Interactions between vertical mixing and UVR effects

The previous section showed how mixing processes determine the way UML constituents (molecules or organisms) enter the photoactive zone where they may participate in a UVR-mediated process. Under strong stratification, such transport is very limited, so UVR effects will only involve those constituents already present in the active zone. Such extreme stratification can be episodically important in systems where diurnal thermoclines form, but more typically the UML extends below the photoactive zone so that constituents in waters below the photoactive zone will also participate in UVR photochemistry and photo-biology. The mixing characteristics of the UML influence photoprocesses in at least two fundamental ways. First, the depth of the UML influences the average UVR exposure that occurs; second, the rate of vertical transport, either due to advection or turbulent exchange, determines the residence time in and out of the photoactive zone (Figure 2). If all biological and chemical effects were linear functions of cumulative exposure over all time scales, then the timing and sequence of exposure would not matter and knowledge of the vertical distribution of irradiance (Chapter 3) would be sufficient to assess UVR effects in aquatic ecosystems. However, most UVR effects are non-linear at least over part of the environmentally relevant range of time scales. Another way of stating that effects are not proportional to cumulative exposure is that there is a failure of reciprocity. When reciprocity fails, effects are dependent on the duration and irradiance of exposure, so that residence time and average irradiance make a difference. Detailed examples of how these dependencies arise are considered in the next sections.

4.3.1 Photochemistry

The effects of mixing on photochemical reactions are not well known. As a corollary of the first law of photochemistry (see Chapters 6 and 8), primary photochemical reaction rates should be directly proportional to the rate of light absorption. However, the previous statement applies only to completely mixed, optically thin water bodies, or where the reactant is the sole absorber [39]. In contrast, in optically thick water columns with competition amongst chromo-phores for photons, differences in mixing rates can affect photochemical reaction rates. Under such circumstances, fast turnover would tend to release the CDOM pool from self-shading, which should translate into higher photoreaction rates [40].

A few studies have addressed the effect of mixing on photodegradation of natural organic matter from a mass transport perspective [40-42]. Both me-socosm experiments [40] and models [41,42] have shown that mixing rates can exert a strong influence on the rates and distribution of photoprocesses in the water column. In general, lack of, or slow mixing confines photoreactions to the very top of the water column in water bodies where there is strong light attenuation of wavelengths involved in the photoreactions and the rates of these primary photoreactions decrease exponentially with depth [41]. With more thorough mixing, fresh photochemical reactants are continually added to the photoactive layer, which results in higher rates of photolysis for the entire water column [43]. In addition, highly reactive species produced photochemically may be transported to deeper waters by vertical mixing. For example, reactive oxygen species (ROS, Chapter 8), produced near the surface may be transported by mixing and oxidize organic molecules several meters below the water surface.

Conversely, lack of mixing (i.e., stratification) may also be important because it isolates a certain CDOM pool within the UML during the stratified period. CDOM in the UML (epilimnion in lakes) becomes photobleached during seasonal exposure to UV, causing differences in CDOM absorbance between surface layer and deeper waters (hypolimnion in lakes). For example, studies in two Pocono lakes [12] found differences in photobleaching and the spectral weight ing function for UVR bleaching of CDOM between epilimnetic and hypolim-netic samples. This suggests that the timing and extent of stratification may affect the annual production of photoproducts, including carbon gases, such as C02 and CO.

The previous paragraphs illustrate how mixing can affect photochemical reactions by resupplying surface waters with fresh, unbleached chromophores and reducing surface concentrations of photoproducts [42]. But mixing may have additional effects that are as yet unexplored. Organic molecules are complex structures having several photoactive sites (see Chapter 6). Absorbed energy decays through a variety of pathways, which may or may not involve molecular rearrangements and fragmentation [44]. In cases where the molecular structure is altered, the resulting new structure may have different absorption and photo-reactivity characteristics than its parent molecule. Thus, in addition to affecting the mass transport along the water column, vertical circulation may influence the sequence of reactions undergone by organic molecules. To our knowledge, there is only one study that addressed the effect of fluctuating levels of solar radiation, on a time scale of minutes, on photodegradation of natural organic matter [45], That study showed differences in (i) photobleaching, (ii) nutrient release, and (iii) subsequent use of CDOM by algae and bacteria, between bottles incubated at fixed depths and bottles rotating within the water column. Although far from definitive, such results suggest that vertical mixing should be explicitly considered in photochemical studies of natural waters.

