Piotr Kowalczuk

Optical characteristics of two contrasting Case 2 waters and their influence on remote sensing algorithms

This paper describes the results of measurements of inherent and apparent optical properties of two contrasting Case 2 waters (Southern Baltic and off the west coast of Ireland). The experiments were carried out over two seasons, both before the peak of the phytoplankton growth period, and during the bloom events. The first study was made when the concentration of chlorophyll (phytoplankton pigment) was similar in both waters and in the range of 0.6–3.23 mg m � 3 . The second study was made when concentrations of chlorophyll were very high, reaching 14 mg m � 3 in the west coast of Ireland and 70 mg m � 3 in Southern Baltic. Optical measurements of surface reflectance were made with a profiling spectroradiometer. The spectral shapes of the particulate absorption spectra and the diffuse attenuation coefficient in both regions and seasons are compared. The contribution of detritus and coloured dissolved organic matter absorption to the apparent optical properties of water have impact on the accuracy of remote sensing retrieval algorithm for chlorophyll a. Differences have been found between algorithms based on conventional spectral bands (e.g. SeaWiFS) and proposed new spectral channels. The most accurate ratio for chlorophyll retrieval (490/550) was for the western Irish shelf (R 2 =76%, and the standard error of the estimate ranged from 30% to 37%), with poor results for this ratio in the Baltic. The Rrs (550/590) ratio gave better results for the Baltic (R 2 =75%, and the standard error of the estimate ranged from 20% to 55%), with poor results for the Irish shelf. The results show that a unique combination of spectral bands needs to be applied in Baltic waters to achieve an acceptable accuracy of the in-water remote sensing algorithm whereas the more commonly accepted band ratios were acceptable for the Irish Shelf. This suggests that for accurate determination of chlorophyll from satellite-borne sensors in different coastal waters a wider choice of spectral bands is needed. r 2003 Elsevier Science Ltd. All rights reserved.

Seasonal variability of yellow substance absorption in the surface layer of the Baltic Sea

The value of the yellow substance absorption coefficient in the visible-light spectrum is a very important characteristic used in the optical classification of seawater. This quantity also needs to be included in algorithms for the remote detection of optically active seawater constituents. An extensive database of measurements of optical parameters has been collected during a series of cruises since 1993 in different water masses of the southern Baltic. The yellow substance absorption coefficient at a wavelength of 400 nm was measured and the yellow substance absorption spectrum slope coefficient calculated. The statistical distribution of values of these parameters in the three various water masses (bay waters, coastal, and open seawater) is presented. The seasonal variability of both parameters in the three regions is analyzed. Results are discussed and compared with the published record of yellow substance absorption properties and variability patterns of other optical parameters in the southern Baltic and other marine basins.

Empirical relationships between coloured dissolved organic matter (CDOM) absorption and apparent optical properties in Baltic Sea waters

Coloured dissolved organic matter (CDOM), also know as yellow substances, dissolved in Baltic Sea waters is a dominant constituent, absorbing light in the blue and green parts of the spectrum. The relative share of CDOM absorption in the total absorption of blue light can reach 80%. This causes a high correlation between the CDOM absorption coefficient a y(400) and irradiance diffuse attenuation coefficient K d(412). The seasonal variations of CDOM and particulate absorption have a combined effect on the magnitude and shape of the irradiance diffuse attenuation coefficient spectra, which is represented by a shift of the maximum transmission wavelength towards the red part of the spectrum. Such an effect has been observed in the Gulf of Gdansk during recent experiments. CDOM absorption also influences the shape of remote sensing reflectance spectra. Appropriate spectral bands for the construction of a local, empirical, remote sensing algorithm for quantification of CDOM absorption are proposed, based on the seasonal variability of remote sensing reflectance spectra. The empirical relationships between CDOM absorption and downwelling irradiance diffuse attenuation coefficient and spectral reflectance are presented and their potential for quantifying CDOM absorption is explored. The CDOM absorption coefficient is highly correlated with downwelling irradiance diffuse attenuation coefficient in the blue. The estimation of CDOM absorption using the downwelling irradiance measurements can be achieved with low errors: −4.6% systematic and 25.3% random. The estimation of CDOM absorption from empirical relationships between absorption and remote sensing reflectance suffers lower accuracy: 4% for systematic and 32% for random errors of estimates.

