My name is Sonia and I study greenhouse gas (GHG) evasion from river reservoirs. My study interest led me to this year’s Students for Rivers camp, which took place on the beautiful Soča River in Slovenia. It was a great pleasure to participate and give a talk about the biogeochemistry of reservoirs. Now, I am glad to share the current scientific knowledge about the biogeochemistry of reservoirs, while increasing awareness of how rivers are actual Earth arteries that should be preserved. I really have a lot of information to share with you, so I’ve decided to write two blog posts. This post will be about the biogeochemical cycle of rivers and how this cycle is affected by dams.
Importance of rivers in nutrient cycles
Rivers can be seen as water conduits that transport organic and inorganic material from the headwaters to the ocean. These materials come from the surroundings of the river along its path. Along the river, microorganisms, plants, and animals transform these materials. Rivers redistribute nutrients coming from inland (leaves and soils) to aquatic systems all the way to the ocean, and they give nutrients back to animals and plants on land through animals that move between rivers and terrestrial systems or with floods. The main three nutrients for plants and microorganisms in rivers are Carbon, Nitrogen, and Phosphorous. In freshwaters, Phosphorous is usually limited and determines the growth and Phosphorous inputs can increase the growth of algae and aquatic plants disproportionally.
Effects of damming on biogeochemical cycles
Damming is one of the biggest threats to rivers worldwide. Dams are barriers for many fishes as they prevent them from migrating and therefore these fishes cannot reproduce. But dams also act as barriers for nutrient cycling. Up to 12% of the phosphorous  that should be transported to the ocean is retained in dams. Also, some dams retain 22% of the nitrogen of their rivers . This leads to the accumulation of nutrients in the reservoirs. The increase in nutrients now triggers algae growth, especially on the surface. Rivers normally have most of their microorganisms in the top layer of sediments and rocks. The accumulation of water in the reservoir allows that microorganisms grow in the water column, as they do in lakes. The accumulation of water also leads to a temperature difference between the warm surface water and a cold bottom.
The excessive growth of algae is commonly known as algae blooms. Algae on the surface use sunlight and do photosynthesis (taking dissolved carbon dioxide (CO2) and producing oxygen). Nevertheless, in the absence of light, algae together with all the other microorganisms and animals also respire oxygen and produce CO2. This causes a negative effect: The reduction of oxygen in the deeper water layers of the reservoir, where photosynthesis cannot counteract respiration, as there is no light. All the microorganisms, plants and animals sink down to the bottom of the reservoir once they die and with time form a layer of organic sediment of the reservoir.
The dam is also a barrier for pieces of wood and other organic materials, which sink and accumulate, creating a highly organic sediment bed in the reservoir. The organic sediments and lack of oxygen create the habitat for anaerobic microorganisms that are able to respire other molecules than oxygen. Among the anaerobic organisms are bacteria and archaea that produce nitrous oxide (N2O) and methane (CH4).
– Sonia Herrero
References – links to main scientific articles used to write the blog
 Maavara, T., Parsons, C.T., Ridenour, C., Stojanovic, S., Dürr, H.H., Powley, H.R. &
Van Cappellen, P. (2015). Global phosphorus retention by river damming. PNAS, 112 (51) 15603-15608. doi:10.1073/pnas.1511797112
 Schoch, A. L., Schilling, K. E., & Chan, K.-S. (2009). Time-series modeling of reservoir effects on river nitrate concentrations.Adv. Water Resour., 32(8), 1197–1205. doi:10.1016/j.advwatres.2009.04.002