Development in phytoplankton assemblages, ecological status and purification effect in Teglverksdammen : a study of a pond the first two growth seasons following deculverting of an urban stream system in Oslo, Norway

Abstract

There is growing consensus that deculverting and restoration of buried urban streams may come with a range of ecological and socioeconomic benefits, including; reduced flood risks, improved water quality, facilitation of biodiversity and decreased habitat fragmentation. Ponds in such systems may constitute appreciated landscape-elements and further function as sedimentation basins, thus facilitating removal of environmental pollutants and nutrients associated with suspended solids. However, as such systems are susceptible to nutrient pollution, ponds may also potentially facilitate large amounts of phytoplankton, which may degrade the ecological status and aesthetic appeal of the system.

This thesis is a case study that covers the first two growth seasons of the pond Teglverksdammen that is a part of a newly deculverted reach in Hovinbekken, Oslo, Norway. Relatively few such urban deculverting projects have been conducted in Norway, and studying the development in Teglverksdammen can therefore offer valuable insights to problems and opportunities for future stream deculverting and restoration projects. The early development of the pond’s phytoplankton assemblages is described, and it was tested how it related to physio-chemical environmental variables. It was also determined what ecological status was indicated by phytoplankton using the water framework directive classification system. Last, upstream and downstream water samples were used to test whether the pond facilitated net retention of nutrients and organic matter. In situ-measurements, phytoplankton samples and water samples were collected with monthly interval May-October the first two growth seasons following opening of the reach, 2016 and 2017. In addition, water discharge data (available for the first year only) and weather data were obtained.

Apart from during the longer stagnation period recorded, phytoplankton concentrations were moderate in the pond. Nutrient concentration and light availability were generally high, and neither could be identified as important controlling factors for the phytoplankton biovolume. As the residence time in the pond was generally short, variations in the rate of phytoplankton loss through flushing was likely of larger importance for the phytoplankton concentration than the growth controlling variables. The short residence time and location of the pond makes it susceptible to disturbances and the physio-chemical variables recorded also showed large alternations. The phytoplankton assemblage succession showed little order in form of seasonality or consistent response to the environmental variables examined here. In additional samples collected in the littoral zone in April 2017, several marine species of phytoplankton were also found, indicating an ecological disturbance likely caused by salt pollution from road runoff. The ecological status as indicated by the phytoplankton quality element alone was moderate in 2016 and good in 2017, but as phosphorus concentrations remained high the overall ecological status was moderate for both years. There was large variation in whether the pond acted like a sink or source for nutrients and organic matter, although there was a general trend for retention. Data for stream discharge was only available for the six data points from the first year, but the results indicated longer residence did not increase net nutrient retention in the pond. The results further indicated longer residence time may result in increase in phytoplankton biovolume and overall trophic state of the pond.