Dale A. Kiefer

Light scattering by microorganisms in the open ocean

Abstract Recent enumeration and identification of marine particles that are less than 2μm in diameter, suggests that they may be the major source of light scattering in the open ocean. The living components of these small particles include viruses, heterotrophic and photoautotrophic bacteria and the smallest eucaryotic cells. In order to examine the relative contribution by these (and other) microorganisms to scattering, we have calculated a budget for both the total scattering and backscattering coefficients (at 550nm) of suspended particles. This budget is determined by calculating the product of the numerical concentration of particles of a given category and the scattering cross-section of that category. Values for this product are then compared to values for the particulate scattering coefficients predicted by the models of GORDON and MOREL (1983) and MOREL (1988). In order to make such a comparison, we have estimated both the total scattering and backscattering cross-section of various microbial components that include viruses, heterotrophic bacteria, prochlorophytes, cyanobacteria, ultrananoplankton (2–8μm), larger nanoplankton (8–20μm) and microplankton (>20μm). Such determinations are based upon Mie scattering calculations and measurements of the cell size distribution and the absorption and scattering coefficients of microbial cultures. In addition, we have gathered published information on the numerical concentration of living and detrial marine particles in the size range from 0.03 to 100μm. The results of such a study are summarized as follows. The size distribution of microorganisms in the ocean roughly obeys an inverse 4th power law over three orders of magnitude in cell diameter, from 0.2 to 100μm. Thus, the size distribution of living organisms is similar to that for total particulate matter as determined by electronic particle counters. For representative values of refractive index, it appears that most of the scattering in the sea comes from particles less than 8μm in diameter, and that most of the backscattering comes from particles less than 1μm. Among the microorganisms that are found in this size range, free-living heterotrophic bacteria may be often most important. These microbes account typically for 10 to 50% of the total particulate scattering and 5 to 20% of backscattering in oligotrophic waters that contain less than 0.5mg chlorophyll per m3. The second most important source of microbial scattering is cyanobacteria (especially in tropical and temperate waters) and ultrananoplankton. Viruses, which may be very abundant, make little contribution because they have extremely small cellular scattering cross-sections. Large microorganisms, which include nanoplankton >8μm and microplankton and which efficiently scatter light, contribute little because of their low cellular concentrations. While a significant fraction of the total scattering coefficient may come from the combined contributions of viable procaryotic and eucaryotic cells, only a small fraction of the backscattering appears to come from these microbes. Instead, it appears that the major source of particulate backscattering is small (

Chlorophyll α specific absorption and fluorescence excitation spectra for light-limited phytoplankton

Methods are described for the measurement of spectral absorption coefficients, fluorescence excitation, and fluorescence yields for pigmented particles retained on filters. The corrections required for absorption coefficients include determining increased optical pathlength while corrections for fluorescence include determining system spectral variability, mean light level and reabsorption. The empirical technique is consistent with and validated by theoretical relationships for light transmission and fluorescence of absorbing particulate material embedded in a medium with intense scattering.These methods were applied to a study of photoadaptation in several phytoplankton species and revealed variations in the blue for chlorophyll α specific absorption [αph*(λ)] and fluorescence excitation [F*(λ)] of greater than 3− and 10-fold, respectively. Variations in the spectral shapes and the magnitude of αph*(λ) and F*(λ) with photoadaptation are determined largely by the effect of pigment absorption in discrete particles, sometimes referred to as the sieve or package effect. A model is presented expressing F*(λ) in terms of α*(λ) which predicts large variability in F*(λ) due to cell size and cellular pigmentation and which may help reconcile the previously reported, but unexplained variations in F*(λ). Spectral variations in the fluorescence yield appear to be caused by variations in the fraction of light absorbed by photosystem II which fluoresces as compared to photosystem I or photoprotective pigments which do not fluoresce. The techniques presented provide a rapid, reproducible, and simple approach for routine analysis, particularly for field applications where particle densities are too low for direct analysis of absorption spectra.

Origins of vertical patterns of phytoplankton and nutrients in the temperate, open ocean: a stratigraphic hypothesis

Abstract A new hypothesis is presented to explain the origins of patterns in the vertical distribution of phytoplankton and nutrients in open, temperate oceans. According to the hypothesis, vertical distributions are largely determined by the dynamics of the formation of the seasonal thermocline. In particular, it is suggested that features such as the chlorophyll and nitrite maxima are formed in the spring when the mixed layer is warming and becoming shallower, and water containing phytoplankton and nutrients form strata within the seasonal thermocline. Once the chlorophyll and nitrite maxima are formed, they remain as relicts until again entrained into the winter mixed layer. The hypothesis is examined quantitatively by constructing a model that combines the features of a one-dimensional model of the seasonal thermocline with a model of phytoplankton growth and metabolic regulation. The physical model ( Gill and Turner , Deep-Sea Research , 23 , 391–401, 1976) predicts seasonal changes in mixed-layer depth and thermocline formation and disappearance. The phytoplankton model is a steady-state description of the cellular concentration of chlorophyll as a function of the light and nutrient fields at a given depth, and the rates of conversion of nitrate, nitrite, and cellular nitrogen. Simulations using the model compare favorably with observations of seasonal distributions of phytoplankton and nutrients at Station S in the Sargasso Sea.

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.

