In the past thirty years, the Elbe River water quality has vastly improved. During the eighties, fertilizers, sewage, and pollutants were discharged into the river almost unfiltered. With the collapse of the GDR, at the latest, came a radical change of thought. But does cleaner Elbe water have a positive effect on the climate?
Conceivably: The Elbe is an important transport route towards the North Sea, also for carbon – a crucial climate factor. If the carbon cycle is unbalanced, however, the CO2 release starts to increase with repercussions on the climate. In large cities, such as Hamburg in northern Germany, the human influence on the river plays a major role. For that reason, my colleagues from the Cluster of Excellence CliSAP and I have examined the Elbe estuary, the passage where the river current still interacts with the tide.
Our particular focus was on the interdependent carbon and silicon cycles with minute algae as key agents. In clean water, which is flooded with light, algae absorb CO2 into their biomass, while at the same time extracting dissolved silicon as building material. When dying, the algae sink into the depth, taking the carbon with them. This is relevant to the global CO2 balance because the CO2 is thus removed from the cycle. Moreover, converted into shell particles, the silicon also subsides to the riverbed.
My investigation is based on water data collected by Hamburg city authorities over the past thirty years. In addition, I took water samples and analyzed them. Until now, only the saline section of the lower Elbe onwards from Glückstadt, a small town in Schleswig Holstein, had been examined. Hence, our study is the first to include Hamburg’s fresh water harbor region. We managed to create a detailed picture of the elements in flux there, in spatial as well as in temporal respect, showing where and when large amounts of CO2 are released. Furthermore, we demonstrated that the fresh water region is highly significant: In the vicinity of the metropolis, the largest CO2 exchange between water and atmosphere – in relation to expanse – takes place.
Surprisingly, although the Elbe water quality has improved over the years, adverse effects have occurred in the estuary. In theory, the cleaner water ought to have generated algal growth and an increase in oxygen. Yet the contrary was the case: Veritable oxygen holes in the harbor area have multiplied. Prevalent CO2 highs plus low oxygen levels even cause an increase in fish mortality. How did this come about?
Human interventions, for instance, the deepening of the Elbe, extirpate shallow water areas. Unfortunately, algae cannot survive in deep, dark currents. They die in such environments, consequently being decomposed by bacteria that produce CO2 and use up large amounts of oxygen in the process. In conjunction with dead algae, the silicon is deposited at the bottom of the Elbe. Subsequently, if not excavated with sediments in the continuous efforts to keep the watercourse navigable, it will disintegrate as well. This may explain why less silicon than expected reaches the North Sea.
Our results prove that the Elbe water quality has bettered with time. Nonetheless, human-induced damage, for example, due to excavations of river channels, is still immense.
Author: Dr. Thorben Amann
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