Bio-dredging
Bio-dredging is a non-invasive, nature-based process that uses bacteria to digest nutrient-rich organic sediment, in order to remove this sediment from eutrophic water bodies. Bacterial digestion of nutrients is a type of Biotechnology that is purported to serve as an eco-friendly alternative to traditional dredging, which has a number of negative impacts on the aquatic environment and associated species,[1][2][3] and other treatments like algaecides and herbicides.
The introduction of a large quantity of bacteria to a pond or lake starts the process as the bacteria feed not only on nutrients in the sediment, but also on decomposing plant material,[4] to remove nutrients before they are available for uptake by vegetative life.
Bio-dredging operations are optimised by complementary processes that de-stratify and re-oxygenate the water column to create benthic zone conditions that are conducive to bacterial digestion. The technique is non-invasive because as the aquatic ecosystem is restored, the bacteria lose the nutrients that are their food source, leading them to die off until they return to normal levels. Bio-dredging is generally available as a service provided by private pond/lake water management companies. Depending on the size of the water body and the number of applications required, it can be expensive.[4] There is also a non-profit organisation[5] in Virginia, USA, that is lobbying for government grants to facilitate more bio-dredging operations to clean up the state's waterways.
The term bio-dredging was also utilised by a Dutch study to refer to the process of sediment remediation via the use of various biological techniques - incorporating microorganisms like fungi and bacteria - to degrade organic pollutants, or chemical techniques to degrade heavy metal pollutants.[6]
The threat posed by the sediment nutrient stockpile
Organic sediments are made up of a variety of nutrients released when different types of biomass starts to decompose. In aquatic environments, this dead biomass is made up of:
- macrophytes
- leaves and vegetation from terrestrial plants
- algae and cyanobacteria
- animal carcasses that have settled to the bottom
Invasive weeds thrive in nutrient-rich organic sediment. The rooting and growth of these weeds spreads along with the spread of this mucky sediment, as the weeds recycle nutrients from the sediment nutrient stockpile to fuel Harmful Algae Blooms (HABs).
As organic sediment accumulates on the bottom of a body of water, eutrophic conditions become self-sustaining via a number of positive feedback loops, without requiring further input of nutrients from external sources.[7] Once this occurs the sediment stockpile may provide enough nutrients to weeds, algae, and cyanobacteria to sustain eutrophic conditions.
Because nutrients are often carried to the sea in rivers, the accumulation of organic sediment is also a major cause of eutrophication in coastal areas like bays and enclosed seas.[8][9]
When mucky, slimy sediment covers the bottom of a lake, it also denies fish suitable spawning grounds. This further constrains animals in the aquatic food web and restricts the food web's capacity to fulfil its function, namely to take up and clear nutrients from the aquatic ecosystem.
Nutrient overloads can be caused by residual nutrients discharged in treated wastewater effluent,[10] agricultural fertilizer runoff and livestock farming waste products,[11] overgrazing and other types of erosion,[12] combined sewer overflows,[13] and untreated or improperly treated sewage,[citation needed] as well as other human impacts like climate change and overfishing.[9] The nutrient overloads are increasingly leading to blooms of invasive weeds and seasonal HABs in source water bodies.[14][15]
A healthy aquatic food web has a hierarchical structure whereby nutrients - in particular phosphorus and nitrogen - are taken up by plant life and algae to produce primary biomass. This conversion of nutrients into biomass is the foundation for progressive levels of animal life. At the bottom of the food web, zooplankton grow by eating primarily algae,[16] thereby producing animal biomass, which is then passed up the levels of the food web as animals from higher levels feed on those beneath.[17] With excess nutrients entering the system and producing vast amounts of primary biomass, animals cannot eat all the plant life before it dies, sinks, and starts to decompose and accumulate in the sediment. The short lifecycles of algae and cyanobacteria, and the use of different lifecycle strategies by various species of cyanobacteria for survival and dispersal[18][19] also contribute to the build-up of nutrients in the sediment once this cycle starts to take hold.
How bio-dredging removes the threat of nutrient accumulation
Bio-dredging applies bacteria to the benthic substrate to clear nutrients biologically. These bacteria digest the nutrients in the organic sediment that promote eutrophic water conditions. By decreasing the volume of organic sediment, bio-dredging also conversely increases the volume of water by increasing the depth profile of the lake or other water body. This removes the anchorage for invasive weeds, while the digestion of the organic sediment helps to return the natural balance of the ecosystem by removing the nutrients supporting the growth of plants, algae and cyanobacteria.[4]
Once organic sediment has been cleared, the animals of the aquatic food web start to return of their own accord, re-establishing the food web's functions so that the system is once more capable of self-regulation.
Quantifying bio-dredging success
Bathymetric scanning accurately measures the depth profile of a water body and enables calculations of the volume of water within that water body. Scans can be run before and after a bio-dredging programme. At the time of the scans being taken, water level reference measurements are also taken. This allows offsets and adjustments to be input to algorithms that can correct for any changes in the water level, calibrate equipment, and calculate water depth and volume.
These measurements allow comparisons to be made so that changes in depth and water volume can be monitored - effectively reflecting the change in the amount of sediment on the bottom of the water body. so that the volume of sediment removed by bio-dredging can be accurately measured.
Results can be presented in a number of ways, including bathymetric charts, changes in water volume and depth profile, and vertical transects.
