Abstract-Two series of laboratory experiments were undertaken to investigate the influences of exposure duration and frequency on the toxicity of short-term pulses of ammonia to rainbow trout (Oncorhynchus mykiss Walbaum) and brown trout (Salmo trutta L.). For the investigation of exposure duration, rainbow trout were exposed to a range of unionized ammonia concentrations in single pulses of 1, 6, or 24 h, and mortality was recorded as the endpoint. For the investigation of exposure frequency, brown trout were exposed to repeated sublethal ammonia pulses of different concentrations either once or three times weekly over a period of 53 d to simulate intermittent pollution episodes. A range of endpoints were measured, including growth, gill condition, blood indices, and organ-to-body weight ratios. The results suggest that exposure duration and frequency are both important factors influencing the severity of effect in fish exposed to short-term ammonia peaks. Growth, gill condition, organ weights, and hematocrit were all significantly affected by repeated exposures, particularly at the higher exposure frequency. The results of this study confirm that the duration and return period of transient pollution events are critical dimensions when considering the development of environmental quality standards for intermittent pollution.

No abstract

Laboratory experiments were undertaken to investigate the influences of exposure duration and frequency on the toxicity of short‐term pulses of low dissolved oxygen (DO) to fish. For the investigation of exposure duration, rainbow trout (Oncorhynchus mykiss [Walbaum]) and roach (Rutilus rutilus L.) were exposed to a range of DO concentrations in single pulses of 1, 6, or 24 h. For the investigation of exposure frequency, brown trout (Salmo trutta L.) were exposed to 24‐h pulses of DO concentrations of 4.0 and 5.5 mg/L at frequencies of once or twice weekly over a period of 75 d. The results suggest that, for a given duration, there is a narrow threshold concentration range above which mortality does not occur and below which mortality rapidly becomes high. This threshold concentration range increases as exposure duration increases. Roach were able to survive lower DO concentrations than trout. Observations on experimental animals following exposure indicated no significant postexposure effects, even at very low DO levels. For the exposure frequencies used here, DO concentration rather than frequency of exposure was the important factor in terms of effect on fish. No significant effects on growth rate were observed but there were significant differences in hemoglobin levels, hematocrit, and organ weights. These results have important implications for the derivation of environmental quality standards aimed at the control of episodic pollution in rivers.

Contact CEH NORA team at noraceh@ceh.ac.ukThe NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. The release of phosphorus (P) from bed sediments to the overlying water can delay , 2012;Spears et al., 2012;Sharpley et al., 2014). (over 3 days, as slurry from a pontoon) to control internal P loading, in an effort to Loch Flemington (Table 1) and likewise, being shallow, does not thermally stratify. 189This site also has sufficient data available and satisfies the BACI requirements for 190 the differences between sites to be relatively constant in the before period and not 191 subject to localised long-term random influences (e.g. large storms) during the study. 192The phytoplankton community in Loch Leven is, primarily, P limited and the loch has and cyanobacterial bloom intensity (using cyanobacteria biovolume) (see also , 1986). The null hypothesis is that there is no change in the difference in 262 the two time periods i.e. no effect of the La-bentonite application and where a 263 significant difference in the differences in the two time periods is detected at the 5% evidence that a large-scale driver i.e. weather, was influencing changes at both sites. 422Hence the increase in annual mean TP concentration may have been due to 423 enhanced internal P loading, associated with the drier and warmer than average 424 summer of 2013, when compared across the five year monitoring period (Table 4) essential and could potentially be discerned using long-term monitoring data. 441The phytoplankton compositional metric (PTI EQR) showed a sustained Implications of the results for future eutrophication management in lakes 468We have demonstrated the use of a BACI approach for detecting responses in shallow lake ecosystems (Hosper & Jagtman, 1990;Søndergaard et al., 2000; 532 Wichelen et al., 2007;Mehner et al., 2002 Mehner et al., & 2008 Duras & Dziaman, 2010). 533Controlling catchment-derived nutrient loading is a prerequisite for lasting lake 534 restoration efforts, otherwise internal stocks of nutrients will be replenished and 535 recovery periods protracted through their sediment release (Cooke et al., 2005). 536We have presented the first comprehensive assessment, using a BACI 546• This initial cyanobacterial response was not sustained beyond post-application 547 year two, implying a short lived reduction in internal P loading as a result of capping 548 sediment release. 549• The resumption in annual mean TP concentration, suggests that the La-bentonite 550 application was insufficient to control internal P loading for longer than two years. 551• Continual improvements in the phytoplankton compositional and abundance 552 metrics, not observed at the control site, may reflect a response to the sustained 553 reduction in annual mean FRP concentration following La-bentonite application. 554• The cyanobacterial bloom intensity metric is sensitive to inter-a...

No abstract