Laying the foundation for our understanding of the Baltic Sea

A van full of sampling equipment leaves Stockholm University at half past five in the morning. Twelve hours later it returns, carrying hundreds of bottles, jars and test tubes filled with water from different locations and depths in the Baltic Sea. Follow along on a day of environmental monitoring at sea!

The so-called CTD probe, which measures temperature, oxygen content and indirect parameters for salinity and depth, will soon be sent down 40 metres to just above the seabed at the legendary Askö B1 monitoring station in the Trosa archipelago. Photo: Michaela Lundell

– We can start from here. Okay, let's go. Five metres!

Engineer Elizaveta Mattsson lets go of the wire, and the probe and the first water sampling bottle are lowered into the water. The research vessel Electra's engineer Henrik Andersson operates the crane. After five metres, he stops, and Elizaveta attaches the next water sampling bottle. The procedure is repeated eight times, with a new water sampler attached at every five metres, until the bottom probe is just above the seabed, at a depth of 40 metres.

Long data series

We are at the Askö B1 monitoring station in the Trosa archipelago south of Stockholm. Here, scientists have taken water samples since the 1960s, and regular environmental monitoring has been ongoing since the late 1970s.

Also, the frequency of sampling determines how valuable the data series will be. Sampling shows but a snapshot, and if it is done too infrequently, important peaks can be missed and the results will be misleading. At B1, samples are taken 25 times a year, most often during the spring bloom in March-April, when diatoms and dinoflagellates, the basis of the Baltic Sea ecosystem, multiply excessively. The sampling personnel are then here once a week. Now, during the winter, they come once a month. This makes B1 one of the few well-attended monitoring stations in the Baltic Sea.

Some of the hundreds of test tubes that the research engineers handle during a day of sampling. These are chlorophyll samples, which will be analysed in the lab the next day using a spectrophotometer. Photo: Michaela Lundell

Backbone of knowledge

The long, uninterrupted, high-frequency series of measurements is invaluable to anyone who wants to learn about the state of the Baltic Sea. Together with other sampling data from the marine environmental monitoring, it forms the backbone of our knowledge about this fragile inland sea.

Once water has been collected from all depth ranges, the large zooplankton net is mounted on the wire and lowered 35 metres into the water. The net has to be moved at a certain, fixed speed through the water. If the sample is taken too quickly or slowly, it has to be redone. Henrik Andersson watches intently at the speedometer as he operates the levers.

– For zooplankton, the net needs to be this big, to capture both common and less common species, says Elizaveta Mattsson. We want to detect if for example invasive species have arrived.

When all the plankton have been washed into the bottle at the bottom of the net, she pours the catch into a jar.

– See how it moves? They're copepods. But they're still small, most of them are still nauplius larvae.

Zooplankton are captured using a large net. Photo: Michaela Lundell

Flow of samples and duplicate samples

Meanwhile, on the foredeck, research engineer Stefan Svensson mounts a plumb bob at the end of a 25-metre hose, which he then lets down into the water. The resulting water sample will be used for analysis of the average chlorophyll content in the water column down to 25 metres’ depth. The chlorophyll content is an indirect measure of the density of phytoplankton.

Once he has hoisted up the long hose, sampling with the phytoplankton net follows. It is smaller and can be handled by hand.

Elizaveta calls out from the aft deck. It's time for one of the day's many duplicate samples. At least one duplicate sample for each parameter is taken, to detect possible errors. Elizaveta has just measured the water clarity, with a so-called Secchi disc and a view tube.

Water transparency is measured using a so-called Secchi disc, if possible using a view tube, to remove the disturbing effects of light reflections from the water surface. Photo: Michaela Lundell

Lifejacket is on, because during that sample the cargo door opens and the person sampling lies flat on the deck, chest down, just over the edge. Henrik Andersson keeps an eye. A man-overboard situation in this temperature, end of January, can quickly become critical. Elizaveta slowly brings the white disc up through the water.

– That's where it becomes visible, she says, reading the mark. Six metres.

The corresponding test with the view tube gives a water transparency of 6.5 metres.

– There is ice inside the tube, Elizaveta says.

Stefan makes the same tests. His results are 5.7 and 7 metres respectively. Everything is carefully recorded.

The Askö Laboratory technician Henrik Andersson and engineer Elizaveta Mattson from the Department of Ecology, Environment and Plant Sciences work together with the sampling on the aft deck. The lines become icy and slippery in the cold. Photo: Michaela Lundell

Reliable data

– Delivering reliable data is what our work is all about, says Stefan Svensson.