This section has called attention to some ways that vertical mixing complicates the photochemistry of natural waters. On the other hand, if the rates of CDOM absorption and photochemistry can be quantified, then the steady state profile of a photochemical product (i.e. dissolved hydrogen peroxide) can be used to infer vertical mixing rates. This was possible in freshwater systems (Canadian Lakes and the St. Lawrence River) that accumulate higher levels of peroxide due to their CDOM content [36,46]. A similar attempt to model the depth-time variation of hydrogen peroxide in the ocean (where CDOM is much lower) was only partially successful in reproducing the observed distribution [47,48].

4.3.2 Photobiology - phytoplankton and bacterioplankton

Phytoplankton and bacterioplankton are small (0.2-100 fim) unicellular organisms that have no, or weak, motility. Some bacterioplankton exhibit chemokinetic motility, but velocities are small (on the order of 0.001-0.01 cm s-1[e.g., 49]) compared to typical vertical transport rates (on the order of 1-10 cm s-1). Phytoplankton have varying properties of buoyancy and motility that can interact with the mixing regime and affect residence time in the photoactive zone (examples [16,18]). Indeed, it has been argued that the diverse life forms of algae reflect the diversity of mixing regimes in aquatic environments [50]. Under strong mixing (cf. Figure 3), variations of cellular physical properties and motility may have only subtle effects on vertical transport rates but significant effects on vertical distribution, e.g. positive or negative buoyancy will result in a

Midnight Noon Midnight

Time of Day

Figure 3. Schematic of the time vs. depth distribution of floats released at various times during a diel cycle of nocturnal mixing and diurnal stratification in the Labrador Sea [E. D'Asaro and G. Dairiki unpublished data as shown in 2], The heavy line (a composite of three separate float deployments) depicts a possible depth history of a non-motile plankton over the 24 h period. Note that trajectories are terminated once they enter the stable diurnal thermocline since they actually start to ascend due to very slight positive buoyancy of the floats.

Midnight Noon Midnight

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Figure 3. Schematic of the time vs. depth distribution of floats released at various times during a diel cycle of nocturnal mixing and diurnal stratification in the Labrador Sea [E. D'Asaro and G. Dairiki unpublished data as shown in 2], The heavy line (a composite of three separate float deployments) depicts a possible depth history of a non-motile plankton over the 24 h period. Note that trajectories are terminated once they enter the stable diurnal thermocline since they actually start to ascend due to very slight positive buoyancy of the floats.

distribution with a larger proportion of plankton in the upper or lower (respectively) part of the mixed layer.

Photosynthesis by phytoplankton is an important process in aquatic environments and is well known to be time dependent. Accounting for the effects of vertical mixing as a determinant of light history is a long standing issue in the study of phytoplankton productivity (see reviews by [2,14]). Vertical mixing can influence productivity through several types of time-dependent responses [51]; however, integral photosynthesis is most often affected when the assemblage is sensitive to inhibition by near-surface irradiance [52].

Phytoplankton respond to light variation over time-scales of seconds to many days [51], with average light exposure over a generation time (ca. 1-3 days) having a strong effect on capacity to resist photoinhibition (see Chapter 10). Induction of these defense mechanisms depends on the balance between PAR and the energetic demands of phytoplankton metabolism. If average light intensity is low, then phytoplankton emphasize efficiency of photosynthesis through accumulation of light-harvesting pigments and have generally weak capacity to defend against high-light exposure (PAR and UVR). When such low-light acclimated phytoplankton are transported near the surface, they can be severely affected by high-light exposure. On the other hand, when light availability is high relative to energetic demands, phytoplankton emphasize protective and defensive mechanisms more than photosynthetic efficiency, accumulating protective pigments like xanthophylls and UVR-screening compounds, and maintaining efficient mechanisms to repair photodamage.

