Long-Term Variability in Bioassessments: A Twenty-Year Study from Two Northern California Streams

Abstract

Long-term variability of bioassessments has not been well evaluated. We analyzed a 20-year data set (1984–2003) from four sites in two northern California streams to examine the variability of bioassessment indices (two multivariate RIVPACS-type O/E scores and one multimetric index of biotic integrity, IBI), as well as eight metrics. All sites were sampled in spring; one site was also sampled in summer. Variability among years was high for most metrics (coefficients of variation, CVs ranging from 16% to 246% in spring) but lower for indices (CVs of 22–26% for the IBI and 21–32% for O/E scores in spring), which resulted in inconsistent assessments of biological condition. Variance components analysis showed that the time component explained variability in all metrics and indices, ranging from 5% to 35% of total variance explained. The site component was large (i.e., >40%) for some metrics (e.g., EPT richness), but nearly absent from others (e.g., Diptera richness). Seasonal analysis at one site showed that variability among seasons was small for some metrics or indices (e.g., Coleoptera richness), but large for others (e.g., EPT richness, O/E scores). Climatic variables did not show consistent trends across all metrics, although several were related to the El Niño Southern Oscillation Index at some sites. Bioassessments should incorporate temporal variability during index calibration or include climatic variability as predictive variables to improve accuracy and precision. In addition, these approaches may help managers anticipate alterations in reference streams caused by global climate change and high climatic variability.

High Variability Is a Defining Component of Mediterranean-Climate Rivers and Their Biota

“…Comparing several studies in California and the Mediterranean Basin, inter-annual variability was the highest for number of EPT and the Northern Coastal California Index of Biotic Integrity (IBI) in non-perennial streams from California during summer across a 20-year period [80]. In contrast, the lowest inter-annual variability was observed for the Iberian Biomonitoring Working Party index (IBMWP) and ICM-11 multimetric index in perennial siliceous streams from Spain across three years [71].…”

“…In the study from Mazor et al [80], the spatial component interacting with year, however, represented the highest source of variability for the IBI index and was mainly associated with the degree of flow intermittence and stream order. Compared to perennial Mediterranean reference streams, intermittent ones present much higher variability in assemblages and biotic indices, complicating the validation of the reference condition criteria using biological communities across space [25,[106][107][108][109].…”

“…For example, results from two long-term macroinvertebrate-based assessments conducted in four stream sites in Northern California (1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003) and in 171 in Catalonia (1996Catalonia ( -2004 showed that variability was mainly attributed to the interaction between years (dry vs. wet) and seasons (spring vs. summer) for most of metrics [80,106]. In contrast, results from a three-year study (2003)(2004)(2005)) in 14 stream sites from Spain did not present significant inter-annual nor seasonal variability in the metrics analyzed, showing a higher stability than the streams studied in California that may be attributed to the shorter duration of the former study.…”

“…There are few datasets available that include more than 10 years of data, which are needed to detect long-term community changes (cf. [26,80,81]). Many of these datasets have been used to relate inter-annual community-changes to climate change, highlighting that the frequency of dry years has increased in more recent years and that communities have changed accordingly.…”

Long-term trends in aquatic ecosystem bioassessment metrics are not influenced by sampling method: empirical evidence from the Niobrara River

“…Relatively few long-term aquatic invertebrate monitoring data sets exist, and the few that do are vital to our collective understanding of how ecosystems change over time, particularly in response to specific events (Jackson and Füreder, 2006;Jackson et al, 2009;Mazor et al, 2009). In this study, four of six bioassessment metrics (HBI, EPT richness, taxa richness and the proportion of EPT taxa) showed significant trends over the 19-year monitoring period, regardless of which data set (HD or Hess) was included for the 2010-2014 timeframe.…”