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While the effects of irradiance on coral productivity are well known, corals along a shallow to mesophotic depth gradient (10–100 m) experience incident irradiances determined by the optical properties of the water column, coral morphology, and reef topography.Modeling of productivity (i.e., carbon fixation) using empirical data shows that hemispherical colonies photosynthetically fix significantly greater amounts of carbon across all depths, and throughout the day, compared with plating and branching morphologies. In addition, topography (i.e., substrate angle) further influences the rate of productivity of corals but does not change the hierarchy of coral morphologies relative to productivity.The differences in primary productivity for different coral morphologies are not, however, entirely consistent with the known ecological distributions of these coral morphotypes in the mesophotic zone as plating corals often become the dominant morphotype with increasing depth.Other colony‐specific features such as skeletal scattering of light, Symbiodiniaceae species, package effect, or tissue thickness contribute to the variability in the ecological distributions of morphotypes over the depth gradient and are captured in the metric known as the minimum quantum requirements.Coral morphology is a strong proximate cause for the observed differences in productivity, with secondary effects of reef topography on incident irradiances, and subsequently the community structure of mesophotic corals., Corals along a shallow to mesophotic depth gradient (10–100 m) experience incident irradiances determined by their morphology and reef topography. Both the substrate angle (i.e., topography) and a hierarchy of colony morphologies have significant, and repeatable, effects on modeled productivity in mesophotic corals.

Background/purpose The purpose of this paper is to estimate the contribution to our erythemal exposure at the coast of solar ultraviolet (UV) both reflected from, and transmitted into, the ocean. Methods The reflection of solar UV radiation from, and transmitted into, seawater was calculated using a numerical model under a number of atmospheric conditions to estimate erythemal exposure on the skin of supine/prone and ambulant people. Results The results were expressed as UV Indices. Even under the most extreme insolation with the sun directly overhead, where the ambient UV Index may be around 14, reflected UV from the ocean contributes an erythemal exposure to the skin equivalent to a UV Index of about 0.7. For typical ocean waters, with the sun high in the sky, the UV index within the water is about 7 at a depth of 2 m. Conclusion Whilst our eyes often sense a high level of reflected sunlight from the ocean, especially when the sun is low in the sky, our skin does not share that experience. The reason people get sunburnt at the seaside has more to do with the absence of shade than with reflectance by the water surface or even beach sand.

Atmospheric and oceanic radiative transfer models were used to compute spectral radiances between 285 and 400 nm onto horizontal and vertical plane surfaces over water. The calculations kept track of the contributions by the sun's direct beam, by diffuse-sky radiance, by radiance reflected from the sea surface and by water-leaving radiance. Clear, hazy and cloudy sky conditions were simulated for a range of solar zenith angles, wind speeds and atmospheric ozone concentrations. The radiances were used to estimate erythemal exposures due to the sun and sky, as well as from radiation reflected by the sea surface and backscattered from the water column. Diffuse-sky irradiance is usually greater than direct-sun irradiance at wavelengths below 330 nm, and reflected and water-leaving irradiance accounts for20% of the UV exposure on a vertical surface. Total exposure depends strongly on solar zenith angle and azimuth angle relative to the sun. Sea surface roughness affects the UV exposures by only a few percent. For very clear waters and the sun high in the sky, the UV index within the water can be10 at depths down to two meters and6 down to 5 m.

Many pelagic species, particularly teleost fish, have silvered lateral surfaces that are thought to primarily serve as a form of camouflage. The underlying argument is that the underwater light field is cylindrically symmetrical around the vertical axis; thus a vertical mirror reflects a region of the water column that matches the region directly behind the mirror. However, the degree of symmetry of the underwater light field itself has not been assessed. Modeled underwater radiances from the surface to a depth of 100 m using measured profiles of inherent optical properties and HydroLight radiative transfer software showed that the horizontal light field under sunny conditions was asymmetrical over a wide range of solar elevations. In addition, the maximum asymmetry at 100 m occurred not when the sun was near the horizon, but when it was 45u above it. We validated these modeled results in Hawaiian waters using a modification of a commercial radiometer. Both modeled and measured radiances showed that the inherent contrast of silvery fish was typically higher at longer wavelengths. However, models of the sighting distances of these surfaces showed that sighting distance was greatest at the peak wavelength of the downwelling irradiance (,480 nm). The modeled and measured asymmetry of the horizontal light field implies that mirror camouflage is not always as successful as originally thought and suggests that there may be further refinements for this form of crypsis that have not been previously considered.

