In Situ Measurement of the Benthal Oxygen
Requirements of Tidal Flat Areas
GENE R. CROOK, Graduate Assistant
DAVID A. BELLA, Assistant Professor
Department of Civil Engineering
Oregon State University
Corvallis, Oregon
INTRODUCTION
A visual comparison between high and low water conditions in most tidal
estuaries reveals large tidal flat regions. The deposits within these regions, which are
exposed during low water conditions, often contain large amounts of organic
material. In this respect, they resemble non tidal benthal deposits. It is the purpose
of this paper to discuss the oxygen demand exerted by these deposits on the
overlying water during the periods in which these deposits are covered.
Laboratory methods of measuring benthal oxygen uptake rates have utilized
carboy enclosures containing hand placed deposits (1,2,3), sealed reaction chambers
with magnetic stiring devices (4,5,6), and tubular devices containing core samples of
benthal deposits taken in the field (7,8,9,10,11,12,13,14). Most field studies utilized
a closed container which was placed over the benthal deposit (15,16,17,18).
Different methods were employed to simulate mixing conditions, however, in
most cases, the mixing within the respirometer was quite different from that of the
undisturbed system. This problem was partially corrected through the use of a long
funnel shaped respirometer in which the velocity of the water passing through the
respirometer provided the mixing (19).
EXPERIMENTAL DESIGN AND PROCEDURE
Light and dark benthal respirometers developed during this research were
constructed from plexiglas half cylinders, 5.64 meters long by 0.152 meters wide.
The resulting long and narrow respirometer covered a benthal area (0.813 sq m) large
enough so that small isolated inconsistencies in bottom muds would not cause great
variabilities in uptake rates. The long, narrow shape was also required for simulation
of actual mixing conditions. Turbulent mixing over tidal flat muds is primarily
caused by the action of water velocities. In the respirometer designed during this
project, velocities typical of a specific test site were generated over bottom muds by
recirculating water in the enclosed long respirometer. A flow development section
was constructed on the inflow end of the respirometer to distribute the flow evenly
over the respirometer cross-section. With the volume to area ratio used, reasonable
oxygen uptake rates could be measured in four to eight hr. Removable flanges were
designed so that the flange portion of the respirometer could be inserted into the
bottom deposit some time before the actual respirometer sections were attached. In
this way, the bottom deposit was allowed to come to equilibrium before attaching
-249-