Using bio-optics to investigate the extent of coastal waters: A Swedish case study

Abstract

In order to develop an optical model to map the extent of coastal waters, the authors analyzed variations in bio-optical constituents and submarine optical properties along a transect from the nutrient-enriched coastal bay, Himmerfjarden, out into the open Baltic Sea. The model is a simple implementation of the “ecosystem approach,” because the optical constituents are proxies for important components of ecosystem state. Yellow substance or colored dissolved organic matter (CDOM) is often a marker for terrestrial freshwater or decay processes in the littoral zone. Phytoplankton pigments, especially chlorophyll a, are used as a proxy for phytoplankton biomass that may be stimulated by fluvial or coastal inputs of anthropogenic nutrients. Suspended particulate matter (SPM) is placed in suspension by tidal or wind-wave stirring of shallow seabeds, and is therefore an indicator for physical forcing. It is the thesis of this article that such constituents, and the optical properties that they control, can be used to provide an ecological definition of the extent of the coastal zone. The spatial distribution of the observations was analyzed using a steady-state model that assumes diffusional transport of bio-optical variables along an axis perpendicular to the coast. According to the model, the resulting distribution along this axis can be described as a low-order polynomial (of order 1–3) when moving from a “source” associated with land to the open-sea “sink.” Order 1 implies conservative mixing, and the higher orders imply significant biological or chemical processes within the gradient. The analysis of the transect data confirmed that the trend of each optical component could be described well using a low-order polynomial. Multiple regression analysis was then used to weigh the contribution of each optical component to the spectral attenuation coefficient K d(490) along the transect. The results showed that in this Swedish Baltic case study, the inorganic fraction of the SPM may be used to distinguish between coastal and open-sea waters, as it showed a clear break between coastal and open-sea waters. Alternative models may be needed for coastal waters in which fronts interrupt the continuity of mixing.