Mixing processes

4.2.1 UML depth

The sun is the primary source of both the stirring energy that drives vertical mixing and heating that dampens vertical mixing. Variations in solar energy over the Earth (see Chapter 2) ultimately result in variations in mixing between different parts of the ocean and lakes in different latitudes. Heating of surface waters is a direct effect of the sun, which stratifies waters and dampens mixing. Surface forcing by winds is an indirect effect, caused by the gradients in solar heating of the atmosphere between the poles and the equator. At large scales in the ocean (104—103 km), prevailing wind fields set up gyres defining oceanic provinces with characteristic UML depths based on convergent or divergent flow within the gyre [2]. At intermediate scales (103-102 km), the horizontal and vertical boundaries of currents associated with gyres and along the continents are shear zones that produce turbulence and eddies which also influence surface mixing.

Given that both surface UVR exposure and vertical mixing are products of the global distribution and seasonality of solar irradiance, it is reasonable to expect these factors to co-vary in some systematic way. Overall, subpolar waters are cooler, fresher, and have strong winter heat losses that favor the development of deep mixed layers which, at least occasionally, transport water to the surface from deeper, UVR protected, layers. On the other hand, tropical and subtropical waters experience strong surface heating, producing warm and salty upper layers that persist for long periods and isolate deep waters from the surface. The overall salinity gradient also affects the strength (i.e., density gradient) of the pycnocline and the probability of a deep mixing event, and this differs between ocean basins ([2], Figure 1). Lakes follow similar trends, though both UML depths and UVR penetration are more variable but generally shallower. Usually, tropical lakes tend to have stable, shallow stratification, whereas seasonal deep mixing is more common in temperate and polar lakes. As a result, where UVR exposure is the highest, i.e. at low latitudes, average exposure over the UML (as a fraction of incident UVR) also tends to be high. Of course, the depth of the photoactive layer will depend on the water transparency (Chapter 3), but significant exposure (> 10% of incident UV-B) rarely occurs deeper than 10 m and in turbid environments is usually < 1 m [5]. In this case the UML sustains a much greater magnitude of chemical and biological effects of UVR than deeper waters. The surface layer of lakes during temperate summer, especially smaller lakes with a surrounding windbreak of trees (e.g., in forested regions of North America and Europe), is a similarly isolated zone of UVR action [6,7]. In contrast, at higher latitudes, or during the temperate zone winter, mixing extends deeper and will periodically flush the near surface zone with water from depths with very low UVR exposure. In this situation, surface UVR is lower, because of the latitude or season, but the constituents of these waters may be more labile to UVR effects since they have received little previous exposure.

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Figure 1. Maps of mixed layer depth in the global ocean as monthly averages for January (upper panel) and July (middle panel). The lower panel shows typical profiles of sigma-t (a measure of density) for a polar (Southern Ocean) versus tropical (Pacific) area of the ocean. The maps use global ocean temperature and salinity data sets compiled by the U.S.

National Oceanic Atmospheric Administration as processed by Kara et al. [94].

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Figure 1. Maps of mixed layer depth in the global ocean as monthly averages for January (upper panel) and July (middle panel). The lower panel shows typical profiles of sigma-t (a measure of density) for a polar (Southern Ocean) versus tropical (Pacific) area of the ocean. The maps use global ocean temperature and salinity data sets compiled by the U.S.

National Oceanic Atmospheric Administration as processed by Kara et al. [94].

Implications of the depth of the UML are discussed in more detail below in the context of specific chemical and biological effects of UVR in the aquatic environment.

A further source of variation in the UML depth occurs in association with fronts and eddies that form along the boundaries between major oceanic mixing regimes. Depending on whether they are cyclonic or anticyclonic, these eddies increase upwelling or downwelling, respectively [2], Coastal upwelling, typically associated with increased productivity due to enhanced nutrient supply, also causes shallow UMLs (Figure 1). Along the equator, there also is upwelling, as well as complex patterns of convergence and divergence. Certain combinations of conditions can lead to extremely high plankton biomass at the surface [8].

In the coastal ocean, estuaries and shallow lakes, the depth of the water column is such that the bottom topography might limit the extent of the UML. For example, in some situations with high wind stress, such as those encountered during spring in the Patagonia region of Argentina, the water column is completely mixed and the UML therefore comprises the entire water column.

