CHAPTER 7 • The Implications of Oceanographic Chaos for Coastal Management
133
lens (Fig. 8). This technique is the preferred option because it minimizes disturbances
to the suspended matter (Eisma et al. 1990). However it has limitations because of four
reasons. First, it necessitates a lot of power to illuminate the field of view, and this requires an electric cable to the main research vessel or a small boat (Fig. 9). Secondly,
in rough weather waves introduce rapid and violent instrument motions that blur the
images. Third, it is impractical in very turbid systems (usually the cut -off point is about
100-200 mg 1-1) when the concentration is so high that excessive floc overlap occurs
on the images. Finally, strong currents cause floc breakage around the camera housing's optical port. In these environments an alternative sampling technique for suspended sediment is to sample the water using a wide Niskin bottle with no rubber cord
inside the tube. This minimizes floc break-up. The suspended matter settles onto a
microscope slide with a well (Anim. 10). Using an externally operated piston, the well
slide is then capped with another slide without disturbing the sample, which can then
be examined with an inverted microscope.
All these techniques reveal an enormous amount of patchiness at all scales. Duplicate casts can yield widely different estimates of sediment concentration.
In clear waters, without mud, nonmotile and asexual zooplankton commonly occur in patches up to 20 cm wide and most oceanic plankton in patches up to 5 m wide
(Davis et al. 1992). Patchiness in this case is generated by the plankton aggregating
by swimming. In muddy waters the instruments reveal that the marine snow is sticky
and traps small flocs of suspended sediment to generate micro aggregates typically
500-2000 11m in diameter (see examples in Figs. 10-12). These micro aggregates have
settling speeds 10-100 times faster than those of the original small mud flocs. This settlement process constitutes a biological filter at the mouth of turbid tropical estuaries
that trap the sediment in patches in the coastal zone (Ayukai and Wolanski 1997).
There is a dynamic feedback in these patches between the physics and the biology.
Previously the common belief was that the main effect of mud was to decrease light
and hence primary production. It was also thought that that bacterial activity was extensive on mud (Alongi 1998). Mud obviously affects both light and bacterial activity,
but our observations reveal that plankton are also strongly affected. Frequently we
found plankton grazing on the surface of the flocs (Anim. 11). At other times we found
plankton actually being killed by the flocs. For instance, plankton can become glued
by a string to a floc and cannot escape even after swimming madly in circles around
the floc} like a dog on a leash (Anim. 12). Plankton can also become trapped in a floc
by its hooks and be unable to escape (Anim. 13). Plankton can also become buried in a
stringy mud floc and trapped like a fly in a spider web (Anim. 14). Because of this apparent direct interaction between plankton and mud, one suspects that patchiness in
mud distribution would introduce patchiness in the plankton distribution, which in
turn would introduce patchiness into the entire food chain. This requires further investigation.
7.4
Turbidity and Seagrass
Mangrove-fringed Hinchinbrook Channel in tropical Australia (Fig. 13) is shallow, with
a series of sand banks, a muddy coast and a number of natural channels several metres deep (Anim. 15). Strong tidal currents prevail"At spring tides the waters inundate
133
lens (Fig. 8). This technique is the preferred option because it minimizes disturbances
to the suspended matter (Eisma et al. 1990). However it has limitations because of four
reasons. First, it necessitates a lot of power to illuminate the field of view, and this requires an electric cable to the main research vessel or a small boat (Fig. 9). Secondly,
in rough weather waves introduce rapid and violent instrument motions that blur the
images. Third, it is impractical in very turbid systems (usually the cut -off point is about
100-200 mg 1-1) when the concentration is so high that excessive floc overlap occurs
on the images. Finally, strong currents cause floc breakage around the camera housing's optical port. In these environments an alternative sampling technique for suspended sediment is to sample the water using a wide Niskin bottle with no rubber cord
inside the tube. This minimizes floc break-up. The suspended matter settles onto a
microscope slide with a well (Anim. 10). Using an externally operated piston, the well
slide is then capped with another slide without disturbing the sample, which can then
be examined with an inverted microscope.
All these techniques reveal an enormous amount of patchiness at all scales. Duplicate casts can yield widely different estimates of sediment concentration.
In clear waters, without mud, nonmotile and asexual zooplankton commonly occur in patches up to 20 cm wide and most oceanic plankton in patches up to 5 m wide
(Davis et al. 1992). Patchiness in this case is generated by the plankton aggregating
by swimming. In muddy waters the instruments reveal that the marine snow is sticky
and traps small flocs of suspended sediment to generate micro aggregates typically
500-2000 11m in diameter (see examples in Figs. 10-12). These micro aggregates have
settling speeds 10-100 times faster than those of the original small mud flocs. This settlement process constitutes a biological filter at the mouth of turbid tropical estuaries
that trap the sediment in patches in the coastal zone (Ayukai and Wolanski 1997).
There is a dynamic feedback in these patches between the physics and the biology.
Previously the common belief was that the main effect of mud was to decrease light
and hence primary production. It was also thought that that bacterial activity was extensive on mud (Alongi 1998). Mud obviously affects both light and bacterial activity,
but our observations reveal that plankton are also strongly affected. Frequently we
found plankton grazing on the surface of the flocs (Anim. 11). At other times we found
plankton actually being killed by the flocs. For instance, plankton can become glued
by a string to a floc and cannot escape even after swimming madly in circles around
the floc} like a dog on a leash (Anim. 12). Plankton can also become trapped in a floc
by its hooks and be unable to escape (Anim. 13). Plankton can also become buried in a
stringy mud floc and trapped like a fly in a spider web (Anim. 14). Because of this apparent direct interaction between plankton and mud, one suspects that patchiness in
mud distribution would introduce patchiness in the plankton distribution, which in
turn would introduce patchiness into the entire food chain. This requires further investigation.
7.4
Turbidity and Seagrass
Mangrove-fringed Hinchinbrook Channel in tropical Australia (Fig. 13) is shallow, with
a series of sand banks, a muddy coast and a number of natural channels several metres deep (Anim. 15). Strong tidal currents prevail"At spring tides the waters inundate