MONITORING AND MODELLING OF THE COASTAL ENVIRONMENT
USING SST, SOLAR FLUXES, CHLOROPHYLL CONCENTRATION AND
INORGANIC SUSPENDED PARTICULATE MATTER
Francis Gohin and Alice Vanhoutte-Brunier
IFREMER Dynamiques de l’Environnement Côtier, Centre de Brest, BP 70, 29280 Plouzané,
Brittany, France
e-mail : [email protected]
ABSTRACT
Remote sensing observations can be used as forcing data into biogeochemical models. We can mention
amongst those data the solar Irradiance at sea surface, as derived from METEOSAT visible imagery, and the
inorganic Suspended Particulate Matter, SPM, which can be derived from the SeaWiFS or MODIS marine
reflectance. These quantities are all essential in modelling the irradiance available for the photosynthesis in
the water column over the continental shelf. Chlorophyll concentration maps can be used for validating the
biogeochemical models or for assimilation and a better estimation of key biological parameters. Doing so,
biologists generalize the procedures already applied by physical oceanographers who use satellite SST
maps to validate the outputs of their physical models.
As for other domains related to the environmental and climatic change, the remote sensing data are
essential for monitoring the variability of the coastal seas. In the different browsers developed at IFREMER,
satellite derived chlorophyll and inorganic SPM are associated to the in situ measurements provided by the
conventional networks (SOMLIT/CNRS/INSU), REPHY (IFREMER). Three browsers are now available at
IFREMER providing SST (AVHRR/ SAF Ocean and Sea Ice and the decadal climatology NOAA-SAF),
chlorophyll and inorganic SPM concentrations (derived form SeaWiFS and MODIS) for the English
Channel/Southern North Sea, the Bay of Biscay and the Occidental Mediterranean Sea. As it is a quantity
used in the models, the solar irradiance, integrated over 10 day periods, will be added to the satellite maps
shown in the browser. These data are also available, colocated, in NETCDF format. All those products are
developed for the validation of the biogeochemical models and the monitoring of the environment quality
(eutrophication risks and Harmful Algal Bloom occurrence) in the spirit of the european program GMES
(Global Monitoring for Environment and Security).
1. Introduction
In the frame of GMES (Global Monitoring for Environment and Security), the European Commission and the
European Space Agency found projects, like ROSES, COASTWATCH, MARCOAST, designed to establish
a European capacity for the provision and use of operational information in the coastal domain. Within
ROSES (Real Time Ocean Services for Environment and Security) two browsers of images have been
defined at IFREMER for monitoring the coastal environment in the Bay of Biscay and the English Channel.
These browsers aim at providing information useful for the monitoring and the development of
biogeochemical modelling (including forecasting in the next years). Nowadays, the most operational systems
using remote sensing data for monitoring the coastal environment are based on sensors operating in the
visible (Ocean Colour, Surface Solar Irradiance) and infra-red (SST) part of the radiative spectrum. SST
(average and anomaly) is a basic parameter of the environment from which is discerned the climatic trend
and the annual characteristics. Ocean colour provides information on the biological and chemical status of
the environment through the chlorophyll concentration and the inorganic suspended particulate matter.
Though chlorophyll concentration is a positive indicator of the productivity of the water, an excess in
chlorophyll is often associated to eutrophication and HABs (Harmful Algal Blooms). Turbidity is a significant
indicator of the environment quality as it affects the amount of light available for the phytoplankton growth in
the water column and the death rate of bacteria with time. Light at the sea surface, the solar irradiance,
which can be derived from METEOSAT or AVHRR, is another major component of the environment
observed from space.
2. National coastal in situ networks in France
Two main national networks have been developed for the monitoring of the coastal environment
The SOMLIT network has been developed by INSU (Institut National des Sciences de l’Univers) with a
double aim :
1- to monitor a common and stable set of locations to assess the mean effect of the climatic and
anthropogenic forcing on the long-term evolution of the coastal environment.
2- to build a minimum reference set of environmental data (including chlorophyll) for each location without
excluding additional measurements for specific-site research
The Phytoplankton and Phycotoxin Monitoring Network (REPHY) monitors particularly the phytoplankton
species which are toxic for humans and marine organisms. This national network, carried on by IFREMER at
numerous stations along the coast, is also measuring chlorophyll-a to evaluate the phytoplankton biomass.
Figure 1. The SOMLIT (left) and the REPHY
(right) stations.
3. The browsers of images
Two browsers of images, for the English
Channel and the Bay of Biscay, are
available through internet for the
scientific community.
Figure 2. View of the Bay of Biscay
Browser.