The patterns of vertical mixing that have been described reflect the global distribution of ocean-atmosphere energy exchange. It should be kept in mind that global climate change is affecting this distribution and changes in these patterns are expected. Indeed, climate induced changes in UML depth and strength of stratification have already been linked to changes in the community composition of plankton in the N. Pacific gyre [9]. Changes in vertical mixing as a consequence of global climate change are likely to have much greater influence on UVR effects on aquatic systems than ozone depletion per se (see Chapter 17).

The depth of vertical mixing may also be directly affected by increased UVR. In many systems (particularly fresh waters), the major component responsible for determining UVR penetration, chromophoric dissolved organic matter (CDOM), is also the primary absorber of visible radiation. Absorption of solar radiation by CDOM results in near-surface heating and shallow stratification [10,11]. However, CDOM absorbance is not constant, due to photobleaching by UVR [12]. As CDOM bleaches, there is deeper penetration of solar radiation and less pronounced surface heating, allowing deeper mixed layers to develop ([13] also see Chapters 3 and 6).

4.2.2 Time scales of vertical mixing

In addition to controlling the vertical structure of the water column, vertical mixing also determines the variability of UVR exposure, i.e. through determining the residence time of constituents (molecules and organisms) in the photoactive zone. Residence times depend on the particular processes contributing to vertical transport, and these can include convective or shear-stress driven turbulence, Langmuir circulation and breaking of internal gravity waves in the pycnocline [2]. Denman and Gargett [14] reviewed measurements of wind speed (U m s_1), the buoyancy frequency (N, an index of stratification, units s_1) and the rate of dissipation of turbulent kinetic energy (e, m2 s~3) from several marine and freshwater mixed layers (for more background on N and £ see reviews [4,15]). Based on this data, they estimated the times of transport over a vertical displacement of 5 m (a nominal depth for the photoactive zone in the ocean) using scaling relationships. The estimated transport time near the surface varied from a few minutes (strong winds, >8 m s_1) to a hundred hours (low winds <5 m s_1). Deeper in the mixed layer the estimated 5 m displacement time varied from nearly a day to weeks over the same range of winds. Although vertical mixing in lakes and the ocean is an active research area, there have been few further estimates of vertical transport rates near the surface since Denman and Gargett's 1983 review. A study of vertical mixing in a turbid shallow lake (photoactive zone probably < 50 cm) estimated mixing times over the upper 50 cm of 1-2 minutes during typical midday-afternoon breezes [16].

Rapid transport into and out of the photoactive zone results in extremely variable UVR exposure for organisms such as phytoplankton that are entrained in these flow features. As an example, we show exposure time-courses during eddy circulation in moderately clear lake which were simulated by rotation of bottles over a 0-4 m depth interval every 8 minutes (Figure 2). Photosynthetic active radiation (PAR) remains saturating for photosynthesis during the whole rotation; however, effective UVR for inhibition of photosynthesis (UVeff) varies between insignificant at maximum depth to very strong at the surface. Such circular trajectories are about the only type of motion that has been practical to use in field studies. In contrast, the most common approach in numerical models is to generate constituent trajectories using a random walk approximation of vertical displacement [17-19], In reality, the water motion is a composite of random displacements (sometimes anisotropic) and advection by large eddies [2], Detailed studies of these processes, particularly turbulence, have been enabled by development of instruments that measure small-scale variations in physical properties [4,15]. The microstructure profiler measures mm-cm scale fluctuations in density (temperature) or velocity, from which e can be estimated [4,20]. Scaling expressions similar to those described by Denman and Gargett [14] are then calculated, e.g. that the largest overturning scale of turbulence having dissipation rate, e, in a background stratification measured by the buoyancy frequency, N, is given by the Ozmidov scale, LQ = (e/N3)1^2. A complementary approach is to estimate large-eddy scales using Thorpe scales [21]. This requires measuring cm-m scale density profiles with a free-fall instrument and sorting the vertical density profiles to stability (i.e., monotonically increasing density with depth). The (minimal) distance that must be moved to achieve stability becomes a measure of the local Thorpe displacement. The Thorpe scale, LT, is defined as a root mean square value of Thorpe displacements, sometimes over fixed depth intervals, but more reliably over individual "overturns" [22]. Oceanic measurements to date have found Lx = c0t Lq, with 0.5 < CqT<2 [23-26]. While it is generally accepted that in stratified water columns the relevant time scale is the buoyancy period Tb = 2n /N, for studies of UVR effects, it is also necessary to determine local large-eddy length scales in the vertical dimension of the light gradient.

