Ecological status monitoring and assessment in Greek rivers: Do macroinvertebrate and diatom indices indicate same responses to anthropogenic pressures?

Biotic indices based on macroinvertebrates and diatoms are regularly used for ecological status assessment in freshwaters, showing variable responses to anthropogenic pressures. In this work, we investigate the response of each biological quality element (BQE) index (i.e. diatoms and macroinvertebrates) to multiple stressors and compare their responses during ecological status assessments of seasonally variable river ecosystems. A total of 421 samples, collected during a four-year survey for the Greek National Water Monitoring Programme from 195 stream sites throughout Greece, were used. The correlation between macroinvertebrate (HESY2), diatom (IPS) and physicochemical (Ph) indices was tested and the environmental parameters affecting biotic indices were examined. All indices were positively intercorrelated, with IPS showing the highest correlation with the Ph index, confirmed by the fact that IPS was also driven by nutrient concentration and dissolved oxygen. HESY2 was driven by PO4, the extent of agriculture and urbanization, river typology, as well as sediment type and water temperature. The two BQEs, hence biological assemblages, respond differently to local and larger scale pressures; diatoms responded to reach-scale variations of water quality driven by large-scale drivers, such as land uses, to which macroinvertebrates responded. Moreover, macroinvertebrates depended on seasonality, whereas diatoms were independent of seasonality. Subsequently, when macroinvertebrates are absent, or haven't yet recolonize a stream after drought, diatoms can reflect disturbance as they respond to water quality, rather than seasonality. Thus, in Mediterranean rivers, where flow variability is pronounced, the complementary use of both BQEs, although important for ecological status assessment, it could rely solely on the use of diatoms when macroinvertebrates are absent.