Temporal variation in ecosystem metabolism in relation to water quality in the Piako River

Report: TR 2015/04

Authors: Joanne Clapcott and Kati Doehring (Cawthron Institute).

About this report

Rates of ecosystem metabolism can be used to provide a functional assessment of stream health to accompany more traditional structural measures such as water quality and biological indicators. Ecosystem metabolism varies temporally driven by daily, seasonal and annual variation in the primary drivers of light, temperature, nutrients and stream discharge. As such, an increased understanding of the temporal trends in ecosystem metabolism can help determine the most appropriate temporal scale to assess stream health.

We explored the temporal and spatial variation in the ecosystem metabolism components of gross primary production (GPP) and ecosystem respiration (ER) in the Piako River (near Morrinsville, Waikato) in relation to measures of water quality. Ecosystem metabolism was estimated from continuous dissolved oxygen data collected from six sites on the Piako River. Monthly estimates from October 2012 to September 2013 were compared to monthly spot measures of water quality.

The Piako River was a net consumer of carbon throughout the year with productivity to respiration (P/R) ratios averaging 0.7. Rates of ecosystem metabolism reflected good to poor stream health longitudinally down the catchment and varying throughout the year. A small upland site with low land-use impacts, Piakonui, had consistently low levels of metabolism. In contrast, downstream sites subject to greater than 85% pasture catchments had increased metabolism, particularly during months of warmer temperatures and stable flow.

Higher rates of average annual metabolism occurred at larger stream sites with macrophyte beds and where there was higher water clarity, lower turbidity, higher conductivity and nutrient concentrations. Monthly metabolism estimates were most strongly related to water quality measured during the same calendar month, even when water quality measurements were made after metabolism estimates. On average water quality measurements were made four days before metabolism estimates. Fewer significant correlations were observed between metabolism estimates and water quality measured the month before (on average 33 days before), and between metabolism and the three-month rolling mean water quality value. A consistent positive relationship between ecosystem metabolism and conductivity regardless of time period suggests conductivity is an indicator of persistent and cumulative land-use impacts at each site; especially given that within each site there appeared to be a negative correlation over time. Similarly, the relationship between GPP and E.coli is more likely to be a correlative rather than causative, with both indicators showing poor stream health in the Piako River.

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