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

摘要

Biogenic silica (BSi) and phosphorous (P) accumulation were investigated insediment cores from Karlsk?rsviken, a bay of Lake M?laren. The aimwas to make use of BSi and P relations in sediment stratigraphies in orderto investigate the historical nutrient trophy in a near shore lakeenvironment since the Middle Ages, with focus on industrial times, and toevaluate anthropogenic influences on the bay's trophic state. The BSiaccumulation in the sediments is a better indicator of former nutrientpelagic trophy than P accumulation in sediments and for this reason a BSiinferred P (BSi-P) water concentration is calculated. This method enablesthe determination of the background total phosphorous (TP) concentration(which is related to the reference conditions) in the investigated bay; thisbackground TP is determined equal to 0.020–0.022 mg L−1. There is anincreasing trend of BSi-P concentration in the bay since the Middle Ages tothe present, about 0.025 mg L−1, with a small decrease in the inner baysection during the last decades. The P accumulation rate is not found tohave changed since the 1960s and 1970s, which indicates that the P loadingto Karlsk?rsviken has not decreased. In Karlsk?rsviken, the shallowinner section of the bay, where the water quality is dominated by loadingfrom the bay catchment area, is less nutritious than the water in the outersection, which is influenced by the main streams from the western part ofLake M?laren.