The main determinants of average exposure in a mixed layer are transparency of the water and the depth of the UML. Thus, in water bodies of relatively stable optical characteristics, sensitivity to UVR would be expected to increase with depth of mixing, but defense mechanisms should be inversely related to mixing depth. This hypothesis is supported by a few field studies. Helbling et al. [53] found an increased inhibition of photosynthesis in natural populations of Antarctic phytoplankton with increased depth of the UML. Whereas samples coming from an UML <25 m showed no significant inhibition, the inhibition due to UVR was about 40% and 75% for samples from mixed layers depths of 35 m and more than 100 m, respectively. In their study, photosynthetic inhibition also decreased from the Antarctic to the Equator, and part of this was attributed to the shallowing of the UML towards tropical areas (see also Figure 1). Vernet et al. [54] found that concentration of UVR screening compounds (one type of acclimation to high-light exposure) was inversely related to UML optical depth (and therefore directly related to average exposure) for waters near the Antarctic Peninsula. Neale et al. [55] reported that the sensitivity to UVR for phytoplankton in the Weddell-Scotia Confluence (Southern Ocean) was related to surface layer density, which was used as an indicator of the overall light history. Higher surface density reflects a greater contribution of deep water and thus deeper mixing. Depth of mixing appears to be an important factor explaining seasonal variation in sensitivity in the North American Great Lakes [56],

A number of studies have suggested that rapid vertical mixing can actually enhance production if the time-scale of surface exposure is short compared to the induction time of photoinhibition [57-60]. On the other hand, under the low vertical mixing of diurnal thermoclines, photoinhibition is quite pronounced [61,62]. Effects on other indicators of photoacclimation also seem to be greatest when the time scales of mixing and photoresponse are comparable [cf. 63,64]. This has led to the commonly expressed view that photoinhibition (by PAR or UVR) as measured in long-term incubations does not apply in situ under the prevalent conditions of vertical mixing and thus can be ignored [e.g. 65].

Early studies of productivity and vertical mixing did not specifically consider responses to UVR, despite some early, order-of-magnitude calculations that suggested it could be important [66]. Certainly, during the presence of diurnal thermoclines, the surface community receives extended UVR exposure and severe inhibition of photosynthesis can occur [36,67,68]. However, another consideration is overall effect of UVR exposure on water column (vertically integrated) production. Helbling et al. [69] were the first to consider the relationship between vertical mixing and UVR effects on integrated production using an experimental approach. Screens were rotated over quartz tubes providing a UVR and PAR gradient to simulate mixing and the results were compared to average rates obtained under static conditions (no screen rotation). Incubations were conducted using phytoplankton samples from near Elephant Island in the Southern Ocean during which the screens covering the tubes were rotated at a rate within the range of estimates for transport in the UML so that UVR varied between 100% and 3% of incident irradiance. Average inhibition of photosynthesis increased when Antarctic phytoplankton were incubated in the variable light regime (rotated screens) compared to a series of control bottles that received a fixed percentage of incident radiation (Figure 4). At low mean irradiances in the UML the phytoplankton that were static (i.e., fixed irradiances) had lower carbon fixation than the samples that were in a simulated UML. At higher irradiances, however, samples that were "mixing" had a rate of photosynthesis that was inhibited (lower) compared to the average of the static treatments.

That vertical mixing could actually enhance inhibition was a surprising result, considering earlier experiments that suggested that vertical mixing decreased the effect of photoinhibition by PAR. However, experiments with phytoplankton from the Weddell-Scotia Confluence (WSC, near Elephant Island) showed that the kinetics of photosynthetic response to UVR were quite different from those assumed in earlier studies. The decrease in photosynthesis occurred rapidly after the onset of exposure, and recovery, after exposure to near-surface irradiance, was absent or slow [55]. The absence of recovery meant that reciprocity of exposure applied over short-time-scales but near-surface inhibition was strong enough that responses were non-linear for exposures exceeding about 30 minutes