– It must be repetitive. Everything has to be done exactly the same way every time. And that's harder than you might think!

Since all data should be comparable, special care is needed when introducing new measurement methods. Just changing the method can cause method-switching effects in the results, which destroy the long data series. Everything must be calibrated and checked to ensure quality and comparability. A new, simpler and faster method for chlorophyll analysis is currently being tested. One of its advantages is that it requires less water, so fewer kilos need to be carried during and after sampling. Stefan and Elizaveta have been taking parallel samples throughout last year. If the results are good, they hope to switch to the new method alone this summer.

Research engineer Stefan Svensson takes water for various analyses from the so-called Niskin water sampling bottles, which have collected water from different depths. Photo: Michaela Lundell

Sampling outside treatment plant

The R/V Electra af Askö heads for four more monitoring stations, at different distances from the large sewage treatment plant in Himmerfjärden, where a large part of the wastewater from southern Stockholm County is treated.

The sampling is part of the treatment plant’s compulsory so-called recipient monitoring programme. In collaboration with researchers from Stockholm University, the treatment plant varied its nitrogen emissions for a period, so that the researchers could see what effect this had on eutrophication in the sea. Nitrogen removal is expensive, so it's important to know what effect different measures have.

The sampling here at Himmerfjärden has contributed to our understanding of how important the supply of nitrogen is for the environment along our coasts.

Weather conditions and samples taken are recorded at each monitoring station. Photo: Michaela Lundell

Ensuring quality and proximity to research

Assigning the environmental monitoring mission to the university, ensures both data quality and the immediate proximity to the highest level of assessing expertise. Stockholm University has specialists in brackish water chemistry, toxic pollutants, phytoplankton and zooplankton, invertebrates, macroalgae and other aquatic plants.

Environmental monitoring may sound unglamorous, but it is an important basis for the university's core activities, both in terms of research and education. While research projects, usually lasting only a few years, can elucidate specific processes, it is environmental monitoring that provides the basic information about the sea, and can reveal the large-scale changes in the environment. Regular sampling also provides an infrastructural platform for researchers and students. Many researchers and PhD students have come along over the years, taking the opportunity to get their own samples for their research projects. Today, biology student Elena García Hernández is onboard to take plankton samples.

Biology student Elena García Hernández with a zooplankton sample. Photo: Michaela Lundell

Westerly winds are the best

Stefan and Elizaveta work fast, smoothly and methodically, keeping a close eye on all the hundreds of test tubes, jars and bottles that are by now filled with sea water from different stations and depths, for analyses of oxygen, hydrogen sulphide, nutrients and particles, among other things. They dip, rinse, inject, filter, shake, turn upside down, label, knock out air bubbles, haul hoses, nets, tubes, water sampling bottles and other sampling equipment in and out of the sea.

– Perfect weather, Elizaveta notes with satisfaction in the raw cold.
At each monitoring station, she takes notes of the weather conditions. The wind is moderate and coming from the west.

– Westerly winds are the best. Then you have the whole landmass of Sweden for protection. When the wind is from the east, you can have the whole sea coming against you. As long as it's not too windy, it doesn't matter if it rains or snows a bit. We do not require sun!

The samples are taken, one by one. Hour is added to hour of intensive fieldwork. The last thing to be done is measuring the water clarity at station H5.

– That's it, we can go home!

Once the water from the last set of sampling bottles have been secured in carefully labelled containers, the sampling team gathers in Electra's kitchenette.

– Now I'm just going to thaw my face a bit, then I will laugh, says Henrik.

Text: Michaela Lundell

Facts about Environmental monitoring

Stockholm University contributes to the national environmental monitoring programme of the Baltic Sea with sampling and analyses in four sub-programmes: the open sea, vegetated bottoms, macroscopic animals in soft bottoms, and studies of the small crustacean Monoporeia affinis (whose malformed embryos reflect the levels of organic pollutants and metals).

The first three are carried out by researchers and research engineers from the Department of Ecology, Environment and Plant Sciences, while the latter is carried out by researchers from the Department of Environmental Science.

The Baltic Sea Centre provides field station and research vessels for the environmental monitoring.

Read more about the marine environmental monitoring here

Read more about the Marine Ecology Laboratory at the Department of Ecology, Environment and Plant Sciences