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-- Seakem
:: "I, Ai rborne Remote Sens i ng of Sea Surface Chlorophyll and Temperature along the B.C. outer coast
(July 12 and August 11,1979)
by
G. Borstad and R. Brown Seakem Oceanography ltd. 9817 West Saanich Road Sidney, B.C. V8l 3S1
SCIENTIFIC AUTHORITY
J.F.R. Gower Remote Sensing
Institute of Ocean Sciences Department of Fisheries and Oceans
9860 West Saanich Road Sidney, British Columbia
DSS FILE NO. 6-1850-000-000-130 CONTRACT SERIAL NO. 15879-00097
011829
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TABLE OF CONTENTS
1. Description of the project 2. Measurement of chlorophyll by Analysis of Water Colour 3. INSTRUMENTATION
3.1 3.2 3.3 3.4 3.5 3.6 3.7
The I.O.S Spectrometer Airborne Radiation Thermometer Aircraft Navigation Personnel Data Processing Sea Truth Measurements
4. MISSION REPORTS
4.1 4.1.1 4.1.2 4.1.2.1 4.1.2.2 4.2 4.2.1 4.2.2 4.2.2.1 4.2.2.2
July 12 Overflight of Ship-of-Opportunity Cruise RV003 Overview Resul ts Temperature Chlorophyll a August 12 Overflight of Ship-of-Opportunity Cruise RV004 Overview Resul ts . Temperature Chlorophyll a
5. DISCUSSION
5.1 5.2 5.3
Oceanography of the area as determined by remote sensing Problems Observation of commercial fishing vessels
Page
1
2
3
3 6 7 7 9 9
10
11
11 11 11 11 16 19 19 19 19 24
29
29 29 32
LIST OF FIGURES
Page
1. Reflectance Spectra for varying chlorophyll concentrations 5 2. Fluorescence line height vis chlorophyll a concentration
(CFOX Operations) . 5
3. Britain Norman Islander aircraft 8 4.· Distribution of Sea surface temperature, July 12, 1979 12 5. Distribution of surface chlorophyll ~, July 12, 1979 13 6a. Ai rborne temperature esti.mates vis shi p temperature measurements,
July 12 Cruise 15 6b. Airborne fluorescence line height measurement vis chlorophyll ~,
July 12 Cruise 15 7a. Continuous airborne fluorescence line height record for line AB,
July 12 flight 17 7b Continuous airborne temperature record for line AS, July 12
flight 17 8a. Continuous record of sea temperature as measured by tne ship,
July 12 18 8b. Continuous record of in vivo fluorescence as measured by the
ship July 12 18 9. Distribution of sea surface temperature, August 11, 1979 20 10. Distribution of surface chlorophyll a, August 11, 1979 21 11a Ai rborne temperature estimates vis ship temperature measurements,
August 11 Cruise 23 lIb Airborne fluorescence line height measurements vis chlorophyll a,
August 11 Cruise - 23
12a Continuous record of airborne fluorescence line height for line DE, August 11 flight 26
12b Continuous record of airborne temperature measurements for 1 i ne DE, August 11 flight 26
13a Continuous record of airborne fluorescence line height for line GH, August 11 fl i ght 27
13b Continuous record of airborne temperature measurements for line GH, August 11 fl ight 27
14a Continuous record of sea temperature as measured by the ship August 11 28
14b Continuous record of in vi.vo fluorescence as measured by the ship August 11 28
15 Tiros-N satellite thermal infra-red image for study area, July 14, 1979 30
Seakem
1. DESCRIPTION OF THE PROJECT
This report outlines the organization,methods and some of the results of the first two airborne missions of a routine remote sensing_ operation off the southern portion of Canada1s west coast. The project is the result of an unsolicited proposal from Seakem Oceanography ltd., Sidney, British Columbia to the Department of Supply and Services, Ottawa. The Scientific Authority is J.F.R. Gower, Remote Sensing, Institute of Ocean Sciences, 9860 West Saanich Road, Sidney, B.C., V8l 4B2.
The project which began in June 1979 is operated in conjunction with another Seakem project in which, a commercial ship-of-opportunity is used to gather oceanographic data. The objectives of the remote sensing project are:
1. to demonstrate the potential usefulness of airborne remote sensing of chlorophyll and sea temperature off the west coast especially in conjunction with commercial ships-of-opportunity.
