Ocean Water Types Research Articles

CHLNET: A novel hybrid 1D CNN-SVR algorithm for estimating ocean surface chlorophyll-a

Developing a unified chlorophyll-a (Chla) inversion algorithm for cross-water types is a significant challenge owing to the insufficiency of input features and training samples. Although machine learning algorithms can build a consistent model for different trophic waters, the accuracy of the inversion is dependent on the quality of the extended features. Here, we designed a novel hybrid framework called CHLNET, which combines a one-dimensional convolutional neural network (1D CNN) and support vector regression (SVR). The 1D CNN is used to extract features from the original band features, and the SVR is used to perform a fit of Chla. CHLNET is trained and tested using match-up pairs of SeaWiFS remote sensing reflectance [Rrs(λ)] in situ with Chla ranging from 0.009 mg/m³ to 138.046 mg/m³, which covers mostly ocean water types. Performance metrics in the log space of CHLNET were better than those of the state-of-the-art algorithms on the testing dataset, and CHLNET had the best overall performance with the largest cover area in the star plot. The frequency distribution of predicted Chla by CHLNET was more consistent with that of in situ Chla. While the spatial pattern was not smooth in low Chla concentration waters, CHLNET demonstrated excellent mapping ability at the global and local scales in high Chla concentration waters. Through the band-shift method, which transfers the Rrs(λ) of MERIS and MODIS-Aqua to the Rrs(λ) of SeaWiFS in the visible spectral range, CHLNET obtained better accuracy than the blended algorithm of OCx and CI on MERIS and MODIS-Aqua matchups, which validates the generalization of CHLNET on cross-sensor types. The results indicate that CHLNET avoids the drawbacks of manually constructing extended features and the need for merging water type-appropriate algorithms for Chla retrieval, as well as provides a new idea for unified Chla concentration inversion across water types. Thus, CHLNET may serve as an alternative approach for Chla inversion.

High-speed long-range multihop underwater wireless optical communication convergent with free-space optical system for optical internet of underwater things and underwater optical wireless sensor network applications

We propose a multihop underwater wireless optical communication (UWOC) convergent with free-space optical (FSO) system for an optical internet of underwater things (O-IoUT) and underwater optical wireless sensor network (UOWSN) applications. A closed-form expression of outage probability was derived for the proposed system using the cumulative distribution functions gamma–gamma and hypertangent log-normal for FSO link and UWOC link, respectively. The outage performance of the proposed multihop UWOC convergent with FSO system was analyzed over various oceanic water types (clear ocean, coastal ocean, and turbid harbor) and different FSO weather conditions (clear air, haze, drizzle, and light fog). The results also depict the end-to-end system performance for different pointing errors (strong, moderate, and weak) and varying the number of hops. The maximum link range of ∼2.5 km, which includes 2 km of FSO link and 450 m of multihop UWOC link (nine hops with each of 50 m clear ocean), is considered.

Real-Time Video Transmission Over Different Underwater Wireless Optical Channels Using a Directly Modulated 520 nm Laser Diode

We experimentally demonstrate high-quality real-time video streaming over an underwater wireless optical communication (UWOC) link up to 5 m distance using phase-shift keying (PSK) modulation and quadrature amplitude modulation (QAM) schemes. The communication system uses software defined platforms connected to a commercial TO-9 packaged pigtailed 520 nm directly modulated laser diode (LD) with 1.2 GHz bandwidth as the optical transmitter and an avalanche photodiode (APD) module as the receiver. To simulate various underwater channels, we perform laboratory experiments on clear, coastal, harbor I, and harbor II ocean water types. The measured bit error rates of the received video streams are 1.0 × 10-9 for QPSK, 4-QAM, and 8-QAM and 9.9 × 10-9 for 8-PSK. We further evaluate the quality of the received live video images using structural similarity and achieve values of about 0.9 for the first three water types, and about 0.7 for harbor II. To the best of our knowledge, these results present the highest quality video streaming ever achieved in UWOC systems that resemble communication channels in real ocean water environments.