If the benthic boundary in optically shallow waters is spatially inhomogeneous or sloping, the underwater light field is inherently three-dimensional (3D). Numerical simulations of 3D underwater radiances were made for environmental conditions observed in shallow Bahamian waters. The simulations show that if the pattern of bottom reflectance for an inhomogeneous but level bottom has a spatial scale much smaller than the bottom area seen by a radiometer, the inhomogeneous bottom can be replaced by a homogeneous bottom whose reflectance is the areaweighted average of the actual bottom reflectances. For large-scale patterns of bottom reflectance, the 3D light fields near the edges of bottom patches of different reflectances can be predicted from analytical models incorporating the sensor geometry and one-dimensional (1D) light fields computed for homogeneous bottoms, with errors of order 10% when compared to the exact 3D solutions. The same holds true for uniformly sloping bottoms, whose 3D light fields can be modeled in terms of the 1D light fields computed for level bottoms, with errors of less than 10% for bottom slopes of 20° or less.

Numerical simulations of radiative transfer within the ocean surface mixed layer have been used to derive a set of simple equations for estimating the absorption, a, and backscattering, bb, coefficients in the blue-green spectral region from measurements of downwelling irradiance, Ed, upwelling irradiance, Eu, and upwelling nadir radiance, Lu. Two relationships are used in this derivation: (i) irradiance reflectance, R 5 Eu/Ed, is approximately proportional to bb/a and inversely proportional to the average cosine of the underwater light field, m; and (ii) radiance reflectance, RL 5 Lu/Ed, is proportional to bb/a but changes little with m. Accordingly, m is first estimated from Lu/Eu and then the absorption coefficient is obtained from Gershun’s equation. Having determined a, bb is estimated from the relation between RL and bb/a. We also show how the estimation of m, a, and bb is improved with additional data of the beam attenuation coefficient, c. The retrieval of a and bb from Ed, Eu, and Lu has been tested using field data collected in the Southern California Bight. The a values predicted by the model show good agreement with absorption determinations from the ac-9 instrument and on-board spectrophotometric method, and the modeled bb agrees well with the backscattering determinations from the Hydroscat-6 instrument. The algorithm has been derived for a broad range of the ocean inherent optical properties that do not necessarily covary with one another like in chlorophyll-based bio-optical models. However, the model should be used with caution if significant departure from the average Petzold particle scattering phase function is expected, for example in waters with high load of mineral particles. The inherent optical properties (IOPs) of seawater are solely the properties of the medium and are fundamental to the understanding of the marine optical environment. The development of instrumentation for measuring IOPs and models for deriving the IOPs from the apparent optical properties (AOPs) have been essential to the objective of achieving accurate determinations of oceanic IOPs. Historically, accurate radiometric measurements of underwater light field

We have investigated the effect of Raman scattering on the average cosine of underwater irradiance, ,%, the diffuse attenuation coefficient of irradiance, K, and the Raman scattering source coefficient, P*, throughout the water column and into the asymptotic field. The Raman scattering source coefficient is the fractional gain in the scalar irradiance from local Raman scattering at a given depth, and it can be found from the Gershun equation with a Raman source term. In particular, by using calculations from a radiative transfer model (Hydrolight 3.0), we compare ,%, K, and P* from simulations that include and that omit Raman scattering. These simulations are performed at wavelengths between 355 and 665 nm in a vertically homogeneous ocean with inherent optical properties (IOPs) determined from case 1 bio-optical models for three different chlorophyll concentrations (0.05, 0.5, and 5 mg m-j). We also investigated several two-layered oceans that assumed different inherent optical properties in the surface and deep layers. Our calculations showed that when Raman scattering is present, there may exist strong vertical gradients in the average cosine and the diffuse attenuation coefficient, especially at long wavelengths for low chlorophyll concentrations. In addition, a quasi-asymptotic field may be a feature of the vertical profiles of the average cosine and the diffuse attenuation coefficient. In the asymptotic field, there is no effect of Raman scattering at shorter wavelengths, there is a strong effect of Raman scattering at longer wavelengths and between these two spectral regions, and there is a transition that increases from 500 to 590 nm as the concentration of chlorophyll increases from 0.05 to 5 mg m-2. Over a broad range of IOPs, the asymptotic parameters pm*, K,*, and P*, are related by a quadratic relationship. Neither the magnitude or angular distribution of surface light nor changes in the absorption and scattering coefficients in the surface layer of a two-layered ocean have auy effect on the values of ,%=*, K,*, and P*,; the values of these parameters depend only upon the IOPs in the deepest layer.

