The researchers studied the nutrient fluxes and biotic communities in a small pond and its main tributary before and after heavy rain. Along the relatively large contact area between land and water, large amounts of organic matter were washed into the system. Thus, the nutrient levels initially increased a hundredfold. The rain reduced the residence time of the water in the pond, changed inflows and outflows and thus also the composition of the entire community of organisms.
Collapse is followed by rapid recovery
The researchers described in detail for the first time the adaptation cycle with four clearly defined phases that followed the heavy rain: In the first phase of “collapse and release”, mass effects dominate: the resident microorganisms in the lake are washed out and new microorganisms from the surrounding area are washed into the pond. This phase is followed by the second phase of “reorganization”, in which specific organisms establish in the system due to the changed environmental conditions. This is where it is decided whether the system will take an alternative steady state or fall back into its old state. The path back is initiated by a third phase of “growth and exploitation” of the available resources. In the fourth phase of “conservation”, the retention time of the water is prolonged and dissolved reactive nutrients are intensively converted by microorganisms. In this phase, the organism community typical for the system is almost completely re-established.
In this study, the first three phases were repeated after another heavy rain – a sign of the high resilience of the system. When and whether an alternative stable state is reached in the reorganisation phase depends on the frequency and strength of the extreme events. Therefore, it is important to know future scenarios and the organisms' responses to them.
Rapid recovery is good for the nutrient balance of the landscape
Nutrient loads returned to their original levels particularly quickly – after only 9 days. This shows how efficiently aquatic microorganisms metabolised the nutrients. The original microbial communities with prokaryotes and eukaryotes were restored after 16 days; due to their longer generation times, the higher phyto- and zooplankton needed somewhat longer.
"We were surprised at the high resilience of the small but complex pond system – we would not have expected such short recovery times. Our results show the important, previously underestimated role of small water bodies and their microbial community for the nutrient balance in the landscape. This is because the rapid nutrient turnover within the water body prevents large amounts of terrestrial nutrients from entering nearby streams. This helps with freshwater management. However, we do not yet know how the increase in such extreme events in climate change will change this resilience. With our detailed study, we have now laid the necessary foundation for this," summarises Professor Hans-Peter Grossart.
The sampling was not a four-phase adaptation cycle but rather a drama in four acts (with a good outcome), as first author Tanja Shabarova describes in Behind the paper >
In larger and deeper freshwaters, the microbial communities also show resilience to heavy rain events. This is the conclusion of another research team with Hans-Peter Grossart at Lake Feeagh in Ireland. Read this article Open Access in Water >