Resumen de The physical control of contaminant distribution in aquatic ecosystems
Water contamination endangers aquatic ecosystems worldwide. The contamination of aquatic ecosystems impairs the system's functioning and its use as a water source. The spread of contaminants within a system is possible due to dispersal, i.e. the transport of a substance or organism by the movements of the ambient water. The knowledge and understanding of these movements is necessary to determine the contaminant's final distribution, its concentration, its fate, and, thus, the potential impacts on ecosystems and human health. Water currents and mixing can be determined mathematically by resolving the advection-diffusion equation. Hydrodynamic transport model have been applied for the study of contaminant transport in coastal systems, rivers, reservoirs and lakes. In lakes, currents are largely forced by winds, waves and convective processes, and are characterized by low magnitude as well as high temporal and spatial variability. However, little is known to date about the distribution of contaminants by wind-driven lake currents.
The objective of this thesis is to analyze the passive dispersal of living contaminants (organisms) by wind-driven lake currents. For this purpose, a mechanistic individual-based model grounded on the reaction-advection-diffusion equation has been developed. The model consists of three modules (Release-Transport-Survival) that represent the basic processes of contaminant dispersal: (R) the incorporation of a given contaminant into the water column through entrainment or shedding, (T) passive transport and dispersal, and (S) contaminant survival (or inactivation) in function of environmental conditions endured during transport. The particular aim is to characterize the mechanisms of dispersal, the temporal and spatial variations of the pathways, and the contaminant's final distribution. The dispersal model has been applied to study the distribution of two contaminants, (i) an invasive bivalve, Asian clam (Corbicula fluminea) and (ii) a human water-borne pathogen (Cryptosporidium parvum), in a deep, alpine lake surrounded by a complex topography - Lake Tahoe, USA.
Asian clam is among the most aggressive freshwater invaders worldwide. Passive (natural) hydraulic transport by water currents is considered to be the main mechanism for the local dispersal of Asian clam. The local dispersal largely occurs during the larval and juvenile stages of their life when, as a result of their low density, larvae may remain suspended in the surface mixed layer even under minimal turbulence. Larvae are not motile, but can travel long distances drifting with water currents. The contribution of currents in the local dispersal of Asian clam larvae, however, is not known. Laboratory studies on the dispersal of Asian clam by water currents have focused on the transport of adults. The local dispersal of planktonic larvae of Asian clam by passive hydraulic transport has been evaluated in Lake Tahoe. The probability of dispersal of Asian clam larvae from the existing high density populations to novel habitats is determined by the wind regime, the magnitude of wind events and their timing. Larvae dispersal and migration occurs in form of pulses in response to episodic events of strong wind forcing. Dispersal of Asian clam occurs along a discrete number of preferential pathways. The impact of ultraviolet radiation during the pelagic stages on Asian clam mortality is low as a result of the short dispersal distances associated with relatively high larval settling velocity and the predominantly weak winds observed. The final distribution of dispersed larvae and the probability of their survival are sensitive to the larval settling velocity. Flow inside bays tends to exhibit recirculating currents, likely as the results of flow separation at the bay boundary. Bays that are characterized by low current velocities and re-circulation act as traps for suspended benthic larvae.
Pathogen contamination of aquatic ecosystems is a severe threats to human health worldwide.
The protozoan parasite Cryptosporidium, or rather its oocysts, is widespread in lakes and reservoirs, even in developed countries. Only in the US, the presence of Cryptosporidium parvum is estimated to 55\% of surface waters and 17\% of drinking water supplies. Cryptosporidium poses a problem to water treatment as it is highly resistant to conventional methods of disinfection. The small size and omnipresence of its oocysts has caused many health outbreaks in drinking as well as recreational waters. To the extent of the author's knowledge, no quantitative studies exist on the risk of pathogens entering drinking water intakes of lakes or reservoirs resolving the nearshore circulation and taking into account temperature as well as light inactivation. The specific aim is to assess the risk of infectious human water-borne pathogens, focusing on the example of Cryptosporidium, released at recreational beaches entering a sample water intake of Lake Tahoe. The risk of contamination by Cryptosporidium is analyzed using a modified version of the dispersal model, including a novel technique of back-tracking pathogens from the point of concern (water intake) toward the source regions (recreational beaches). This technique allows studying non-point contaminations at a modest computational cost and identifying time periods of potential drinking water contamination. For the case of Cryptosporidium dispersal in Lake Tahoe, the results reveal that for this particular lake (1) the risk of human water-borne pathogens entering drinking water intakes is low, but significant; (2) this risk is strongly related by the depth of the thermocline in relation to the depth of the intake; (3) the risk of increases with the seasonal deepening of the surface mixed layer; and (4) the risk increases at the night when the surface mixed layer deepens through convective mixing and inactivation by ultraviolet radiation is eliminated.
