Our data from the southern Baltic Sea study area exhibit considerable variability in all the particle concentrations measured. In the case of the basic suspended particulate matter characteristic – its mass concentration (SPM) – the corresponding coefficient of variation (CV, defined as the ratio of the standard deviation to the average value and expressed as a percentage) is more than

90%. There is a > 40-fold variability between the measured maximum and minimum values (see Table 1). In the case of other biogeochemical quantities like concentrations of POC and POM, which characterize the organic fraction of suspended matter, the recorded variability is of the same order (with CV reaching > 90% in both cases, PARP inhibitor drugs and with > 50-fold and > 30-fold variability between the extremes respectively). In the case of the phytoplankton pigment concentrations found within the suspended matter the variability is even greater. The concentration of the primary pigment, chlorophyll a (Chl a), has a CV of almost 130% and there is a > 190-fold variability between the maximum and minimum values; the variability in the overall concentration of all accessory pigments is of the same CX-5461 mouse order of magnitude. Although most of the particle populations encountered were composed primarily of organic matter, the different particle

concentration ratios suggest that particle composition varied significantly. For example, the average POM/SPM ratio is about 0.8 but the corresponding CV is 22% (see the data in Table 1). In the case of the two other composition ratios – POC/SPM (av. = ca 0.25) and Chl a/SPM (av. = ca 3.5 × 10−3) – the corresponding CVs are even greater (41% and 81% respectively). As these three composition ratios can provide insight into the variable proportions between the organic and inorganic fractions in the total suspended matter, there are two other ratios worth mentioning, which suggest that the composition of the organic fraction of suspended matter is itself subject to significant variability. The CV of the Chl a/POC

ratio (av. = 1.3 × 10−2) is 74%, while for the ratio of total accessory pigments to STAT inhibitor Chl a it is 29%. The relations between the different biogeochemical measures characterizing suspended matter are illustrated graphically in Figure 2. This also shows (with the aid of the colour coded data points – see Figure caption for details) that, on average, lower suspended matter concentrations were typical of the open southern Baltic waters rather than of the Gulf of Gdańsk. The high variability in different concentration measures of particulate matter in southern Baltic waters had to yield a high variability in IOPs. Relationships between particle concentrations and optical properties will be described in detail below, but at this point it is appropriate to emphasize the general variability ranges in particle IOPs. The absorption coefficient of particles at 440 nm varied between < 0.