Characterizing the phytoplankton soup: pump and plumbing effects on the particle assemblage in underway optical seawater systems

Many optical and biogeochemical data sets, crucial for algorithm development and satellite data validation, are collected using underway seawater systems over the course of research cruises. Phytoplankton and particle size distribution (PSD) in the ocean is a key measurement, required in oceanographic research and ocean optics. Using a data set collected in the North Atlantic, spanning different oceanic water types, we outline the differences observed in concurrent samples collected from two different flow-through systems: a permanently plumbed science seawater supply with an impeller pump, and an independent system with shorter, clean tubing runs and a diaphragm pump. We observed an average of 40% decrease in phytoplankton counts, and significant changes to the PSD in 10-45 µm range, when comparing impeller and diaphragm pump systems. Change in PSD seems to be more dependent on the type of the phytoplankton, than the size, with photosynthetic ciliates displaying the largest decreases in cell counts (78%). Comparison of chlorophyll concentrations across the two systems demonstrated lower sensitivity to sampling system type. Observed changes in several measured biogeochemical parameters (associated with phytoplankton size distribution) using the two sampling systems, should be used as a guide towards building best practices when it comes to the deployment of flow-through systems in the field for examining optics and biogeochemistry. Using optical models, we evaluated potential impact of the observed change in measured phytoplankton size spectra onto scattering measurements, resulting in significant differences between modeled optical properties across systems (~40%). Researchers should be aware of the methods used with previously collected data sets, and take into consideration the potentially significant and highly variable ecosystem-dependent biases in designing field studies in the future.

Review of PAR parameterizations in ocean ecosystem models

Commonly-used empirical equations for calculating downward ‘photosynthetically available radiation’ or PAR were reviewed in order to identify a more theoretically-sound parameterization for application to ocean biogeochemical models. Three different forms of broadband PAR parameterization are currently employed in biogeochemical models, each of them originating from the downward irradiance formulations normally applied to ocean circulation models, which produce poor attenuation estimates for PAR. Two of the PAR formulations, a single-exponential function and a double-exponential function, are parameterized by multiplying surface irradiance by a coefficient determining the portion of underwater PAR. The third formulation uses the second term of the double-exponential function. After elucidating the theoretical problems of modeling PAR using these parameterizations, we suggest an improved, R-modified double-exponential PAR formulation, including Paulson and Simpson's (1977) parameter values. We also newly estimate PAR penetration via least-squares fitting of values digitized from Jerlov's (1976) observations in different oceanic water types, and compare this PAR-observation derived parameterization with our new, theoretical, R-modified parameterization. Finally, we discuss a universal limitation inherent in current theoretical approaches to PAR parameterization.

Performance Evaluation of Short Range Underwater Optical Wireless Communications for Different Ocean Water Types

This paper has presented our interesting in wireless underwater communications for different ocean water types. Recent interest in ocean exploration has brought about a desire for developing wireless communication techniques in this challenging environment. Due to its high attenuation in water, a radio frequency (RF) carrier is not the optimum choice. Acoustic techniques have made tremendous progress in establishing wireless underwater links, but they are ultimately limited in bandwidth. In traditional communication systems, constructing a link budget is often relatively straight forward. In the case of underwater optical systems the variations in the optical properties of ocean water lead to interesting problems when considering the feasibility and reliability of underwater optical links. The main focus of this paper is to construct an underwater link budget which includes the effects of scattering and absorption of realistic ocean water. As well as we have developed the underwater optical wireless communication systems to have shorter ranges, that can provide higher bandwidth (up to several hundred Mbit/s) communications by the assistant of exciting high brightness blue LED sources, and laser diodes suggest that high speed optical links can be viable for short range application. The received signal power, signal to noise ratio, bit error rate, transmitted signal bandwidth, and transmission bit rates are the major interesting parameters for different ocean water mediums as a criteria of the best signal transmission characteristics of short wireless optical communications over wide range of the affecting parameters.

Modeling the onset of photosynthesis after the Chicxulub asteroid impact

We do a preliminary modelling of the photosynthetic rates of phytoplankton at the very beginning of the Paleogene, just after the impact of the Chicxulub asteroid, which decisively contributed to the last known mass extinction of the Phanerozoic eon. We assume the worst possible scenario from the photobiological point of view: an already clear atmosphere with no ozone, as the timescale for soot and dust settling (years) is smaller than that of the full ozone regeneration (decades). Even in these conditions we show that most phytoplankton species would have had reasonable potential for photosynthesis in all the three main optical ocean water types. This modelling could help explain why the recovery of phytoplankton was relatively rapid after the huge environmental stress of that asteroid impact. In a more general scope, it also reminds us of the great resilience of the unicellular biosphere against huge environmental perturbations.