The beam spread function (BSF) of sea ice is of interest for several reasons. The BSF characterizes beam propagation through sea ice. Its equivalent, the point spread function, is essentially the optical impulse response of the medium, which has many useful connections to radiative transfer theory. In-ice measurements of the BSF over the full angular range 0–180°, using a novel method, were made of first- and multiyear ice off the shore of Barrow, Alaska. All of the measured sea ice BSFs were drastically different than the BSF of ocean water, and they strongly indicated that sea ice is a highly scattering medium, with a single scattering albedo generally >0.97 over the visible spectrum. At pathlengths >30 cm, the BSF was found to be nearly identical to the computed asymptotic radiance distribution. The rapid approach to the asymptotic state and the high single scattering albedo of sea ice suggest that photon diffusion theory should accurately describe radiative transfer in sea ice away from boundaries. Equating the results of diffusion theory with asymptotic radiative transfer theory yields a simple expression that relates the asymptotic attenuation coefficient K∞, to the inherent optical property coefficients and the asymmetry parameter g of the scattering phase function. It is shown that the necessary optical parameters for computing g can be obtained from the measured BSE Thus, all the information necessary for modeling optical propagation in sea ice can be obtained from the BSF measurements using the method described here.

Radiative transfer numerical modeling provides a powerful means of separating and understanding the effects of various types of microbial particles on oceanic light fields. We illustral:e the methodology for achieving this separation and provide various idealized examples. We begin with a database of the single-particle optical properties of marine microbial particles and then construct the total absorption and! scattering properties of natural waters for given concentrations of the various particle types. When used as input to the Hydrolight radiative transfer numerical model, these absorption and scattering properties generate underwater light fields. Because this modeling process rests upon the optical properties of the individual microbial components, rather than just on the total absorption and scattering properties of a water body, it is possible to analyze the e:ffects of the various particulate components on quantities such as diffuse attenuation coefficients and remote-sensing reflectances. We find that different microbial compositions in two water bodies can give considerably different optical properties, even though the chlorophyll concentration is the same in each of the water bodies. Our simulations show how variability in chlorophyll concentrations obtained from remotely sensed ocean color signals can be explained by the detailed composition of the water body. This approach suggests that it may be possible to distinguish between blooms of small picophytoplankton and larger nanophytoplankton by using remotely sensed signals. Stramski and Mobley (1997, hereafter referred to as SM) described a database of single-particle optical properties for five types of microbial particles. These particle types were marine viruses (VIR), heterotrophic bacteria (BAC), cyanobacteria (CYA), small nanophytoplanktonic diatoms (DIA), and nanophytoplanktonic chlorophytes (CHLO). In the present paper, the particular microbes found in the database are viewed as representatives of broader classes of particles. For example, the particular species of DIA in the database (Thalussiosiru pseudonana) has cells with diameters of -4 Frn, and the particular species of CHL0 (Dunaliella tertiolecta) has cells -8 p,m in size. We thus take DIA as representative of small nanophytoplankton of size 4 pm, and we take CHL0 as representative of nanophytoplankton of size 8 pm. The particular species involved are not important for the purposes of this paper. Moreover, it is sufficient for our present purposes to consider only five types of microbial particles, even though realistic simulations of actual water bodies might require more microbial types (such as prochlorophytes, larger bloom-forming species, or coccolithophores). We do include detritus (DET) and colored dissolved organic matter (CDOM) in some of our examples. The database includes, among other things, the single-particle absorption and scattering cross sections, o,(X) and a,(X), respectively, and the scattering phase functions p(0

