Phosphate is an important nutrient that restricts microbial production in many freshwater and marine environments. The actual concentration of phosphate in phosphorus-limited waters is largely unknown because commonly used chemical and radiochemical techniques overestimate the concentration. Here, using a new steady-state radiobioassay to survey a diverse set of lakes, we report phosphate concentrations in lakes that are orders of magnitude lower than estimates made spectrophotometrically or with the frequently used Rigler radiobioassay. Our results, combined with those from the literature, indicate that microbes can achieve rapid turnover rates at picomolar nutrient concentrations. This occurs even though these concentrations are about two orders of magnitude below the level where phosphate uptake is estimated to be half the saturation level for the pico-plankton community. Also, while phosphate concentration increased with the concentration of total phosphorus and soluble reactive phosphorus in the lakes we sampled, the proportion of phosphate in the total phosphorus pool decreased from oligotrophic to eutrophic lakes. Such information, as revealed by the phosphate assay that we use here, should allow us to address hypotheses concerning the concentration of phosphate available to planktonic microorganisms in aquatic systems.

We analyzed the 21-yr dynamics of dissolved organic carbon (DOC) in 55 lakes during ice-free periods in five regions across eastern Canada in relation to total solar radiation (TSR), precipitation, air temperature, sulfate deposition (SO 4 ), Southern Oscillation Index (SOI), North Atlantic Oscillation, and Pacific Decadal Oscillation (PDO). A synchronous pattern in DOC was found among lakes within each region; however, a synchronous pattern in DOC was not found among regions, except for Kejimkujik and Yarmouth. Long-term trends of increasing or decreasing DOC concentration were not evident except at the Experimental Lakes Area (ELA), where an increase in DOC correlated with a decrease in summer TSR and an increase in summer precipitation. Annual mean temperature increased at the Nova Scotia and Turkey Lakes Watershed regions (TLW) over the study period, but there was no corresponding change in DOC. TSR and precipitation were important explanatory variables across all regions, except for the TLW. Summer TSR, or annual TSR, had a negative relationship, while summer precipitation had a positive relationship with the temporal DOC pattern in all regions except TLW. TSR and precipitation explained 78%, 49%, and 84% of the variation in the long-term DOC patterns at Dorset, ELA, and Nova Scotia (NS) regions, respectively. In contrast, the long-term pattern in DOC at TLW was only weakly related to SOI and PDO.

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Previous work has shown that the photo-oxidation rate of dissolved organic carbon (DOC) exposed to solar radiation was significantly enhanced in acidic stream waters and that the average photo-oxidative loss of DOC in the UVA region was more than 60%. This study examined the effect of pH on photobleaching and the photo-oxidative loss of DOC and the degree of photo-oxidative loss by hydroxyl radicals ( ∑ OH) over the pH range 4-9 in a coloured, softwater stream. Binding iron with NaF lowered the photo-oxidation rate constant by about 50% at pH 4 and by 10-20% at pH 4.6 and 4.9. Hence, iron plays an important role in the photo-oxidative loss of DOC at pH 4 with declining importance as the pH increases. The importance of ∑ OH in photo-oxidative loss of stream DOC was evaluated by adding KI to quench ∑ OH. Aquatic SciencesAfter 69 hours of artificial irradiation without KI, DOC loss decreased as pH increased from pH 4 to 9 whereas the minimum loss of absorbance at 300 nm occurred at pH 7. KI significantly reduced loss of DOC at pH 4, 5 and 7 but not at pH 9 with the fraction of DOC lost by non-∑ OH mechanisms gradually increasing from 58% to 75% between pH 4 and 7, and 100% at pH 9. Since many northern freshwater lakes on the Precambrian Shield receive large fluxes of UV-labile humic matter from peatlands and have pH < 7.5, photo-oxidative loss of DOC by both non-∑ OH and ∑ OH mechanisms is important in C dynamics in northern freshwaters but with photo-oxidative loss dominated by non-∑ OH mechanisms likely operating in the UVA region.