D. magna in situ bioassay responses depicted in Fig. 2 did not
always followthe observed detrimental effects on benthic communities.
Post-exposure feeding ratesweremainly affected negatively
by PAHs and positively by nitrogen sources (NO3, NO2), although T,
SS, and TRZ also contributed marginally to the observed responses.
There is reported evidence that PAHs inhibit the feeding rates of
Daphnia at lower concentrations than those impairing survival
(Barata and Baird, 2000). Furthermore, environmental variables
such as UV radiation, which was likely to be maximal in summer
in the studied rivers owing to longer days and reduced flow rates,
and hence water column depths, may increase the toxicity of PAHs
several orders of magnitude (Hatch and Burton, 1999; Nikkila et al.,
1999). Therefore, the estimated lethal toxicity of PAHs measured
in the studied river sites (Table 3), although low (