We describe a database of the single-particle optical properties cd marine microbial particles. This database includes representatives from five classes of particles: viruses (VIR), heterotrophic bacteria (BAC), cyanobacteria (CYA), small nanoplanktonic diatoms (DIA), and nanoplanktonic chlorophytes (CHLO). The optical properties of VIR, whose mean size is 0.07 pm, were determined from Mie scattering calculations using reasonable approximations about the size distribution and refractive index of viral particles. The database for BAC, CYA, DIA, and CHL0 was created from laboratory measurements of microbial cultures and modeling of particle optics. BAC are represented by a mixed natural population of bacterial species (-0.55 pm in size), CYA by Synechococcus (clone WH 8103, - 1 pm), DIA by Thalassiosira pseudonana (-4 Fm), and CHL0 by Dunaliella tertiolecta (-7.5 pm). The database includes the single-particle optical properties that are useful in radiative transfer modeling: the absorption and scattering cross sections and scattering phase functions. Additionally, the database includes the attenuation cross sections, optical efficiency factors, single-scattering albedos, and backscattering properties. For phytoplankton species, chlorophyll- and carbon-specific optical coefficients are also available. The optical quantities are generally determined at I-nm intervals in the spectral region from 350 to 750 nm. The scattering phase function is determined at 5-nm intervals in wavelength and 1” intervals in scattering angle. The size distribution and refractive index of the particles are also included. This database, when combined with radiative transfer modeling, provides a powerful approach to advancing our understanding of oceanic optics. Achieving a detailed understanding of the roles played by various types of particles in oceanic radiative transfer is a prerequisite to advancing our knowledge of the marine optical environment and applying optical techniques to the study of the ocean. It is the fundamental processes of absorption and scattering of light by many individual particles and soluble materials present in seawater that determine the magnitudes and variability of the bulk optical properties and light field characteristics in the ocean. To achieve a comprehensive understanding of the marine optical environment and to construct reliable predictive and inverse optical models, we must first understand how each of these various water components absorbs and scatters light, and hence affects underwater radiative transfer. It is well known that there is great variability in the inherent optical properties (IOPs) of seawater in the world’s oceans. This variability induces a correspondingly large variability in oceanic apparent optical properties (AOPs), even after accounting for variability in the AOPs associated with the incident lighting and sea state. The optical properties of

The average cosine of the underwater light field (i) is a simple quantity that describes the angular distribution of radiance at a given point. A model of the rate of vertical change of ji in the ocean was developed in order to examine the influences of light absorption and scattering. We made calculations of radiative transfer based on invariant imbedding theory assuming an optically homogeneous ocean with a typical scattering phase function and the simple boundary conditions of the sun overhead in a black sky and a flat ocean surface. Under such conditions, the decrease of ji throughout the water column is well approximated by a single exponential function. The dependence of the parameter P7, which describes the rate of change of ji with optical depth, on the single-scattering albedo wo, is well approximated by a quadratic function. By applying a linearization technique to the P, vs. w. relationship, we identified the contributions of absorption and scattering to P,. Our results indicate that scattering is the more important process, contributing >50% to P, for typical situations when w. > 0.1. Absorption dominates P, when w. 6 50 nm). Our analysis of the effect of scattering phase function shows that the scattering into the middle angles, approximately between 20” and 45”, largely determines the magnitude of P,. Using spectral bio-optical models with several Chl concentrations, we also examined the rate of change in ji with geometric depth P, for various water types with realistic values of the absorption and scattering coefficients. This analysis shows large variations in both the magnitude and the spectral behavior of P, with varying Chl concentration. The average cosine of the light field (fi) is a simple and convenient quantity that describes the angular distribution of the underwater radiance at a given point. This quantity is defined as

A numerical technique is presented for computing radiance distributions in natural waters that have wind-blown surfaces and depth-dependent inherent optical properties. Input to the numerical model consists of the radiance distribution incident on the air-water surface from above, the wind velocity, which specifies the state of randomness of the air-water surface via a wind speed-wave slope spectrum, the volume scattering and volume attenuation functions of the water body as functions of depth and wavelength, and the type of bottom boundary. Primary output from the model consists of directionallv discretized radiances as functions of wavelength, direction, and depth throughout and above the water body. General knowledge of the radiance distribution in a water body is a prerequisite for solving more specific problems, such as those occurring in studies of underwater visibility, remote sensing of the oceans from aircraft or satellites, heating of the upper layers of the oceans, or photosynthesis. Moreover, since radiance is the fundamental radiometric quantity, if the radiance is known then all other quantities of interest, such as the irradiance, are easily obtained. Unfortunately, measurement of radiance is a difficult and expensive task in the ocean environment, and few systematic studies of radiance have been made in natural waters. However, a numerical model has been developed for computing radiance distributions in natural hydrosols, such as lakes and seas, given the inherent optical properties of the hydrosol itself and appropriate boundary conditions at the surface and bottom of the water body, along with the radiance incident on the water surface. This