A regional comparison of particle size distributions and the power law approximation in oceanic and estuarine surface waters

The particle size distribution (PSD) is commonly used in studies of sediment fluxes, phytoplankton dynamics, and optical scattering from particulates, but little is known about the spatial and temporal variability of this parameter. Here, we analyze in situ laser diffraction measurements of the PSD from a variety of estuarine and open ocean systems. The power law or “Junge‐type” distribution provided a good fit to surface ocean particle size distributions measured from 6 to 250μm. PSD slopes ranged from 2.7 to 4.7 with a mean of 3.63. Consistent with theory, high particle slopes (3.78) and low particle concentrations characterized the open ocean waters of the Atlantic and the Southern Ocean. Lower PSD slopes (3.63) and high concentrations were characteristic of estuarine waters. River plumes consistently showed the lowest PSD slopes (3.30) and highest particle concentrations characteristic of larger bloom‐forming phytoplankton and production of particle aggregates. In the North Atlantic, an inverse relationship was found between PSD slope and chlorophyll concentration. Such results follow the biological paradigm that larger phytoplankton are prevalent in bloom conditions and smaller phytoplankton dominate in oligotrophic water. Large temporal variability in PSD was observed in near coastal regions prone to sediment resuspension from storms and tidal flow. Dense sea grass beds consistently had lower particle concentrations compared to surrounding waters due to reductions in current flow and sediment resuspension. Simple power law approximations of particle size distributions can be successfully used to describe and assess particle distributions in a wide array of oceanic and estuarine water types.

Short-term effects of gamma ray bursts on oceanic photosynthesis

We continue our previous work on the potential short-term influence of a gamma ray bursts on Earth’s biosphere, focusing on the only important short-term effect on life: the ultraviolet flash which occurs as a result of the retransmission of the γ radiation through the atmosphere. Thus, in this work we calculate the ultraviolet irradiances penetrating the first hundred meters of the water column, for Jerlov’s ocean water types I, II and III. Then we estimate the UV flash potential for photosynthesis inhibition in the whole photic zone, showing that it can be important in the first tens of meters of the water column.

Eddy transport of Western Mediterranean Intermediate Water to the Alboran Sea

During the second cruise of the EU funded OMEGA project the towed undulating vehicle SeaSoar, was deployed to survey the upper 350 m of the water column in the eastern Alboran Sea and extreme western Algerian basin. With an effective along‐track resolution of 4 km, the data sets enabled a detailed description of the different upper ocean water types and the fronts that separate them. The Almeria Oran front forms at the eastern boundary of the Alboran Sea gyre system, in the upper 150–200 m of the water column, and separates waters of predominantly Atlantic origin from those formed in the Western Mediterranean Sea. Below these surface waters, but above the Levantine Intermediate Water, Western Mediterranean Intermediate Waters, believed to be formed to the north of the Balearic Sea, are normally observed in this region. However, to our knowledge, this is the first time a discrete eddy of Western Mediterranean Intermediate Water, a “weddy,” has been described in the extreme western Algerian basin. Repeated surveys of the region allowed us to observe the evolution of the eddy over a period of 40 d. A climatological analysis of historical data in the MEDAR/MEDATLAS database provides evidence for the repeatability of this observation and the significance of the estimated transport.

Low dissolved DMSP concentrations in seawater revealed by small‐volume gravity filtration and dialysis sampling

