| The measurement of low levels of ferrous and total reactive iron (TRFe) in freshwaters using three common colourimetric methods was evaluated. In the absence of reducing agent, TPTZ reagents alone reduce ferric to ferrous iron (autoreduction), while ferrozine reagents have negligible autoreducing properties. Humic dissolved organic carbon (DOC) produced significant autoreducing effects as well. The temporal relationships between anoxia, internal loading of Fe and TP, and shift in dominance to large colonial and filamentous cyanobacteria was investigated in three Muskoka lakes of differing trophic status, and eutrophic Hamilton Harbour at the west end of Lake Ontario. The results indicate that the shift to cyanobacteria dominance is dependent on a high supply rate of ferrous iron from anoxic sediments to hypolimnetic waters adjacent to the metalimnion. In situ YSI fluorometric measurements of phytoplankton communities are shown to be limited in utility because of species-specific differences in cross-sectional area and fluorescence intensity. |
