Long-term recovery of macroinvertebrate biota in grossly polluted streams: re-colonisation as a constraint to ecological quality

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Description/Abstract

To meet targets imposed by the European Water Framework Directive (2000/60/EC) it is vital that
measures to improve the status of rivers are both effective and economically viable. Achievement of such
aims needs robust understanding of biological responses to changes in water quality vis-a` -vis
mechanisms of and constraints to the colonization of previously polluted sites. This study therefore
examined the long-term chemical and biological changes in historically polluted rivers to elucidate the
responses of macroinvertebrate biota to improvements in chemical water quality. For three historically
polluted sites in the English Midlands, data from surveys over a period of ca. 50 years were analysed.
Ammonia (NH3) and 5-day biochemical oxygen demand (BOD5) were used as chemical water quality
indicators. Variations in the ecological recovery of the study sites were assessed using an average
pollution sensitivity score (Average Score Per Taxon) and the number of taxa present (usually to family
level) present in hand-net samples. Ecological recovery varied widely and was influenced by the
intensity and spatial extent of the pollution and the proximity of available sources of potential
colonisers. At the site most isolated from potential sources of colonizing taxa, no clean-water
macroinvertebrate taxa were recorded 30 years after the major sources of pollution ceased. Where cleanwater
colonisers were more readily available, significant improvements in ecological quality followed
within 2–5 years of the improvements in chemical quality. Macroinvertebrate communities and hence
monitoring data may thus be indicative of long past conditions or of biological isolation rather than
contemporaneous chemical conditions. Combined chemical and biological data were used to explore a
generic model for predicting recovery rates and success. Neither BOD5 nor NH3 were found to provide a
consistent and meaningful prediction of either average pollution tolerance of macroinvertebrate taxa or
of the number of taxa present. Long-term relationships between macroinvertebrate variables and
chemical water quality variables, however, were non-linear, suggesting that water quality thresholds
may have to be exceeded before biological recovery can occur. Even when chemical water quality has
been improved substantially, the apparent ecological status of macroinvertebrate communities may not
reflect reduced pollution levels attained until adequate time to allow for re-colonisation (possibly
decades) has elapsed.