Abstract. Predicting the effects of climate change and atmospheric deposition on water quality requires predicting the effects of landscape form on export of substances downstream. In this paper, we present dissolved organic carbon (DOC), total phosphorus (TP), and iron (Fe) export data (1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992) suggesting that Fe export is dependent to a large extent upon either the export of organic material or the reducing conditions. The long-term export models worked well when export of a substance was dominated by peatlands but was not very sensitive to the influence of mineral soils. The long-term average TP/DOC ratio was remarkably constant among most whole-lake catchments, ranging from 1.4 to 2.0 mg P/g C, the exception being the catchment of anthropogenically acidified Plastic Lake with a ratio of 0.8. Fe export to Plastic lake was also enriched relative to TP export compared with the other lakes. Therefore TP export to Plastic Lake may be limited by some mechanism related to acidification.
Abstract:This study examines the relationship between wetlands hidden beneath the forest canopy ('cryptic wetlands') and dissolved organic carbon (DOC) export to streams and lakes in forested ecosystems. In the Turkey Lakes Watershed (TLW), located in the Algoma Highlands of central Ontario, Canada, there is substantial natural variation in average annual DOC export (kgC ha 1 year 1 ), ranging from 11Ð4 to 31Ð5 kgC ha 1 year 1 in catchments with no apparent wetlands. We hypothesized that the natural variation in DOC export was related to cryptic wetlands. Cryptic wetlands were derived manually from geographic coordinates that were surveyed with a differential global positioning system, and automatically from identification of topographic depressions and flat slopes (<1Ð5°) within a digital elevation model (DEM) in a geographic information system. For the TLW catchments, which are characterized by shallow soils over bedrock, a significant correlation (r 2 ½ 0Ð9, p < 0Ð001) between manual and automated methods was observed for scales up to 50 m when a light detection and ranging DEM was used for the topographic analysis. Regression models indicated that cryptic wetlands (%) explained the majority of the natural variation in DOC export (kgC ha 1 year 1 ), with r 2 D 0Ð88 (p < 0Ð001) for the model based on the manually derived wetlands and r 2 D 0Ð85 (p < 0Ð001) for the model based on the automatically derived wetlands. The strength and significance of the automatically derived wetlands (%) versus DOC export (kgC ha 1 year 1 ) regression model diminished when other sources of DEMs were used. This study emphasizes the importance of including cryptic wetlands in predictive models of DOC export, particularly in catchments where the topography includes depressions and flat areas but no apparent wetlands.
SUMMARY1. A novel conceptual model linking anoxia, phosphorus (P), nitrogen (N), iron (Fe) and sulphate to the formation of noxious filamentous and colonial cyanobacteria blooms is presented that reconciles seemingly contradictory ideas about the roles of P, N and Fe in bloom formation. 2. The model has several critical concepts: (i) P regulates biomass and productivity in fresh waters until excessive loading renders a system N-limited or light-limited, but it is the availability of ferrous ions (Fe 2+ ) that regulates the ability of cyanobacteria to compete with its eukaryotic competitors; (ii) Fe 2+ diffusing from anoxic sediments is a major Fe source for cyanobacteria, which acquire it by migrating downwards into Fe 2+-rich anoxic waters from oxygenated waters; and (iii) subsequent cyanobacterial siderophore production provides a supply of Fe 3+ for reduction at cyanobacteria cell membranes that leads to very low Fe 3+ concentrations in the mixing zone.3. When light and temperature are physiologically suitable for cyanobacteria growth, bloom onset is regulated by the onset of internal Fe 2+ loading which in turn is controlled by anoxia, reducible Fe content of surface sediments and sulphate reduction rate.4. This conceptual model provides the basis for improving the success of approaches to eutrophication management because of its far-reaching explanatory power over the wide range of conditions where noxious cyanobacteria blooms have been observed.
Abstract. We examined the extent to which photolytic and nonphotolytic decomposition rates of dissolved organic carbon (DOC) could account for the annual retention or loss of DOC inputs in lakes (retention is equal to stream inputs plus atmospheric inputs minus stream discharge which is equivalent to storage in sediments plus degassed to atmosphere). Losses of DOC inputs to sediments and the atmosphere were large, averaging 38 to 70% of total inputs in seven study lakes between 1980 and 1992. Up to 50% of stream DOC was lost as inorganic C when exposed to solar radiation during 6 to 11 day surface exposures in bottles whereas lake DOC concentration was unaffected by solar radiation. Stream DOC loss was significantly less in the dark suggesting a low microbial consumption rate. Photodecay constants, extrapolated to each of the study lakes after correction for in situ mixing conditions and extinction of UVA and UVB, were similar to corresponding mass balance rate constants representing sediment storage and losses to the atmosphere. This suggests that photodecay is potentially large enough in situ to account for all of the DOC losses to the atmosphere and sediments in the low DOC lakes (<4 mg L 'l) but cannot account for all of the DOC lost in the high DOC lakes (>4 mg L'l). The mass balance and photodecay approaches employed in the study of carbon budgets show that UV degradation is probably an important mechanism in transfer of stream DOC to the sediment particulate C pool and to the atmosphere.
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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