"The current disregard of ecosystem processes is akin to assessing the performance of a car without checking whether the engine runs and without performing a road test," Mark Gessner of EAWAG (the Swiss Federal Institute of Aquatic Science and Technology), ETH Zürich, IBG (Leibniz Institute of Freshwater Ecology and Inland Fisheries) and TU Berlin, Germany told environmentalresearchweb. "It is the fundamental argument that ecosystems need to be assessed in a comprehensive manner that, above all, led us to evaluate the usefulness of litter breakdown as an indicator of stream ecosystem health."
As Gessner explained, ecosystem processes are not included in environmental legislation such as the EU Water Framework Directive. "Ecosystem processes have been almost entirely ignored in stream bioassessment, in spite of – or perhaps because of – a long successful tradition of stream bioassessment for over 100 years," he said.
To carry out the study, Gessner and colleagues placed leaves of of pedunculate oak (Quercus robur L.) and black alder (Alnus glutinosa [L] Gaertn.) into streams in litter bags made with either fine or coarse mesh. Oak leaves tend to decompose slowly whereas black alder rots faster.
"Given the vital importance of leaf litter for stream ecosystem functioning and the ease with which breakdown rates can be determined in the field, we were struck that litter breakdown (or any other ecosystem process) had been essentially ignored in stream management legislation and practice," said Gessner.
The streams under study exhibited nutrient concentrations that varied 1000-fold; leaf-litter decomposition rates tended to peak at intermediate nutrient concentrations.
Although the results did not show a simple relationship between nutrient concentrations and leaf-litter breakdown rate, Gessner believes litter breakdown could be an important indicator of stream health "at the low end of the nutrient gradient, where traditional metrics would classify streams as healthy".
"Low breakdown rates are a hint, though not a guarantee, that a key functional component of stream ecosystems is in order," he said. "More important, accelerated breakdown rates at relatively low to moderate nutrient concentrations would correctly indicate impairment where classic approaches would classify them as healthy. It might be impossible to reconcile different goals in stream management (i.e., low nutrient levels and efficient resource use)."
Gessner said that the next steps are to develop metrics related to litter breakdown that go beyond breakdown rates, for example sporulation rates of fungal decomposers; test responses of litter breakdown to impacts other than elevated nutrient loads; test where functional metrics based on other ecosystem processes such as nutrient transformations, algal production, etc, provide useful information about stream health; compare process-based metrics with traditional metrics relying on biological community structure and, increasingly, geohydromorphology; and develop a comprehensive framework for stream and river assessment that is similar in approach to "differential diagnosis", the gold standard in medicine.
The researchers reported their work in Science.