Experiences in making acoustic measurements in a mesocosm with Calanus finmarchius
Original version
This report is not to be cited without prior reference to the authorsAbstract
In connection with a larger study of the effects of nutrification and turbulence on
primary and secondary production, acoustic measurements have been made in the
water volume defined by floating, vertically oriented, cylindrical enclosures, with
flexible but impermeable walls, called mesocosms. Nominal diameters and
greatest depths in the tapered bottom region are 2 and 9 m, respectively. The
mesocosms contain rather small quantities of zooplankton, of which Calanus finmarchicus is the organism of primary interest in the acoustic work. In fact, a
range of scatterers were visualized, including krill, smaller organisms, and, most
likely, hydrographic features as well as the wall of the mesocosm, complicating
recognition and quantification of Calanus finmarchicus. The density of this
copepod in stages I-VI, reckoned in orders of magnitude, is 10-1000 animals per
cubic meter, the precise figure depending on the particular mesocosm. An attempt
was made to quantify the scatterers by echo integration. Measurements were made
with the SIMRAD EK500 echo sounder, with narrowband transducers operating at
119, 200, and 714 kHz. Postprocessing was accomplished with the Bergen Echo
Integrator. While preliminary, limited results are not demonstrably unambiguous,
the observations and associated measurement experiences may prove valuable in
like future investigations.