SCI. MAR., 61(4): 431-438
SCIENTIA MARINA
1997
Sources of acoustic scattering near a halocline
in an estuarine frontal system*
A. MADIROLAS1, E.M. ACHA2, R.A. GUERRERO1 and C. LASTA1
1
Instituto Nacional de Investigación y Desarrolo Pesquero, Paseo Victoria Ocampo Nº 1, 7600 Mar del Plata, Argentina.
2
INIDEP and Dto. Ciencias Marinas, UNMDP, Funes y Peña, 7600 Mar del Plata, Argentina.
SUMMARY: Salt wedge is a quasi permanent feature at the Río de la Plata estuary, generating strong vertical density gradients. Zooplankton and ichthyoplankton organisms aggregated below the halocline were insonified utilizing a scientific
echosounder operating with a 120 kHz single beam transducer. From acoustic data and CTD measurements, a strong coincidence between the depths of the scattering layer and the halocline was proven. The organisms from the zooplankton and
the ichthyoplankton were caught with a Nackthai sampler and classified into four functional groups according to their scattering properties. A multiple regression analysis shows fish larvae as the most significant source of sound scattering at the
halocline. Finally, the salt wedge across the estuarine regime is described employing an echochart. The halocline here
showed a quasi constant depth at 7-8 m with the salt water intruding 100 km upstream.
Key words: Acoustics, halocline, estuarine zooplankton, Río de la Plata.
RESUMEN: FUENTES DE REVERBERACIÓN ACÚSTICA EN INMEDIACIONES DE UNA HALOCLINA EN UN SISTEMA FRONTAL ESTUARI– El estuario del Río de la Plata se caracteriza por una cuña salina casi permanente, que genera fuertes gradientes verticales de densidad. Los organismos del zooplancton y del ictioplancton agregados por debajo de la haloclina, fueron insonificados empleando una ecosonda científica operada con un transductor de haz simple de 120 kHz. A partir de los datos acústicos y observaciones CTD, se probó una marcada coincidencia entre la profundidades de la capa dispersora y de la haloclina. Los organismos del zooplancton y del ictioplancton fueron capturados con un muestreador Nackthai y clasificados en
cuatro grupos funcionales de acuerdo a sus propiedades acústicas. Empleando un análisis de regresión múltiple se demuestra que las larvas de peces fueron la fuente más significativa de reverberacion acústica en la haloclina. Finalmente se
describe por medio de un ecograma la cuña salina a través del régimen estuarial. La haloclina aquí presentó una profundidad casi constante entre los 7 y 8 m, y la intrusión río arriba del agua salina fue del orden de los 100 km.
NO.
Palabras clave: Acústica, haloclina, zooplancton estuarino, Río de la Plata.
INTRODUCTION
The Río de la Plata, draining the second
largest hydrologic basin of South America,
defines at the confluence with the continental
*Received August 22, 1996. Accepted April 14, 1997.
**Contribución INIDEP nº 978
shelf waters an important estuarine system. It is
situated at 35ºS between Uruguay and Argentina,
covering an area of 35,000 Km2. This estuarine
system is characterized by a highly dynamic
salinity front, extending about 230-250 km. High
salinity water flows below the less dense riverine
waters, producing a salt wedge with strong vertical gradients in salinity.
SOURCES OF ACOUSTIC SCATTERING NEAR A HALOCLINE 431
FIG. 1. – The study area and the analyzed transects (A and B). Dots indicate CTD stations, solid stars show CTD and plankton stations, and
empty stars on transect A indicate stations where CTD and two level plankton tows were performed. P.E.: Punta del Este; P.P.: Punta Piedras;
S.A.C.: San Antonio Cape.
It has been shown that planktonic organisms aggregate in frontal zones (Le Févre, 1986; Bakun, 1994).
This association has been explained by either passive
concentration or behavioral response of the organisms.
Acoustic techniques have proven to be a useful tool
for monitoring the spatial structure of these highly
dynamic frontal zones (Nash et al., 1987).
In several cruises carried out in this region, zooplankton and ichthyoplankton layers were acoustically recorded in the frontal area. The depth of these
scattering layers coincided with the depth of the
halocline detected at the oceanographic stations.