Scaling relationships have been developed from theories and observations of

Relative Irradiance

Relative Irradiance

Time

Figure 2. Time variable exposure to UVR and PAR in a simulated mixed layer of a temperate lake. Data is from Lake Lucerne, September 15,1999 (adapted from [79]). (A) Profiles of UV-B (290-320 nm), UV-A (320-400 nm) and PAR (400-700 nm) as irradiance relative to the surface. A line shows the bottom of the "photoactive zone" as defined by 10% of surface UV-B (1.7 m in this case). (B) Temperature profiles showing an approximately 5 m thick upper mixed layer. Experimental mixing was conducted over the upper 4 m (circle). (C) Exposures for PAR (thick line) and UVeff (circles, UVR weighted for inhibition of photosynthesis) obtained when samples were mixed with a 8 min rotation time. Photosynthesis was estimated to be about 60% lower at the surface compared to the maximum depth [79].

Time

Figure 2. Time variable exposure to UVR and PAR in a simulated mixed layer of a temperate lake. Data is from Lake Lucerne, September 15,1999 (adapted from [79]). (A) Profiles of UV-B (290-320 nm), UV-A (320-400 nm) and PAR (400-700 nm) as irradiance relative to the surface. A line shows the bottom of the "photoactive zone" as defined by 10% of surface UV-B (1.7 m in this case). (B) Temperature profiles showing an approximately 5 m thick upper mixed layer. Experimental mixing was conducted over the upper 4 m (circle). (C) Exposures for PAR (thick line) and UVeff (circles, UVR weighted for inhibition of photosynthesis) obtained when samples were mixed with a 8 min rotation time. Photosynthesis was estimated to be about 60% lower at the surface compared to the maximum depth [79].

turbulence and large eddy scales for several cases. If turbulent intensity is zero, the vertical position of neutral particles in the upper mixed layer (UML) is affected by the vertical motions associated with the surface and internal wave fields, including near the bottom of the mixed layer where internal waves dissipate with buoyancy frequency near that of the pycnocline immediately below. In both cases, vertical displacements can be estimated with some accuracy [27]. If turbulence is so intense that the UML is completely mixed, reasonable scalings exist for surface shear-stress and/or convection driven motions [14,28], as well as for Langmuir circulation [29]. No good scaling exists for the more complex case in which turbulence of intermediate intensity proves insufficient to mix the upper layer thoroughly in the presence of stabilizing influences like solar heating and/or surface freshwater input from melting ice, the latter being important in polar seas. Direct measurements are the best approach under these conditions. Because vertical motions resulting from mixing may be highly intermittent in such cases, they offer a particular challenge for studies of the biological effects of UVR, as the balance between damage and repair may shift depending on the time and length scales involved.

Both the depth and rate of mixing reflect a dynamic balance over all time scales between kinetic energy transfer and dissipation, as well as heat gain and loss. The daily time scale is particularly important for UVR effects. Mixing processes operating on the daily scale, and the diel cycle of turbulent mixing and stratification, have been extensively studied in the ocean [30-32] and lakes [16, 33-36]. An illustration of the interplay between these processes is provided in the observations made by the serial release over a daily cycle of near neutrally buoyant floats that are acoustically tracked ([Figure 3, D'Asaro and Dairiki unpublished, as cited in 2]). The floats can be viewed as showing the depth-time variation of neutrally buoyant constituents (molecules or non-motile organisms). These observations show a typical cycle of nocturnal convective mixing, followed by heat gain during the morning leading to a shallow diurnal thermocline. As surface irradiance declines, wind stress and cooling again dominate and deep mixing resumes. This is a dramatic illustration of how vertical mixing processes control the time scale of UVR exposure - at times the tracers remain confined near the surfaces, in other instances there are rapid (time scale minutes) ascents (or descents) between the surface and 30 m. In another field deployment of floats in the Pacific ocean off" of Vancouver during January, the principal source of energy for vertical mixing was the wind and the 5 m transport times of the floats varied between 17 minutes during low winds to 3 minutes during high winds [37]. Similar time scales were obtained during strong convective mixing [38].

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Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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