There are key differences between the processing and flux of carbon, nitrogen and phosphorous in aquatic systems. In aquatic systems, the carbon cycle entails invertebrates leaching and fragmenting the litter only after it enters the stream, becomes lodged, and can be colonized (Chapin et al. 2011). In contrast to carbon, the phosphorous cycle is slower because it is a limiting nutrient only found in rocks that are worn down overtime and leached into the water (Chapin et al. 2011). Alternatively, nitrogen enters aquatic systems from atmospheric deposition, leaching, and sediment erosion (Chapin et al. 2011). Photosynthesis and the transformation of carbon into a version that is transferred throughout the trophic structure still occurs in aquatic systems. The phosphorous cycle is similar to the carbon cycle in that vegetation acquires it and then the phosphorous is transferred throughout the aquatic trophic structure. Unlike carbon or phosphorous, nitrogen is transferred through the system in a variety of forms and oxidation states (Chapin et al. 2011). The differences are significant as we consider the health of aquatic ecosystems.
The imbalance of the three is not healthy for aquatic ecosystems. Groundwater that enters the aquatic system has high CO2 concentrations, showing that much of the CO2 released from aquatic systems stems from terrestrial accumulation of carbon (Chapin et al. 2011). In streams, while carbon can be carried throughout the system by the current, there are large fluxes in carbon metabolism (Chapin et al. 2011). However, excess carbon is a different issue from excess nitrogen and excess phosphorous because carbon is more easily cycled through. So, there is not a delay in the absorption phase which would cause a build-up before being able to continue through the process (Chapin et al. 2011). While excess release of carbon is a major issue, excess presence of unabsorbed nitrogen and phosphorous is a different problem. Nitrogen and phosphorous are introduced into aquatic systems through agricultural and wastewater runoff and industrial debris (Chapin et al. 2011). The excess amounts are offloaded into the water which leads to eutrophication. The imbalance between the carbon, phosphorous and nitrogen results in a decrease in aquatic diversity.
Runoff from farms is a major source of excessive phosphorus and nitrogen entering rivers, lakes, and coastal waters (Carpenter et al. 1998). I would start by explaining to the farm how a high density of livestock is directly related to the nutrient flows in aquatic ecosystems because it creates situations in which manure production exceeds the needs of crops to which the manure is applied (Carpenter et al. 1998). I would go on to convey manure production causes a phosphorous surplus which accumulates in soil as well as transports in runoff to aquatic ecosystems (Carpenter et al. 1998). Further, I would explain how the manure production also creates a nitrogen surplus on agricultural lands which leaches into surface waters or percolates into groundwater (Carpenter et al. 1998). In aquatic ecosystems, over-enrichment with phosphorous and nitrogen causes a wide range of problems, including toxic algal blooms, loss of oxygen, fish kills, loss of seagrass beds and other aquatic vegetation, degradation of coral reefs, and loss of biodiversity including species important to commercial and sport fisheries and shellfish industries (Carpenter et al. 1998). I would emphasize to the farmer over-enrichment seriously degrades marine and freshwater resources which impairs use for further farming purposes and drinking water.
Carpenter, Stephen, Nina F. Caraco, David L. Correll, Robert W. Howarth, Andrew N. Sharply, and Val H. Smith. 1998. “Nonpoint Pollution of Surface Waters with Phosphorus and Nitrogen.” Issues in Ecology 3:1-12.
Chapin III, F. Stuart, Pamela A. Matson, and Peter M. Vitousek. 2011. “Principles of Terrestrial Ecosystem Ecology”, 2nd Ed. New York: Springer.
Response by Professor Fenton Kay:
Good stuff, Mary. Your farmer operates on the Great Plains with nop major rivers or streams nearby. His wastewater runoff spreads out and runs into a nearby gulley. What would you tell this farmer with respect to the potential impacts of his wastewater drainage?
Great question. While a new gully may be narrow and 2 or 3 feet deep, older gullies can grow to enormous size – 40 feet deep and as much as 100 feet from wastewater drainage. Gullies are relentless destroyers of good farmland. They can cut up a field into small parcels. Further, if young livestock go to drink water from the gulley, excess nitrate contained in the wastewater would be harmful because it results in restriction of oxygen transport in the bloodstream. Gullies can also threaten nearby buildings, which may have to be moved before they are undermined. Stopping a gully often requires extensive earthmoving and construction of dams or other measures.
Original Post by Sofia Maldonado:
I find the topic of phytoplankton and their ability to consume and convert carbon dioxide so interesting! Just as the phytoplankton helps control the CO2 in the water and atmosphere, it also depends on the availability of it for its growth (Lindsey and Scott 2010). There can be very hardy species of phytoplankton that can grow in tougher areas such and ones where nitrate concentrations are low (Lindsey and Scott 2011). Their resilience is important because by intaking carbon they also help regulate the temperature, salinity, and trophic structures in the water.
Lindsey, Rebecca, Michon Scott. “What are Phytoplankton?” NASA. July 13, 2010. Accessed October 31, 2020. https://earthobservatory.nasa.gov/features/Phytoplankton
It is fascinating the phytoplankton regulates temperature and salinity while simultaneously being impacted by those factors.
The Arctic Ocean has been experiencing rapid warming, which accelerates sea ice melt. A shorter sea ice season, as a result of earlier sea ice retreat and later freeze-up, leads to increases in the exposure of the sea surface to sunlight and wind stress, which has altered the productivity and phenology of phytoplankton blooms (Sugie et al. 2020). Also, decreased salinity significantly increased the growth of phytoplankton (Sugie et al. 2020).
Sugie, Koji, Amane Fujiwara, Shigeto Nishino, Sohiko Kameyama and Naomi Harada. 2020. “Impacts of Temperature, CO2, and Salinity on Phytoplankton Community Composition in the Western Arctic Ocean”. Frontiers in Marine Science. 6: 1-17.