2. to extend the quasi-synoptic nature of the ship-ofopportunity project into a second geographical dimension, thereby providing more extensive coverage of the west coast.
1
One full time oceanographic technician, a part-time computer-programmer and part-time oceanographer are involved in monthly survey flights over Juan de Fuca and along the west coast of Vancouver Island. Sea surface temperature and chlorophyll ~ concentration are measured using an infra-red radiometer and the I.O.S. colour spectrometer. The aircraft in use is a rented twin-engine Britain Norman Islander.
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2 Seakem
2. MEASUREMENT OF CHLOROPHYLL BY ANALYSIS OF WATER COLOUR - Summary of Water Colour Theory
The amount of radiation emerging from the sea surface is controlled by absorption and scattering processes within the upper layers of the water column, and the downwelling incident radiation above the surface. The downwelling incident radiation may vary both spectrally and in absolute amount with sun elevation and sky state. Within the water, the spectral character of the incident radiation is altered by absorption due to water itself and a variety of other dissolved and suspended materials, including planktonic algae, and the remaining energy is scattered in all directions. Part of that which is scattered upwards and emerges can be utilized as an indicator of the water content. The emerging signal (E ) can be normalized u for the variations of the incident radiation (I) by dividing by I to give the reflectance (R)[(Clarke et al 1970), Arveson et al 1973, Morel 1978]. Since for our application we wish to measure phytoplankton concentration we make use of the fact that chlorophyll ~, the major phytoplankton pigment, absorbs strongly in the blue portions of the visible spectrum, while the backscatter from phytoplankton is more or less spectrally constant. On the other hand, the absorption by water itself is relatively weak in the blue and increases towards the red wavelengths (see Morel & Prieur 1977 for a thorough discussion of the theoretical aspects of the analysis of variations in water colour). The most generally used method of computing water chlorophyll content from water reflectance spectra, makes use of the ratio of reflactancesin the green (540 nm) and blue (460 nm) (Clarke et al. 1970; Curran, 1972; Arvesen et al, 1973; Miller et al, 1976; Morel, 19]8; and many others). This ratio (which we call p) takes advantage of the fact
that with increasing chlorophyll concentration, the absorbance at 460 nm will increase much more strongly than that at 540 nm. Figure 1 illustrates the changes in the reflectance spectrum as the chlorophyll concentration of
the upper 20 m of the water column changes. A second approach to the remote measurement of chlorophyll content
has also studied using the 1.0.5. spectrometer (Neville and Gower, 1975, Gower and Neville, 1977, Borstad and Gower, in prep.). The technique is
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-- Seakem 3
based on the ~ vivo fluorescence by phytoplankton at wavelengths around 685 nm. In British Columbia coastal water outside areas of high turbidity, ~ vivo chlorophyll ~ fluorescence is sufficiently strong to be detected from above the surface. While absorption of red wavelengths in the water limits the depth from which fluorescence from phytoplankton can be measured to about 2 or 3 m, very weak attenuation in the atmosphere allows the signal to be measured almost equally well at 2 m or 300 m altitude.
The height of the very strong peaks at 685 nm on the reflectance spectra in Figure 1 are closely related to the amount of chlorophyll in the uppermost layer of the water. By way of example, Figure 2 illustrates the agreement between Fluorescence Line Height (FLH) and chlorophyll a for a series of aircraft and ship measurements made by us in July and August in Georgia Strait as part of the Canada-France Optics Experiment. The three open circles, which indicate points having a wide departure from the indicated relationship are samples which also showed low ~ vivo fluorescence per unit chlorophyll a.
3. INSTRUMENTATION
3.1 The I.O.S. Spectrometer (chlorophyll measurements)
The Institute of Ocean Sciences (I.O.S.) spectrometer has been described in detail elsewhere (Walker et al., 1974, 1975) and it will only be briefly described here. It is a lightweight instrument which uses as its sensing element a 256-channel silicon diode array. The array is scanned in approximately 0.1 second with a variable frequency of 4, 2, 1 or 1/2 (second)-l. The integrated output from each diode is digitized and recorded on computer compatible tape via a custom built computer. After each scan the diodes are re-zeroed, and where several seconds data are averaged, the signal-to-noise ratio is very high.
