Human Impacts on Stream Microbial Ecology: Urbanization and Agriculture

Human activities profoundly alter stream ecosystems, modifying their physiochemical conditions and, consequently, their microbial ecology. These impacts are often multifaceted, such as a channelized stream receiving multiple pollutant sources, which complicates the discernment of cause and effect. This section highlights selected, well-studied anthropogenic impacts, including urbanization and agriculture, that significantly disrupt microbial communities and their functions. While some topics, like acid mine drainage, are extensively documented, the following provides a concise overview of key mechanisms and consequences.

Urbanization and the Urban Stream Syndrome. Urbanization impacts watersheds through direct pathways, such as effluent from Wastewater Treatment Plants (WWTPs), and indirect pathways, including increased impervious surface cover. These changes manifest as the Urban Stream Syndrome, a well-documented condition characterized by altered stream channels, degraded riparian zones, and altered hydrology due to wastewater and stormwater inputs. Urban streams typically exhibit degraded water quality, elevated temperatures, flashier hydrology, and shifted organic matter sources. Furthermore, the burial or "daylighting" of streams creates unique ecological circumstances, and complex hydrologic interfaces connect streams to infrastructure like sewer systems.

Studies specifically focused on the microbial ecology of urban streams are less common than those on biogeochemistry. However, research indicates that urbanization significantly alters bacterial community composition and the activity of extracellular enzymes. Key ecological questions for urban streams, as outlined by Wenger et al., directly involve microbial processes. These include understanding the paradox of accelerated leaf decomposition and the capacity of the microbial nitrogen cycle to process variable, high nutrient loads that can overwhelm the ecosystem's retention capacity.

The Role of Wastewater Treatment Plants (WWTPs). Wastewater Treatment Plants (WWTPs) are point sources that introduce a complex mixture of contaminants into receiving waters, including nutrients, pharmaceuticals, metals, microplastics, and microbes. The problem is exacerbated by Combined Sewer Overflows (CSOs), which release untreated sewage during heavy rain. While the impacts on general stream biota are well-known, the specific effects on native microbial ecology are a growing field of study.

Many studies focus on introduced enteric bacteria and the spread of Antibiotic Resistance Genes (ARGs). However, research shows WWTP effluent also reshapes native stream microbial communities. For instance, studies have documented shifts in bacterial community composition and the abundance of key nitrogen-cycle genes downstream of WWTP outfalls. Furthermore, experimental work by Rosi-Marshall et al. demonstrated that pharmaceuticals from WWTPs can reduce bacterial respiration in biofilms by up to 90% and alter community structure. The accumulation of ARGs downstream of WWTPs poses a significant public health concern.

Microplastics represent a unique pollutant from WWTPs that serves as a novel microbial habitat. Research in streams, though limited, shows that microplastics are abundant downstream of WWTPs and are colonized by distinct bacterial communities with lower diversity compared to those in the surrounding water or natural substrates. This creates new ecological niches and potential pathways for microbial dispersal and interaction.

Agricultural Impacts on Stream Ecosystems. Agriculture impacts streams through hydrological modifications and non-point source pollution, primarily from excess nutrients and pesticides. The effects on key biogeochemical processes like denitrification are clear; agriculturally impacted streams often have high nitrate concentrations that drive elevated denitrification rates. While much attention is paid to microbial inputs from livestock, cultivation practices also fundamentally alter the stream ecosystem.

Agricultural runoff delivers complex pollution, including organic matter, fertilizers, and pesticides, with the nature and intensity of impact varying seasonally. Case studies from the US Midwest, such as the Sugar Creek and Leary Weber Ditch watersheds in Indiana, illustrate these dynamics. These systems, dominated by row crops, show how hydrology controls microbial function. Researchers found that denitrification rates correlated clearly with hydrological patterns, but the underlying denitrifier community structure showed no such clear relationship, suggesting functional redundancy within the microbial community.

Pesticides have diverse and compound-specific effects on stream microorganisms, as reviewed by DeLorenzo et al. These impacts depend on the pesticide's chemical structure and its physiological target within the microbial cell. Current understanding is limited by the narrow range of microbial species tested, and expanding this scope is crucial to fully assessing the ecological consequences of agricultural pesticides on stream ecosystem function.