To assess whether the acoustic and the optical method of investigation of the
distribution of zooplankton and suspended particles give similar results in terms of
distribution in space, as well as to quantify the concentration of zooplankton, the
comparison of both methods was performed.
In most cases there was a good agreement between measured and modeled
values of backscattering strength S V . The lack of agreement was in conditions of
very low zooplankton abundance when the values of S V fell to the level of subsurface noise. The peaks of the measured S V values visible on Fig. 3 are probably
caused by the presence of fish, which could not be detected by LOPC and thus
could not be modeled. There were cases when the modeled results of S V occasionally reached higher values then measured ones (Fig. 3). This could be caused by
the fact that both instruments sampled different water volumes. Acoustic transducer
was mounted to the ship hull, while LOPC was hauled alongside the ship—the
particles detected at a given moment by optical sensor could have been omitted by
acoustic sounding. The other reason is that backscattering strength is very sensitive
to changes in density and sound speed contrasts, which in turn depend on size and
properties of each zooplankton individual. Acoustic model assumed homogenous
contrasts for whole collection of zooplankton individuals, which was obviously a
simplification.
Except the occasional fish presence, the modeled values were also much lower
than the measured ones in the case of hydrological front (Fig. 5). LOPC measurements detected high abundance of suspended matter in the colder waters
(Fig. 6). Elevated levels of acoustic backscatter were also recorded in the same
fragment of the study section (Fig. 7). A comparison of measured and modeled
values of backscattering strength indicates that in the glacial waters there is lack of
agreement between two methods (Fig. 8). This could suggests that the density and
sound speed contrasts used in the model were too low. With use of environmental
properties, it could be assumed that the detected suspended matter is most likely
mineral of glacial origin and thus having different properties than zooplankton
assemblages (i.e. higher contrasts giving higher backscattering strength S V ).
The combined use of acoustic and optical techniques improves the resolution of
zooplankton measurements, while additional sensors enable insights into the
dynamics of the ecosystem, including environmental forces that regulate zooplankton communities.
6 Conclusions
Acoustical and optical methods supplemented by environmental parameters measurements can identify water masses of different origin. They can also detect
zooplankton distribution in time and space as well as differences in zooplankton
abundance and size structure.
Backscattering strength measured by echosounder and predicted by “high-pass”
model using zooplankton size distribution provided by the LOPC agrees very well,
58
L. Hoppe and J. Szczucka
distribution of zooplankton and suspended particles give similar results in terms of
distribution in space, as well as to quantify the concentration of zooplankton, the
comparison of both methods was performed.
In most cases there was a good agreement between measured and modeled
values of backscattering strength S V . The lack of agreement was in conditions of
very low zooplankton abundance when the values of S V fell to the level of subsurface noise. The peaks of the measured S V values visible on Fig. 3 are probably
caused by the presence of fish, which could not be detected by LOPC and thus
could not be modeled. There were cases when the modeled results of S V occasionally reached higher values then measured ones (Fig. 3). This could be caused by
the fact that both instruments sampled different water volumes. Acoustic transducer
was mounted to the ship hull, while LOPC was hauled alongside the ship—the
particles detected at a given moment by optical sensor could have been omitted by
acoustic sounding. The other reason is that backscattering strength is very sensitive
to changes in density and sound speed contrasts, which in turn depend on size and
properties of each zooplankton individual. Acoustic model assumed homogenous
contrasts for whole collection of zooplankton individuals, which was obviously a
simplification.
Except the occasional fish presence, the modeled values were also much lower
than the measured ones in the case of hydrological front (Fig. 5). LOPC measurements detected high abundance of suspended matter in the colder waters
(Fig. 6). Elevated levels of acoustic backscatter were also recorded in the same
fragment of the study section (Fig. 7). A comparison of measured and modeled
values of backscattering strength indicates that in the glacial waters there is lack of
agreement between two methods (Fig. 8). This could suggests that the density and
sound speed contrasts used in the model were too low. With use of environmental
properties, it could be assumed that the detected suspended matter is most likely
mineral of glacial origin and thus having different properties than zooplankton
assemblages (i.e. higher contrasts giving higher backscattering strength S V ).
The combined use of acoustic and optical techniques improves the resolution of
zooplankton measurements, while additional sensors enable insights into the
dynamics of the ecosystem, including environmental forces that regulate zooplankton communities.
6 Conclusions
Acoustical and optical methods supplemented by environmental parameters measurements can identify water masses of different origin. They can also detect
zooplankton distribution in time and space as well as differences in zooplankton
abundance and size structure.
Backscattering strength measured by echosounder and predicted by “high-pass”
model using zooplankton size distribution provided by the LOPC agrees very well,
58
L. Hoppe and J. Szczucka