Figure 4. Integrated primary production in an incubator with 12 light levels (100% to 3% of incident) when samples are rotated (Pr) to simulate vertical movement in an UML as a percent of static (fixed) samples (Pf). Percent change is given as a function of total UV-B (290-320 nm, J m~2) over the incubation period. Total UV-B was estimated from measured irradiance at 320 nm (Em) by applying the ratio of measured to modeled Em to a full UVR spectrum calculated by a radiative transfer model for clear sky conditions. [Adapted from Helbling et al. 69],

Figure 4. Integrated primary production in an incubator with 12 light levels (100% to 3% of incident) when samples are rotated (Pr) to simulate vertical movement in an UML as a percent of static (fixed) samples (Pf). Percent change is given as a function of total UV-B (290-320 nm, J m~2) over the incubation period. Total UV-B was estimated from measured irradiance at 320 nm (Em) by applying the ratio of measured to modeled Em to a full UVR spectrum calculated by a radiative transfer model for clear sky conditions. [Adapted from Helbling et al. 69],

[55]. Thus, high-rates of photosynthesis were not maintained during brief exposures to near-surface irradiance as had been found for PAR effects. The consequence of these different kinetics were examined in a numerical model of varying UVR exposure in the mixed layer of the WSC [70]. Similar to the experimental results of Helbling et al. [69], vertical mixing enhanced inhibition of integrated water column production for moderate depth mixed layers (zmix < 40 m) (Figure 5). Under static conditions, inhibition by UVR is severe but effects are limited to the near surface. Photosynthesis decreases with time according to an exponential [or survival curve, 71] relationship [55]. The implication of these kinetics is that UVR has the greatest absolute effect at the beginning of the exposure period, i.e. 75% of the inhibition occurs during the first half of the day. Thus, in static layers the decrease in productivity is small after the first hour or two of exposure in the morning. Vertical mixing results in a flux of phytoplankton from depths where exposure is low to the high exposure surface layer. Under these conditions, there will always be a relatively unexposed component of the surface phytoplankton that will experience large, UVR induced, decreases in photosynthetic rate. Since recovery is low (or nonexistent) when previously exposed phytoplankton are transported away from the surface layer, effects accumulate and integrated inhibition is higher than under strongly stratified conditions. This situation has analogies with the mixing enhancement of CDOM bleaching as discussed in the previous section (4.3.1).

The rate of mixing not only affects the overall impact of UVR, but also the response to ozone depletion (Figure 5). Stratospheric ozone depletion, which is

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Figure 5. Results of a model for inhibition of photosynthesis in the surface layer of the Weddell-Scotia Confluence of the Southern Ocean (modified from Neale et al. [70]). (a) Interactive effects of mixing depth and mixing time scale on daily water column productivity relative to the uninhibited rate (curves labeled with imix = time scale of mixing). Langmuir circulation can mix the upper water column rapidly, so mixing times < 1 h are modeled for mixing depths <42 m. (b) Interactive effects of 03 depletion (150 DU vs 300 DU) and mixing time as a function of mixing depth: proportional change in PT for the same curves as in (a).

most severe over the polar regions, has a spectrally specific effect on the solar UVR spectrum in which UV-B (280-315 nm) radiation is particularly enhanced (Chapter 2). The calculated effect of the Antarctic "ozone hole" (i.e. 50% ozone depletion) on water column phytoplankton production was around 1.5% reduction for the static case, but as much as 8% reduction in the mixing scenarios [70]. Again, this appears to be a consequence of the non-linear response of photosynthesis to cumulative exposure. Attenuation of UV-B with increasing depth is always more rapid than attenuation of UV-A (in the WSC, as elsewhere, Figure 2 also see Chapter 3). As a result the enhancement of exposure of organisms to UVR by ozone depletion is always greater near the surface. This increment in exposure results in more loss of photosynthesis for the relatively unexposed phytoplankton transported to the surface in the mixed case relative to the static case, where the enhanced UV-B exposure occurs mainly to the near-surface phytoplankton that have low photosynthetic rates even under normal UV-B conditions.

