The effect of tDOM and nutrients pulses with different intensity and frequency on bacterial production, respiration and nutrient limitation – A mesocosm study in lake Bolmen, Sweden

Abstract

Lakes with increased water colour levels have been observed in many locations in the northern hemisphere, a phenomenon that is called brownification. The driving components of brownification are dissolved organic matter (DOM) and iron, which are affected by rainfall patterns as a large fraction originates from terrestrial sources. Brownification has many detrimental effects for the ecosystem, among which increased costs for drinking water production. Lake Bolmen, southwestern Sweden, is a drinking water source for nearly 600.000 citizens in Skåne. This study simulated three run-off scenarios with DOM and nutrients pulses of different intensity and frequency, and specifically looked at the effect on bacterial production, respiration and nutrient limitation. For this, a mesocosm experiment was executed on lake Bolmen during April and May 2023. The study found that different precipitation patterns do not affect bacterial production and respiration differently, but the addition of DOM and nutrients increased bacterial production and respiration. However, after a two-week recovery period the bacterial respiration dropped and bacterial production increased significantly. Moreover, bacteria were phosphorus limited at the start of the experiment but their response to P addition disappeared as soon as DOM and nutrients were added. A possible reason for the lack of differentiation between the treatment responses could be that lake Bolmen has already been suffering from brownification since the 1980’s and the bacteria are therefore more tolerant to DOM additions.

Popular Abstract

A lake as brown as tea

Have you ever seen a lake in which the water almost looks like black tea, clear but definitely very brown? This phenomenon is called brownification and it means that there is a lot of coloured dissolved organic matter present in the water. This brown colour is not only not nice to look at or unpleasant to swim in, it also holds negative consequences for the aquatic ecosystem. Visual predators such as pike have a harder time hunting for food and since darker coloured water holds heat better, the water temperature goes up. This may sound quite nice, especially in colder countries such as Sweden, but it has a big effect on the (micro)biological processes going on in the lake. There are several causes for brownification, for example higher temperatures and rainfall levels because of climate change and changes in land-use from agriculture to forestry.

In this study I looked at the effect different rainfall patterns have on bacterial production, respiration and nutrient limitation in lake Bolmen in southwestern Sweden. Bacterial production is the new biomass produced by bacteria. Bacterial respiration is the “breathing” of bacteria, they take up organic carbon and oxygen and “breathe out” CO2. Bacteria need nutrients to grow, specifically nitrogen and phosphorus. If one of these is not sufficiently available in the water, it limits the extent to which bacteria are able to grow. When we know which nutrient is limiting the bacteria in a lake, we can predict what will happen if these circumstances change. To study all these things, we set up a mesocosm experiment in Lake Bolmen. The mesocosms are big containers which we suspended from a platform in the lake. We filled them up with lake water and over 6 weeks during April and May 2023 we added dissolved organic matter (DOM) and nutrients in different amounts and at different frequencies. During this period we took water samples and analysed them in the lab for bacterial production and respiration, we also investigated if they were still nutrient limited.

We found that DOM and nutrient additions stimulate bacterial production and respiration but it does not really seem matter how the DOM gets added to the ecosystem. So the different rainfall patterns do not seem to have an effect on bacterial production and respiration. We also saw that in the beginning, before we added any nutrients, the bacteria were limited by phosphorus, meaning that the extra addition of phosphorus boosted bacterial production. However, after any DOM and nutrient additions were made, this limitation disappeared. All of this makes more sense when put into a bigger context. It seems that lakes which are already brown do not experience much change from different DOM pulses. So whether the rainfall patterns change and become more extreme, this should not make much difference for already brown lakes. But there are also lakes that are still clear, and they might react quite differently to varying rainfall patterns. More studies will follow that explore the effect of DOM and nutrients on clear lakes.

Master’s Degree Project in Biology, 30 ECTS, 2023

Supervisors: Johanna Sjöstedt and Kevin Jones
Department of Biology, unit Aquatic Ecology, Lund University

• author: van Dam, Eline
• supervisor: Johanna Sjöstedt; Kevin Jones
• organization: Degree Projects in Biology
• course: BIOM02 20231
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9145089
• date added to LUP: 2024-01-15 16:58:42
• date last changed: 2024-01-15 16:58:42