Spectral characterization of chromophoric dissolved organic matter (CDOM) in a fjord (Doubtful Sound, New Zealand)

Abstract.Doubtful Sound, Fiordland National Park, New Zealand has a stable low salinity layer (LSL) at the surface due to the high annual rainfall and an additional freshwater input from the discharge of a hydroelectric power plant. Chromophoric dissolved organic matter (CDOM) distribution dynamics in this LSL were analyzed using Excitation Emission Matrix (EEM) fluorescence and UV/Vis absorption measurements. The levels of CDOM in surface water in Doubtful Sound were much higher than previously reported for coastal zones. CDOM analyzed by UV/Vis absorbance and EEM fluorescence decreased by about 90% within the first 5 m depth. In the first 5 m, the salinity gradient was extreme (5 – 34 salinity), creating a vertical mixing gradient, which appeared to be stable over time. The spectral slope S and the fluorescence index increased during vertical mixing within the first 5 m depth indicating changes in the CDOM chromophores. Observations during a strong rain event revealed that CDOM in a freshwater stream initially increased, but decreased with additional rain (same flow rate), which is an indication that most CDOM was quickly exported within these catchments. Furthermore, the lower CDOM input after substantial rain diluted the surface CDOM level and created a subsurface CDOM maximum. However, shortly after the rain event (<24 h) the surface CDOM maximum with a steady decrease with depth was restored. The combination of extreme rainfall, pristine temperate rain forest and steep mountains with distinct watersheds creates a highly dynamic environment with very high terrestrial CDOM input. These CDOM dynamics and the high degree of scatter in the linear correlations between CDOM levels and salinity cannot be explained solely in terms of conservative mixing, but suggest that biophysical controls in the catchments and mixing with marine derived CDOM are involved in the dynamics of CDOM in this system.

Effect of sea‐ice melt on inherent optical properties and vertical distribution of solar radiant heating in Arctic surface waters

The inherent optical properties (IOPs) of Polar Waters (PW) exiting the Arctic Ocean in the East Greenland Current (EGC) and of the inflowing Atlantic waters (AW) in the West Spitsbergen Current (WSC) were studied in late summer when surface freshening due to sea-ice melt was widespread. The absorption and attenuation coefficients in PW were significantly higher than previous observations from the western Arctic. High concentrations of colored dissolved organic matter (CDOM) resulted in 50–60% more heat deposition in the upper meters relative to clearest natural waters. This demonstrates the influence of terrigenous organic material inputs on the optical properties of waters in the Eurasian basin. Sea-ice melt in CDOM-rich PW decreased CDOM absorption, but an increase in scattering nearly compensated for lower absorption, and total attenuation was nearly identical in the sea-ice meltwater layer. This suggests a source of scattering material associated with sea-ice melt, relative to the PW. In the AW, melting sea-ice forms a stratified surface layer with lower absorption and attenuation, than well-mixed AW waters in late summer. It is likely that phytoplankton in the surface layer influenced by sea-ice melt are nutrient limited. The presence of a more transparent surface layer changes the vertical radiant heat absorption profile to greater depths in late summer both in EGC and WSC waters, shifting accumulation of solar heat to greater depths and thus this heat is not directly available for ice melt during periods of stratification.

Colored Dissolved Organic Matter in Frontal Zones

Dissolved organic matter (DOM) includes a broad range of organic molecules of various sizes and composition that are released by all living and dead plants and animals. Measuring the fraction of DOM that absorbs light (colored or chromophoric DOM; CDOM) and fluoresces (referred to as CDOM fluorescence or FDOM) at specific wavelengths is diagnostic of DOM source and amount. The composition and dynamics of CDOM and FDOM across estuarine and coastal mixing zones, eddies, upwelling, and nepheloid layers are discussed in relation to the anomalies in physical (e.g., salinity and temperature), chemical (e.g., nutrients, δ 18O, dissolved oxygen), and biological properties (e.g., chlorophyll-a, primary production) reported in the frontal zone. In situ observations using profiling sensors and gliders, and remote sensing across coastal and oceanic fronts are described.

