CDOM as a mediator of climateUV interactions

In addition to the effects of climate change on ozone depletion, it is now widely recognized that climate change can influence the UV transparency of inland waters by changing the concentration and characteristics of UV-absorbing compounds. This means that underwater UV environments will continue to change even if ozone depletion is arrested or reversed. Climate change effects on underwater UV are largely mediated by CDOM, the light-absorbing component of the yellow-brown "gelbstoff" that results from the incomplete decomposition of living organisms. CDOM is a useful metric of ecosystem level processes in inland waters [14] and it exhibits strong variation in its chemical composition and spectral slope related to variation in source and prior exposure to microbial or photolytic breakdown [15,16]. This complex nature of CDOM has led to a variety of both optical and chemical methods of quantification. The simplest optical characterization of CDOM involves measuring the absorbance of a water sample at a given wavelength (320 and 440 nm have recently been proposed) in a laboratory spectrophotometer [14,17]. The simplest chemical measurement involves measuring DOC. Recent advances in methods of chemical characterization can also provide useful information on CDOM source [18,19] as discussed further below and in Chapter 6.

The bulk of the CDOM in inland waters is generally derived from alloch-thonous sources in the surrounding catchment areas or littoral zone [20]. Plant matter from terrestrial ecosystems and wetlands in particular are of primary importance [21-23], along with the type of vegetation and hydrology of the catchment area [24-26]. Seasonal variations in hydrology that may be influenced by climate change are also important in determining CDOM inputs as indicated by strong peaks in CDOM in rivers just preceding the peak of the spring flow [27]. Seasonal increases in CDOM in rivers can lead to an increase in heterotrophic microbial activity even at very low temperatures, and increase bacterial biomass [28]. These spring pulses of CDOM can also depress the pH of river waters and contribute to mobilization of aluminium as well as changes in aluminium speciation [29]. The mechanism of action of CDOM in controlling the UV environment in surface waters is largely through its selective absorption of the shorter, more damaging wavelengths of UV [30,31]. The UV absorptivity and the elemental composition of CDOM change with the source of the DOC

and the chemical characteristics of the fulvic acid [28,32]. In particular, the C: N ratio of the fulvic acid may vary from < 10 to >90, and algal-derived fulvic acid tends to be less aromatic and thus less color-absorbing than terrestrially-derived fulvic acid [28,32], Recent advances in fluorometric characterization allow these sources of fulvic acids to be distinguished by comparing emission intensity at 450 and 500 nm during excitation with 370 nm [19].

CDOM also influences the underwater UV environment by altering temperature and water mixing depths (see section 17.3.2). In the process it may also alter other important ecosystem processes such as CDOM biolability (lability to microbial processing) and photolability (lability to photolysis), and bacterial biomass and growth efficiency (Figure 1, see also Chapter 6). This central role of CDOM in regulating water transparency in inland waters is in contrast to the open oceans where terrigenous CDOM is at low concentrations, and transparency is regulated largely by biogenic sources in the water column [33,34]. Although coastal and estuarine regions may be highly influenced by terrigenous CDOM from runoff and river plumes that may be altered by climate change, much of the CDOM is precipitated or biodegraded when it reaches the sea water [31,35].

17.3.1 DOC and water transparency

Several studies have clearly demonstrated a strong predictive empirical relation

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Figure 1. Conceptual diagram showing the role of CDOM in regulating solar radiation and temperature regimes in aquatic ecosystems, as well as the processes by which UV alters CDOM biolability, photolability, and photomineralization. The width of the arrows approximates the strength of the effects.

ship between DOC concentration and underwater UV transparency wherein DOC accounts for approximately 90% of the variation in UV attenuation [15,23,36,37]. The strong relationship between terrestrially derived DOC and UV attenuation is supported by studies along a chronosequence of recently deglaciated lakes in Glacier Bay Alaska [38], and by paleolimnological studies of fossil algal pigments [4,39] (see Chapter 16). UV attenuation depths (1% of surface irradiance at a given wavelength) estimated from DOC concentrations across North America vary greatly among geographic regions (see also Chapter 3). For some regions in the western and northwestern U.S.A. and Newfoundland, 25% of the lakes are estimated to have 320 nm attenuation depths of 4 m or more [2]. In other regions, such as Florida, the upper Midwestern USA, and Nova Scotia, 75% of the lakes have estimated 320 nm attenuation depths less than 0.5 m. Streams and rivers with low CDOM concentrations may be even more vulnerable to climate change than are lakes due to the fact that they are generally much shallower [2].