The diode array measures light focused by a 35 mm focal length f/2 Nikon camera lens onto a 100 ~m wide slit. This slit is in turn in the focal plane of a 35 mm local length f/2 Erfle eyepiece. Collimated light from this lens is reflected from a 300 line per millimeter grating blazed
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4 -- Seakem
Figure 1. Reflectance spectra of waters with varying surface chlorophyll a concentrations. ( ... = 1.9 vg/L; ---= 8.4 ~g/L; ___ = 19.6 ~g/L)
Figure 2: Fluorescence Line Height as measured from the aircraft versus surface Chlorophyll a for stations occupied during the CFOX operations. The open circles represent samples showing very different values of in vivo fluorescence per chlorophyll a. These points were not used in the regression.
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.. ~ " .. ~
GO :I:
GO C
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-= u..
1.0,.------------------,
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0.8
~ 0.6 Iu llJ ...I LL
~ 0.4
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400 500 600 WAVELENGTH (om)
400r------r--------~------~--------~
0 300
200
100 0
30 40
Chlorophyll ~ (pg/l)
5
Oceanog}4aphy --
6 -- Seakem
at 760 nm and refocused by a Littow arrangement back onto the diode array. The grating is set so that the diode array registers the spectral range 380 to 1065 nm. Attached to the objective lens is a polarizing filter aligned so as to block the light reflected from the sea surface, which at the Brewster Angle (530 from the vertical) is almost completely horizontally polarized. This arrangement reduces the spurious effects of reflected light by about a factor of 5, although variations may result from increases of surface roughness.
During normal survey operations measurements of the upwelling light signal and the incident light are recorded in conjunction with periodic measurements of the electrical Idarkl current signal. Incident irradiation
is measured through an opal glass diffuser plate and a fibre-optic light pipe with quick release fitting.
The spectrometer electronics package and radiometer are packaged together with a dual pen chart recorder and oscilloscope to facilitate loading and unloading the aircraft. The spectrometer optics are secured in a revolving mount such that they look down at a front surface mirror and out at the water at an angle of 530 from the vertical. The spectrometer can be rot~ted about the vertical axis to allow the water to be viewed at all azimuths to the aircraft line of flight. This allows the operator to select an azimuth such that sun glitter is minimized (in practice we look away from the sun at 150-170 0
). The spectrometer is mounted in a Ibaywindow l door built of plywood and metal to fit the Islander aircraft. This was necessary because the aircraft used does not have a photo-hatch. InstalJation prior to each flight takes less than 1 hour; removal of the equipment after each flight can be accomplished in 15 to 20 minutes. Most flights are
of 4-5 hours duration.
3.2 Airborne Radiation Thermometer (Barnes PRT-5) Temperature
The Barnes radiometer measures naturally emitted radiation in the 9.5 to 11.5 ~m range by continually comparing it to the energy emitted by an internal temperature controlled reference enclosure. Incoming radiation is interrupted at a frequency of 100 Hz by a highly reflective optical chop-
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-- Seakem 7
per thus enabling the detector to alternately view the sea and the temperature reference. The detector itself is an immersed thermister which converts the energy received into a voltage directly related to the temperature difference of the sea and the reference. The field of view of the instrument is 20 which at 90 m altitude means that the footprint on the sea surface is 3 m in diameter.
The instrument is calibrated by using the blackened interior of a copper cone which is immersed in a well-circulated water bath. Its sensitivity in the laboratory is approximately O.loC and its accuracy is better than 0.50 C. In the field, however, there are several factors which may introduce error. The most important of these is absorption by water vapour in the air.
3.3 Aircraft
. The aircraft used here is a twin engine Britain Norman Islander \figure
3) rented locally for each flight. All flights are made at 90 m altitude and at an airspeed of between 90 and 100 kn. At this speed the aircraft has a range of 6 hours.
3.4 Navigation
On the first two flights navigation was from point to point in Juan de Fuca, and by compass and radio beacons along the west coast. The accuracy of this system of navigation is less than would have been achieved if an inertial navigation system was used, but we consider it sufficient for the purpose of this project. Original plans called for use of the I.O.S. inertial navigation system, but this has been out of service since April, 1979. Such a system should give an accuracy of 2-3 km over flights of 50 km while navigation by compass and radio beacons probably results in errors twice as great. Where
neither end of a leg starts or ends over land, errors may be even greater.
