Small freshwater bodies are supercharged ecosystems with regards to carbon turnover within a landscape. They often form the lowest point of catchments and, due to their high surface area to volume ratios, they receive significant external carbon and nutrient input through leaf litter, surface runoff, fertilizer seepage and other anthropogenic effluents.
Consequently, such small, shallow systems are often highly rich in nutrients (eutrophic) and often boast high production rates. They are also extremely abundant. Aquatic systems of a surface area less than 0.1 km2 compose about 20% of the global lake surface area. Yet, despite all this, for a long time they’ve been neglected in global carbon budget calculations and greenhouse gas emission rates, but researchers have recently started looking closely into the processes that occur within these systems.
Most recently, a project called Landscales, a joint collaboration of researchers from the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) and Leibniz Centre for Agricultural Landscape Research (ZALF), looked into the processes and pathways of carbon sequestration in small ponds in Germany, otherwise known as kettle holes (or pot holes). The aim of the project was to calculate the primary production rates of these systems and compare them to that of larger aquatic bodies. Furthermore, the researchers investigated how the different groups of producers affected the overall fate of the sequestered carbon in an effort to understand whether these systems acted as carbon sinks (storing excess carbon in their sediments) or carbon sources (processing organic carbon and releasing it back to the atmosphere in gaseous form as carbon dioxide (CO2) or methane (CH4)).
However, the study was not bereft of major challenges. These kettle holes function exactly as bigger lakes don’t. Essentially, they are highly unpredictable. Given their small volume, they are susceptible to sudden changes in weather conditions and external loading, thus exhibit strong water-level fluctuations and are even prone to drying up completely in very warm or dry summers.
“The biggest challenge was finding out the right methods to measure the productivity of these kettle holes,” said Garabet Kazanjian, lead author of the study. The most common way to estimate aquatic ecosystem production: the oxygen diel technique developed by Odom in 1956, based on recording day-night oxygen fluctuations to calculate daytime production and night-time respiration rates, was inapplicable in this study as the kettle holes were characterized by long periods of anoxia. Kazanjian added:
Instead, we had to sample all the primary producer groups (aquatic plants, algae, and phytoplankton) and calculate their production rates separately using pulse-amplitude modulated fluorescence techniques, a much more time-consuming, intensive effort.
But the results were rewarding. Overall, summer gross primary production (GPP) rates in the kettle holes were found to be quite high, comparable to the most productive aquatic systems in similar temperate regions. Macrophytes (aquatic plants), in particular, emergent ones, had the greatest contribution to the overall system production during the warmer months. In winter, the dropping temperatures hastened the senescence of the macrophytes, the overall system GPP dropped significantly, wherein the majority of the system’s production came from periphyton (the complex mixture of algae, cyanobacteria, and bacteria attached to the submerged surfaces).
The authors also found a strong correlation between carbon sediment deposition and primary production rates during summer and autumn. This indicates that, contrary to initial expectations, the majority of the carbon in the sediments (which potentially gets buried there) are from internal (autochthonous) and not external (allochthonous) sources, at least during the seasons the measurements were made. The high sediment deposition rates, along with the prevailing anoxic conditions in the water column that limit sediment mineralization, create a high potential for carbon burial, thereby removing and locking the carbon from likely returning back to the atmosphere as CO2.
However, on the occasion that a kettle hole starts drying up, mineralization rates might increase due to the sudden abundance of oxygen, leading to the partial loss of the buried carbon. With projected lower future rainfall in the region (and a higher frequency of drying up), the carbon burial potential of the kettle holes may sharply decrease.
This study highlighted how unique and interesting kettle holes can be in understanding complex carbon-cycling processes. Due to their characteristic high primary production rates and unique features, it would be erroneous to automatically group them with other freshwater systems in global carbon budget estimates or ignore them altogether. On the contrary, more efforts should be made to corroborate these results and to further dig deep into the processes of these shallow waters.
This study, Primary production in nutrient-rich kettle holes and consequences for nutrient and carbon cycling was recently published in the journal Hydrobiologia. This work was led by Garabet Kazanjian from the Humboldt University of Berlin.