A variety of filtration and passive dialysis protocols were tested with seawater for determination of the dissolved concentrations of dimethylsulfoniopropionate (DMSPd), a compound originating in the cytosol of many marine phytoplankton. Commonly used sampling procedures, such as in‐line filtration, syringe pressure filtration, and gravity filtration of relatively large‐volume samples (5‐50 mL) through glass fiber filters, caused release of DMSPd from particulate material as evidenced by increasing DMSPd concentrations with volume filtered. Exposure of filters to air at the end of filtration caused particularly severe DMSPd release. Dialysis bags or Slide‐A‐Lyzer dialysis cassettes equilibrated with seawater DMSPd within 4 h, even in polar waters (−1.8 °C), and appeared to give accurate DMSPd concentrations in some circumstances. However, incubation of seawater in laboratory containers (e.g., glass jars) during dialysis sometimes caused artifactual release of DMSPd. We therefore adopted a small‐volume gravity drip filtration (SVDF) procedure, the essential elements of which were: (i) collection of 20 to 50 mL seawater directly from the primary sample container (e.g., Niskin bottle) into a dry, all‐plastic filtration tower; (ii) use of a 47‐mm‐diameter Whatman GF/F filter; (iii) rapid (< 3 min) filtration by gravity (hydrostatic) pressure; (iv) collection of only the first 3.5 mL filtrate for DMSPd analysis; and (v) never exposing the filtered plankton to air. The SVDF procedure appeared to yield the same low DMSPd concentrations as dialysis samplers incubated in the ocean water column. Using the SVDF procedure, DMSPd was found to be < 2.8 nM over a broad range of ocean water types and particulate DMSP concentrations. The maximum DMSPd concentration observed in our study (2.8 nM) was far lower than the reported worldwide average DMSPd concentration of 16.9 nM, raising the possibility that past data collections may have been influenced by filtration artifacts.

Hydrogen peroxide method intercomparision study in seawater

Hydrogen peroxide (H 2O 2) has been studied extensively in the atmosphere and natural waters employing numerous techniques based on absorbance, voltammetry, fluorescence or chemiluminescence. Here we compare the 10-methyl-9-( p-formylphenyl)-acridinium carboxylate trifluoromethanesulfonate chemiluminescent-based (AE-CL) flow-injection analysis (FIA) method to the p-hydroxyphenylacetic acid (POHPAA) batch fluorescence method on 108 paired samples representing a wide range of ocean water types. Over the oceanographically relevant concentration range of 0–250 nM, the two methods are indistinguishable. Since the AE-CL method does not respond to t-butyl hydroperoxide, and no other species including trace metals have been found to cause interferences, it an ideal method for quantification of H 2O 2 in natural waters. The POHPAA method, which cannot distinguish organic peroxides from H 2O 2, was also compared to the HPLC method of Lee et al. [Lee, M., Heikes, B.G., Jacob, D.J., Sachse, G., Anderson, B., 1997. Hydrogen peroxide, organic hydroperoxide, and formaldehyde as primary pollutants from biomass burning. Journal of Geophysical Research, [Atmospheres], 102 (D1), 1301–1309.]. The latter method separates organic peroxides from H 2O 2 and then measures the concentration of each species. These two methods produced results that are not significantly different from each other, demonstrating that hydrogen peroxide is the only peroxide detected in seawater. It can be inferred from these studies that previous H 2O 2 measurements made with non-selective techniques measure H 2O 2 in seawater, and that organic peroxides comprise an insignificant fraction of the total peroxide signal. This study also validates the use of the HPLC method for seawater analyses, as the method was originally developed for atmospheric gas-phase studies.

Application of the small-angle approximation to ocean water types

The small-angle approximation to the radiative transport equation is applied to particle suspensions that emulate ocean water. A particle size distribution is constructed from polystyrene and glass spheres with the best available data for particle size distributions in the ocean. A volume scattering function is calculated from the Mie theory for the particles in water and in oil. The refractive-index ratios of particles in water and particles in oil are 1.19 and 1.01, respectively. The ratio 1.19 is comparable to minerals and nonliving diatoms in ocean water, and the ratio 1.01 is comparable to the lower limit for microbes in water. The point-spread functions are measured as a function of optical thickness for both water and oil mixtures and compared with the point-spread functions generated from the small-angle approximation. Our results show that, under conditions that emulate ocean water, the small-angle approximation is valid only for small optical thicknesses. Specifically, the approximation is valid only for optical thicknesses less than 3.