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8 Seakem
Figure 3. Rented Britain Norman Islander used for these operations.
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3.5 Personnel
On each mission three people are involved. One person acts as navigator sitting in the co-pilot's seat, instrijcting the pilot and keeping notes of times, locations and radio fixes.
A second person operates the data acquisition system and monitors the real-time record of the strip charts. He keeps a log of all spectrometer operations and features visible on or in the sea, such as lines of flotsam, colour changes, congregations of fishing boats, sea birds or medusae as well as subjective information concerning sea state and cloud cover.
A third person is required to rotate the optics package to look away from the sun after each turn, to mount the light pipe and the lens cap when 'incident ' and 'dark' records are required. He also keeps notes on phenomena visible on or in the sea.
3.6 Data Processing
The flights usually begin around 1000 hrs and end near 1600 or 1700 hrs. Preliminary data processing begins the next morning and an uncalibrated record of various parameters such as PRT-5 output in millivolts or fluorescent line height can be plotted by computer directly onto a chart overlay. This allows a roughly contoured map of the temperature or chlorophyll fields to be generated by noon the following day. Such a Imapl was generated for the August flight and made available to the Ocean Ecology group just prior to their
August cruise. It proved useful in directing them to areas of high chlorophyll concentration Ci n other operati ons thi s summer where shi p-to-ai rcraft communications were available, the aircraft was used to direct a research ship on the basis of the real-time data on the strip charts}.
During these flights data was written to tape at approximately 2 second
intervals- the combination of this integration time and the movement of the aircraft results in a footprint on the sea surface which is approximately
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10 -- Seakem
2m x 100m. During computer processing, this data is corrected for variations of incident light and plotted as strip charts for detailed inspection (figures 7 a,b, 12 a,b, 13 a,b). The contour maps (figures 4,5,9 ~10 and ) have been produced by subjectively contouring Calcomp plots of the strip charts which were averaged over 50 second periods. This averaging, plus the subjectivity of the contouring means that features must be larger than 2 or 3 km to appear on these maps. Thi s process admi ttedly results in very conservati ve representations of structures actually observed. Much greater resolution is possible, but for our purposes here and beca~se of the size of the survey grids, we have chosen to simplify and emphasize structures greater than about 2 km.
3.7 Sea Truth Measurements
Sea truth measurements were provided by a commercial ship-of-opportunity operating between Howe Sound and the Nootka Sound-Esperanza Inlet area of the west coast of Vancouver Island (Seakem Oceanography Ltd. Ship-of-Opportunity Project, D.S.S. #lSB78-00255 , Scientific Authority R.O. Brinkhurst). Oceanographic measurements were made from this ship on one return trip each month between April and October. Water drawn from near the ships keel at 3m depth, was analysed continuously for temperature, conductivity, chlorophyll ~ . .!!!. vivo fluorescence and zooplankton particles, (figures 8 and 14 illustrate examples of.. the continuous data). Discretest:lmples were also taken for measurement of extracted chlorophyll and nutrient concentration, salinity and phytoplankton analysis. A complete discription of this project, including details of methods and results is given by Borstad et al (1979) in a report to the scientific authority. It should be noted that the ship data used in the present report as sea truth measurements of surface temperature is not of high quality. At the time of writing, data concerning warming of the water as it enters the ship and final sensor calibrations is not available. We feel that these two uncertainties are such that they tend to cancel each other, and the measured temperature are within .50C of the true temperatures.
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Seakem
4. Mission Reports
4.1 July 12 overfl ight of Ship ... of-Opportunity Crui se RV003
4.1.1 Overview
Figures 4 and 5 illustrate the flight plan followed on the July 12 overflight in conjunction with the Ship-of-Opportunity cruise RV03. This
11
flight was attempted using only a navigator and a single spectrometer operator. This was found to be insufficient personnel, so at least three persons took part in the airborne measurements in following flights.
This was also the first flight using the IIbay window ll spectrometer mounting. Problems with this structure necessitated some in flight repairs and thus caused a change in the intended flight plan. The ship was not observed from the aircraft, but was between station 29 and station 31 during the flight.
