The roles of pH and cation concentration in apparent phosphate inhibition of stream biofilms

  • Funded By: Notre Dame Environmental Change Initiative

The roles of pH and cation concentration in apparent phosphate inhibition of stream biofilms

Nutrient limitation strongly influences the ability of stream networks to retain nutrients exported from the terrestrial landscape. Increased nutrient limitation makes streams strongly ‘retentive,’ potentially alleviating eutrophication in downstream ecosystems. In headwater streams, which comprise the majority of river length throughout a watershed, nutrient retention is driven by biofilms colonizing every surface of the stream. Because these biofilms control nutrient retention in headwaters, nutrient limitation of biofilms has been a key area of study in stream ecology for years.

Although there are multiple methods used to quantify nutrient limitation of stream biofilms, they all involve elevating the availability of nitrogen and phosphorus and measuring the algal community response. One such method is deploying nutrient diffusing substrata (NDS), which release nutrients into the water column over time through a porous substrate, promoting algal colonization. The NDS technique is inexpensive, relatively easy to implement, and has helped ecologists understand patterns of and controls on biofilm nutrient limitation. Nutrient limitation is assessed by biofilm growth on nutrient amended substrata relative to controls, where increased growth indicates nutrient limitation. However, apparent inhibition of biofilm growth by phosphorus also occurs. This research at ND-LEEF provides insights into mechanisms responsible for apparent phosphorus-inhibition and may have changed interpretations of NDS data in the future.

The team also examined the effects of reduced pH of nitrogen and phosphorus NDS treatments in order to determine the influence of environmental pH on biofilm nutrient limitation, which was crucial to predicting ecosystem responses to global climate change. As CO2 concentrations continue to rise, aquatic ecosystems become acidified, which substantially alters their function. Understanding nutrient limitation in a changing climate is essential to protecting water quality and ecosystem health, with important implications for both
organic matter processing and the global carbon cycle.

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