This paper presents an example of utilization of
acoustic techniques to detect zooplankton and
ichthyoplankton associated with the vertical stratification of the water column. Sound scattering due to
the density gradient is considered a minor source
compared with the presence of biota and hence the
recorded scattering levels are mainly attributed to
these organisms. There is a high correlation between
depths of scattering layer and halocline. Acoustic
and Conductivity-Temperature-Depth (CTD) data
corresponding to one particular transect are compared. To identify the sources of biological scattering, zooplankton and ichthyoplankton were sampled
and classified into functional groups according to
their scattering properties.
432 A. MADIROLAS et al.
MATERIAL AND METHODS
Data from two cruises carried out at the Río de la
Plata Estuary in June and November 1991 are employed.
The study area and the sampling stations are shown in
Figure 1. The surveying platform was the R/V “Cap. Oca
Balda”, a 67 m stern trawler equipped for fishing,
acoustic, oceanographic and plankton sampling. The vessel is run by the National Institute of Fisheries Research
and Development (INIDEP, Argentina). In both cruises
the sampling activities were arranged along transects.
The oceanographic sampling was performed using
a Neil Brown MK III CTD (conductivity-temperature-depth profiler), with a sampling rate of 32 scans
per second, lowered at a speed of 0.5 m s-1. Data were
processed to achieve a one meter vertical resolution.
Distance between stations was in the order of 10 km
along the transects. As stated by Kjerfve (1989), in
shallow estuaries salinity alone determines the water
density. The effect of changes in temperature affecting water stratification are negligible compared to
those from salinity, consequently any further analysis
will be based only on the last parameter. Salinity data
are reported with a precision of 0.05 psu (practical
salinity units UNESCO, 1981), resulting adequate to
define the observed estuarine front, where salinity
varies between 15 and 32 psu.
Acoustic sampling was performed to locate the
depth of the scattering layer associated to the halocline. The echosounders (SIMRAD EK400 and
EK120) were operated at 120 kHz with hull mounted single beam transducers. To quantify the sound
scattering, echointegration (Forbes and Nakken,
1972) was performed during the November cruise.
A SIMRAD QD digital echointegrator connected to
a computer for dataloging, was employed. Echointegration was done over several independent depth
strata. The averaging interval was set to 0.9 km and
10 depth layers of two meters height were defined.
Eight layers were surface referenced and two layers
were bottom referenced. Starting depth for the first
surface referenced layer was 2 m from the transducer face, thus the total starting depth resulted 5.5 m
below sea surface. Echosounder was calibrated after
the survey by following ICES recommended procedures using standard targets (Foote et al., 1987;
Foote, 1990). In order to avoid unwanted echoes
(ship’s noise), the lowest possible threshold was set
in the echointegrator and echorecorder. System setup and echosounder calibration values are shown in
Table 1.
Zooplankton and ichthyoplankton organisms
were collected employing a Nackthai sampler.
This high speed plankton collector is a modified
Gulf V plankton sampler (Nellen and Hempel,
1969). It is a torpedo shape sampler with an open,
heavy metal frame of 2.75 m in length and 0.40
m in diameter and having a metal nose cone with
a diameter aperture of 0.20 m. The sampler has a
single net with 405 µ mesh aperture, without an
opening-closing device. It was equipped with a
TABLE 1. – Echosounder system set-up and calibration data.
Echosounder
Transducer
Frequency
Ψ
Pulse duration
Bandwidth
SL + VR
Echointegrator
Threshold
: SIMRAD EK-400
: SIMRAD 120-25-E
: 120 kHz
: -18.1 dB (nominal)
: 0.3 ms
: 10 KHz
: 120.3 dB
: SIMRAD QD
: 30 mV(peak)
digital flowmeter and performed oblique tows at
3-4 knots. Sampling depth was estimated using
an angle indicator (inclinometer) with a wheelout
meter on the cable. Volumes filtered during a tow
varied from approximately 26 to 48 m3. Numerical density of the organisms was estimated after
they were classified into four groups (Table 2).
These groups were defined according to the
organisms scattering properties, mainly size,
shape and body composition (Clay and Medwin,
1977).