There is a general consensus that climate change and other factors that influence CDOM concentrations in streams, rivers, lakes, and coastal marine systems will have a greater effect than ozone depletion on future underwater U V environments [2,4,40,41]. However, to date no comprehensive studies have actually documented changes in underwater UV transparency related to climate change. Rather, they examine changes or differences in DOC and infer UV changes from empirical relationships between DOC and UV. However, a space-for-time substitution study was carried out in lakes of different age along a deglaciation chronosequence in Glacier Bay Alaska where UV as well as DOC and biotic response were directly measured. This study revealed a relationship between the timing of changes in terrestrial inputs of DOC from the surrounding watershed, UV transparency of the water, and zooplankton community structure over time (Figure 2) [38]. Very young, recently deglaciated lakes with little terrestrial vegetation in their watersheds have low DOC concentrations and high UV penetration into the water column, with consequent low diversity of zooplankton. As lakes age, terrestrial inputs of CDOM increase, UV transparency of the water decreases, and zooplankton communities become correspondingly more diverse (Figure 2). While the deglaciation that has occurred in Glacier Bay is more related to regional hydrologic balance than to global climate change, similar deglaciation events may be expected in alpine and polar regions if climate warming continues.

A major mechanism thought to link CDOM to climate change is drought, which reduces the export of terrestrially-derived CDOM to aquatic ecosystems [3,43,44]. Drought may also influence water export from wetlands or peatlands that are an important determinant of CDOM inputs to other downstream aquatic ecosystems [21-25]. Acidification can also reduce CDOM, as well as increase toxic metals [3,42,45]. When DOC is low (< 1-2 mg 1_1) UV transparency increases very rapidly with declines in DOC (Figure 3). This close relationship between DOC and water transparency in low DOC systems suggests that water transparency in these is a particularly sensitive indicator of climate change, acidification, and other anthropogenic stresses in high latitude ecosystems

UV and Zooplankton Lake Age DOC (mg/u Lake Name 10 years 1.0 Little Esker

35 years 2.6 Plateau 1

90 years 4.2 Klotz Hills

Figure 2. Diagrammatic representation of the UV exposure and low temperature constraints placed on zooplankton at different depths in lakes of different ages in a déglaciation chronosequence of lakes in Glacier Bay, Alaska [38]. The presence of 320 nm UV from the surface to the 1 % (of surface irradiance) UV attenuation depth is indicated by shading of the lake basin, while the depth of the thermocline (<1°C change m-1) is indicated by a horizontal line. The presence of the zooplankton species in the lakes is shown diagrammatically where filled organisms represent established populations ( > 1 l-1), and unfilled organisms represent sparse populations (< 1 l-1). All three lakes have thermal gradients that offer a demographic advantage to zooplankton that reside in the surface waters. In the oldest lake, where all five zooplankton species have established populations, potentially damaging UVR is attenuated rapidly, providing a potential refuge from both constraints of cold temperatures and high UV in the warm surface waters. In lakes of intermediate age, zooplankton are faced with either high U V exposure in the warm surface waters, or lower temperatures in the deeper, low-UV strata. In the youngest lake, where only two species exist in very low numbers, substantial UV exposure occurs throughout most of the water column.

In high elevation lakes where terrestrial vegetation is often highly reduced, and productivity low due to a combination of low temperatures, low nutrient inputs, and short growing season, terrestrial CDOM inputs are reduced and in-lake concentrations often extremely low [15,48]. Climate change may have different effects on these high elevation lakes depending on their position along the elevation gradient and the response of terrestrial ecosystems and hydrology to climate change. For example, lakes above the vegetation zone that have extremely low CDOM concentrations may actually have reduced transparency if the vegetation zone moves up to higher elevations with climate warming [48,49]. On the other hand, for lakes in high elevation mountain meadows, where temperature keeps evaporation low, soils are water saturated, and CDOM concentrations moderately high, increased temperature may alter the hydrologie balance to reduce soil saturation, decrease CDOM inputs to the lakes, and thus elevate

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Figure 3. Relationship between the attenuation depth (depth to which 1 % of surface 320 nm UV-B radiation penetrates the water column) and dissolved organic carbon (DOC) concentrations in 65 glacial lakes in North and South America [2,15]. Note the rapid increase in the depth of penetration of UV below DOC concentrations of 1-2 mg 1_1. [Modified from [2] with permission.]

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DOC (mgr1)

Figure 3. Relationship between the attenuation depth (depth to which 1 % of surface 320 nm UV-B radiation penetrates the water column) and dissolved organic carbon (DOC) concentrations in 65 glacial lakes in North and South America [2,15]. Note the rapid increase in the depth of penetration of UV below DOC concentrations of 1-2 mg 1_1. [Modified from [2] with permission.]

