Suspended particulate matter (SPM) in the Baltic Sea-New empirical data and models

This work presents new data on suspended particulate matter (SPM) in the Baltic Sea (from the part called the Baltic proper). Previous reports on SPM from the Baltic Sea and other marine areas are very scarce in spite of the fact that SPM is of fundamental importance in aquatic ecology and management since it regulates the transport of pollutants and influences water clarity and phytoplankton production as well as bacterioplankton production, and hence also secondary production (of zooplankton and fish). It is, evidently, very demanding to collect data at sea during storm events and this paper presents data from 5 sites and 17 sampling occasions in 1999 and 2001 collected along gradients concerning temperature, salinity, exposure, depth and winds (also during storms). The data have been used to address problems related to the variability of SPM, and, hence also the predictive power of models for SPM. The characteristic coefficients of variation (CV = standard deviation/mean value) for SPM among and within these sites are very high, 0.69 and 0.67, respectively. Using statistical methods, the factors influencing the SPM-variability have been quantified and ranked and regressions for SPM under stratified and mixed conditions presented. The model for stratified conditions is based on the following variables (1) exposure (a measure of wind-induced wave influences), (2) number of days with wind speeds lower than 5 m/s, and (3) mean salinity. The following factors are important for the vertical SPM-variability within sites (1) water temperature and (2) distance from the bottom. SPM increases with increasing temperatures (related to increased production and stratification) and decreases with increasing water depth, wind exposure, salinity, and with the number of calm days. The reasons for this are discussed. The factors regulating the SPM-variations among and within these sites should also apply to other marine areas. Studies in other areas cannot, however, falsify these empirical results but they could clarify the boundary conditions for these models and improve our knowledge about the factors regulating SPM. This model for SPM is primarily meant to be used as a sub-model within more comprehensive models, which aim to predict key ecosystems variables, e.g., toxins in fish or production and biomass of functional groups of organisms. We have also introduced and motivated a new concept “coastal focusing” analogous to “sediment focusing” in lakes.