Historical changes in optical properties of Roskilde Fjord, during a period of decreasing nutrient load
Abstract
Loss of water clarity is a common consequence of coastal eutrophication. Efforts have been made to reduce the nutrient load of the inner Danish waters, hence increasing water column clarity. Measurements of Light attenuation, Particulate Organic Matter (POM) and Chlorophyll a (Chl a) concentrations from one of the major Danish estuarine systems have been conducted from April to September in 1985 and 2008. In an effort to assess the dynamics controlling light attenuation linear models are made of light attenuations as a function of Chl a and POM concentration. An average decrease in light attenuation of 34% is observed along with deceases in Chl a and POM concentrations of 71% and 84%, respectively. Analysis of simultaneous changes in light attenuation and secchi depth indicates that changes in scatter-toabsorption ratio accompanied the nutrient and Chl a reductions implying reduction in particulate detritus. Furthermore the managerial effort to increase water clarity seems to have a larger impact on the POM concentration than the Chl a concentration throughout the estuary, which helps to explain the decrease in scatter-toabsorption ratio.