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9 Measured daily mean wind speeds at Blelham Tarn, UK, between August 1999 and October 2000 (continuous line), and the mixed depth, as calculated from wind speed and temperature of the water column. Redrawn with permission from Davis etal. (2003).

spring growth, averaged over a number of consecutive years, is really a response to the weakening 'dilution' of the incoming light in a diminishing mixed depth, even though no conventional thermal stratification is yet established. In reality, this is not at all a particularly smooth temporal progression. The day-to-day variability in cloud cover and wind forcing continues but the frequency of the sunnier, less windy days does accelerate, just as the days are lengthening significantly. It follows that, as the year advances, there are more days on which photosynthetic gain, growth and recruitment in the upper water layers is possible.

Another decade of improvements in monitoring approaches and in simulation modelling techniques permits us to derive a greater resolution on the variability in the underwater pho-tosynthetic environment. In Fig. 3.15, the fluctuations in the mixed depth of Blelham Tarn (between 0.5 and 12 m) through a winter-spring period reveal that there is neither a smooth nor single abrupt switch between fully mixed and stably stratified conditions, more a changing frequency of alternation. The contemporaneous compensation depth varied between 2 and 8 m (Davis et al., 2003), net growth being possible when compensation depth exceeds mixing depth.

Two other modelling approaches anticipated similarly modified views of the Sverdrup critical depth model. Woods and Onken (1983) constructed a 'Lagrangian' ensemble, in which the responses of each of a number (at least 20) of plankters, being simultaneously 'walked randomly' through a simulated light gradient, were summated to derive an aggregate for the population. Huisman et al. (1999) used Okubo's (1980) turbulent-diffusion model and a concept of residual light at the base of the turbulent layer (devised by Huisman and Weissing, 1994) to demonstrate the importance of a critical light threshold and, incidentally, to show up the shortcomings of the literal 'critical depth' model. Though this view has not passed unchallenged, it has been supported independently in the theoretical consideration of Szeligiwicz (1998). He also verifies the point that a critical depth is not the same for all species simultaneously, that those with a lower critical light compensation will perform better in deeply mixed layers and that their own adaptive behaviours may modify the critical light and critical depth while the environmental conditions persist.

The nature of these adaptive responses to low aggregate light doses in mixed layers is, in many ways, similar to those arising from the low aggregate light exposure of plankters residing deep in the light gradient. However, mixed-layer entrain-ment offers short bursts (a few minutes) of exposure to relatively high light intensities separated by probabilistically relatively long periods (of the order of 30-40 minutes) in effective darkness. It would seem important for these organisms to undertake as much photosynthesis as possible in the exposure 'windows' to non-limiting irradiance fluxes, which requires an enhanced light-harvesting capacity rather than a wide spectrum of absorption. Phytoplankton reputed to grow relatively well under deep-mixed conditions include the diatoms (especially those with attenuate cells or that form filaments) and certain Cyanobacteria and chlorophyte genera with analogous morphological adaptations (high msv-1: Reynolds, 1988a); all project substantial carbon-specific areas (10-100 m2 per mol cell C) but they vary their contents of chlorophyll a more than accessory pigments.

Mostly these effects have been detected through population growth and recruitment in culture. Turning off the light for a part of the day soon brings growth limitation into regulation

Figure 3.16

Growth-rate responses of Cyanobacteria in culture at two different temperatures (10 °C, 20 °C) to various and two photoperiods: (a) continuous light at 27 |imol photons m-2 s-1 and (b) under a 6 h : 18 h light-dark alternation. (c) shows the daily growth rate in (b) extrapolated to 24 h, to show the improved efficiency of energy use. Data of Foy et al., (1976), redrawn from Reynolds (1984a).

Figure 3.16

Growth-rate responses of Cyanobacteria in culture at two different temperatures (10 °C, 20 °C) to various and two photoperiods: (a) continuous light at 27 |imol photons m-2 s-1 and (b) under a 6 h : 18 h light-dark alternation. (c) shows the daily growth rate in (b) extrapolated to 24 h, to show the improved efficiency of energy use. Data of Foy et al., (1976), redrawn from Reynolds (1984a).

Experimentally imposed fluctuations in lightexposure levels on periodicities of days to weeks also affect the composition and diversity of phyto-plankton assemblages (Floder et al., 2002). These operate through the replication, recruitment and attrition of successive generations to populations and will be considered in a later chapter (see Sections 5.4.1, 5.4.2). However, to simulate in the laboratory some of the extreme behaviours observed in the field required observations relating to photoperiods rather shorter than an hour. Robarts and Howard-Williams (1989) described the response of a low-light-adapted Anabaena species in a turbid, mixed lake (Rotongaio, New Zealand) whose rate of photosynthesis could accommodate exposure to light at the water surface for 6 minutes but was slowed abruptly under further exposure. In this instance, the productive advantages were to be gained only in the photoperiods of less than 6 minutes, to which the organism had clearly adapted. These observations are considered in the context of photoprotection and photoinhibition, in the next section.

by light-dependent photosynthesis more than light-independent assimilation - exposing algae to saturating light for only 12 or 6 h out of 24 h always results in a reduced daily growth rate. However, photoadaptative responses to shortened photoperiod raise the rate of biomass-specific energy harvest to the extent that growth normalised per light hour is raised. This principle was memorably demonstrated by Foy et al. (1976) (Fig. 3.16).

Litchman (2000, 2003) has taken this approach further, exploring the effect of shorter experimental photoperiods and their discrimination among the performances of the test algae. Fluctuation periods were varied between 1 and 24 h and intensities were varied between 5 and 240 |imol photons m-2 s-1. Photoperiod evoked little photoadaptation at the higher intensities but, at lower intensities, differences in species-specific responses became evident. In general, the effect of fluctuating light tended to be greater when irradiance fluctuated between levels alternately limiting and saturating growth requirements.

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Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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