Data shown in these browsers are :
1) Ocean colour data from SeaWiFS, MODIS, (MERIS) and derived products :
Chl-a
Inorganic Suspended Matter concentrations
Attenuation coefficient of light K
2) SST from AVHRR
SST from NOAA-16 and NOAA-17 (4 daily images)
SST 10 day climatological averages 10 days (obtained from SAF O&SI an derived from
SST/Pathfinder 1985-1995 )
3) Solar Irradiance from METEOSAT
4) In situ data
Chl-a and inorganic SPM (mostly deduced from turbidity) obtained from coastal networks,
instrumented buoys and cruises
4. The satellite-derived maps of chlorophyll and Suspended Particulate Matters
Despite their well-known limitations in coastal waters, the ocean colour sensors provide a unique means for
observing the phytoplankton distribution over the continental shelf. However, optical techniques from space
platforms are hampered by clouds and can not be used alone for the monitoring of the phytoplankton
throughout the year. For that purpose, they need to be associated to biogeochemical models to be fully
efficient. Satellite maps, calibrated on field measurements, can be used to validate and constrain the
physical and biological parameters of the models or provide data for assimilation. In coastal waters, light is
very often a key limiting factor for the phytoplankton growth and the light attenuation coefficient in the
euphotic layer is a major parameter in ecological modelling. The chlorophyll (Chl), as an indicator of the
biological particles, and the inorganic suspended Particulate Matter (SPM) govern a large part of the
absorption and scattering properties of the coastal waters. Both quantities are simulated in the coastal
ecological models and can also be retrieved from ocean colour data.
The chlorophyll concentration and inorganic SPM in the Bay of Biscay and the English Channel have been
routinely retrieved, since the launch of SeaWiFS in August 1997, by using a look-up table described in Gohin
et al (2002, 2005). MODIS is now currently processed by a daily provision of water leaving radiance data
from CLS/Toulouse.
In complement to daily data, averaged chlorophyll and SPM maps, on the 1998-2004 period, have been
defined to be compared to the mean SST maps provided by the SAF O&SI. These averaged maps give us
the most accurate overview on the suspended sediments and the coastal primary productivity throughout the
year.
Averaged chlorophyll concentration in the middle of the spring months is shown on Figure 3. The resolution
is 1.1 km but it may be coarser along the coast as land contaminated pixels may be flagged. The seasonal
trend in chlorophyll concentration appears very clearly on the images. In the English Channel, the first
blooms occur in the Pas de Calais in March, and later in April-May in the vicinity of the Seine plume. The
maximum chlorophyll concentration is observed in May in the North-Sea and in June in the Bay of Seine. In
the vicinity of the river plumes, local structure can be different from this overall situation. For instance high
chlorophyll concentration can still be observed in August near the shore in the vicinity of the Seine plume.
Chl March 11-20
Chl May 11-20
1 Chl April 11-2020
Chl June 11-20
Figure 3. Averaged chlorophyll maps
on 10 day periods.
The maps shown on Figure 4 present the mean chlorophyll concentrations calculated at each location of the
different monitoring networks. More field data are available in April than in March as the monitoring focuses
on the growing season. Mean satellite and field maps are in agreement but the satellite information provides
a spatial overview which is much better than what can be derived from the field stations.
Chl March 10-20
Chl April 10-20
Figure 4. Mean chlorophyll concentration from in situ data.
Figure 5. Chlorophyll concentration at the beginning of April in 2002 and 2003
(in situ data are displayed with coloured disk).
5. Annual variability observed from satellite data
The chlorophyll maps (Figure 5) observed on 2002/04/08 (left) and 2003/04/07 (right) show two contrasted
situations at the beginning of the production season. Early phytoplankton growth has been observed in
spring 2003. The spring conditions in 2003 had a similar impact on the phytoplankton growth of any water
body, located in the Bay of Seine or in Western Normandy/Eastern Brittan
6. Use of METEOSAT solar irradiance for understanding the conditions enabling the
growth of a late winter phytoplankton bloom in the Bay of Biscay
At the end of winter, blooms often occur in the distal plumes of the rivers when the solar irradiance is
sufficient. Bloom are enhanced when the depth of the upper layer is under 30 meters and the water clarity is
sufficient. Such conditions occur in the outer part of the river plumes as the haline stratification enables the
formation of an upper layer with low concentration of suspended sediments. The March 2000 bloom is
characteristic of those blooms. The conditions of its development have been detailed in Gohin et al. (2003).
The relatively high solar irradiance at the surface water (Figure 6) has been essential to explain this strong
development (Figure 7) leading to the exhaustion of phosphates in those waters.
(a)
(b)
Solar irradiance in Watt m-2
Figure 6. The solar irradiance derived from METOSAT. (a) Mean solar irradiance at sea surface
derived from METEOSAT imagery during the 5-12 March period . (b) Daily solar irradiance in the
bloom area.
