are neglected, large errors, exceeding 25 percent in some cases, can result from modeling the optically shallow scalar irradiance profile as a function of a constant diffuse attenuation coefficient.

Ocean Optics Revisited

In 1991, Optics & Photonics News published a review article on ocean optics1 that discussed the field of ocean optical research. Many advances have occurred during the past five years. The emphasis of much of the work has also shifted from the deep clear ocean to the coastal environment. The present status and future directions of the field are discussed.

Earth observations during space shuttle flight sts‐49: Endeavor's mission to planet earth (May 7–16, 1992)

A brief mission overview of STS-49 is given, and some of the pictorially outstanding and scientifically interesting photographs obtained during the mission are presented. The Earth observations are described and include the following: the Southwestern Pacific Ocean -- wind and water; the Southwestern Pacific Ocean -- coasts and volcanoes; the US; Cuba and the Bahamas; South America; Africa; the Red Sea and Western Indian Ocean; and the Indian Subcontinent.

Earth observations on space shuttle flight 43, it was a dirty world, August 2–11, 1991

Attention is given to earth observations made by the crew of Space Shuttle Flight 43, during which atmospheric haze and its various aerosols were investigated. The length-of-record time series from the Space Shuttle Earth Observation Program was used to photograph some of the sites under active investigation. It was found that Lake Chads northern basin remained dry, and the southern basin showed no appreciable changes in water level. The Okavango Swamp areas in Botswana showed agricultural burning at several sites, and increasing evidence of land use activity in the areas of the swamp that previously had been under natural cover. Consideration is also given to Hurricane Fefa and Tropical Storm Guillermo, the smoke from the Kuwaiti oil fires, Mount Pinatubo, dust storms in the Sahara, biomass burning, South Asian monsoon runoff, land use patterns, landforms and geology, and oceans and coastal features.