Marlon R. Lewis

Global phytoplankton decline over the past century

In the oceans, ubiquitous microscopic phototrophs (phytoplankton) account for approximately half the production of organic matter on Earth. Analyses of satellite-derived phytoplankton concentration (available since 1979) have suggested decadal-scale fluctuations linked to climate forcing, but the length of this record is insufficient to resolve longer-term trends. Here we combine available ocean transparency measurements and in situ chlorophyll observations to estimate the time dependence of phytoplankton biomass at local, regional and global scales since 1899. We observe declines in eight out of ten ocean regions, and estimate a global rate of decline of ∼1% of the global median per year. Our analyses further reveal interannual to decadal phytoplankton fluctuations superimposed on long-term trends. These fluctuations are strongly correlated with basin-scale climate indices, whereas long-term declining trends are related to increasing sea surface temperatures. We conclude that global phytoplankton concentration has declined over the past century; this decline will need to be considered in future studies of marine ecosystems, geochemical cycling, ocean circulation and fisheries.

Vertical Nitrate Fluxes in the Oligotrophic Ocean

The vertical flux of nitrate across the thermocline in the upper ocean imposes a rigorous constraint on the rate of export of organic carbon from the surface layer of the sea. This export is the primary means by which the oceans can serve as a sink for atmospheric carbon dioxide. For the oligotrophic open ocean regions, which make up more than 75% of the worlds ocean, the rate of export is currently uncertain by an order of magnitude. For most of the year, the vertical flux of nitrate is that due to vertical turbulent transport of deep water rich in nitrate into the relatively impoverished surface layer. Direct measurements of rates of turbulent kinetic energy dissipation, coupled with highly resolved vertical profiles of nitrate and density in the oligotrophic eastern Atlantic showed that the rate of transport, averaged over 2 weeks, was 0.14 (0.002 to 0.89, 95% confidence interval) millimole of nitrate per square meter per day and was statistically no different from the integrated rate of nitrate uptake as measured by incorporation of 15N-labeled nitrate. The stoichiometrically equivalent loss of carbon from the upper ocean, which is the relevant quantity for the carbon dioxide and climate question, is then fixed at 0.90 (0.01 to 5.70) millimole of carbon per square meter per day. These rates are much lower than recent estimates based on in situ changes in oxygen over annual scales; they are consistent with a biologically unproductive oligotrophic ocean.

Photosynthetic characteristics and estimated growth rates indicate grazing is the proximate control of primary production in the equatorial Pacific

Macronutrients persist in the surface layer of the equatorial Pacific Ocean because the production of phytoplankton is limited; the nature of this limitation has yet to be resolved. Measurements of photosynthesis as a function of irradiance (P-I) provide information on the control of primary productivity, a question of great biogeochemical importance. Accordingly, P-I was measured in the equatorial Pacific along 150°W, during February-March 1988. Diel variability of P-I showed a pattern consistent with nocturnal vertical mixing in the upper 20 m followed by diurnal stratification, causing photoinhibition near the surface at midday. Otherwise, the distribution of photosynthetic parameters with depth and the stability of P-I during simulated in situ incubations over 2 days demonstrated that photoadaptation was nearly complete at the time of sampling: photoadaptation had not been effectively countered by upwelling or vertical mixing. Measurements of P-I and chlorophyll during manipulations of trace elements showed that simple precautions to minimize contamination were sufficient to obtain valid rate measurements and that the specific growth rates of phytoplankton were fairly high in situ, a minimum of 0.6 d−1. Diel variability of beam attenuation also indicated high specific growth rates of phytoplankton and a strong coupling of production with grazing. It appears that grazing is the proximate control on the standing crop of phytoplankton. Nonetheless, the supply of a trace nutrient such as iron might ultimately regulate productivity by influencing species composition and food-web structure.

Ocean primary production and available light: further algorithms for remote sensing

Abstract In the context of remote sensing of the ocean, the general problem of estimating water column production from surface irradiance and column chlorophyll concentration is examined, and some refinements are made to the linear theory presented by Platt (1986, Deep-Sea Research , 33 , 149–163). Further empirical evidence is presented to show the stability of the relationship between surface light and biomass-normalized primary production of the ocean water column. A theoretical explanation is given for the non-zero intercept often obtained when these two variables are regressed. The systematic errors in the estimation of primary production by remote sensing, due to non-uniformity in the biomass profile, are examined through sensitivity analyses on a generalized biomass profile. The errors are shown to be functions of the parameters of the biomass profile, of the photosynthetic parameters and of the optical properties of the water. The probable random errors in the estimation of water column primary production using remotely sensed data are evaluated. Some general issues related to the collection and assimilation of data on ocean primary production are addressed.

Effects of Penetrative Radiation on the Upper Tropical Ocean Circulation

Abstract The effects of penetrative radiation on the upper tropical ocean circulation have been investigated with an ocean general circulation model (OGCM) with attenuation depths derived from remotely sensed ocean color data. The OGCM is a reduced gravity, primitive equation, sigma coordinate model coupled to an advective atmospheric mixed layer model. These simulations use a single exponential profile for radiation attenuation in the water column, which is quite accurate for OGCMs with fairly coarse vertical resolution. The control runs use an attenuation depth of 17 m while the simulations use spatially variable attenuation depths. When a variable depth oceanic mixed layer is explicitly represented with interactive surface heat fluxes, the results can be counterintuitive. In the eastern equatorial Pacific, a tropical ocean region with one of the strongest biological activity, the realistic attenuation depths result in increased loss of radiation to the subsurface, but result in increased sea surface te...