References
- ↑ Bray, R. N., ed. (2008). Environmental Aspects of Dredging. CRC Press. doi:10.1201/9780203894897. ISBN 978-0-203-89489-7. Search this book on
- ↑ Lu, Jianzhong; Li, Haijun; Chen, Xiaoling; Liang, Dong (2019-11-21). "Numerical Study of Remote Sensed Dredging Impacts on the Suspended Sediment Transport in China's Largest Freshwater Lake". Water. 11 (12): 2449. doi:10.3390/w11122449. ISSN 2073-4441.
- ↑ Aldridge, David C. (2000-10-01). "The impacts of dredging and weed cutting on a population of freshwater mussels (Bivalvia: Unionidae)". Biological Conservation. 95 (3): 247–257. doi:10.1016/S0006-3207(00)00045-8. ISSN 0006-3207.
- ↑ 4.0 4.1 4.2 "Excess Aquatic Vegetation in Your Pond | Northern Virginia Soil and Water Conservation District". www.fairfaxcounty.gov. Retrieved 2021-12-11.
- ↑ "Clean Streams, Rivers and Lakes". cleanstreamsriversandlakes.org. Retrieved 2021-12-11.
- ↑ Ferdinandy-van Vlerken, Marijke M. A. (1998). "Chances for biological techniques in sediment remediation". Water Science and Technology. 37 (6–7): 345–353. doi:10.2166/wst.1998.0771. ISSN 0273-1223.
- ↑ Ni, Zhaokui; Wang, Shengrui; Wang, Yuemin (2016). "Characteristics of bioavailable organic phosphorus in sediment and its contribution to lake eutrophication in China". Environmental Pollution. 219: 537–544. doi:10.1016/j.envpol.2016.05.087. ISSN 0048-9691 Check
|issn=value (help). PMID 27268756. - ↑ Turner, R. Eugene; Rabalais, Nancy N. (1994). "Coastal eutrophication near the Mississippi river delta". Nature. 368 (6472): 619–621. Bibcode:1994Natur.368..619T. doi:10.1038/368619a0. ISSN 0028-0836. Unknown parameter
|s2cid=ignored (help) - ↑ 9.0 9.1 "Climate change worsens effects of nutrient pollution on marine ecosystems". UPI. Retrieved 2021-09-16.
- ↑ US EPA, OW (2013-03-12). "The Sources and Solutions: Wastewater". www.epa.gov. Retrieved 2021-09-16.
- ↑ Reicks, G. W.; Clay, D. E.; Carlson, C. G.; Clay, S. (2008). "Better Management Practices for Improved Profitability and Water Quality". SDSU Extension Fact Sheets. 143.
- ↑ Chen, Xiaofeng; Chuai, Xiaoming; Yang, Liuyan; Zhao, Huiying (2012). "Climatic warming and overgrazing induced the high concentration of organic matter in Lake Hulun, a large shallow eutrophic steppe lake in northern China". Science of the Total Environment. 431: 332–338. Bibcode:2012ScTEn.431..332C. doi:10.1016/j.scitotenv.2012.05.052. ISSN 0048-9697. PMID 22705868.
- ↑ Barone, Laura; Pilotti, Marco; Valerio, Giulia; Balistrocchi, Matteo; Milanesi, Luca; Chapra, Steven C.; Nizzoli, Daniele (2019). "Analysis of the residual nutrient load from a combined sewer system in a watershed of a deep Italian lake". Journal of Hydrology. 571: 202–213. Bibcode:2019JHyd..571..202B. doi:10.1016/j.jhydrol.2019.01.031.
- ↑ Gatz, Laura. Freshwater harmful algal blooms : causes, challenges, and policy considerations. ISBN 2-01-823136-7. OCLC 1057446860. Search this book on
- ↑ US EPA, OW (2013-03-12). "Sources and Solutions". www.epa.gov. Retrieved 2021-09-16.
- ↑ Thorp, James H.; Rogers, D. Christopher (2011), "A Primer on Ecological Relationships among Freshwater Invertebrates", Field Guide to Freshwater Invertebrates of North America, Elsevier, pp. 37–46, doi:10.1016/b978-0-12-381426-5.00026-0, ISBN 9780123814265, retrieved 2021-09-16
- ↑ Gaedke, U. (2021), "Trophic Dynamics and Food Webs in Aquatic Ecosystems", Reference Module in Earth Systems and Environmental Sciences, Elsevier, doi:10.1016/b978-0-12-819166-8.00009-8, ISBN 9780124095489, retrieved 2021-09-16 Unknown parameter
|s2cid=ignored (help) - ↑ "Life Cycle – Harmful Algal Blooms". Retrieved 2021-09-16.
- ↑ Azanza, Rhodora V.; Brosnahan, Michael L.; Anderson, Donald M.; Hense, Inga; Montresor, Marina (2018), Glibert, Patricia M.; Berdalet, Elisa; Burford, Michele A.; Pitcher, Grant C., eds., "The Role of Life Cycle Characteristics in Harmful Algal Bloom Dynamics", Global Ecology and Oceanography of Harmful Algal Blooms, Cham: Springer International Publishing, 232, pp. 133–161, doi:10.1007/978-3-319-70069-4_8, ISBN 978-3-319-70068-7, retrieved 2021-09-16
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