Satellite mapping of Baltic Sea Secchi depth with multiple regression models

Secchi depth is a measure of water transparency. In the Baltic Sea region, Secchi depth maps are used to assess eutrophication and as input for habitat models. Due to their spatial and temporal coverage, satellite data would be the most suitable data source for such maps. But the Baltic Sea’s optical properties are so different from the open ocean that globally calibrated standard models suffer from large errors. Regional predictive models that take the Baltic Sea’s special optical properties into account are thus needed. This paper tests how accurately generalized linear models (GLMs) and generalized additive models (GAMs) with MODIS/Aqua and auxiliary data as inputs can predict Secchi depth at a regional scale. It uses cross-validation to test the prediction accuracy of hundreds of GAMs and GLMswith up to 5 input variables. A GAM with 3 input variables (chlorophyll a, remote sensing reflectance at 678nm, and long-termmean salinity) made themost accurate predictions. Tested against field observations not used formodel selection and calibration, the bestmodel’smean absolute error (MAE) for daily predictionswas 1.07m (22%), more than 50% lower than for other publicly available Baltic Sea Secchi depth maps. The MAE for predicting monthly averages was 0.86m (15%). Thus, the proposed model selection process was able to find a regional model with good prediction accuracy. It could be useful to find predictive models for environmental variables other than Secchi depth, using data fromother satellite sensors, and for other regions where non-standard remote sensingmodels are needed for prediction andmapping. Annual and monthly mean Secchi depth maps for 2003–2012 come with this paper as Supplementary materials. © 2015 Elsevier B.V. All rights reserved.