2022 > 05

Text by Antonia Liess (Halmstad University)

Lake Bolmen, the drinking water reservoir for southern Sweden, as well as a source of local fish production and a popular tourist destination, is becoming increasingly browner. Since this trend is expected to continue, it is important to understand the ecosystem consequences of increased brownification as well as potential concurrent nutrient loading. In summer 2021 a mesocosm experiment was conducted in Lake Bolmen using Aquanet mesocosms and the SITES platform and infrastructure at Sydvatten’s Research Station Bolmen in Tiraholm. The aim of this five-week mesocosm experiment was to untangle the effects of increasing brownification and nutrient runoff for the pelagic food web (microbes to zooplankton) of Lake Bolmen. This experiment was a collaboration between Lund University (Johanna Sjöstedt with PhD students) and Halmstad University (Antonia Liess and Lars Gunnar Franzén).

The Lake Bolmen mesocosm platform on a calm day. Photo by Juha Rankinen

At the beginning of June 2021, twenty mesocosms were filled with lake water containing a natural pelagic communality without fish. Five treatments with four replicates each were applied to the mesocosms and comprised of (1) control without any addition, (2) nutrient addition at high N:P ratio, (3) a nutrient addition with low N:P ratio, (4) a brown treatment where soil extract was added and (5) a brown high N:P treatment where soil extract was added as well as nitrogen. Soil extract was produced using dried and homogenized Lake Bolmen shore soils. Soil extract was added as a “pulse” once in the beginning of the experiment to the appropriate mesocosms to produce the state of brownification expected in 50 years. Soil extract also adds phosphorous (P) but very little nitrogen (N) to the mesocosms. Therefore N:P availability was adjusted in the brown high N:P treatment by adding dissolved inorganic nitrogen (DIN) and in the high N:P and low N:P treatment by adding appropriate amounts of soluble reactive phosphorous (SRP) and (DIN).

During the experiment, absorbance, pH, dissolved oxygen, dissolved and total nutrients, dissolved and total organic carbon, bacteria biomass, phytoplankton pigment concentration and composition, zooplankton abundance and species composition were recorded. Brown and brown high N:P treatments had lower pH, and dissolved oxygen, and higher DOC and absorbance than the control, high N:P and low N:P treatments. Phytoplankton pigment composition showed soil addition caused a succession of blooms. Chlorophyll a peaked during the first (week 1) in the brown and the brown high N:P treatment and later (week 4) in high N:P treatment. Early blooms in week 1 were likely driven by mixotrophs such as chrysophytes, cryptophytes, and dinoflagellates as indicated by patterns in chlorophyll c and carotin concentrations. Later blooms in weeks 2 and 4 were driven by green algae, as indicated by patterns in chlorophyll b concentrations. In the control and low N:P treatment, no pigment peaks were observed. Zooplankton composition and abundance showed similar time x treatment interactions, where copepods increase in dominance during week 2 in the brown and brown high N:P treatments and cladocerans dominate in week 4, but only in the brown high N:P treatment. 

Overall, results from this experiment indicate that increased brownification will lead to shifts in phytoplankton community composition from autotrophic to mixotrophic phytoplankton groups, but that these shifts will be easily reversed once the pulse addition of terrestrial subsidies is over. The effects of brownification and nutrient subsidies propagate to the zooplankton levels and influence the relationship between copepods and cladocerans. Cladocerans, which are better food for fish, are disadvantaged shortly after a terrestrial subsidy pulse, a high frequency of these pulses is likely to have consequences for the fish production and fish community composition in Lake Bolmen.

Close up photo of the mesocosms used in Lake Bolmen. Photo by Tatyana Barnes

The bathymetry of a lake is a crucial cornerstone to be able to conduct hydrological and ecological research in lakes. Existing bathymetric data for Lake Bolmen is now more than 30 years old and very limited in detail. A recently purchased echo sounder, which is able to measure the water depth with the help of acoustic waves, will be used in 2022 and 2023 to update current maps. Other than improved resolution for the bathymetric map, the new echo sounder will provide information on submerged aquatic vegetation cover as well as sediment composition in the lake.
The creation of a new bathymetric map for Bolmen will be very time demanding since it is necessary to measure within each little bay. Moreover, driving speed is limited to prevent measurement failures. Currently, the possibility to involve local citizens is discussed, which would significantly reduce the time of completion as well as increase local commitment to ongoing research projects.

