Determination of the historical variation of the trophic state in lakes using sediment stratigraphies

Biogenic silica (BSi) and phosphorous (P) accumulation were investigated in sediment cores from Karlskärsviken, a bay of Lake Mälaren. The aim was to make use of BSi and P relations in sediment stratigraphies in order to investigate the historical nutrient trophy in a near shore lake environment since the Middle Ages, with focus on industrial times, and to evaluate anthropogenic influences on the bay’s trophic state. The BSi accumulation in the sediments is a better indicator of former nutrient pelagic trophy than P accumulation in sediments and for this reason a BSi inferred P (BSi-P) water concentration is calculated. This method enables the determination of the background total phosphorous (TP) concentration (which is related to the reference conditions) in the investigated bay; this background TP is determined equal to 0.020–0.022 mg L−1. There is an increasing trend of BSi-P concentration in the bay since the Middle Ages to the present, about 0.025 mg L−1, with a small decrease in the inner bay section during the last decades. The P accumulation rate is not found to have changed since the 1960s and 1970s, which indicates that the P loading to Karlskärsviken has not decreased. In Karlskärsviken, the shallow inner section of the bay, where the water quality is dominated by loading from the bay catchment area, is less nutritious than the water in the outer section, which is influenced by the main streams from the western part of Lake Mälaren.