Until now, the identification of water layers with distinct optical properties was attempted using RAMSES transmittance and VIPER absorbance data. The first approach imple-mented different fitting methods to estimate broadband extinction coefficients. The sec-ond method utilized a small number of direct measurements which were unevenly dis-tributed along the depth profiles. Current VIPER measurements are insufficient for the desired analysis. In contrast, the number of RAMSES transmittance measurements along the depth profiles is adequate. However, the analysis showed that the methods selected for the investigation of distinct layers using the RAMSES data, are still premature and require improvements of the measurement and data processing techniques. Results from individual stations indicated changes in broadband extinction coefficients along the depth profiles, but the identification of distinct layers is not yet possible. More importantly, the direct determination of the boundaries between adjacent layers is not achievable.
Following the analysis, it is evident that additional information is required for the desired investigation. While, PS101 temperature, salinity and pH measurements from the ROV are unreliable, backscatter, FDOM, and chlorophyll concentration data from the ECO-Triplet provide additional information essential to this work’s analysis.
Figure 5.5 shows downcast values of chlorophyll concentration, backscatter and FDOM for all PS101 ice stations. A decrease in backscatter and chlorophyll concentration from the first (057) to the last (171) station is evident. Stations 057, 096 and 114 show compa-rably high chlorophyll concentrations (approximately 0.7, 0.5 and 0.4µg/l, respectively) and backscatter values (approx. 2.2×10−4, 1.5×10−4 and 1.4×10−4, respectively) in the upper part of the cast. At depths between 20-30 m, values for both parameters drop noticeably to match the values measured at stations 142, 162 and 171. These results reveal water property changes associated to the change in season. While measurements at station PS101- 057 were taken right after the sea- ice minimum (September 13, 2016) on September 15, 2016, PS101-171 measurements were conducted later in the freeze-up
PS101: downcast - ECO-Triplet
Figure 5.5: Values of chlorophyll concentration ([µg/l],left), backscatter ([m−1sr−1], mid-dle) and FDOM ([ppb],right) retrieved from the ECO-Triplet sensor. Measurements are taken during the downcasts of the six stations of PS101.
to 171 (between 3.5 to 4ppb), while FDOM was noticeably lower at station 057 (between 1.5 and 2ppb).
Chlorophyll concentration and backscatter values indicate pronounced biological activity at the first three stations that reduced towards the end of the campaign. Within the first three stations, different chlorophyll concentrations and backscatter signals are apparent along the downcasts. The tendency of biological activity reducing throughout the sam-pling period of PS101 is also apparent in the downcast RAMSES broadband extinction coefficients estimated for each of the six stations (Table 4.1). Broadband extinction coef-ficients decrease from station 096 (0.2378m−1) to station 142 (0.0694m−1).
Higher chlorophyll concentrations are associated with more extinction in the water col-umn. However, the water column at station 057 shows the highest chlorophyll concen-trations of all stations, nevertheless, the broadband extinction coefficient (0.1396m−1) is smaller than for station 096. This is most likely caused by the low FDOM values in the water column of station 057. These results indicate that the optical properties of seawater are controlled by multiple biological parameters that have very different impacts on the extinction within the medium. The decrease in extinction coefficients with a decrease in biological matter confirms findings of Taskjelle et al. (2016). They found that increased biological activity has the potential to increase absorption in the upper 20 m of the water column noticeably.
Figure 5.6 a shows downcast chlorophyll concentration and backscatter values obtained at station PS101-114. Chlorophyll concentration remains at approximately 0.37µg/l throughout the upper 30 m of the cast. A distinct drop to roughly 0.16µg/l is appar-ent between 30 and 32 m depth. The layer extending below this depth shows chlorophyll concentrations of 0.15µg/l down to end of the profile. The same distinct boundary is ev-ident in the backscatter data. Backscatter values are centred around 1.3×10−4m−1sr−1 throughout the upper 30 m and drop to roughly 1×10−4m−1sr−1 below. Both data sets suggest a layer with higher particle concentrations located above a layer that is less con-taminated. These layers only occur at the first couple of stations and are caused by the accumulation of biological matter under the ice cover during the melt season. This activity is favoured by the fact that the ice is more light-permeable during the summer months. During the freeze-up less light reaches the under-ice water layer, and biological activity reduces.
The presented ancillary information is used to specifically select data points from optically different layers for the fitting of the RAMSES broadband transmittance data. Figure 5.6 b shows the exponential fits plotted onto the broadband transmittance. The upper (lower) layer estimate shows a κbb-value of 0.1142m−1 (0.0636m−1). While R2 = 0.9513 for the upper fit, R2 = 0.2261 for the fit below 30 m. These changes in the optical properties of individual layers are strongly linked to the amount of biological matter in the water
0 0.5 1 1.5
PS101-114/1 - ECO depth plots
downcast
55 0 0.005 0.01 0.015 0.02 0.025
Transmittance
Figure 5.6: Values of (a) chlorophyll concentration ([µg/l], left) and backscatter ([m−1sr−1],right) retrieved from the ECO-Triplet sensor. (b) exponential fits (red lines) projected on RAMSES broadband transmittance data (black dots). κbb-values indicate the estimated broadband extinction coefficients of the respective layer. R2-values repre-sent the fraction of total broadband transmittance variability that is explained by the fit. All measurements are taken during the downcast of station PS101-114.
column, however, the given data set is insufficient to provide a quantitative description of this relationship.
The presented results indicate that the identification of distinct layers is possible using the given RAMSES data. Estimating broadband extinction coefficients from broadband transmittance data with the selected exponential decay model is achievable, but with-out extra data from other instruments the presented methods are too limited to find the boundaries between layers with distinct optical properties. The presented exponen-tial fitting methods are limited in parts of the profile where broadband transmittance values approach zero. Moreover, the quality of the RAMSES transmittance data limits the quality of broadband extinction coefficient estimates and the availability of VIPER measurements is not sufficient.