4.1. 2 Results
4.1.2.1 Temperature
The relationship between sea-surface temperature as measured with the ART and the ship measurement of sea temperature at 3m depth is presented in figure 6a. The solid line indicates the line of perfect agreement with dashed
lines indicating the + 0.50C accuracy specified by the manufacturer of the ART. About half this error is expected in the ship measurements due to differences in temperature between the sea skin and 3m depth, warming of the water as it enters the ship, and uncertainties pertaining to the shipboard temperature sensor which are unresolved at the time of writing.
The ship was in the vicinity of Station 30 during the flight and these
measurements are therefore the most comparable. No synchronous observations were made on this flight as the ship was not sighted from the air and fuel remaining did not allow a search to be made. Stations 26 and 34 are those ship stations for which the airborne measurements are most separated in time from surface
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ESTEVAN PT,
12W OO'W
JULY 12. 1979
Sea Surface Temperature
{as determined by Airborne Radiation Thermometer measurements}
AIRCRAFT FLIGHT PATH
@
49·~(i .. N~· ~_\~·.::::~.~~:·~:"}6 A. o
SHIP MEASUREMENTS (3m depth)
AIRCRAFT TRANSECT SHOWN IN FIGURE 7
CONGREGATIONS OF FISHING VESSELS
........
100 talhoma
.'.
\.
4S-00'N __ _
Figure 4:
A-U
1241' oo'w
)15
" ~". -,', "
Map af the study area showing contours. of sea surface temperature for July 12, 1979 overflight.
ROCKS
I-' N
ESTEVAN PT.
125'- 00' W JULY 12.1979
Distribution of Chlorophyll a
(as determined by airborne measurements of Fluorescence line Height)
AIRCRAFT FLIGHT PATH
@ SHIP MEASUREMENTS (3m depfh)
" B
A. 11;;7
*.)'0'/../1'" W; AIRCRAFT TRANSECT SHOWN IN FIGURE 7
CONGREGATIONS OF FISHING VESSELS
_ 48- 0 O' N ____ _
1241' oo'w
100 'Olhom,
\.
Figure 5: Map of the study area showing FLH isogleths for July 12 1979 measurements. Units are FLH x 10 and are related to chlorophyll ~ as shown in figures 2 and 6b.
I-' W
'~~~7.·~_._~~~~T·~~._~ ___ ,""·_" -~ ..... """~~_~_""'-_-.r.~''''''''' •. ''''''''''''':=-'~ <"...,.. .. ::>-"'""'.,..r=~-:'.~ .... ··.a=?".rl-"-'';O' .... :l.IK\·n:Q.,.."'='~l!::fn~.:'i-..::I2".~:-..~.,.-aw.i:.@~
14 __ Seakem
Figure 6(a): Relationship between sea temperatures as measured on board the ship and estimates of sea surface temperature from aircraft measurements. The solid line indicates a line of perfec6 agreement. The dashed lines indicate the ±O.S C accuracy specified by the ART manufacturer.
Figure 6(b): Relationship between Chlorophyll a (from samples taken on the ship) and the estimated Fluorescence Line Height for the July 12, 1979 overflight. The dashed line indicates the FLH.vs Chlorophyll a relationship from CFOX operations.
Oceanography --
-- Seakem
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12 14
Shipboard Sensor Temperoture (·C)
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31 "30
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16
-
15
15
Oceanography ---
16 -- Seakem measurements. As would be expected in a dynamic area, the agreement between ai rborne and shipboard observat~ons __ decrease~ as the interval between the tWQ .
.. measur~m~nts increases.
A temperature map of the study area is presented in Figure 4. The main feature of the area is a sharp temperature front located to the west of Sheringham Point. At this front the sea surface temperature shows a rapid
increase from 11-120C in eastern Juan de Fuca to >140C to the west. This temperature front was extremely sharp when observed from the aircraft when returning down Juan de Fuca (flight line A-B). Figure 7b shows the rapid decrease in temperature as the aircraft passed through the front. This rapid change in temperature is also evident in the continuous temperature trace from the ship, between stations 32 and 33 (Figure Sa).
A second temperature feature, a IItongue ll of warm water off Observatory Point was missed by the ships track.
