As pollution control measures have reduced the loadings from continuous discharges to receiving waters, episodic pollution has become increasingly detectable and important. A pollution episode involves a toxicant discharge to rivers (or other surface waters) of short duration and high concentration which is attenuated by physicochemical processes. After an episode has passed downstream, the timeconcentration profile of exposure to pollution and previous ecological status of the river are difficult to determine. After an episode, and over the course of time, recolonisation and succession will occur. This contrasts with the response of rivers to continuous discharges of pollution where recovery is spatial through the dilution and degradation of pollutants. Such spatial recovery has led to the description of recovery zones (e. g. Kolkwitz and Marsson, 1908). Pollution from farm waste accounted for almost 20% of all reported pollution incidents in 1988 (NRA 1989), so the investigation focussed on those aspects of pollution closely associated with strong organic wastes. A range of responses of freshwater macroinvertebrates and fish was investigated during simulated episodes of reduced dissolved oxygen (D. 0. ), increased ammonia and increased sulphide concentrations. These episodes were achieved by dosing streams after acertaining their biological condition, monitoring the impact upon captive animals and invertebrate drift during events, and by describing patterns of recovery. Catastrophic increases in drift occurred under conditions of reduced D. O. and mortalities in caged animals occurred, species exhibiting a range of sensitivities. Invertebrate responses to increased ammonia and sulphide concentrations (drift and mortality) were less dramatic, however, behavioural changes in captive animals were observed during episodes. With captive Gammarus pulex (L. ), the addition of substrate (sand and stones) to the container reduced mortality and a positive correlation was found between toxicity and ambient water velocity. Parasitised individuals of G. pulex were more susceptible to the simulated episodes than unparasitised ones. Seasonal differences occurred in the response of benthic invertebrates to low D. O. In November the response of drift density to low D. O. was about 1/50th of that in July despite broadly comparable benthic abundances. The recovery of benthic invertebrates following pollution events was rapid (2-3 months). In one series of experiments, which assessed the recolonisation route, 45% came from upstream as drift. Within substrate migration (from deep to surface layers of sediments), upstream movements and aerial sources contributed 31.5%, 15.6% and 7.9% respectively. From the measurements of pollutant concentrations within sediments during episodes and from tracer studies using sodium chloride, the penetration of pollutants into stream substrates was spatially patchy. In consequence some organisms survived in the deeper layers of sediments and were available for local recolonisation. Laboratory investigations showed that it is for possible animals to migrate downwards within a substrate in response to pollution episodes.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:527558 |
| Date | January 1993 |
| Creators | Turner, C. |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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