A transect from November cruise was chosen
(transect A in Figure 1) to show visually the relationship between the halocline and the sound scattering. Then, echorecordings taken at CTD stations
from both cruises, were employed to probe statistically the coincidence between the oceanographic
data and the acoustic records. A simple correlation
analysis among depth of the halocline (defined as
the maximum vertical salinity gradient) and depth of
the scattering layer (obtained graphically from the
echocharts) was performed.
TABLE 2. – Functional groups of zooplankton and ichthyoplankton organisms, defined in accordance to their scattering properties. Density is
expressed as number of individuals per m3, A: above the halocline; B: below the halocline.
Functional Group
Fish larvae
(FL)
Decapoda larvae
+
Holoplanktonic Crustacean
(Cr)
Medusae + Ctenophora
(GP)
Copepods
+
Chaetognath
(C+Q)
Body Length
Body Composition
10-20 mm
Fleshy body with
swimbladder
4-10 mm
Quitinous body
Range Density of
Positive Stations
A: 0.126 - 0.342
B: 0.106 - 0.814
A: 4.64 - 29.50
B: 11.20 - 68.93
10-100 mm
Watery body
<2 mm
quitinous body
A: 0.70 - 5.02
B: 0.80 - 12.20
A: 107.73 - 2,613.92
2-10 mm in length
semi gelatinous body
B: 189.53 - 1,352.38
SOURCES OF ACOUSTIC SCATTERING NEAR A HALOCLINE 433
To identify the contribution of each group to the
total sound scattering, a multiple regression analysis
was performed. This analysis was based only on
those stations where the halocline was observed
from the CTD data, and acoustic information was
available. The stepwise method was employed with
the numerical densities of each planktonic functional group, and the values of the column scattering
coefficient Svc (the integration of the volume scattering coefficient Sv over the water column) as the
dependent variable. The volume scattering coefficient Sv is defined as the sound energy scattered per
unit volume, measured at a reference distance (usually 1m). For a given species and size class, Sv is
proportional to the density of organisms per cubic
meter (Clay and Medwin, 1977).
To describe vertical distribution of the functional
groups, two oblique tows were performed at six stations along transect A (Figure 1), with a mean distance of 15 km between stations. The first tow from
bottom to surface, and the second one only above
the halocline crossing the surface mixed layer. The
comparative analysis of both samples provides a
rough insight of the association of the zooplankton
and ichthyoplankton with the surface and bottom
layers, defined by the halocline.
Finally, to bring a general view of the scattering
layer across the whole estuary, an echochart
obtained along transect B (Figure 1) is presented
together with the corresponding values of the vertical salinity gradients.
RESULTS
A typical temperature (T), salinity (S), and density (σt) profile obtained at the stratified regime for
springtime, is presented in Figure 2. The thermocline (typical at this area) and a sharp salinity gradient of about 4.5 psu m-1, are seen at a 9 m of depth.
Mean gradients (± sd) measured along the transect A (Fig. 1), at a mean depth of 9.6 m (2.11), were
1.12ºC m-1 (± 0.43) for temperature and 4.58 psu m1
(± 2.73) for salinity. The contour lines in Figure 3.a
represent the vertical distribution of salinity along
this transect. The halocline depth shallows with
increasing distance towards the coast. An outer and
an inner front at the surface are also seen. Acoustic
data obtained along this transect are shown in Figure
3.b, where the size of the symbols is proportional to
the measured values of the volume scattering coefficient. The sound scattering due to the plankton also
434 A. MADIROLAS et al.
FIG. 2. – Example of a temperature (T), salinity (S), and density (σt)
profile, characterizing the vertical structure of the study area.
presents a tendency to shallow towards the coast.
Also high scattering values from the near bottom
layer, seen at the end of the transect, are mainly
associated to the benthic crustacean (Peisos
petrunkevitchi). In general terms, a good visual correlation between the halocline depth and the scattering layer due to the plankton, is observed.
The simple regression analysis performed
between the depth of the halocline and the depth of
the scattering layer, showed a highly significant correlation (r2= 0.964). A linear model, fit without a
constant, resulted in a slope of 1.02 which indicates
that both halocline and scattering layer depths were
coincident (Table 3).
