Ambient parameters and their interaction with UV

There are a number of biotic and abiotic factors that may be of particular relevance for the pelagic environment with regard to UV-effects. Some of these have already been touched upon, such as concentrations of DOC, which will not only be the main determinant of UV-attenuation but also may promote the formation of harmful photo-products. Oxygen, temperature and ionic strength (in freshwaters) may also be determinants of UV-effects. System productivity may be important in various ways. High primary production (and thus increased cell numbers of phytoplankton) will aim at reducing UV-exposure. Food access could also affect the physiological status of the animals and thus their UV-susceptibility. There is little general information on such physiological responses, however. Rather than screening the long list of potential effects, we shall focus on a few parameters of particular relevance.

12.4.1 Oxygen and temperature

Oxygen and temperature in several ways affect UVR susceptibility. High po2 could aim at increasing the oxidative stress, and low temperature could slow down kinetic reactions such as anti-oxidant expression in poikilotherm animals. Shallowness and wind exposure promote oxygen saturation in surface waters. High primary production may cause pronounced vertical gradients in 02-con-centrations, and diurnal oscillations. Since oxidative damage such as lipid peroxidation is intimately linked with 02-concentrations both inside and outside cells [47], and UVR is a major mediator of such oxidative damage, this parameter may be of vital importance for aquatic organisms. Temperature may also show strong vertical gradients, and naturally also shifts along altitudinal and longitudinal gradients. An a priori assumption would be that low temperatures in particular could increase the UV-susceptibility in heterotrophs as it does for autotrophs [81,82]. These two key parameters may also interact with each other.

Studies on phototrophic organisms show that UV-effects may be balanced between photochemical damage and biosynthetic repair. This balance will shift increasingly towards damage with decreasing temperature [83]. There are very few data on corresponding temperature induced trade-off in eukaryotic heterotrophs. Borgeraas and Hessen [67] tested the UV-susceptibility in Daphnia

3 h low

3 h low

Figure 5. Post-treatment recovery in grazing rates of Daphnia after different exposure to UVR. Filled bars represent UVR exposed individuals, open bars are unexposed controls. Light was provided by a 100 W xenon lamp with spectral properties close to daylight after passing through a cellulose acetate filter. At "high light" the dose-rate over 300-315 nm and 300-400 nm band was 1.1 and 35.9 W m~2 respectively, while 282 W m~2 for PAR (400-700 nm). 'Low light" represents dose-rates reduced by 50%. All data are means (±SD) of three replicates, each consisting of 10 animals. Grazing rates were measured as the decrease in algal cell number over time by use of a Scatron flow cytometer. [Hessen and Brudevoll, unpublished.]

Figure 5. Post-treatment recovery in grazing rates of Daphnia after different exposure to UVR. Filled bars represent UVR exposed individuals, open bars are unexposed controls. Light was provided by a 100 W xenon lamp with spectral properties close to daylight after passing through a cellulose acetate filter. At "high light" the dose-rate over 300-315 nm and 300-400 nm band was 1.1 and 35.9 W m~2 respectively, while 282 W m~2 for PAR (400-700 nm). 'Low light" represents dose-rates reduced by 50%. All data are means (±SD) of three replicates, each consisting of 10 animals. Grazing rates were measured as the decrease in algal cell number over time by use of a Scatron flow cytometer. [Hessen and Brudevoll, unpublished.]

magna along gradients of oxygen saturation and temperatures. Increased oxygen concentrations over the range 5.6-14 mg 021~1 did not cause increased mortality under UVR, however, and a lowered temperature (range 6-18 °C) did in fact decrease UV-susceptibility. Neither oxygen nor temperature caused any significant effects on anti-oxidant expression. Low temperatures may slow down UV-induced mortality in several ways. Although repair and detoxification mechanisms may be impaired at low temperatures, so may also activation processes such as ROS metabolism and lipid peroxidation. Low temperatures may also change the physiological status of the animal. D. magna is not a cold adapted species, however, and the tested clone had been raised to high temperatures (18°C for years). Thus these tests may not be relevant for cold adapted species, but for Daphnia they are in support of those of Abele et al. [84], who reported that exposure to elevated temperatures and hydrogen peroxide elicits oxidative stress and anti-oxidant response in the Antarctic intertidal limpet Nacella concinna.

12.4.2 UV and water chemistry

A number of different water quality parameters such as concentration (and type) of DOC, pH, salinity and sub-optimal or insufficient concentrations of specific ions or minerals could affect the biotic responses to UVR. The presence of DOC is in most cases the main determinant of UV attenuation and effects on freshwater metazoans (see Chapter 3), and yet increased DOC also yields increased production of ambient free radicals and oxidants; the net effect on the biota is in general positive. The role of inorganic parameters is less well known. Under laboratory conditions, swollen body tissue may be observed in UV treated freshwater invertebrates animals, which could indicate osmotic disorder [15]. This could indicate that ionic content or salinity per se could be one determinant to UV tolerance, but Hessen [15] found no effect of salinity in the range from 250 to 1000 fiS cm~1 in D. pulex exposed to artificial UVR (peak wavelength 312 nm).

Low levels of calcium is another potential co-stressor that could severely increase the UV-susceptibility of Zooplankton. Calcium concentrations of < 10 mg 1_1 seem sub-optimal for Ca-demanding Daphnia species and UV-B tolerance was significantly reduced over a gradient from 10 to 0.5 mg Ca l-1 in both D. magna and high Arctic D. tenebrosa [85]. Whether this effect could be accredited to some physiological interaction between UV stress and Ca metabolism (i.e., membrane damage and distorted uptake of Ca) or merely to the additional effect of two physiologically independent stressors remains unresolved. Nevertheless, it is obvious that the highly variable ionic content and Ca concentration in freshwater localities, ranging from values around 0.1 mg Ca 1_1 in very dilute soft-water localities to commonly >20 mg Ca l-1 in hardwater lakes, could also be a determinant of the effects of UVR on Zooplankton communities.

0 0

Post a comment