Unfortunately, urban lakes that in the past were valued and used by the population for fishing, swimming or boating, due to their inadequate management, have seen the services they provided curtailed by markedly deteriorating the quality of their waters, to the point that they were limited in many cases to just a scenic use, even prohibiting contact with the water.[30].
Urban lakes are threatened by various threats or disturbances that impact both their recreational qualities and their biological or educational value, due to their own location and their morphometric features, which make them very vulnerable to anthropogenic aggression.
The greatest threats are the entry of toxins, nutrients and the excessive increase in the physical-chemical parameters of its waters, which creates hostile conditions for the development of the aquatic ecosystem itself. This may be due to different causes.
It may be related to a low or non-existent degree of water evacuation, which results in a slow accumulation of variables, exacerbated by: feeding by pumping from underground layers (with harder waters), the incorrect isolation of the washing - which produces rainwater - of the streets and vehicle parking areas around it (carrying away the waste generated by the vehicle fleet - fuels, lubricants, battery fluids or coolants, precipitated soot, etc.—),[31] the presence of sewage drains (from homes, recreational facilities, public toilets around them, etc.), the lack of waste baskets or their non-use (with the consequent arrival of garbage directly or by the action of the wind in the water), smog and any other atmospheric pollution that arrives through the wind from points far from the city, etc. In the case of urban lakes positioned on river courses, the quality of their water is also determined by the actions carried out in the portion of their basin located upstream; among the most harmful are the discharge of sewage or industrial wastewater, the intensive or incorrect use of agrochemicals, the discharge of intensive swine, poultry or livestock production, etc.[3].
The structure of the urban lake itself can influence the degree of dissipation of physicochemical imbalances. A deeper central area allows the contained volume to be increased, which helps to mitigate the thermal rise of the summer; On the contrary, if the aquatic body is very shallow, the sun will heat the entire volume more quickly, causing the water to reach temperature levels that cause a dangerous drop in oxygen levels, with the consequent mortality of the most sensitive fish species. The air currents move the surface of the mirror, which improves the internal circulation in the water column, its oxygen content and the release of heat, something that is diminished if the orientations of the prevailing winds in the summer season are densely forested. To improve the conditions of the aquatic body, a more artificial, but increasingly common, intervention is the installation of one or more fountains within the mirror itself; The water is taken from the lake and, pumping it, it is thrown several meters above the surface, making it fall in the form of simulated rain, already highly oxygenated, a fall that at the same time produces the movement of the surface, which is also very positive.[32][33].
If the arrival of available nutrients (mainly nitrogen and phosphorus) is high, the biotope is eutrophic. Animal feces or leaf litter from the trees on its margins (plus that which is transported by the wind from other sectors of the park) are usually some of the causes of the eutrophication of these aquatic bodies.[9] Hence, privileging the planting of evergreen tree essences over deciduous ones, reduces the annual load of leaves that reach the basin.[31] An adequate intervention of the terrestrial areas of the perilago allows minimize the arrival of nutrients to it by stopping the leaves and garbage that would arrive carried by the wind and runoff, for example, raising the soil on the edges of the lake with a topographic redesign, placing dense mesh fences around it, installing trellises and mud retention chambers in the stormwater inlets, creating restricted access areas with parallel strips of shrubs - on the coastal edge - and marsh vegetation - in the immediate sectors of shallow waters - (prioritizing native species of the region), etc.[3].
The levels of nutrients accumulate and intensify, so, in addition to generating unpleasant odors that give it a negative reputation among the general public, the appropriate conditions will be given for the uncontrolled growth of aquatic macrophytes,[4] along with the triggering of outbreaks of dangerous algal blooms,[34] due to the excessive proliferation of phytoplanktonic algae and cyanobacteria, which produce toxins that are very dangerous for animals and plants. people.[8][15][35][36].
If the urban lake has waters in a eutrophic state, very good levels of transparency and excess macrophytes, the usual intervention is the reduction of these by mechanical means - with periodic harvesting - or through the use of biocidal chemicals - algaecides and herbicides.[37][3].
For urban lakes to have an environmental quality that satisfies all their potential users, it is mandatory that a correct diagnosis of their problems and threats be carried out on them, in addition to remediation techniques and management policies, carried out by professionals. External management techniques (those applicable to the perilago surfaces) are less expensive and simpler and more durable than internal ones (interventions on the water itself and the bed), but in emergency cases both must be applied at the same time, for example, in lakes with turbid, green, hypereutrophic waters.[38].
Among the internal interventions is the cleaning of the surface of the lake (thus intercepting the garbage, leaves or branches that would otherwise precipitate to the bottom) and, especially, the dredging of the organic and inorganic garbage accumulated in the sediments of the bed or resting on it, using the wet technique (without drying the lake) or dry, that is, completely removing the liquid volume contained in the lake, which also allows the disposal of already eutrophicated waters and the incorporation of new waters.[28] This is a Proceeding is catastrophic with regard to the aquatic biota, but this damage can be minimized if the lake contains at its bottom (from coast to coast on one of its margins or in a small interior area in the shape of a basin) a raised hill or cement wall that, when the water level drops, causes a fraction of it to remain isolated, which would thus conserve the water and maintain the biocenosis, while the remaining part is heavily intervened.[12] If structurally it does not contain that overelevation, the temporary installation of any type of barrier can perform the same function. In parallel to this, sealing or waterproofing of the glass can be carried out.[27]
Another way it is done is to capture specimens of each species of fish (and other animals) and then move them to a temporary site until the pond water is replenished.[27][28].
Unfortunately, both dredging and mechanical harvesting tend to be the techniques most used by municipal administrations, since they produce a greater impact on the electorate than less visible methods, but more effective and beneficial for the biota.[3].
Other actions are the application of biomanipulation techniques, for example, by introducing species that fulfill a niche that is not well covered by the existing biota but that will be beneficial for the health of the biotope.[3] An example of this is the release of specimens of the so-called grass carp (Ctenopharyngodon idella) to control the profusion of macrophytes.[39].
Another option is the incorporation of native fish species with piscivorous habits so that they prey on species with omnivorous-planktivorous trophic habits, in order to reduce the pressure on the zooplankton and which can keep the level of herbivory on the phytoplankton high and thus clarify the water column; this may be accompanied by the selective removal of this last guild of fish.[40][41].