Recent experimentation conducted in temperate Patagonian marine waters [72] also indicated different responses of phytoplankton to UVR during simulated mixing. Vertical movement (i.e., change in irradiance) and mixing within the euphotic zone depth (£ph) were simulated using a moving system in combination with neutral density screens, and compared with a fixed system. The rates of production of winter and summer samples were compared under the same simulated UML (UML/ £ph<0.5). In terms of UVR-dependent reduction of carbon fixation rates under a variable ("mixed") irradiance field, species adapted to low irradiance levels (i.e. winter samples, generally dominated by microplankton) were more sensitive than summer samples (dominated by nanoplankton). In addition, when summer samples were exposed to the irradian-ces encountered in shallow UMLs (UML/ £ph<0.5), vertical mixing enhanced the inhibition by UVR. However, when a deep simulated UML condition (UML/ £ph>0.8) was imposed on the samples, the phytoplankton that were in the moving system had a higher integrated carbon fixation rate than samples in the fixed system. In these studies, the kinetics of inhibition were dependent on the composition of the phytoplankton population; there was a tendency for samples dominated by nanoplankton to have relatively slow kinetics of inhibition. This was also observed in a study conducted in various Andean lakes [73], where samples dominated by smaller cells had slower inhibition rates as compared to larger cells.

The kinetics of photosynthetic response are clearly the key in predicting how vertical mixing will affect the impact of UVR exposure, as seen for the cases discussed so far. Contrasting kinetics have been observed for phytoplankton assemblages in other temperate environments. In these assemblages, recovery appears to be much more active and thus reciprocity is not observed [74-77]. While inhibition is still rapid, a steady-state rate is obtained after a short transition. This steady-state is the result of an equilibrium between damage and repair processes. When exposure decreases, photosynthetic activity is restored. When the time-scale of mixing is sufficiently long that steady-state is established, UVR inhibition of photosynthesis is a function of irradiance alone and thus should be independent of vertical mixing rate. However, the interaction of vertical mixing and UVR inhibition of photosynthesis has received little experimental examination in assemblages with active repair. An example of an assemblage with active repair is the phytoplankton in the surface layer of a temperate Swiss lake (Vierwaldstattersee or Lake Lucerne) [78]. The effect of fluctuating UVR on photosynthesis was measured in bottles that were circulated by a rotating lift system through a surface mixed layer at relatively fast rates (complete bottle rotation every 4-20 minutes, exposure conditions shown in Figure 2) [79]. The observed effect of UVR could be accounted for using a steady-state (irradiance dependent) relationship developed using laboratory incubations (Figure 6). Since the model of UVR effects is not time-dependent, this indicated that the effect of vertical mixing, if present, was too small to cause a deviation from the profile predicted under static conditions. While more studies are needed of how mixing interacts with UVR effects on phytoplankton production, the first indications are that results depend strongly on the kinetics of the UVR response.

Figure 6. Effects of UVR on photosynthesis (total C assimilation) of phytoplankton moved through different mixing depths, presented as per cent photosynthesis in quartz (UVR transparent) relative to glass (partial UVR exclusion) bottles. Measured rates are for bottles that were circulated over the indicated depth ranges at the rate of once per 4 min (0-2 m), once per 8 min (0-3.9 m) and once per 20 min (0-10 and 0-14 m) for a 4 h midday incubation period. The modeled rates are the average of the steady-state (irradiance based) photosynthesis predicted using a biological weighting function and photosynthesis irradiance (BWF/P-I) curve applied to in situ irradiance estimated from recorded surface irradiance, depth of the bottles and measured vertical extinction coefficient. Model and measurements agree within measurement variability (ca. 10%) except for the 0-10 m incubation. Experiments were conducted in Lake Lucerne on September 13,1999 (no asterisks) and September 15, 1999 (asterisks, see exposure data in Figure 2). [Modified from Köhler et al. 79.]

Figure 6. Effects of UVR on photosynthesis (total C assimilation) of phytoplankton moved through different mixing depths, presented as per cent photosynthesis in quartz (UVR transparent) relative to glass (partial UVR exclusion) bottles. Measured rates are for bottles that were circulated over the indicated depth ranges at the rate of once per 4 min (0-2 m), once per 8 min (0-3.9 m) and once per 20 min (0-10 and 0-14 m) for a 4 h midday incubation period. The modeled rates are the average of the steady-state (irradiance based) photosynthesis predicted using a biological weighting function and photosynthesis irradiance (BWF/P-I) curve applied to in situ irradiance estimated from recorded surface irradiance, depth of the bottles and measured vertical extinction coefficient. Model and measurements agree within measurement variability (ca. 10%) except for the 0-10 m incubation. Experiments were conducted in Lake Lucerne on September 13,1999 (no asterisks) and September 15, 1999 (asterisks, see exposure data in Figure 2). [Modified from Köhler et al. 79.]