Echo charts showed that adult fish were rare or
absent and that plankton concentrations dominated
the midwater recordings. This situation was supported by trawling and plankton catches. The dominant larval fishes in the plankton samples were Gobiosoma parri (Gobiidae), Engraulis anchoita and
FIG. 3. – Salinity (a) and acoustic (b) sections for the transect A in Figure 1. Contour map of salinity expressed in psu units. Size of the square
symbols are proportional to the volume scattering strength coefficient of the plankton. Symbol (t) on the top axis indicates the location of the
CTD profile shown in Figure 2.
Anchoa marinii (Engraulidae). The most abundant
Crustacea groups were Acartia tonsa (Copepoda),
decapoda larvae (mainly Porcellanidae zoeas),
Mysidacea, Amphipoda and Cladocera. The gelatinous zooplankton was composed by small Medusae
(primarily Liriope tetraphylla) and Ctenophora
(mainly Mnemiopsis maccradyi). Sagitta friederici
was the highly dominant Chaetognath.
The ranges of the density estimations for the four
plankton groups here defined, above and below the
halocline, are presented in Table 2.
As stated previously, sound speed changes due to
the water density gradient appears as a minor source
of scattering compared to scattering from the biota
present. According to sound speed equation by
Mackenzie (1981), the sound speed change originated by the picnocline itself (due mainly to the halocline) was very low, 1497.4 m s-1 and 1500.0 m s-1
above and below respectively.
The multiple regression analysis between the
density of the plankton groups in each station, and
the corresponding average column scattering, shows
fish larvae as the most significant source (Table 4).
This regression is illustrative of the importance of
fish larvae, but should not be considered predictive.
TABLE 3. – Simple regression analysis between halocline depth
(independent variable) and depth of the scattering layer. Y-intercept
value was forced to zero.
Constant
Std. Err. of Y Est.
R Squared
R
No. of Observations
X Coefficient(s)
Std. Err. of Coef.(X)
0
0.443671
0.964624
0.982153
30
1.018132
0.008822
The resulting model can be written as follows:
Svc = 158.32 δf + 0.025 δc-q
where:
Svc : column scattering coefficient, in m2 m-3
δf : fish larvae numerical density, in m-3
δc-q : copepods plus chaetognath numerical density, in m-3
The observed and estimated values are shown in
Figure 4.
SOURCES OF ACOUSTIC SCATTERING NEAR A HALOCLINE 435
FIG. 4. – Estimated values of the column scattering coefficient (Svc)
obtained from the multiple regression analysis, and the observed
values at the stations.
In those stations employed for the multiple
regression analysis, Gobiosoma parri (average total
length 12.11 mm; standard deviation 3.06) was the
most abundant larval fish. As other Gobiidae, G.
parri larvae are characterized by a conspicuous
swimbladder at all developmental stages (Acha,
1994). For individuals of mean size, swimbladder
length was in the range 1.0-1.2 mm. According to
Holliday and Pieper (1980) the individual target
strength for similar sized fish larvae is well above
the detection threshold of the system, which is estimated at -104 dB as the minimum individual target
strength for a single target detection. It must be
noted that at the employed sound frequency and
FIG. 5. – Vertical distribution of the functional groups of plankton,
defined as percentages of organisms in each group, above and
below the halocline (F.L.= fish larvae; Cr.= crustaceans; G.P.=
gelatinous plankton; C+Q: copepods and chaetognath).
regarding the size/wave-length ratio of these organisms, their acoustic response falls in the Rayleigh
zone where a high frequency dependence may be
expected.
TABLE 4. – Multiple regression analysis employing stepwise method, between the column scattering coefficient (dependent variable) and the
numerical densities of the planktonic functional groups (Fish larvae; Decapoda larvae + holoplanktonic crustacean; Medusae + Ctenophora;
Copepods + Quetognats)
Statistics of the multiple regression analysis
Independent Variable
Coefficient
Std.Error
t-Value
Sig.Level
δf (fish larvae)
δc-q (copep.+ quetog.)
158.325195
0.024623
30.77219
0.012272
5.1451
2.0065
0.0013
0.0848
R-squared (Adj)= 0.8615
Standard Error= 31.69160
N= 9
Analysis of Variance for the Full Regression
Source
Sum of Squares
DF
Mean Square
F-Ratio
P-value
Model
Error
50969.5
7030.51
2
7
25484.7
1004.36
25.3742
0.0006
Total
58000.0
9
436 A. MADIROLAS et al.
FIG. 6. – Composed echo chart of the transect B in Figure 1, showing typical zooplankton and ichthyoplankton concentrations in the estuary
of the Río de la Plata. Numbers on the top axis indicate the gradient value at the halocline, expressed in psu m-1. On the bottom axis, daytime
is indicated.
