Increased utility of the Secchi disk to assess eutrophication in coastal waters with freshwater run-off

Abstract

Changed local supply of nutrients in coastal waters can lead to changed content of phytoplankton. This influences the visibility, and thereby the water quality in the surface layers, which can be observed by e.g., the Secchi disk. However, the translation of Secchi depth observations to plankton content is complicated by the existence of light attenuating matter entering with run-off from land. To increase the utility of Secchi depth measurements to assess local eutrophication in coastal waters, a formula for calculating the Secchi depth in areas influenced by freshwater was developed. The study includes data analysis of Secchi depth, chlorophyll content, wind speed, and freshwater height estimated from salinity at two stations in the Gullmar Fjord in Sweden. The correlation coefficient between calculated and observed Secchi depths at the inner station close to the freshwater source increased from 0.63 using chlorophyll as the only dependent parameter, to 0.75 ( p < 0.001) including the accumulated freshwater in the surface layer from run off, and wind speed. At the outer station close to the mouth of the fjord it increased from 0.68 to 0.70 ( p < 0.001). The correlation between observed and calculated Secchi depth decreased considerably, if the restriction to observations made at sun elevations higher than about 30° above the horizon was relaxed.