Lakes and other water bodies have been an important part of civilisation all around the world. Freshwater sources have been the cardinal for settlements historically, as these settlements required a continuous and dependable source of fresh water for subsistence. Dense network of lake ecosystems were maintained by dedicated group of people and designated communities. Storm water was effectively managed to recharge aquifers and ground water. A purposeful, intricate water system worked as the understructure of civilisations where lakes were linked through a web of canals, connected to surrounding agricultural and wetland landscapes. Water sourced from managed small ponds or tanks were used for drinking, domestic purposes and so on.
However, between then and now, the meaning of water bodies have been interpreted in a myriad of ways. Over time, it seemed lakes were no longer seen as essential for human use as in the modern cities of the 20th century, piped water started being imported from rivers and the value of lakes were lost. So much so, that lakes were particularly perceived as hosts of disease, floods and bad omen. In many cities, lakes were drained and filled to be converted for other land uses. Such practices became prevalent and common practice once the pressures of population growth and the crunch for space for residential settlements increased. The few lakes that continued to exist, started to be used as end points of a city’s metabolism, wherein it began to be filled with sewage and effluents of the surrounding area (by ways of letting toxic storm water runoffs enter the water body and other times as intended endpoints of unfiltered outlets of industries). The drastic change in the processes of lack of care and management of lakes, and the range of disturbances endured by the lakes has resulted in the altered state of equilibrium tending towards degradation for these water bodies. Disruption/fragmentation in the previously connected lake system (water table and canal systems) has pushed the resilience beyond their thresholds, and the capacity of lakes to bounce back to its healthy/balanced form has been severely compromised in most cases. One of the most common outcomes of this specific kind of widespread ecological perturbation is the environmental problem of eutrophication, indicating large scale deterioration in water quality of lakes all over the world. Eutrophication is understood as the process of enrichment by plant nutrients in water bodies. On the hydrological map of the world eutrophication has become the primary water quality issue (Costa-Böddeker, 2012)
The worst perpetrator of nutrient enrichment and the primary cause of eutrophication of water bodies is anthropogenic activity. Sewage, industrial discharges, agricultural runoff, building sites, and urban areas are the most common sources of excess nutrient inputs to water bodies. Eutrophication is generally linked to increased human activity in lake catchments and increased nitrogen and phosphate loading from residential, agricultural, and industrial sources. As a result of land use change in the catchment, water quality decline in urban lakes can be relatively rapid and intensive (Johnes 1999). Shallow lakes are highly vulnerable to eutrophication, and biotic feedback processes are particularly powerful in tropical/subtropical environments, making restoration measures more difficult to implement (Bicudo et al. 2007).
Trophic State Index of a Water Body
The trophic state of a water body is the presentation of the levels and qualities of biologically relevant nutrients like phosphorus, nitrogen that are dissolved in the water. The increase and decrease in the trophic levels are determined by these nutrients’ increase/decrease. The higher the trophic state, the more the plant growth in the body of water. According the Carlson’s index: the trophic state of a water body is defined as the total weight of the biomass contained in it.
Oligotrophic lakes have low productivity due to the low level of dissolved nutrient content. In appearance, these lakes are quite clear and have a scanty algal growth. Such water bodies are potable and supports aquatic species that have high oxygen requirement. Oligotrophic lakes are usually common in cold regions of the world because of low temperatures which do not encourage fast nutrient growth.
Mesotrophic lakes have a relatively intermediate level of productivity, which makes these lakes are translucent with submerged aquatic plants.
Eutrophic lakes have high levels of productivity. Because of the presence of excessive nutrients. the plant growth in such water bodies crosses the carrying capacity, often leading to decrease in lake fauna due to competition for oxygen with living vegetative matter.
Hypereutrophic lakes suffer are degrading lakes with negligible transparency and dense overgrowth of aquatic flora. At this stage, the water body can have dead zones beneath the water surface.
In some cases, the trophic levels of a water body may be natural. But with the variation and intensity of perturbations that impact water bodies in the city, it is evident that most of the rapidly degrading lakes are a result of anthropogenic activities. According to survey data of a 2017 lake study of 25 lakes in China: the proportion of eutrophic lakes in China has surged considerably over the last decade (from 5.0 to 55 percent of lake area examined), whereas the extent of oligotrophic inland waters has reduced (3.2-0.5 percent ). Lakes in far-flung locations like Inner Mongolia and Xinjiang have also been affected. Because most of China has been densely inhabited or has been substantially transformed by millennia of human activity, restoration of eutrophic or hypertrophic lakes is challenging. As a result, flows entering into lakes tend to have large sediment and nutrient loads.
Chance at Restoring City Lakes
Knowledge of an ecosystem’s natural baseline condition prior to disturbance is critical for the development of effective recovery methods, as it allows restoration practitioners to establish a realistic target and assess the success of their restoration efforts (Bennion and Battarbee 2007; Dixit et al. 2007; Bennion et al. 2011). Long-term monitoring data is critical for comprehending the complexities of environmental change over time and space, but they are unfortunately scarce (Battarbee et al. 2005). Lake sediments, on the other hand, can maintain a drainage basin’s environmental history and provide vital information about a lake’s response to exogenous factors.
Reference
Costa-Böddeker, S., Bennion, H., de Jesus, T. A., Albuquerque, A. L. S., Figueira, R. C., & Bicudo, D. D. C. (2012). Paleolimnologically inferred eutrophication of a shallow, tropical, urban reservoir in southeast Brazil. Journal of Paleolimnology, 48(4), 751-766.
Johnes PJ (1999) Understanding lake and catchment history as a tool for integrated lake management. Hydrobiologia 395(396):41–60
Bicudo DC, Fonseca BM, Bini LM, Crossetti LO, Bicudo CEM, Araújo-Jesus T (2007) Undesirable side-effects of water hyacinth control in a shallow tropical reservoir. Freshw Biol 52:1120–1133
Xiang Can Jin (1994) An analysis of lake eutrophication in China, SIL Communications, 1953-1996, 24:1, 207-211, DOI: 10.1080/05384680.1994.11904038
