Particle-size distributions in the Baltic

Abstract

Over 160 particle size distributions were measured with the use of a Coulter Counter in the particle diameter D range of 2 to 32 μm in the Baltic waters in different seasons during 1975–78. The particle-size distributions were found to be fairly well described by the following sum of Gaussian and hyperbolic functions:

where α,α,D₀, k₁, m₁, m₂ and D_b are coefficients of fit and K₂ = k₁D_b^{m1 + m2}. The Gaussian term was found to be related to phytoplankton and present when phytoplankton was at sufficient concentration. The hyperbolic term, always present, appears to be related to particles formed by non-biological processes. The average values of the parameters were found to be: α = ∼ 800 cm^-3 μm^-1, γ = ∼0.7 μm^-2, D₀ = ∼6 μm for June samples, while k = 10⁵ (0.99 × 10⁵) cm^-3 μm^-1, m₁ = 3.2 (0.5), m₂ = 5 (0.6) and D_b = 7.5 (1.8) μm for the whole open Baltic data set, with one standard deviation in parentheses.

The particle-size distribution was found to vary greatly in the Baltic, depending on season, depth and location. The greatest variations were encountered in the layer extending from the surface to the seasonal thermocline located at about 20–30 m in summer. The concentrations of all particles counted (with diameters > 2 μm) in this layer was found to vary between 2000 and 35,000 particles/cm3, corresponding to late winter and late summer, respectively. The layer extending between the thermocline and permanent Baltic halocline (60–70 m) was found to be the clearest water mass in the Baltic throughout entire year, with minimum concentrations of all particles counted as low as 1000–1500 particles/cm³. Below the halocline, the concentration of suspended particles was found to rise steeply with depth down to ∼10 m above bottom not exceeding, however, ∼ of the maximum surface value.

The major part of the variability of the particle-size distribution was found to be related to variations in particle concentration. The shape of the non-biological particle-size distribution function was found to be fairly stable and similar to that of other seas and oceans.