Variability of light absorption coefficients by different size fractions of suspensions in the southern Baltic Sea
Abstract. Measurements of light absorption coefficients by particles suspended in seawater (ap (λ)), by phytoplankton (aph(λ)) and detritus (ad (λ)) were carried out in the Baltic Sea waters. Measurements were performed for the original (unfiltered) seawater samples and the four selected size fractions: pico-particles with diameters (0.2–2 μm), ultra-particles with diameters (2–5 μm), nano-particles with diameters (5–20 μm) and micro-particles (20–200 μm). Chlorophyll a (Chla) concentrations and total suspended particulate matter (SPM) concentrations were determined. The proportions of particles from the size classes (micro, nano, ultra and pico) in the total light absorption by particles, phytoplankton and detritus were determined. Particles with sizes < 5 μm (i.e. pico and ultra-particles) had the largest contribution to the total particles absorption – an average of 38 % and 31 %. Particles of 5–20 μm accounted for approximately 20 % of all particles and phytoplankton and 29 % of the detritus. The contribution of large particles > 20 µm averaged 5–10 %.
The average chlorophyll-specific and mass-specific light absorption coefficients, i.e. light absorption coefficients normalized to Chla and to SPM concentration, were determined for all size fractions. The determined average chlorophyll-specific light absorption coefficients ap(Chla)(λ), ad(Chla)(λ) and aph(Chla)(λ), along with standard deviations, do not allow clear separation of the individual fractions. For mass-specific light absorption coefficients, ap(SPM)(λ), ad(SPM)(λ) and aph(SPM)(λ), it is possible to distinguish between large particle fractions (microplankton – 20–200 μm) and small and medium particle fractions (0.2–20 μm).
Justyna Meler et al.
Justyna Meler et al.
Justyna Meler et al.
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Meler et al. present a descriptive study documenting measurements of particulate, phytoplankton, and detrital absorption coefficients obtained from size-fractionated water samples collected from nearshore and offshore waters of the Baltic Sea. The water samples comprise a significant range in biogeochemical properties based on SPM and Chla observations, although the sampling locations were confined to a southern sub-region of the Baltic sea. The dataset contains 38 elements. The results do not indicate significant differences between size fractionated samples in terms of absorption properties. Further, the results indicate differences in mass-specific (but not Chla-specific) light absorption coefficients between larger (micro) versus smaller (nano and pico) organic and inorganic particles.
Although the general topic could potentially be compliant with the journal's scope, the manuscript does not satisfy the journal's citeria to merit publication, as follows:
Scientific Significance: The study is not sufficiently comprehensive based on insufficient number of observations (n=38) and small spatial extent of the sampling area, compared with the variety of oceanic conditions, physical forcings, biological conditions, and the terrestrial, riverine, and anthropogenic inputs to the Baltic Sea. The key-finding (that Chla-specific absorption properties of size-fractionated samples are not significantly different from eachother within the authors' n=38 dataset from southern Baltic Sea waters near Poland) would be more compelling if the study was more expansive, or if the authors could better extablish the significance of their null results. The study may also not be generalizable outside of the Baltic Sea, and becuase the observations span a small geographical sub-region of the Baltic sea (the southern waters around Poland) the results may also not be representative of the optical properties elsewhere in the Baltic Sea; and
Presentation Quality: Comprehension of the manuscript is inhibited by low presentaiton quality. In particular, the authors' combination of the Results and Discussion materials into a single section significantly detracts from the presentaiton of each, and at times made comprehension of the manuscript difficult, or resulted in ambiguity in elements of the methods or results. I suggest that the authors separate the results and discussion in order to add clarify.
Additional (General) Comments:
The authors do not adequately demonstrate the other dimensions of variability in their dataset, e.g., due to seasonal factors, site-specific differences like onshore vs offshore, biomass, or total particle content. One way that the authors could have helped with this would be to color the markers in the scatter plots to indicate other parameters, e.g., by seasons or by whether the site was nearshore or offshore.
Comparing the overlap in mean +/- std between data points is most useful when uncertainties due to environmental or methodological variability are well described (uncertain measurements of moderately disimilar parameters can easily overlap). The authors do not convey uncertainty in their absorption, Chla, or SPM measurements, which would help to identify the extent to which overlap in absorption properties is or is not meaningful.
The authors did not identify differences in Chla-specific optical properties between size fractionated samples. I'd suggest that the authors investigate or discuss what other factors (e.g., distance from shore, biomass, wind-driven mixing, contribution of inorganic particles) may have been associated with the variability in observed Chla-specific absorption properties within size fractions.
Minor (Specific) Comments:
Table 1: Is the section "Nano+ultra particles (2-20um)" intended to be Pico + nano particles (based on the sampling difficulty of the first 14 samples; L200-202)?
Lines 303-312 and figures 5-7: I'd suggest that log scale R2 values are reported as well. These datasets are mostly log-normally distributed in both axes, and R2 calculated on the linear axes is strongly influenced by the points in the upper-right corner of the plot. For exmaple, consider the high R2 despite low association of points in Fig 6 panel G.