24-May-2012

EO Data Access
		How to Apply

		How to Access


3rd ERS SYMPOSIUM Florence 97 - Abstracts and Papers

Remote Sensing and in situ measurements of Bio-optical Variability in a Mediterranean Mesoscale System

Remote sensing and in situ measurements of bio-optical variability in a mediterranean mesoscale system

Services

Site Map

Frequently asked questions

Glossary

Credits

Search
Advanced Search

Remote sensing and in situ measurements of bio-optical variability in a mediterranean mesoscale system

Trevor H. Guymer, Steven G. Alderson, Peter G. Challenor, Paolo Cipollini, Thomas N. Forrester, Jill N. Schwarz, Helen M. Snaith		Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, U.K. email: thg soc.soton.ac.uk web: http://www.soc.soton.ac.uk/JRD/OMEGA/

Alison R. Weeks		Southampton Institute, East Park Terrace, Southampton SO9 4WW, U.K. email: weeks_a southampton-institute.ac.uk web: http://www.southampton-institute.ac.uk

Giovanni Corsini		Department of Information Engineering, Univ. of Pisa, via Diotisalvi 2 56126 Pisa, Italy email: gcorsini iet.unipi.it web: http://radar.iet.unipi.it/OMEGA/index.html

Abstract

This paper deals with some results of a study of the biological variability in a strong mesoscale system (the Almeria-Oran front and the Algerian Current) by means of remote sensing and in situ data. CTD data from transects by SeaSoar (a towed, undulating profiler) and stationary deployments have given us a detailed picture of the vertical structure of the ocean both at the front and on the gyre scale. A fluorometer on the SeaSoar has helped to resolve the chlorophyll patchiness which is believed to be the result of strong vertical motion. Discrete measurements of total suspended matter and dissolved organic matter (gelbstoff) have still to be analysed in detail but will contribute to the definition of the optical variability across the front and to the direct comparison of in situ and satellite ocean colour data. These include OCTS and MOS imagery, Secchi disc observations and underwater light measurements within the euphotic zone gathered by a new, multispectral instrument at a number of stations. Some early results of the interpretation of such a comprehensive dataset are discussed. They indicate the potential and capabilities of the combined satellite/in situ approach in rendering a picture of bio-optical variability at the mesoscale.

Keywords: Alboran, Western Mediterranean, Biological variability, phytoplankton, MOS

1. Introduction

The eastern Alboran Sea exhibits strongly varying oceanographic signatures within a small geographic area (Tintoré et al., 1988), allowing a comprehensive fine scale survey to be completed in few days. To resolve and characterize the biological and bio-optical variability across the Almeria-Oran front and in the Algerian Current, and the effects of the intense mesoscale activity on the optically active parameter distribution, a synergetic, multiplatform approach has been adopted, using data from a ship, and remotely-sensed data from an aircraft and a number of satellites, including ERS-2. The orientation of the survey itself has been chosen to include transects along ERS sub-satellite tracks, which facilitates a direct comparison of in situ geophysical measurements with satellite-derived quantities. This work has been undertaken in the framework of the OMEGA project in parallel with a study of the physical circulation at the front which is described in Snaith et al. (1997). Altimeter and IR SST data, combined with continuous in situ bulk and radiometric surface temperatures, have helped to define the structure and evolution of the front's surface expression. ATSR-2 IR images show that the front remained within the fine scale survey pattern throughout, running from NW to SE. In this paper we focus our interest on those observations more directly related to the biology and we show the potential and capabilities of the combined satellite/in situ approach in rendering a picture of bio-optical variability at the mesoscale.

2. Data

2.1 In situ measurements

The RRS Discovery Cruise 224 took place in December 1996 and January 1997 in the eastern Alboran Sea, across the Almeria-Oran front and in the westernmost part of the Algerian basin. The first part of the cruise (1/12/96 to 11/12/96) was devoted to a large scale survey of the whole area (fig. 1a) where some legs coincided with TOPEX/POSEIDON or ERS-2 ground tracks. Then, a fine scale survey across the Almeria-Oran front was repeated five times in the period 11/12/96 to 17/01/97 (fig 1b shows the tracks of the first fine scale survey) to monitor the evolution of the front. The second repeat of leg e was conducted as ERS-2 passed directly overhead. Moreover, a survey further east along the Algerian coast was performed (25/12/96 to 28/12/96), to map the physical and biological variability in the Algerian current (fig. 1c).

Figure 1 - Ship tracks during the Discovery 224 cruise -a) large scale survey; b) first fine scale survey (letters in italics identify the single transects); c) survey in the Algerian Current. The dot and arrow in b) mark the position of station DS13050

A number of biological and bio-optical measurements were taken during the cruise (for their physical counterpart, see Snaith et al. (1997), which includes an ATSR-2 image showing the location of the front). The SeaSoar towed by the ship carried a transmissometer to measure light attenuation and a fluorometer by which it has been possible to measure chlorophyll fluorescence along the ship track. The SeaSoar itself profiled from the surface to a depth of 350 m and back within few minutes. That corresponds roughly to a resolution of 4 km along the ship track. So this instrument sampled a virtually complete vertical section of the water body up to a depth of 350 m along the ship track.