Bio‐optical properties of Arctic drift ice and surface waters north of Svalbard from winter to spring

We have quantified absorption by CDOM, aCDOM(k), particulate matter, ap(k), algal pigments, aph(k), and detrital material, aNAP(k), coincident with chlorophyll a in sea ice and surface waters in winter and spring 2015 in the Arctic Ocean north of Svalbard. The aCDOM(k) was low in contrast to other regions of the Arctic Ocean, while ap(k) has the largest contribution to absorption variability in sea ice and surface waters. ap(443) was 1.4–2.8 times and 1.3–1.8 times higher than aCDOM(443) in surface water and sea ice, respectively. aph(k) contributed 90% and 81% to ap(k), in open leads and under-ice waters column, and much less (53%–74%) in sea ice, respectively. Both aCDOM(k) and ap(k) followed closely the vertical distribution of chlorophyll a in sea ice and the water column. We observed a tenfold increase of the chlorophyll a concentration and nearly twofold increase in absorption at 443 nm in sea ice from winter to spring. The aCDOM(k) dominated the absorption budget in the UV both in sea ice and surface waters. In the visible range, absorption was dominated by aph(k), which contributed more than 50% and aCDOM(k), which contributed 43% to total absorption in water column. Detrital absorption contributed significantly (33%) only in surface ice layer. Algae dynamics explained more than 90% variability in ap(k) and aph(k) in water column, but less than 70% in the sea ice. This study presents detailed absorption budget that is relevant for modeling of radiative transfer and primary production.

Optical properties of waters around Svalbard and Franz Josef Land

The light attenuation coefficient c((lambda) ) is a key parameter which gives information about basic optical properties of marine environment. A set of measurements of c((lambda) ) together with suspension concentration (ms), Secchi disc (z(sigma )) and CTD were done during cruises in 1988 - 1992 in Spitsbergen fiords and Franz Josef Land. Mutual relationships between measured parameters were examined. The layer of horizontal suspension transport was estimated based on vertical profiles of ms and CTD.

Optical characteristic of humic acids from lake sediments by excitation-emission matrix fluorescence with PARAFAC model

PurposeThe study aimed to find out whether fluorescence parameters of humic acids (HA), obtained from excitation-emission matrix (EEM) fluorescence spectra, could be used to explain the origin and properties of lake sediments organic matter.Materials and methodsThe research material was HA extracted from lake sediments. Sediment samples were collected in the summer stagnation, from maximum depth of each lake. Humic acids extraction was performed using the method proposed by the International Humic Substances Society. In extracted HA, the elemental composition, free radical content, UV-Vis, and EEM fluorescence spectra were determined. EEM spectral modeling was performed using a multidirectional statistical method—parallel factor analysis PARAFAC.Results and discussionFour specific, two protein-like (C2 and C4) and humic- and fulvic-like (C1 and C3) components, components responsible for emission of fluorescence were identified in HA extracted from lake sediments. In the obtained EEM spectra, the dominant component was C2, while the smallest participation have C4 component. The mean contribution of C3 and C1 components to the total intensity of fluorescence was almost the same and was about 25%. Significant correlations were obtained between the concentration of EEM components and parameters characterizing the chemical properties of sediments and humic acids.ConclusionsThe study showed that the EEM fluorescence spectroscopy can be successfully used to characterize natural organic matter in aquatic environments. This method may provide supplementary information on the origin and transformation of organic matter in the water environments. By applying the parameters obtained from PARAFAC modeling, the relative degree of organic matter transformation (component C1) and autotrophic productivity (component C3) may be assessed. The analysis of the component C1 and C4 complete the information about the organic matter origin.

Bio-optical water quality assessment

1. The colour of the sea, i.e. its spectral reflectance, depends on the absorbing and scattering properties of substances in the water. 2. The main optical in-water constituents are chlorophyll a (Chl a), coloured dissolved organic matter (CDOM) and suspended particulate matter (SPM). 3. Optical data can be obtained from sensors deployed into the water or by remote sensing imagers on aircrafts or satellites. 4. With remote sensing, the optical properties of large geographical areas can be surveyed with high temporal and spatial resolution. 5. Chl a can be used as a proxy of phytoplankton biomass, CDOM as a marker of terrestrial freshwater and decay processes of marine primary producers and SPM as an indicator of land runoff and wind-driven resuspension of sediments. 6. Remote sensing of Chl a, CDOM and SPM can assist in the evaluation of water quality, e.g. the state of eutrophication, the extent of freshwater runoff, the depth of the photic zone and the breadth of the coastal zone. 7. The bio-optical characteristics of the brackish Baltic Sea differ from those of other seas. Due to the large overall freshwater influence, CDOM is usually the dominant optical in-water constituent not only near river discharges, but also in the open waters of the Baltic Sea. 8. The CDOM concentrations in the open waters of the Baltic Sea are inversely related to the large-scale Baltic Sea salinity gradient, with CDOM absorption highest in the northern Baltic Sea and lowest in the southwestern Baltic Sea. 9. Due to the high CDOM absorption regional Baltic Sea algorithms are required to derive water quality parameters that can be used as indicators of ecosystem health.