Results of a Monte Carlo investigation of the diffuse attenuation coefficient

There has been a large effort to relate the apparent optical properties of ocean water to the inherent optical properties, which are the absorption coefficient a, the scattering coefficient b, and the scattering phase function rho(theta). The diffuse attenuation coefficient kdiff' has most often been considered an apparent optical property. However, kdiff' can be considered a quasi-inherent property kdiff' when defined as a steady-state light distribution attenuation coefficient. The Honey-Wilson research empirically relates kdiff' to a and b. The Honey-Wilson relation most likely applies to a limited range of water types because it does not include dependence on rho(theta). A series of Monte Carlo simulations were initiated to calculate kdiff' in an unstratified water column. The calculations, which reflected open ocean water types, used ranges of the single-scattering albedo omega(0) and the mean forward-scattering angle theta(m) for two analytic phase functions with different shapes. It was found that kdiff' is nearly independent of the shape of rho(theta) and can be easily parameterized in terms of a, b, and theta(m) for 0.11 <or= theta(m) <or= 0.48 rad and 0.5 <or= omega(0) <or= 0.95. kdiff' is an asymptotic quantity; that is, a steady-state distribution is reached only after many scattering lengths.

Relation between underwater irradiance and quantum irradiance in dependence on water transparency at different depths in the water bodies

The ratio between underwater quantum irradiance (q) and irradiance (E) for Jerlov's [1976] oceanic and coastal water types and for 11 Estonian and Finnish lakes was studied. This ratio was found to depend on the depth in the water body and the transparency of the water. The ratio q/E for the photosynthetically active radiation (PAR) region of the spectrum may differ from its value in air by up to 24%. The results of the present paper can be used to convert the underwater radiation data from units of W m−2 to μmol s−1 m−2 (or vice versa) and to estimate the errors caused by applying the “air” value of q/E instead of its real value in the water.

Short‐term sediment trap fluxes from Chatham Rise, southwest Pacific Ocean

Sediment trap, nephelometer, and particulate matter (PM) data were collected in the vicinity of the Subtropical Convergence, north of Chatham Rise (42–43°S), southwest Pacific Ocean, in austral autumn 1992. Free‐floating cylindrical sediment traps were deployed below the euphotic zone at 200‐, 300‐, and 500‐m water depths. Increases in total mass flux and concomitant proportional decreases in other particulate fluxes (total carbon, nitrogen, and phosphorus) with depth reflected the collection of relatively refractory material, enriched in particulate carbon, as evinced by increases in C: N and C: P ratios below 300 m. Nephelometer and PM concentration profiles indicate that resuspension of sea‐floor sediments from the nearby submarine high (Chatham Rise) probably contributed to the observed increase in total mass flux with depth. Published pCO2 estimates, biological productivity data, and moderate particulate fluxes, as indicated by the present study suggest that oceanic water types east of New Zealand may be a biologically mediated regional sink for atmospheric CO2.

A comparison of optical and biochemical classifications of ocean waters

Multivariate statistical analyses have been used to examine Jerlov's optical classification of ocean waters and to compare it with a classification derived from other oceanographic properties. The data came from two cruises in three oceans and were representative of all of Jerlov's oceanic water types. It is shown that the water masses sampled can be allocated to their optical water types using criteria related to levels of primary production.

PENETRATION OF UV‐B AND BIOLOGICALLY EFFECTIVE DOSE‐RATES IN NATURAL WATERS*

Abstract— Spectral irradiance measurements. from 310 to 650 nm. have been made in low and modcrately productive ocean waters. These new data and selected earlier clear ocean water data are used as a basis for extrapolating the diffuse attenuation coefficient for irradiance into the 280 nm region. This allows a quantitative calculation of the penetration of UV‐B (280–340 nm) and of biologically (DNA) effective dose‐rates as a function of depth into various ocean water types. The model of Green et al. (1974a) for various atmospheric ozone thicknesses has been used to obtain input surface irrddiancc for this calculation. Our purpose is to provide a basis for estimating the penetration of possible increased UV‐B into natural waters due to possible changes in the ozone concentration of the stratosphere. Given appropriate biological data, this method allows a quantitative evaluation of radiation effects on aquatic organisms as a function of depth. As a specific example, our results have been graphically compared with the dosage‐response results on anchovy larvae obtained by Hunter et al. (1978).

Irradiance and beam transmittance measurements off the West Coast of the Americas

Measurements of total irradiance versus depth and beam transmission versus depth were made at stations near shore along the west coast of the North and South American continents. The water types at each station were optically classified according to the system of Jerlov (1976), thus providing additional information for the description of the distribution of the world's ocean water types. In addition, the parameter k/c, where k is the irradiance attenuation coefficient and c is the beam attenuation coefficient, has been shown to be a useful parameter for determining the relative particle concentrations of ocean water.