On board ship there was a second fluorometer which was fed through the continuous non-toxic water supply, thus giving a continuous reading of fluorescence at the surface. A third fluorometer was employed on a CTD at a number of stations for a more accurate study of the vertical distribution of biological activity.

The readings of all these instruments are being calibrated by means of discrete chlorophyll measurements from water samples collected at the surface and from the CTD bottles at various depths. These samples were first filtered using 0.7 痠 glass fiber filters, chlorophyll was extracted from the filters with acetone and then measured on a Turner fluorometer with respect to a known chlorophyll standard. Suspended sediments were measured at a number of stations with the gravimetric method on 0.7 痠 glass fiber filters. Weighing before and after incineration of the filters provided a measure of the organic and inorganic fractions. Dissolved Organic Matter (also known as gelbstoffe or'yellow substance') was estimated by filtering the water samples through 0.2 痠 cellulose nitrate membrane filters and then measuring the absorption spectrum of the filtrate in the region 300-700 nm with a Camspec M350 spectrophotometer. For these measurements we used two 40 mm quartz cuvettes where the reference cuvette was filled with fresh milli-Q water. The absorption curves obtained were corrected for residual scattering and fitted to an exponential model of the form

a_y(lambda)=a_y(440) exp (-s_y (lambda-440))

thus yelding the values of absorption at lambda=440 nm due to yellow substance a_y(440) and of the slope s_y for every sample.

A contribution to the study of the biological and bio-optical variability was given by the SUMOSS (Southampton Underwater Multiparameter Optical-Fibre Spectrometer System) (Robinson et al., 1996; Schwarz and Weeks, 1997). This is a self contained fibre optic spectrometer which can be lowered down to several tens of meters and whose use was possible in some stations during the cruise. The SUMOSS is a prototype instrument capable of measuring both the inherent optical properties (scatterance and transmission) and the apparent optical properties (upwelling and downwelling irradiance, and thus reflectance). As the measurements are taken to a high degree of spectral resolution, this instrument is highly appropriate for validating remotely sensed measurements from the new generation of ocean colour sensors. During the cruise the SUMOSS was used to collect up- and downwelling irradiance, beam transmission and scattering, hyperspectrally. Transmission and scattering were also measured using a red laser diode (670nm). Scattering levels were below the detection limit of the SUMOSS in its current configuration. Optical measurements were made at a range of depths, from 0 to 70m, and profiles of reflectance and Kd, the diffuse attenuation coefficient, were generated.

2.2 Remotely sensed data

During the cruise and immediately before and afterwards, a number of satellites were gathering data which will be extremely useful to depict the physical, biological and bio-optical variability of the region, once they are compared to and calibrated with the in situ measurements. Those data which are more directly related to the physical variability of the region (such as infrared SST data and altimeter data) are discussed in Snaith et al. (1997). Here we will focus our attention on ocean colour data. These have been gathered in broad spectral bands in the visible by the AVHRR on board NOAA-12 and NOAA-14 satellites and by the ATSR (Along-Track Scanning Radiometer) on board ERS-2. The high temporal repetition rate of the observations from these satellites makes them useful for observing the dynamical evolution of the water masses, but the width of their spectral bands in the visible makes it difficult to use the data from these bands for a quantitative estimation of the optically active parameters in the water body.

Sensors with a much better spectral resolution like the MOS (developed at DLR Berlin and on board the Indian Remote Sensing Satellite IRS-P3) and the OCTS on board ADEOS can be used to estimate the concentrations of the optically active parameters by means of algorithms relating the water leaving radiance in different bands of the spectrum to these parameters (bio-optical algorithms). Such algorithms can be calibrated and validated whenever in situ data (sea truth) are available which are simultaneous with the satellite overpass. Frequent upper air measurements of wind, temperature and water vapour were made by launching radiosondes from the ship and these will be used to check the atmospheric corrections for the satellite data.

Although OCTS data have been gathered by NASDA at the time of the cruise, they will not be available for some time. Thus, in the following section, we will concentrate our attention on a MOS image to show how this instument can successfully detect oceanographic features characterized by variability in the phytoplankton content and thus in the colour of the sea. These structures are in turn linked to the circulation and physical variability of the area.

3. Some results

The fluorescence signal measured by the fluorometer on SeaSoar during the five passes along the ERS-2 track (track e in figure 1b) is shown in figure 2. Chlorophyll patchiness and subsurface maxima can be clearly observed, especially close to the front. In survey 3 there is clear evidence of phytoplankton extending in a tongue down to 200 m. This tongue coincides with the frontal zone defined by SeaSoar temperature and salinity (Snaith et al., 1997). Similar, but less well-marked tongues can be seen in surveys 1, 2 and 5. This is in perfect agreement with the findings by Strass (1992) that chlorophyll patchiness can be due to the vertical circulation at fronts.

