Sławomir Sagan

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.

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.

Visual prey availability and distribution of foraging little auks (Alle alle) in the shelf waters of West Spitsbergen

As diving seabirds use vision underwater, it is presumed they should preferentially select sites where their preferred food items are not only abundant but also clearly visible. To test this, we studied the optical properties of the seawater in the West Spitsbergen Shelf, in combination with zooplankton abundance in the feeding grounds of the planktivorous little auks from the nearby colonies in Hornsund. We estimated the relative attractiveness of the foraging sites using a novel parameter—visual prey availability (VPAv), which relates density and proportion of the preferred food item (Calanus glacialis) of the little auk, in total zooplankton, to the optical properties of the seawater. We found a significant positive correlation between the density of foraging little auks and VPAv values. Birds chose areas where C. glacialis was both abundant and clearly visible, because of the clarity of the water and low proportion of other zooplankton species. The birds avoided foraging over the warmer Atlantic-type waters, characterised by a high abundance of zooplankton taxa mostly ignored by birds and where VPAv values were low. VPAv values could potentially also be applied to other visual planktivores for which prey preference and visual acuity are known.

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.

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.

Characteristics of chromophoric and fluorescent dissolved organic matter in the Nordic Seas

Optical properties of chromophoric (CDOM) and fluorescent dissolved organic matter (FDOM) were characterized in the Nordic Seas including the West Spitsbergen Shelf during June–July 2013, 2014, and 2015. The CDOM absorption coefficient at 350 nm, aCDOM(350) showed significant interannual variation (T test, p< 0.00001). In 2013, the highest average aCDOM(350) values (aCDOM(350)= 0.30± 0.12 m−1) were observed due to the influence of cold and low-salinity water from the Sørkapp Current (SC) in the southern part of the West Spitsbergen Shelf. In 2014, aCDOM(350) values were significantly lower (T test, p< 0.00001) than in 2013 (average aCDOM(350)= 0.14± 0.06 m−1), which was associated with the dominance of warm and saline Atlantic Water (AW) in the region, while in 2015 intermediate CDOM absorption (average aCDOM(350)= 0.19± 0.05 m−1) was observed. In situ measurements of three FDOM components revealed that fluorescence intensity of protein-like FDOM dominated in the surface layer of the Nordic Seas. Concentrations of marine and terrestrial humic-like DOM were very low and distribution of those components was generally vertically homogenous in the upper ocean (0–100 m). Fluorescence of terrestrial and marine humic-like DOM decreased in surface waters (0–15 m) near the sea ice edge due to dilution of oceanic waters by sea ice meltwater. The vertical distribution of protein-like FDOM was characterized by a prominent subsurface maximum that matched the subsurface chlorophyll a maximum and was observed across the study area. The highest protein-like FDOM fluorescence was observed in the Norwegian Sea in the core of warm AW. There was a significant relationship between the proteinlike fluorescence and chlorophyll a fluorescence (R2= 0.65, p< 0.0001, n= 24 490), which suggests that phytoplankton was the primary source of protein-like DOM in the Nordic Seas and West Spitsbergen Shelf waters. Observed variability in selected spectral indices (spectral slope coefficient, S300–600, carbon-specific CDOM absorption coefficient at 254 and 350 nm, SUVA254, a CDOM(350)) and the nonlinear relationship between CDOM absorption and the spectral slope coefficient also indicate a dominant marine (autochthonous) source of CDOM and FDOM in the study area. Further, our data suggest that aCDOM(350) cannot be used to predict dissolved organic carbon (DOC) concentrations in the study region; however the slope coefficient (S300–600) shows some promise in being used.