Remote sensing of oceanic primary production: computations using a spectral model

Abstract A spectral model of underwater irradiance is coupled with a spectral version of the photosynthesis-light relationship to compute oceanic primary production. The results are shown to be significantly different from those obtained using the conventional non-spectral approach. The problem of non-uniform vertical distribution of biomass is investigated next, from the point of view of estimation of water-column primary production using satellite data. The errors in estimated production are shown to be functions of parameters of the biomass distribution; of the photosynthesis parameters; and of the optical properties of the water column. Some examples are given to illustrate the comparison of model results with thee observed data.

Influence of bubbles on scattering of light in the ocean

The scattering and backscattering properties of bubble populations in the upper ocean are estimated with Mie theory and a generalized bubble size spectrum based on in situ observations. Optical properties of both clean bubbles and bubbles coated with an organic film are analyzed; the results are compared with the corresponding optical properties of micro-organisms of similar size. Given a bubble number density (from ~10(5) to ~10(7) m(-3)) frequently found at sea, the bubble populations significantly influence the scattering process in the ocean, especially in oligotrophic waters. Bubbles appear to make a large contribution to the missing terms in constructing the observed total backscattering coefficient of the ocean. This contribution to backscattering is strongly enhanced if the bubbles are coated with organic film. The injection of bubbles will shift ocean color toward the green, resembling phytoplankton blooms, and hence introducing error in ocean color remote sensing if its effect is not corrected.

Satellite ocean-color observations of the tropical Pacific Ocean

The Coastal Zone Color Scanner (CZCS) data set provided some insights into biological processes in the equatorial Pacific, but the sampling was too sparse to address questions on temporal and spatial variability. Since late 1996, the Ocean Color–Temperature Sensor (OCTS), the Polarization Detection Environmental Radiometer (POLDER), the Sea-viewing Wide Field-of-View Sensor (SeaWiFS), and the Moderate-Resolution Imaging Spectroradiometer (MODIS) have provided a nearly continuous record of biological processes in this region for the first time. This study summarizes the SeaWiFS observations of the tropical Pacific from September 1997 through March 2000, with particular emphasis on equatorial and mesoscale variability, the influence of biological processes on penetrating irradiance, and the performance of primary production algorithms in this region. Specific mesoscale phenomena described are the phytoplankton blooms along the west coast of Central America, in the vicinity of the Costa Rica dome, and south of the equator. The coastal Central American and Costa Rica dome blooms result from orographically steered coastal winds and Ekman divergence, respectively. An unusual bloom event occurred south of the equator and persisted for several months in 1999; specific mechanisms that would have sustained the bloom could not be identified. Also, the time-evolution of the equatorial bloom during the May–August 1998 transition from El Ni * no to La Ni * na is discussed. Again, no concise and broadly accepted explanation of the bloom’s genesis and migration has yet emerged. During this transition, the monthly mean diffuse attenuation coefficient decreased by a factor of 3 at some locations along the equator. This change in water transparency, coupled with large changes in mixed-layer depth, resulted in significant changes in surface layer heating rates that were substantiated with field observations. Finally, certain primary production algorithms designed to use remotely sensed chlorophyll-a concentrations are evaluated. None of the algorithms capture the observed variability in primary production, and all appear to underestimate the total primary production of the tropical Pacific. r 2002 Elsevier Science Ltd. All rights reserved.

A New Method for the Measurement of the Optical Volume Scattering Function in the Upper Ocean

Abstract A new method to measure the optical volume scattering function (VSF) of seawater is presented. The VSF is a fundamental property used in the calculation of radiative transfer for applications as diverse as upper-ocean heating by solar radiation to laser ranging of the sea bottom. The approach differs from traditional ones and involves use of a special periscope prism that allows the direct determination of the VSF over a wide range of angles (0.6°–177.3°) with an angular resolution of 0.3°. Measurements taken in the laboratory using Barnstead International, Inc., Nanopure water, cleaned seawater, and known additions of defined scatterers indicate close correspondence between experimental data and theoretical simulations based on Mie theory over the angle range from 12° to 170°. Field deployments in the Atlantic Ocean continental shelf water are shown to produce high quality measurements of the VSF in the angle range from 0.6° to 177.3°; such observations have not been available before. The new da...

Biological processes and optical measurements near the sea surface: Some issues relevant to remote sensing

The advent of remote sensing, the development of new optical instrumentation, and the associated advances in hydrological optics have transformed oceanography: it is now feasible to describe ocean-scale biogeochemical dynamics from satellite observations, verified and complemented by measurements from optical sensors on profilers, moorings, and drifters. Only near-surface observations are common to both remote sensing and in situ observation, so it is critical to understand processes in the upper euphotic zone. Unfortunately, the biological principles that must be used to interpret optical variability near the sea surface are weaker than we would like, because relatively few experiments and analyses have examined bio-optical relationships under high irradiance characteristic of the upper optical depth. Special consideration of this stratum is justified, because there is good evidence that bio-optical relationships are altered near the surface: (1) the fluorescence yield from chlorophyll declines, leading to bias in the estimation of pigment from fluorometry; (2) the modeled relationship between solar-stimulated fluorescence and photosynthesis seems to deviate significantly from that presented for the lower euphotic zone; and (3) carbon-specific and cellular attenuation cross sections of phytoplankton change substantially during exposures to bright light. Even the measurement of primary productivity is problematic near the sea surface, because vertical mixing is not simulated and artifactual inhibition of photosynthesis can result. These problems can be addressed by focusing more sampling effort, experimental simulation, and analytical consideration on the upper optical depth and by shortening timescales for the measurement of marine photosynthesis. Special efforts to study near-surface processes are justified, because new bio-optical algorithms will require quantitative descriptions of the responses of phytoplankton to bright light.