Brunnsviken : Utvärdering av åtgärder för förbättrad vattenkvalitet

Sammanfattning: Periods of hypoxia are a recurring issue in Brunnsviken. Main factors that trigger hypoxia in Brunnsviken are eutrophication  due to anthropogenic emission of nutrients, stratification in the water column due to salinity and temperature differences  that prevents mixing and a sill inlet that prevents water exchange. As an attempt to improve the oxygen levels, bottom water is pumped from Brunnsviken causing aerated surface water to flow back into the basin.  The objective of this Master’s thesis was to study the change in dissolved oxygen levels due to pumping.  Since  the question is  complex,  two areas  were  chosen  to focus on; oxygen variability and the impact of water exchange on dissolved oxygen.  Conductivity, temperature, depth  and oxygen  measurements, which were performed  by Stockholm Vatten, were studied.  This type of data set, with high resolution, is very rare and the data set from Brunnsviken  has not been  evaluated  thoroughly  before. The result of the evaluation was that high resolution data of oxygen are  required in order to describe  the dynamics of oxygen  in Brunnsviken and other aquatic systems properly. Measurements of oxygen once a month is not sufficient. For that reason, it is not possible to use monthly measurements to evaluate  the  effect of the pumping. However, high resolution data for approximately one year were available. Episodes of hypoxia occurred during that period. This was not surprising, as the main purpose of  the pumping was to keep the oxycline as deep as possible.  The water exchange was determined from a water balance of Brunnsviken and compared with oxygen data. The water exchange did not seem to influence the oxygen level in Brunnsviken. The reason for this is most likely that the density of inflowing water was less than the density of  the  bottom water in Brunnsviken  during  most of  the period  studied.  This  result in combination with the fact that the pump flow is small compared to the water exchange suggests that the most important result of the pumping is that water with a low oxygen level is removed.