The constant intensification of anthropogenic activity has resulted in nutrient inputs which continuously threaten the health of marine and coastal estuarine ecosystems. An estimated 24% of N (nitrogen) and P (phosphorus) that originate from agriculture and sewage treatment plants are leached into watersheds are estimated to be discharged into coastal ecosystems, increasing their productivity. Malone and Newton (2020) expounds on this further, indicating how these inorganic anthropogenic deposits have caused the ocean to become ‘net autotrophic’ where the primary production of organic carbon exceeds the ‘respiratory metabolism’ of organic carbon (Deininger and Frigstad, 2019 as cited in Malone and Newton, 2020: 2). These increased concentrations of of CO2 have the ability to alter photosynthesis and facilitate the growth of phytoplankton (Schulz et al., 2017).
Hence, excess anthropogenic deposits that are double that of prehuman epochs increase the fertility of coastal ecosystems. Increased productivity thus facilitates the growth of harmful algal blooms (HAB) in a phenomenon termed cultural eutrophication (Cultural Eutrophication, undated). The proliferation of HAB is a destructive phenomenon as it mainly grows on the surface and obstructs light penetration, resulting in hypoxia where little to no oxygen exists in bottom columns of water. Hence, this causes massive fish kills which have damaging ramifications for the food chain and local economies.
To reverse eutrophication and to induce oligotrophication, several governments have released guidelines to fisheries and industries to reduce their nutrient deposits to coastal environments. However, Duarte and Krause-Jensen (2018) argue that these measures are insufficient and require more comprehensive strategies. Therefore, interventions that are not only ecologically sound but enhance nutrient export and dilution should be implemented. This can come in the form of hydrological engineering of coastal lagoons to enhance the inundation of coastal waters. Coastal waters facilitate the growth of mangroves, seagrasses and salt-marshes which are instrumental in removing nutrients through nutrient sequestration. For instance, the incorporation of nature-based solutions such as seaweed aquaculture has proven to be an effective measure as one hectare effectively leached out nitrogen nutrient-inputs of 17.8 hectares and phosphorus nutrient-inputs of 126.7 hectares from Chinese coastal waters. Moreover, they also generate climate change mitigation, ecosystem and economic benefits.
However, it should be noted that research on these interventions still remain insufficient to ensure that they accelerate complete recovery from coastal eutrophication. Moreover, Duarte and Krause-Jensen (2018) assert that these interventions should be implemented with care as eutrophied coastal waters may still be flushed into the lagoons. Moreover, seaweed aquaculture may in turn introduce exotic species, oxygen depletion as a result of organic inputs and the seeding of green tides (ibid.)
Therefore, a more comprehensive eco-system scale approach alongside robust policy action that actively curtail harmful anthropogenic activity should be administered especially with eutrophication rates worsening due to climate change (the topic of our next post!)
Till next time! 🙂
References
https://www.frontiersin.org/articles/10.3389/fmars.2018.00470/full
https://www.frontiersin.org/articles/10.3389/fmars.2020.00670/full
https://www.britannica.com/science/cultural-eutrophication
https://www.frontiersin.org/articles/10.3389/fmars.2017.00064/full