Sextio år av Mälarövervakning: historisk återblick av hur programmet startade och har förändrats för att möta ändrade behov

Extensive investigations began in Lake Mälaren in 1964, initiated by the Natural Resources Committee, a part of the National Natural Science Research Council. The motivation was the lake's central location as a source of raw water, recipient and recreation area in combination with a declining water quality with large algal blooms in summer. The survey program was seen as the beginning of in-depth studies of Sweden's larger lakes. The objective was to clarify scientific questions, analyse causal relationships, describe practical water conservation issues and educate expertise in the water area. In the beginning, tributaries and Lake Hjälmaren were also included. When the Swedish Environmental Protection Agency was formed in 1967 it became the principal for the project. The Lake Mälaren’s Water Conservation Association (MVVF) was formed in 1998, and the program was included in national environmental monitoring as the sub-program Large lakes comprising Lakes Mälaren, Vättern and Vänern. In 2011, the aquatic sections of Swedish Environmental Protection Agency were moved to the newly formed Swedish Agency for Marine and Water Management. Which, since then, accounts for part of the funds for the program, while the remaining part comes from the MVVF's membership fees. When the investigations started, around a hundred sites were sampled for analysis of water chemistry, phyto- and zooplankton. The number of sampling sites was reduced when results were similar enough to reduce the number of sites. The lowest number of sampling sites was in 2001–2005, with only eleven long-term stations remaining in the program. In 2008–2010, a wide surface coverage sampling for water chemistry was carried out similar to that at the start, at 56 sites. The surface-covering information and new needs linked to the Water Framework Directive led to an increase in the number of sampling sites to around 20 in 2006 and to just over 40 after 2017. Sampling in the straits between different basins ended in 2000 because the results were difficult to evaluate as the flow varied greatly and even changed direction. At that time, the technology for continuous measurements did not exist. However, the transport between basins was modelled. At the start of the program, only surface samples were taken, but known, poor oxygen conditions in the bottom water indicated that depth profiles were important, thus during many years sampling was done at every five meters depth. After 1995, the intervals were reduced to samples only at the surface, at 15 meters and near the bottom. In recent years, when both oxygen and temperature measurements are made easily directly in the field via analysers on a long cord, these measurements are again made at many sites, and more frequently, especially in August. However, water chemistry is only analysed on samples from surface, 15 m and close to the bottom. Regarding sampling intervals, the program started with summer and autumn sampling, then added months, usually sampling was done seven times a year until 1995 when only the four seasons were covered for a few years: But in 2001 at least the eleven long-term stations were sampled six times a year. The program now has the highest number of sites covered in August because phytoplankton results can then be used for status assessment and reporting. The water chemistry parameters are many, 79 are reported in the database. Several common ones such as colour, conductivity, alkalinity, many common ions such as calcium, magnesium, sodium, potassium and chloride, ammonium nitrogen, pH, oxygen, chlorophyll-a, zooplankton and benthic fauna have been determined using similar methods throughout the years, which makes them workable to evaluate with time series analysis. Other common parameters such as total phosphorus, organic substances, Secchi depth, sulphate, many nitrogen compounds, phosphate and phytoplankton have changed methods during the 60 years. In the report, we highlight this so it can be likely that step changes in time-series of some parameters may be due to such changes. Turbidity was added around 2010 because it became increasingly important to follow how high flows affect the water with particles from land. Substances that usually are present in low concentrations such as heavy metals and semi-metals (so-called trace elements), were measured periodically, most recently in 2017–2012 focusing on heavy metals to assess chemical status. Parameters deleted early were heterotrophic bacteria, primary production and the sediment content of various substances. In the beginning, mapping of the hydrological conditions was also done, something that was later refined with new measurements of depth for different purposes not included in the environmental monitoring program. Following the species abundance and biomass of phytoplankton in different parts of Lake Mälaren has been a primary task since the beginning. Early results show that both cyanobacteria and diatoms contribute to high production in the lake; diatoms with high biomass during spring and autumn, while the cyanobacteria are typical summer plankton. Some chrysophytes did also have large biomass, and all three groups were linked to taste and odour problems. Maps were published showing which phytoplankton group dominated in each basins together with the total phosphorus concentration (see picture on the cover of the report or figure 3). Many articles were published in the daily press and professional magazines during the late 1960s with an aim to support the debate that Mälaren needed help to continue being the “queen of lakes instead of a sewer tank”. This contributed to the introduction of a chemical precipitation step in the treatment plants to decrease phosphorus levels and a rather rapid decline in both phosphorus concentrations and phytoplankton was seen in the lake, but the levels were still too high, and the remedial work continued. For other organism groups, we give examples of studies that include cold water adapted species of zooplankton and benthic fauna from earlier lake stages (brackish water) as well as the large aquatic plant inventories made in the first decades using a combination of orthophotos and in situ inventories and suggest that such an inventory should be repeated. The report finishes with several suggestions for adjustments of the current environmental monitoring program.