Seasonal changes in the concentration and colour of humic substances in some aquatic environments

301

261

Climate warming in North America is likely to be accompanied by changes in other environmental stresses such as UV-B radiation. We apply an empirical model to available DOC (dissolved organic C) data to estimate the depths to which 1% of surface UV-B and UV-A radiation penetrate for several major regions of North America. UV attenuation depths are also estimated from DOC data collected from treatment and reference basins during the experimental acidification of Little Rock Lake, Wisconsin. In some regions of North America 25% of the lakes have 1% attenuation depths for UV-B radiation on the order of 4 m or more (western and northwestern U.S., Newfoundland). In other regions, 75% of the lakes have 1% attenuation depths for UV-B shallower than 0.5 m (Florida, upper midwestern U.S., northwestern Ontario, Quebec, and Nova Scotia). Attenuation depths for UV-A radiation are -2.5 times as deep as those for UV-B. Experimental acidification approximately doubled the estimated 1% attenuation depths for UV radiation in Little Rock Lake.The strong dependence of 1% attenuation depth on DOC below the l-2 mg liter-l DOC range suggests that UV attenuation in low DOC lakes is highly sensitive to even very small changes in DOC. We conclude that changes in climate, lake hydrology, acid deposition, and other environmental factors that alter DOC concentrations in lakes may be more important than stratospheric ozone depletion in controlling future UV environments in lakes.

Section: Resultsmentioning

confidence: 99%

Ultraviolet radiation in North American lakes: Attenuation estimates from DOC measurements and implications for plankton communities

Williamson

Stemberger

Morris

et al. 1996

301

261

“…The humic acid from the Rio Negro appears to be removed in part by adsorption onto particles. Studies on small streams indicate that humic acid concentrations vary seasonally while those of fulvic acid, the dominant fraction in the Amazon River, remain relatively constant (Visser 1984). Although our data give only a glimpse of the seasonal cycle, we can estimate the annual transport of carbon and lignin by Amazon humic substances using the annual DOC flux and humic substance concentrations from Cruise VII, since humic substances represent a fairly constant portion of the total DOC in the various tributaries sampled.…”

Section: Discussionmentioning

confidence: 99%

Dissolved humic substances of the Amazon River system1

Ertel

Hedges

Devol

et al. 1986

412

227

Aquatic humic and fulvic acids from nine mainstem and seven major tributary sites in the Amazon River Basin are characterized by their elemental and lignin phenol compositions. Combined humic substances represent 60% of the riverine dissolved organic carbon (DOC), with fulvic to humic acid (FA : HA) ratios in the mainstem averaging 4.7 -t 1 .O. All dissolved humic and fulvic acids have clearly recognizable lignin components at levels (8 and 3% of the carbon) suggesting a predominantly allochthonous source. Lignin compositional characteristics are dominated by diagenetic transformations, which include preferential loss of methoxylated structural units and oxidative degradation of lignin side chains. Fulvic acids have consistently lower lignin levels, lower lignin phenol methoxylation, higher acid : aldehyde ratios, and higher C : N ratios than coexisting humic acids, all indicative of greater aerobic degradation of the fulvic acid fraction. An extreme example of lignin degradation is seen in the Rio Negro humic and fulvic acids, which differ significantly in lignin and elemental compositions from other blackwater tributary and mainstem samples.Over most of the mainstem humic and fulvic acids behave conservatively. However, downstream of the Rio Negro confluence, humic acids appear to be selectively adsorbed onto fine suspended particles. Approximately 25 and 40% of the total fluxes of lignin and of carbon are represented in the dissolved humic substances. The annual flux of dissolved, chemically recognizable lignin in the Amazon River at 6bidos is calculated to be 1.2 x 10" g yr-*.

“…The proposed model could be extended to three dimensions with any of these additional axes to provide further refinements. For example, despite the strong correlation between color and DOC concentration, the optical quality of DOC can show strong variation among lakes (Cuthbert and del Giorgio 1992;Morris et al 1995;McKnight et al 1997) and even among seasons within a single system (Visser 1984;Morris and Hargreaves 1997). These differences could be resolved by adding a third axis that includes DOC concentration as well as UV absorption.…”

Section: Increases Attenuation Of Solar Radiationmentioning

confidence: 99%

Dissolved organic carbon and nutrients as regulators of lake ecosystems: Resurrection of a more integrated paradigm

Williamson

Morris

Pace

et al. 1999

359

298

The primary interpretive paradigm used to study lakes is their trophic status. Oligotrophic lakes have low nutrient loading and low productivity, while eutrophic lakes have high nutrients and high productivity. The strong empirical relationship between nutrient loading and productivity is a valuable tool for teaching, for research, and for management of lakes. In order to incorporate the variety of other known anthropogenic impacts on lakes, however, lake characterization needs to extend beyond the nutrient-productivity paradigm. For example, acid precipitation, heavy metal and toxic organic contaminants, increases in UV radiation, and global warming are all recognized threats to lake ecosystems. One of the key characteristics of lakes that determines how they respond to disturbances such as these is their concentration of colored dissolved organic carbon (CDOC). Here we argue that a paradigm that includes CDOC (using the absorption coefficient at 320 nm as a proxy) as well as nutrients will be useful in predicting and understanding the response of lake ecosystems to multiple stressors. We propose to resurrect the CDOC axis that was proposed by investigators earlier this century and to extend it by adding some operational definitions to permit placing some of the major lake types on the axes in a way that will help us to better understand the structure, function, and response to disturbance of lake ecosystems that are subject to natural and anthropogenic environmental changes at the local, regional, and global scales. Data from a few diverse lakes and a successional sequence in Glacier Bay, Alaska, are used to illustrate the potential utility of the 2-axis model in separating lake types.The most commonly used paradigm for studying lake ecosystems defines lakes in terms of their trophic status. Lakes with low nutrients and low organic production are considered oligotrophic, while those with high nutrient inputs and organic productivity are eutrophic (Wetzel 1983). This paradigm has been adopted by the general ecological community as well as limnologists and is in general use in introductory ecology and limnology textbooks. The trophic status paradigm is supported by a particularly strong empirical relationship between chlorophyll and phosphorus (Vollenweider 1968;Dillon and Rigler 1974;McCauley et al. 1989), by comparative studies and whole-lake experiments that have clearly demonstrated the role of nutrients and grazers in eutrophication Pace 1984; Carpenter et al. 1991), and by the widespread success of remediation of AcknowledgmentsWe thank Dan Engstrom for stimulating discussions on the role of DOC in the succession of lakes in the Glacier Bay chronosequence, Sheri Fritz, Tom Frost, and Bruce Hargreaves for providing feedback on some of these ideas in their early stages of development, Dave Krabbenhoft for discussions on Hg, and Philip Singer and James Symons for introducing us to the literature on water treatment disinfection by-products and DOC. Sheri Fritz provided the unpublished data on Brush La...