m disappears from view is noted as the Secchi depth or Secchi-disk depth. It is easy to use, has no working parts to malfunction and, in the hands of a single operater, it can give fairly consistent results. However, these attractions are countered by difficulties of quantitative interpretation of its measurements (see Box 3.1). However, documented records of Secchi-disk depth (zs) span a wide range, from 0.2 m to 77 m (Berman et al., 1985) and are adequate to separate clear waters (zs >10 m) from the turbid (zs < 3 m) and to be sensitive to temporal changes in the clarity of any one of them.
This section considers the adaptive responses to turbulent entrainment and vertical transport beyond the column compensation point. Vertical mixing per se is not necessarily problematic for a microplanktic photoautotroph and, in entraining plankters to and from the high solar irradi-ances that may obtain near the top of the water column, may help to avoid the photoinhibition response observed in phytoplankton captured in static bottles (Jewson and Wood, 1975). Entrain-ment resists the development of other restricting gradients (for instance, of diminishing dissolved carbon dioxide or accumulating oxygen).
It is likely to be wholly beneficial in maintaining photosynthetic vigour, just so long as the vertical extent of entrainment is within the depth range offering irradiances between I0 to Ik.
With relatively deeper mixing, however (either as a result of stronger physical forcing or of greater underwater light attenuation), entrained plankters are carried beyond the depth of Ik and, in many circumstances, beyond the productive compensation point. During a period of time (probabilistically, the mixing time, tm: see Section 2.6.5), the individual plankter may be successively exposed to light intensities that are saturating, sub-saturating or altogether inadequate to support net photosynthetic production. These effects are represented in Fig. 3.14, where a notional 'Lagrangian' path of a single alga, moved randomly in the vertical axis by turbulent entrainment through an equally notional light gradient (a), is exposed to a predictably fluctuant photon flux (b). From the prediction of light-dependent photosynthesis (c), the instantaneous rate of photosynthesis of the phytoplankter is also now predictable (d). Integrating through time, it is clear that the net photosynthetic gain is impaired below the potential of Pmax. Moreover, the deeper is the mixed depth (hm) with respect to the depth of the column in which net
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