Each stream reacts differently to the addition of nutrients, said researchers who studied three rivers in the Andes, Mediterranean and Pampas. The effects depend on factors such as initial nutrient levels, local geography, food-web structure and the "window of opportunity" for response to nutrients, for example when light levels and temperatures are high.

"Human activities are resulting in global environmental change," Sergi Sabater of the University of Girona, Spain, told environmentalresearchweb. "One of the most worrying trends is an increase in nutrient contents worldwide, derived from agricultural fertilizers and urban wastewater. High nutrient levels threaten freshwater ecosystems, although their response can depend on climatic and geographic constraints."

Sabater and colleagues from France, Spain, Argentina and Colombia wanted to investigate the long-term effects of small changes in nutrient levels, and how these depend on stream geomorphology or habitat type. With this in mind, they increased the concentrations of nitrogen and phosphorus between 1.6 and 4-fold in three different streams.

The first stream, Fuirosos in the mountains of north-east Spain, experiences a Mediterranean climate and is surrounded by forest. La Choza, meanwhile, runs through the lowland prairies of Buenos Aires province in Argentina, where a temperate continental climate brings wet springs and autumns to the surrounding grassland and agricultural fields. In Colombia's Easter Mountains the Tota stream experiences cold and dry conditions. It drains a basin partly used for agriculture and pasture – the rest is secondary forest and sparse vegetation.

"The three systems had differences in forest cover, nutrient background and food web complexity," said Sabater. "We compared the response of the main biological components – bacteria, algae, invertebrates and fish – to that of upstream control reaches."

Despite having similar nutrient levels before the experiment, the Andean and Mediterranean streams responded differently. The Andean stream showed the strongest biomass response, while the Argentinian stream, where basal nutrient concentrations were already high, showed the weakest effect.

According to Sabater, the climatic and geographic differences between the sites resulted in contrasting “windows of opportunity” for biomass growth. The Andean stream, with favourable light and temperature conditions for most of the year, had the widest window of opportunity, while in the Mediterranean "the response was limited to short temporal windows of light availability" in the spring before leaf growth on the surrounding trees.

"Algae, for instance, only responded in early spring in the Mediterranean stream, whereas in the Andean one they responded throughout the year," said Sabater. The team also found that enrichment stimulated food webs based on algae but not those based on detritus.

"Nutrient enrichment tends to globally enhance the biomass of stream biological assemblages, but its magnitude and reach within the food web depends on environmental factors and on the structure of the ecosystem," said Sabater. "Therefore some systems are more prone than others to suffer consequences from nutrient enrichment."

Sabater believes understanding of the response of organisms and ecosystem functions to pollutants such as organic contaminants is also urgently needed. "Nutrients are not the only hazard that streams and rivers need to face," he said.

Sabater and colleagues reported their work in Environmental Research Letters (ERL).