Spring this year has been very odd so far - nights have been really cold, days have been warm and sunny, and there has been very little precipitation. One outcome of this strange spring is that staff at Lönnstorp had to start irrigating their apple trees and hedges in the SAFE (SITES Agroecological Field Experiment) agroforestry system a couple of weeks ago, much earlier than usual. The odd spring has also negatively affected the sugar beet trials at Lönnstorp, with low and variable emergence as a result, which has forced them to reestablish trials. Irrigation measures and reestablishments are very time-consuming activities so everyone is hoping for a more “normal” late spring, summer, and autumn.
There are many activities ongoing at the station at the moment. Aside from the preparation and establishment of trials, there have been many visitors. Last week more than 20 students and researchers from Kiel University visited the station. They were, among other things, looking at a new long-term experiment that investigates what effects a combination of different plant protection measures have on the development of grey mold on strawberries. They also had the chance to see experiments that are searching for agricultural traits in several hundred pea and faba bean varieties that could be suitable for Swedish breeding programs, and a trial that focuses on the effect that catch crops have on nutrient leakage. People connected to the NAPERDIV project have also spent a lot of time at the station during the spring, performing several assessments and measurements in the SAFE infrastructure. Students from SLU have had several lectures and exercises at the station, and this week there were visitors from Väderstad AB, a company developing agricultural machines.

Students from Kiel University are studying the soil profile from a field at SITES Lönnstorp research station. Photo: Johannes Albertsson

People using the infrastructures at SITES Lönnstorp have recently published several scientific papers. Three of the papers are about the perennial grain crop (Thinopyrum intermedium) predominantly commercialized as Kernza® and two are about emissions of nitrous oxide from cover crops and crop residues (see below).

The Perennial Grain Crop Thinopyrum intermedium (Host) Barkworth & D.R. Dewey (Kernza™) as an Element in Crop Rotations: A Pilot Study on Termination Strategies and Pre-Crop Effects on a Subsequent Root Vegetable

Agronomic performance, nitrogen acquisition and water-use efficiency of the perennial grain crop Thinopyrum intermedium in a monoculture and intercropped with alfalfa in Scandinavia

Perennial cereal grain cultivation: Implication on soil organic matter and related soil microbial parameters

N2O emissions from decomposing crop residues are strongly linked to their initial soluble fraction and early C mineralization

Frost killed cover crops induced high emissions of nitrous oxide

Is there life on Jupiter's moon Europa? An ocean suspected to be under miles of ice gives science hope. But how could it be reached and explored?

In March, a six-person research group from the Robotics Innovation Center of the German Research Center of Artificial Intelligence (DFKI) spent two weeks at Abisko Scientific Research Station. 
Their goal was to test an autonomous underwater vehicle (AUV) beneath the ice of lake Torneträsk. The researchers are working on developing a robot that can independently explore the water and stay below the surface for an extended period of time. The technology will be used to explore the subsurface ocean on Jupiter's moon, Europa.

The AUV outside Abisko Scientific Research Station. Photo: DFKI-EurEx-Team

A primary objective of the field trials was to test the AUVs docking behaviour with different levels of autonomy, and the AUV spent about 50 hours actively in the water. The docking station will serve as a "home base" for the AUV during long-term missions, and this is where it can recharge and transmit data. The team looks forward to returning to Abisko in a few years to expand their tests on autonomous under-ice navigation.

You can read more about the project and find cool underwater photos and videos from the AVU on the German Research Center for Artificial Intelligence website

The AUV was deployed through a hole in the ice. Photo: DFKI-EurEx-Team

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