(a)
(b)
(a)
Chlorophyll concentration on March 5 with the data of
the cruise MODYCOT 00
Chlorophyll concentration on March 7
(d)
(c)
Chlorophyll concentration on March 12
Chlorophyll concentration on March 16
Figure 7. Images of the Chlorophyll concentration in
the initial phase of the bloom, (a) and (b), in its
Chlorophyll concentration in mg m-3
maximal extent (d).
7. The heat-wave event during the “canicule” in summer 2003
7.1 The SST observations
The SST was approximately 2 degrees above the average in August 2003 (see Figure 8 in the Bay of
Biscay). The effect of the heat wave are observed as early as June and was still significant in September
2003
Figure 8. The “canicule” episode in august 2003 observed through the browser of the Bay of Biscay
(a) Mean climatological temperature. (b) Night temperature during the heat wave.
7. 2 Use of satellite-derived products in the 3-D modelling of a Harmful Algal Bloom
The browser of images gives an overview of the growth of a spectacular phytoplankton bloom in the Western
English Channel during summer 2003. Thanks to the multi-annual data set of NetCdF data in the browser,
the bloom of July 2003 can be compared to that of 2002 one (Figure 9).
July 2002
July 2003
The bloom was larger and the chlorophyll
levels were higher during the 2003 event. A
cruise managed by the CEFAS and the Martin
Ryan Institute (Lyons et al., 2004) highlighted
that the high chlorophyll concentrations shown
by the satellite SeaWiFS was due to a
monospecific bloom of the dinoflagellate
Karenia
mikimotoi.
Following
these
observations, a model of Karenia mikimotoi,
first developed in the context of the bay of
Biscay (Loyer et al., 2001) has been
introduced in a general model of the primary
production in order to reproduce this bloom
and to investigate the growth controlling factors
(Vanhoutte-Brunier et al., 2004). Many type of
satellite-derived products are used to force the
model (see also Piollé et al. in this issue). First,
mineral SPM derived from the OC5 algorithm
is used to compute the penetration of light in
the water column. This strongly control the
phytoplankton dynamic. The heat exchanges
at the sea surface interface and the available
light for phytoplankton are provided by the
hourly SSI METEOSAT-AJONC product (Brisson et al., 1996).
Figure 9. Monthly average of the surface chlorophyll a concentration for July 2002 an 2003, from
SeaWIFS/OC5 data.
The climatological conditions that occurred during the bloom have been obtained from the satellite imagery.
Monthly averages of the SSI and the SST are shown on Figure 10. The water column received higher heat
fluxes during June 2003. The SST images reflect this heat input ; the difference of SST between the two
years reached 2-2.5°C in the Western English Channel. This over-input of energy at the top of the ocean
favoured the installation of a marked thermocline in the western English Channel. In such conditions, the
growth of Karenia mikimotoi is enhanced at the thermocline depth. At this depth (15-20 m), the shearing
stress is lower and nutrient-rich bottom waters outcrops. Due to exceptional climatological conditions, the
installation of a marked thermocline favoured the development of this microalgal species.
Mean Sea Surface Irradiance
(METEOSAT/SAF)
Mean Sea Surface Temperature
(AVHRR)
June 2002
June 2002
June 2003
June 2003
Figure 10. Monthly mean of satellite-derived products : SSI and SST of June 2002 and June 2003.
June 2003
8. Discussion and conclusion
June 2003
June 2003
The combination of a large spatial coverage by satellite data with detailed and accurate observations from
field networks builds up an efficient tool for the monitoring of coastal waters.
The best spatial coverage is obtained from the satellite data but the temporal frequency of acquisition
(basically everyday) is very variable as the ocean colour method is hampered by clouds. Field measurement
frequency is more regular but cannot be reasonably available at many sites with a high sampling frequency,
for instance at one on two days which is theoretically possible from satellite, except in case of persistent
cloud cover. However, field data sets often present the flora composition which is still difficult or impossible to
obtain from satellite, as well as nutrient concentrations. In conclusion, both types of data are necessary for
coastal monitoring. Satellite data (NOAA/AVHRR, SeaWiFS (OSC/USA), MODIS(NASA/USA), MERIS(ESA)
provide a spatial overview of the environmental conditions while field measurements give a better insight in
the water column content, in nutrient concentrations, and in fauna and flora distributions. They also provide a
long series of field references for calibrating the different space sensors. From these seven years of
experience in satellite data processing, it comes out that both sets of data can be presented and
manipulated together.
Satellite data and field measurements are also of major interest in the development and the use of
biogeochemical models, both for the parameterisation and for the validation of these models. Modelling is a
very powerful tool to provide forecasting both at short term (eutrophication, HAB occurrence and distribution)
and at long term (effects of trends in anthropogenic inputs, climatic change, on the coastal ecosystem).
9. References
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