Production by bacterioplankton is also inhibited by UYR exposure (see review, [80]), but the relationship of inhibition to the depth and rate of vertical mixing has not been studied. However, the vertical distribution of damage to bacterial DNA, an indicator of the UVR effects, has been studied for water columns with different mixing regimes. The most common form of UVR-me-diated damage is the formation of cyclobutane pyrimidine dimers (CPDs) (see Chapter 9). There are dramatic differences in the vertical profile of CPDs depending on whether calm conditions or strongly mixed conditions prevail (Figure 7). Boelen et al. [81] observed an in situ diel cycle of accumulation and decrease of near surface CPDs, while in parallel surface incubations CPDs increased mid-day but were not repaired in the afternoon. This suggested that vertical mixing was necessary to exchange severely damaged organisms (due to surface UVR exposure) out of the near-surface layer.

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Figure 7. Measured and modeled profiles of cyclobutane pyrimidine dimers (CPD) for water columns with varying intensities of vertical mixing (a) Gulf of Mexico, 8 September 1994, mixed layer 20 m, wind speed low (2 ms-1) (b) Gulf of Mexico, 7 September 1994, mixed layer 20 m, wind speed high (8 ms-1) (c) Gerlache Strait, Antarctica, 6 October 1996, mixed layer 25 m, wind speed high (8 ms"1). Modeled profiles are for damage (CPD formation) only, damage plus photoreactivation only, and damage and all repair mechanisms (photoreactivation and excision repair). [From Jeffrey et al. and Huot et al. 82.]

Figure 7. Measured and modeled profiles of cyclobutane pyrimidine dimers (CPD) for water columns with varying intensities of vertical mixing (a) Gulf of Mexico, 8 September 1994, mixed layer 20 m, wind speed low (2 ms-1) (b) Gulf of Mexico, 7 September 1994, mixed layer 20 m, wind speed high (8 ms-1) (c) Gerlache Strait, Antarctica, 6 October 1996, mixed layer 25 m, wind speed high (8 ms"1). Modeled profiles are for damage (CPD formation) only, damage plus photoreactivation only, and damage and all repair mechanisms (photoreactivation and excision repair). [From Jeffrey et al. and Huot et al. 82.]

Vertical profiles of bacterial CPDs reflect the in situ dynamics of stratification and mixing, and thus present an opportunity to test our understanding of how the processes of vertical mixing, damage and repair interact. Such a test was done by Huot et al. who constructed a mathematical model of the depth profile of DNA damage in the UML [82]. In this model, the initiation of DNA damage (CPDs) was linearly related to exposure, and DNA repair was linearly related to accumulated CPDs at rates developed from laboratory studies. However, photo-reactivation (PR) had to be arbitrarily reduced to 10% of the response predicted from laboratory measurements of repair in Escherichia coli in order for the model predictions to fit observed profiles of DNA damage (Figure 7). Under these assumptions, mixing rates affected the vertical distribution DNA dimers but had little effect on average dimer concentration [82,83], The authors recognized that more data on DNA repair in natural assemblages of bacteria is needed before realistic modeling can be done. Both rates and linearity need to be examined. Observations that damage is rapidly repaired when total CPDs are low, but very slowly repaired when CPDs are high ([81], Jeffrey et al. unpublished), suggest that the relative effectiveness of PR decreases when damage exceeds a threshold. If this is the case, average damage within the water column will be higher when mixing slows to the point that the rate of CPD accumulation in the photoactive zone exceeds repair capacity.