UV levels. For example, lakes that one of us has sampled (C.E.W.) in the Beartooth Mountains of Wyoming and Montana, USA at about 3000 m, include meadow lakes with DOC concentrations in the 2-4 mg 1_1 range, and UV attenuation depths at 320 nm of less than 0.2 m.

17.3.2 Transparency, temperature and thermal stratification

A large number of studies have demonstrated that CDOM is also an important mediator of visible water transparency and thermal stratification in lakes [22,23,47,50,51]. This suggests that climate change can alter both the mixing regime in the water column as well as the temperature at which organisms are exposed to UV. Some of the most compelling data have come from studies on Canadian lakes that have the advantage of either long-term data [1,52,53] or large numbers of lakes [22,47]. These studies have demonstrated clear relationships between CDOM, water transparency and mixing depth [22,47], as well as climate variables such as air temperature and precipitation [52]. Increases in air temperature and decreases in precipitation are associated with lower CDOM, greater water transparency, and increased heating of deeper waters. Simultaneous UV measurements in one of these studies indicate that the 1 % attenuation depth for UV-B and UV-A may exceed the depth of the thermocline in some lakes [22]. In another study in small (<500 ha) Canadian Shield lakes, CDOM was found to be the major regulator of water transparency and the depth of the summer mixed layer [54]. Climate change (a 2 x C02 scenario) was predicted to increase mixing depths of these small lakes by as much as 1-2 m. This is important because variation in vertical mixing is considered one of the most fundamental factors in regulating the exposure of phytoplankton and zooplankton to ambient UV in freshwater and marine systems [40,55,56] (see Chapter 4). Simulation modeling efforts based on 40 years of data from 71

shallow Dutch lakes revealed significant increases in lake temperature as well as increases in the probability and timing of periods of high water transparency, the so-called "clear-water phase" events [57]. While these clear-water phase events have been well demonstrated for visible light in many lakes [58-60], only recently have these seasonal patterns of increasing transparency also been demonstrated for UVR [16,61]. These seasonal changes in transparency are largely mediated by UV photolysis [16] and can result in increases in the 1% (of surface irradiance) attenuation depths for 320 nm UV from 3 m in the spring and autumn to 17 m in the early summer [61].

Climate change is also altering the timing of ice cover in both marine and freshwater systems. Between 1978-1998 the extent of multiyear ice cover in the Arctic Sea has been reduced by about 14%, with simultaneous reductions in the thickness of the ice cover [62,63]. The 20-year time scale of these studies is, however, inadequate to be able to distinguish whether these changes are related to short-term climate fluctuations or longer-term climate trends. Longer-term records are available for ice cover on Northern Hemisphere lakes where ice-out is occurring an average of 6.5 days earlier per 100 years [64]. These changes in the pattern and timing of ice cover may be important for several reasons. Reductions in the extent or duration of ice cover may increase exposure to ambient UV because ice is less transparent to UV than is water [65], particularly when there are many air bubbles in, substantial snow cover on, the ice. In spite of this reduced transparency of ice to UV, in some systems such as Lake Vanda in Antarctica, enough UV to cause inhibition of algal growth can penetrate several metres of ice [66], In temperate lakes following early ice-out, vertical mixing may be sustained for longer periods in lakes due to lower solar irradiance or greater winds earlier in the year. The cooler temperatures at which increased UV exposure will occur following ice out may also reduce the effectiveness of enzyme-driven, temperature-dependent photorepair of UV-damaged DNA [67,68].

Temperature and UV generally vary along the same environmental gradients, but changes are not parallel over space and time. Seasonal changes in temperature generally lag behind changes in UV in a way that creates an early season peak in the UV : temperature (UV : T) ratio (Figure 4). Inverse relationships between temperature and UV with elevation can cause even wider variation in seasonal UV : T ratios. For example, UV increases and temperature decreases with increasing elevation [69] such that alpine lakes are likely to experience some of the highest UV : T ratios of any aquatic ecosystems.

Seasonal patterns in both temperature and UV are also being altered by regional and global climate changes. Observed increases in UV-B that are related to ozone depletion at north temperate latitudes tend to be most severe during the late winter to early spring. A 1.6°C warming has been reported for North American boreal forests during the period between 1970-1990 [70]. Increases in CDOM photobleaching [71] and declines in CDOM concentrations [1,70] that further elevate underwater UV may accompany such regional climate change. Photobleaching of CDOM can lead to substantial seasonal increases in water transparency to damaging UVR [16]. The timing of the peak

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