Advances in Understanding Phytoplankton Fluorescence and Photosynthesis

Significant technological and scientific advances were made during the last decade in the measurement of fluorescence from the photosynthetic pigments of natural marine populations of phytoplankton and cyanobacteria. Field studies have begun to employ a diverse array of fluorescence sensors. Airborne Lidar has been used to obtain synoptic, one-dimensional transects of the concentration of chlorophyll a and phycoerythrin. Towed fluorometers have provided rapid, two-dimensional transects of the distribution of chlorophyll a. Moored active and passive fluorometers have given continuous, long-term records of the concentration of chlorophyll with unprecedented temporal detail. Flow cytometers have measured the fluorescence and scattering cross-sections of individual cells. As conventional spectrofluorometers provided fluorescence excitation and emission spectra for entire assemblages of cells and particles, microspectrophotometers provided such data for individual cells.

Vertical distribution of phaeopigments—I. A simple grazing and photooxidative scheme for small particles

Abstract The vertical distribution of phaeopigments in the open temperate ocean generally parallels the patterns in the vertical distribution of chlorophyll a . When the euphotic zone is thermally stratified the principal difference between the vertical distribution of chlorophyll and its degradation product is that the concentration of phaeopigment relative to that of chlorophyll increases with depth. We suggest that particles containing phaeopigments are so small that their sinking is not significant in determining their distribution. The suggestion is supported by the particle size distribution; the majority of phaeopigment is in particles μ in diameter. We also suggest that the distribution of such small particles results from two balancing processes, the production of phaeopigment by the grazing of microzooplankton herbivores and the loss of the pigment by photooxidation.

Estimation of Seasonal Primary Production From Moored Optical Sensors in the Sargasso Sea

A bio-optical mooring, which was deployed for 240 days during 1987 in the western Sargasso Sea (∼35°N, 70°W), provided among other things a detailed record of the seasonal distribution of chlorophyll a fluorescence and the scalar irradiance of photosynthetically available radiation. This data base was augmented by shipboard measurements of chlorophyll a concentration, chlorophyll a fluorescence, scalar irradiance, and net photosynthetic rate during four seasonal cruises to the mooring. The combined data base clearly shows a spring bloom in the surface mixed layer associated with initial stabilization of the water column, formation of a subsurface chlorophyll maximum caused by enhanced stratification of the water column, and disappearance of this feature in the winter caused by deepening of the surface mixed layer. The moored and shipboard data were applied to a detailed description of the seasonal variability in the vertical distribution of chlorophyll a and photosynthetic rate. Specifically, photosynthetic rate at a given depth was calculated as the product of scalar irradiance, chlorophyll concentration, the specific absorption coefficient of the phytoplankton crop, and the quantum yield of photosynthesis. Values of scalar irradiance and chlorophyll a concentration were obtained directly from the moored sensors, values for the specific absorption coefficient of the phytoplankton were obtained by linear interpolation of shipboard measurements, and the quantum yield of photosynthesis was calculated as a function of scalar irradiance. Comparisons of photosynthetic rate calculated from such a formulation with measured rates of carbon assimilation indicated good agreement, with no apparent or easily explained biases in the predictions. Surprisingly, daily values in both the crop of phytoplankton and gross photosynthetic rate varied by only a factor of 2 during the year. The annual rate of gross photosynthetic production at the mooring was 12 mol C m−2. An analysis of errors suggests that the precision of this estimate is about ±20%.

Vertical distribution of phaeopigments—II. Rates of production and kinetics of photooxidation

Abstract In a previous study, we suggested that patterns in the vertical distribution of phaeopigments originate from a relatively simple pathway of pigment degradation. In the pathway, phaeopigments of chlorophyll a are produced by microzooplankton grazing and subsequently compartmentalized in small fecal particles that have negligible sinking velocities. Phaeopigments within such small particles are then lost by photooxidation. Thus, the concentration of phaeopigment at a given depth results from a balance between local rates of grazing and rates of photooxidation. To examine such a mechanism, rates of photooxidation of phaeopigments contained in marine particles were measured. Rates of photooxidation are adequately described by first-order kinetics and are strongly dependent on temperature. The mechanism was examined further by constructing a model of grazing and photooxidation in the upper waters of the temperate ocean.

The role of reabsorption in the spectral distribution of phytoplankton fluorescence emission

A theoretical model has been developed to describe an experimentally observed spectral shift in the fluorescence emission from phytoplankton as a result of the internal reabsorption of that emission. This model accounts for both the absorption of the primary excitation and the modification of the fluorescence through the reabsorption of the emitted light by the chloroplast and by the surrounding medium. Comparisons are made between the results of the theoretical model and data derived from experiments using a number of different phytoplankton species, each adapted to varying light conditions. The details of the model are discussed, and the consequences of its interpretation on the spectral distribution of the fluorescence emission from phytoplankton are examined

Towards a General Description of Phytoplankton Growth for Biogeochemical Models

The growth of phytoplankton is fundamentally important to biogeochemical cycling in the sea, and models of this process are essential to describing the fluxes of carbon, nitrogen, and many other elements in the ocean. We address here nitrogen-based models that predict the photosynthesis and growth of phytoplankton for use in basin-scale simulations of marine biogeochemical processes. These models describe light absorption by photosynthetic pigments and biological transformations of carbon and nitrogen, so they must specify, implicitly or explicitly, the cellular chemical composition of phytoplankton (i.e., chlorophyll a, C and N) and photosynthesis per unit chlorophyll a as a function of irradiance (P B vs. E).