4.3.3 Aquatic biota - zooplankton andfish

Another important biological effect of UVR is increased mortality in multicellular aquatic organisms, particularly zooplankton and ichthyoplankton. As for phytoplankton, sensitivity of zooplankton species to UVR varies (Chapter 12), and differences in sensitivity seem to be mostly related to variations in the ability to repair UVR-induced DNA damage (see Chapter 9) and the presence of UVR screening compounds (Chapter 10). However, an important distinction needs to be made between non-motile or slow-moving organisms (e.g., fish eggs) and active swimmers, e.g. crustacean zooplankton and fish larvae. The former may be viewed as passive constituents, responding to the average UVR over the whole mixed layer, but responses for the latter category will be more complicated. Most zooplankton can migrate vertically over long distances, and it is now becoming apparent that they can adjust their vertical position in response to UVR, and perhaps even to UV-B [84-86, Chapter 14]. These zooplankton may be able to seek a deep refuge from UVR effects depending on whether they inhabit shallow or deep environments.

In shallow environments the water vertical velocity induced by turbulent mixing may exceed the maximum swimming speed of organisms at any depth in the water column. Under calm conditions, the organisms may or may not be able to reach a UVR refuge in deep water (depending on water transparency and swimming speed), but under turbulent conditions the organisms may not avoid being cycled throughout the water column (cf. Figure 3). The latter would be the only case for strong swimmers in which it would make sense to relate average survival over the whole water column to the rate of vertical mixing. Under the above scenario (shallow and windy), the average exposure corresponds to the mean biologically effective irradiance over the water column and the biological effects depends on whether the organisms are able to repair UVR damage. When repair is absent or low, reciprocity applies though there is a non-linear relationship between the mortality and exposure, either as a simple survival curve [as for phytoplankton, see 87,88] or a somewhat more complicated (but also non-linear) logistic curve [89]. When repair is present, reciprocity is no longer obeyed and the actual effect will depend on irradiance and period of exposure.

This type of shallow environment is common in the Patagonian steppe and the Pampean regions of Argentina. A study of zooplankton mortality in lakes in these areas found that the presence or absence of reciprocity strongly affected whether data from fixed depth incubations could be used to predict the survivorship in samples undergoing simulated mixing (Figure 8). A single logistic relationship with cumulative exposure was successful in predicting survivorship of a copepod, Boeckella gracilipes, which lacked photorecovery, whether the copepod was exposed in fixed or rotating (varying UVR) frame [89,90]. In contrast, another species, Ceriodaphnia dubia, which is able to repair DNA damage by photoreactivation, did not obey the same exposure-response relationship in fixed vs. variable (rotated) exposures. Indeed, survivorship was higher in the rotated samples compared to fixed samples receiving the same cumulative exposure [90]. These results show that presence or absence of reciprocity needs to be taken into account in predicting zooplankton survivorship under mixed conditions.

Unlike the previous work on phytoplankton productivity these zooplankton studies did not directly address the issue of whether average survivorship over the water column is affected by the rate of vertical mixing. This question was examined in more detail for a pair of slow moving zooplankton for which responses to UVR appeared to obey reciprocity: eggs of the Atlantic Cod (Gadus morhua) and embryos of a marine copepod (Calanus finmarchicus). For these zooplankton, survivorship was modeled using an exponential function of cumulative dose, similar to the inhibition of photosynthesis in Weddell-Scotia Confluence (WSC) phytoplankton [87,88]. A similar result was obtained as for the WSC phytoplankton (see section 4.3.2): When averaged over the water column, survivorship was higher for low mixing or static cases compared to rapid mixing [83]. Similar results (increased average survivorship in low mixing) could be expected for other zooplankton which lack repair mechanisms (e.g. Boeckella gracilipes). However, when repair is active, vertical mixing could increase average survival in a surface layer, as is suggested by the Ceriodaphnia dubia case already discussed (rotated samples had better survival than fixed samples receiving the same dose).

In deep environments, strong swimmers are able, in principle, to escape from the turbulent upper layer, and avoid damaging UVR exposure by adjusting their vertical position. Consequently, the net UVR-induced mortality is probably minimal. In this case, the costs, if any, are likely to be indirect, i.e., as a result of the trade-off between UVR exposure and risk of predation in surface waters

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versus suboptimal temperature and food availability at depth. This trade-off is explored in more detail in Chapter 12.

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