River macrophyte indices: not the Holy Grail!

Abstract

International audience1. Recent studies have demonstrated that there is generally no unambiguous relationship between plant species composition and specific environmental conditions in rivers. Nevertheless, indices of environmental pressures based on macrophytes are flourishing, because of the requirements of the Water Framework Directive (WFD). 2. We first reviewed nine such indices against 13 criteria for bioindicators. Then, using data from France and England, we tested whether the IBMR (Macrophyte Biological Index for Rivers) and LEAFPACS (predictions and classification system for macrophytes) methods could reliably indicate nutrient and hydromorphological pressures. Finally, we used an improved bootstrapping method to estimate accuracy. 3. Currently, most indices lack ecological meaning for a variety of reasons, including partial sampling (backwaters are excluded); reliance on list of taxa (there are identification difficulties) rather than structure and functions; correlation rather than causation; application within a limited biogeographical area; reliance on 'expert' judgement; high precision but poor accuracy; poorly defined reference conditions; lack of independent tests; and an inability to discriminate reliably between the target pressures of interest from confounding background variables. 4. IBMR was a far better indicator of pH (or HCO3-pCO2) than it was of soluble reactive phosphorus, SRP (or SRP-NH4). While there was a highly significant correlation between IBMR and SRP after removing the effect of pH, the relationship was weak (r2 = 0.08, n = 215, P < 0.001). 5. LEAFPACS is a multi-metric method summing up five individual indices, each compliant with the WFD. Its individual metrics were not better correlated with nutrient and hydromorphological pressures (with r2 < 0.1, n = 62, P < 0.05) than was the IBMR. The meaning of the overall metric is questionable. 6. There are problems in determining the precision of the indices, owing to uncertainties in recording, but they are less than the uncertainties in determining accuracy (because species optima and tolerances are sometimes poorly known). 7. Reliable information is needed to improve the state of our rivers. Macrophyte indices are able to detect statistically significant pressures from a large population of sites but cannot be applied at specific sites, as required by the WFD, owing to large uncertainties and low explanatory power. Typically, more than 90% of the variability in macrophyte indices is attributed to factors other than human pressure. The WFD would be better served by a simpler, holistic approach based on our current mechanistic understanding of river processes. These findings are likely to apply also to other taxonomic groups (macroinvertebrates, diatoms, fish) used in the assessment of purported ecological quality and to palaeolimnological measures of reference status

Macrophyte and macroinvertebrate patterns in unimpacted mountain rivers of two European ecoregions

“…In the case of rivers, such conditions are very rare today, due to the scale of river degradation in the biosphere (Moss, 2008;Demars et al, 2012). Human impact causes a variety of problems leading to the deterioration of river habitats, one of the most serious include eutrophication, acidification, and changes in hydrology and river morphology (Malmqvist & Rundle, 2002;Johnson & Hering, 2009;Manfrin et al, 2013).…”

“…The ecological status of a water body is defined as the deviation between the organisms identified there and the biological communities detected under reference conditions. Therefore, precise and efficient monitoring requires the correct identification of biological communities in reference conditions for each existing river type (Moss, 2008;Nõges et al, 2009;Demars et al, 2012;Mao & Richards, 2012). Ecological assessment based on reference conditions is used to measure streams' health, to identify degraded conditions and to monitor human impact or determine restoration effectiveness; it is also applied outside the European Union member states (Pond, 2012;Gerth et al, 2013;Kosnicki et al, 2014).…”

Artificial neural network modelling of macrophyte indices based on physico-chemical characteristics of water

“…Habitat evaluation based on macrophytes is associated with a number of physico-chemical characteristics of water quality, mainly nutrients (Holmes et al, 1999;Haury, 1996;Schneider et al, 2000;Haury et al, 2006) and morphological degradation . Recently some approaches in monitoring have been critically examined and the existence of strong correlations between aquatic plants and several quality determinants has been disputed (Demars et al, 2012). Therefore macrophyte monitoring requires more support utilising new databases and implementing additional statistical techniques.…”

Aquatic biomonitoring: Lessons from the past, challenges for the future

“…Macrophytes are a further key element in monitoring programs, however additional investigations are needed to refine their use in biomonitoring because the complex interactions between aquatic primary producers and ecological drivers (Demars et al, 2012;. In addition, alien plants may be considered one of the most critical causes of the functionality loss of aquatic ecosystems.…”

“…Along with the lack of knowledge about biology and ecology at several organization levels, methodological issues such as sampling effort and imperfect detection of species, or the difficulty to exactly define the reference conditions if not properly taken into account can severely bias the results of biomonitoring (Bouleau and Pont, 2015;Baattrup-Pedersen et al, 2017). Additionally, for example, the strong capability of primary producers to modulate the physical and chemical conditions in which they live can explain in part: i) the non-linear responses of many aquatic macrophyte communities to external perturbations, or ii) the clear space-and time-dependence of the evaluations provided by some macrophyte multi-metric indices (Demars et al, 2012;. Furthermore, the increasing spread of exotic species on a global scale is another critical factor that can alter the responses of biological communities to the rising impairment of ecosystems.…”