Figure 2 - Fluorescence signal along the ERS-2 track (transect e) for all the five fine scale surveys (as measured by the fluorometer on SeaSoar) - the black arrows mark the position of the front at the surface as seen in the 27.5 kg/m³ isopycnal (Snaith et al., 1997)

Subsurface maxima of chlorophyll were also observed from CTD casts in a number of stations, and in the SUMOSS data. Figure 3 shows the irradiance reflectance profile computed from SUMOSS observations of the upwelling and downwelling irradiance at different depths at station DS13050 (see fig. 1b). The station was occupied on 21/12/96 between 13:27 and 15:13, and was located in the vicinity of the front, while the front itself was rapidly moving northwards between the second and the third fine scale surveys (Snaith et al., 1997). As reflectance is approximately inversely proportional to absorption (Morel and Prieur, 1977), the trough at 30-50 m in the region 400-450 nm (marked as A in the figure) may indicate a high absorption by chlorophyll contained in the phytoplankton (which has its absorption maximum around 440 nm). This corresponds to a sub-surface maximum of biological activity.

Figure 3 (34 Kbytes)

The variability in the concentration of phytoplankton is also evident in ocean colour imagery from satellites. Figure 4 shows a MOS image (22 Dec 1996 at 11:05) in the region of the Algerian Current. The image, which has been radiometrically corrected, refers to the second visible band of the sensor, centred around 443 nm which is the maximum of chlorophyll absorption. It shows very clearly an intrusion of Mediterranean Water (marked with MW in the figure, and darker at 443 nm indicating it is richer in phytoplankton) between two masses of less trophic Modified Atlantic Water (MAW) (Tintoré et al., 1988). This corresponds wery well to what is observed in a nearly-simultaneous AVHRR image, shown in figure 5, where the two different water masses appear to have a distinct difference in temperature. This highlights the potential of ocean colour sensors in general (and of MOS in particular) in giving a quite detailed picture of the biological variability, once this type of data is calibrated and merged with the in situ observations.

Figures 4 and 5 (100 Kbytes)

The results of the yellow substance measurements (not shown here) indicate low levels of this optically-active component (absorption by yellow substance at 440 nm is everywhere less than 0.1 m^-1). Nevertheless, it may be necessary to take its effects into account in the bio-optical algorithms, to improve the accuracy in the estimation of phytoplankton content.

4. Summary and conclusions

This paper shows some preliminary data from a large dataset which has been collected during the OMEGA observational phase. Although the short time from the end of that phase has not allowed any detailed merging and integration of the various datasets, it should be clear from the few examples presented that a large quantity of information has been gathered, from in situ measurements and satellite observations, which has now to be interpreted to render a detailed picture of the biological variability in the region under investigation, and especially of the effects of physics on biology. It is also hoped that the datasets collected will help to understand the surface signature of subsurface structures such as the deep maximum of phytoplankton.

5. Acknowledgements

The OMEGA project is funded by the European Commission MAST programme (MAS3-CT95-001).

We thank Andreas Neumann and the team at DLR - Institute of Space Sensor Technology in Berlin for providing the MOS data. Paolo Cipollini is supported by the Commission of European Communities through a Research Training Fellowship (HCM contract no. ERBCHBGCT930440). Jill Schwarz is partly funded by the Defence Research Agency.

References

Morel, A. and L. Prieur, 1977:: Analysis of variations in ocean colour. Limnology and Oceanography , 22, 4, pp. 709-722.
MOS home page:: http://www.ba.dlr.de/NE-WS/ws5/mos.html
OCTS home page:: http://www.eoc.nasda.go.jp/guide/guide/satellite/sendata/octs_e.html
Robinson, I. S., K. Trundle, A. R. Weeks and J. Dakin, 1996:: NERC final report, Grant No. GST/02/0672.
OMEGA Project home page:: http://formentor.uib.es/~cristina/Omega/
Schwarz, J. N. and A. R. Weeks, 1997:: Toward optical closure in coastal waters. Proc. SPIE vol. 2963, Ocean Optics XIII , Halifax (Canada), 22-25 October 1996, pp. 614-619.
Snaith, H. M., T. H. Guymer, T. N. Forrester, P. Cipollini, J. T. Allen, S. G. Alderson, and D. A. Smeed, 1997:: ERS, shipborne and aircraft observations of circulation in the region of the Almeria-Oran front. Proc. of 3rd ERS Symposium - Space at the Service of our Environment , 17-21 March 1997, Florence, Italy, European Space Agency.
Strass, V. H., 1992:: Chlorophyll patchiness caused by mesoscale upwelling at fronts. Deep-Sea Research , 39, pp. 75-96.
Tintoré, J., P. E. La Violette, I. Blade, A. Cruzado, 1988:: A study of an intense density front in the Eastern Alboran Sea: the Almeria-Oran front. Journal of Physical Oceanography , 18, pp. 1384-1397.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry