by narrowing the boundary layer of each strand. Boundary layers are less extensive at
greater relative velocity. Because of boundary layers, drag tending to tip over benthic
animals extending up into passing currents or to pull them out of the sediment is much
reduced. As already stated, they mean that the supply of nutrients to algal cells
depends upon molecular diffusivity, that is on the background concentrations, the
potential cell-surface uptake rates and the solute-specific diffusivity constants. This
list of boundary-layer effects is far from exhaustive.
(^) For an extensive discussion of hydrodynamic effects on biological processes, refer
to Steven Vogel’s (1996) book Life in Moving Fluids.
Effects from Having Sun Above, Water Below
(^) Ocean water is held against the Earth by gravity, filling basins and with surfaces
almost parallel to the so-called geoid, parallel apart from mild, long-range slopes
created by the dynamics of flow on the curved and rotating form of the planet and, of
course, except for surface waves. These sheets of water, thin relative to the Earth’s
diameter, are thus illuminated from above by sunlight and on some nights by
moonlight. Light that is not reflected back into the atmosphere (and in part back into
space) is progressively absorbed by the water and by both dissolved and particulate
substances. Absorption increases with depth (z), following Beer’s Law: dE/dz = –kE,
for which the solution is Ez = E 0 e−kz. That is, irradiance, E, declines exponentially
with depth. The constant, k, an extinction rate for the overall spectrum of sunlight, has
a value of 0.067 m−1 for just seawater, and actual values in oligotrophic subtropical
gyres are remarkably close to that when chlorophyll concentrations are on the order of
0.05 μg liter−1 or less. More pigment-containing phytoplankton or more suspended
sediment increase k, shoaling the levels reached by specific levels of irradiance.
However, k also varies with the wavelengths of light. The wavelength of maximum
transmission in pure water and in clear oceanic waters is around 435 nm (blue). Other
wavelengths are more rapidly stripped out, eventually leaving only blue, with the only
color vision distinguishing shades of blue below about 100 m, and systems of
photosynthetic and visual pigments must absorb near 435 nm. They do, mostly shifted
toward the green at 465 nm (the extinction rate is almost constant from 410 to 475
nm). In neritic regions the inclusion of larger amounts of colored, dissolved organic
substances (yellow transmitting, termed Gelbstoff or gilvin) and of phytoplankton
(green transmitting) causes a shift in the wavelength of maximum transmissivity
toward the green. Absorbance rapidly increases for longer wavelengths.
(^) Actually all absorption of light by water has a minimum with respect to wavelength,
a “window of clarity” (Yentsch 1980), right at the peak range of wavelengths of solar
irradiance (Fig. 1.5) the wavelengths of visible and photosynthetically active light.
This match of window and available light is one of the remarkable coincidences that