The vertical distribution of the four functional
groups was studied by towing the plankton net separately above and below the halocline. Results are
presented in Figure 5. Since the Nackthai net is not
equipped with an opening-closing device, some
degree of mixing with organisms located above the
halocline may be expected in the samples taken at
the bottom layer. Nevertheless, it may be seen that
larval fishes are more abundant below the halocline.
Scattering layers due to zooplankton and ichthyoplankton concentrations are frequently observed in
the whole estuarine system of the Río de la Plata.
Figure 6 shows an echo chart (compressed in the
horizontal scale) taken across the estuary during the
June cruise (transect B). In this figure, planktonic
scatterers reveal the saline intrusion below the fresher river waters. The halocline here has a quasi constant depth at about 7-8 m, and the salt wedge reaches 100 km in length. The salt wedge inner boundary
is mostly defined by the intersection between the
halocline and the shoaling bottom. The numbers on
top indicate the maximum vertical salinity gradient
in psu m-1, showing that weakening of the halocline
begins at segment 11. No planktonic scatterers were
detected in the upper layer. They were always located in the lower layer close to the halocline. However, in some segments with a strong halocline (segments 3 and 8 in Figure 6), planktonic scatterers
seems to be less concentrated, probably as a result of
the patchy nature of the plankton distributions. This
also supports the fact that most of the sound scattering at the halocline is due to the presence of organ-
isms concentrated at it. Because of the association
between plankton and the salinity stratification,
when the halocline became weaker the planktonic
scatterers are evenly distributed in the lower layer
(segments 12 and 13), or disappear (segment 14).
Since most of transect shown in Fig. 6 happened
during daytime, possible effects of night arrival can
not be seen. However, in spite of the halocline weakness the presence of scatterers only in the lower
layer can still be seen in segments 12 and 13 (night).
Other acoustic recordings taken at night show that
the scatterers remain restricted to the lower layer.
DISCUSSION
The spatial coincidence between the halocline
and the scattering layer has been demonstrated.
Most of the scattering from this layer is explained by
the presence of zooplankton and ichthyoplankton,
with larval fishes being the most significant scattering source.
This study must be seen as a qualitative analysis
about the probable sources of scattering. Catches
obtained with a Nackthai sampler are biased respect
to the smallest and largest range of planktonic
organisms. Copepods are expected to be underrepresented to some extent, but given their low target
strength (Holliday and Pieper, 1980) it should not
affect these results. The presence of other organisms
(e.g. larger medusa and micronekton) contributing
to the scattering could not be discarded. A more
SOURCES OF ACOUSTIC SCATTERING NEAR A HALOCLINE 437
complete identification of the acoustic targets and
their contribution to the total sound scattering could
be obtained by employing a plankton sampler
equipped with a depth indicator (allowing to tow the
sampler exactly at the halocline), and with an opening-closing device (to avoid mixing with organisms
from the other levels). Complementary sampling
with a midwater trawl to collect macroplankton and
micronekton would also be desirable.
Biota at the Río de la Plata estuary is highly
dynamic, both in time and spatial scales. Hence
changes in the taxonomic composition and relative
density of the planktonic scatterers are expectable.
Acoustic sampling techniques are characterized
by high resolution, both in time and space, allowing
one to obtain quasi-synoptic images of the organisms’ distribution over extended and highly dynamic regions like frontal areas.
Continuous acoustic profiles can be used as a
monitoring strategy to describe the spatial structure
of this frontal system by employing zooplankton and
ichthyoplankton organisms as a link between
acoustics and physical processes.
The physical properties of the targets will determine the scattering from a certain aggregation. By
employing a higher frequency sound even smaller
components of the zooplankton and ichthyoplankton
could be detected and hence make feasible the
acoustic monitoring of these areas.
ACKNOWLEDGMENTS
We are grateful to K.G. Foote for his encouraging comments and guiding our efforts.
438 A. MADIROLAS et al.
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