4.1.2.2 Chlorophyll ~
The relationship between airborne observations of ~ yivo chlorophyll fluorescence (FLH) and extracted chlorophyll for cruise 3 is illustrated in Figure 6b. It was very similar to that observed in September (to be reported later) and during the Canada France Ocean Optics Experiment (CFOX) which took place in late July and early August. The dashed line is a regression of the CFOX data illustrated in Figure 2. Very little chlorophyll was encountered during the July mission - the range being from 1.S ~g/L at Station 26 to 2.S ~g/L at Stations 29, 30 and 31. The main feature of the chlorophyll distribution as inferred from aircraft measurements of FLH (Figure 5) was a strong discontinuity near Sheringham Point. Figures 7a and 7b, the continuous records for FLH and ART temperature, illustrate that the very large changes in temperature and fluorescence did not occur simultaneously. The 120
, high fluorescence water in mid-channel was surrounded by water slightly colder and containing much less chlorophyll. The survey grid was not adequate to define this feature more fully.
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200
-... ~ ~
'" .,. 0; :J:
OJ 100 c: ::; .. u c: .. u .. ~ 0 .: II..
0 A
10 20 30 40 n Distance (km)
Figure 7a. Continuous record of airborne Fluorescence Line Height measurements for line AB, July 12, 1979 (see figures 4 and 5).
Figure 7b. Continuous airborne temperature measurement record for line AB, July 12, 1979 (see figures 4 and 5).
17
--- OceanogJ'aphy --
c ~ ~ ~ :: Q
CQ ~ Q "b ~
c:::
Figure 8: Strip chart of shipboard measurements of temperature.as the ship passed through the study area on July 12, 1979. Approximate location of stations and land marks are indicated.
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Fi gure 8 (b) : Strip chart of shipboard measurements of in vivo fluorescence as the ship passed through the study area. Approximate locations of stations and landmarks are indicated.
...... ex>
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Seakem 19
Continuous measurements of chlorophyll .~ fluorescence from the ship (Figure 8b) also shows a region of high fluorescence east of Sheringham Point, but on the south side of the strait, this region extends nearly to Race Rocks. Two other features observed by the aircraft, but not evident in the ship data are the small tongue of warm hi gh f1 uorescence water near Observatory Poi nt and the band of elevated fluorescence west of Cape Flattery.
4.2 August 12, 1979. Overflight of Ship-of-Opportunity, Cruise RV004
4.2.1 Overview
This flight involved coincident measurement of water reflectance by French scientists using a 6 channel spectrometer. The aircraft was therefore carrying five sc;;entific observers and nearly 300 kg equipment plus the pilot. Refueling in Port A1berni allowed the survey grid to extend southwatd, .but approximately one hour was lost in the transit to/from A1berni. Because the aircraft was required for another flight late in the day it was not possible to survey Juan de Fuca more comp1~te1y on the return flight. This is unfortunate in view of the interesting structure observed off Neah Bay.
August 12th was clear and c10udfree over the entire study area. The pattern of the survey grid completed is illustrated in Figures 9 and 10. Note that lines down the centre of Juan de Fuca, and those parallel to the coast (DE and GH) may suffer from larger errors in position than other lines because of navigating. Line GH was based on a radio fix on the beacon at Cape Flattery and thus the distance off the coast was known quite well. The data lead us to believe however, that our positions along the line were wrong by about 5 km.
4.2.2 Results
4.2.2.1 Temperature
The relationship between ART temperature and the shipboard sensor
temperature on cruise RV004 is illustrated in Figure 11a. The aircraft intercepted the ship between Stations 5 and 6. Note that the agreement between ART temperature and ship sensor temperature decreases as the time interval between
the two measurements increases. --- Oceanography --
t . !
ESTEVAN PT.
1251- 00· W
D
AUGUST 11.1979
Sea Surface Temperature·
(as determined by Airborne Radiation Thermometer measurements)
AIRCRAFT FLIGHT PATH
@
E 4S· O'i .. N .. \.)-... \i.i~=<:"
G H
SHIP MEASUREMENTS (3m depth)
AIRCRAFT TRANSECT SHOWN IN FIGURE 12
AIRCRAFT TRANSECT SHOWN IN FIGURE 13
CONGREGATIONS OF FISHING VESSELS
......
.....
\
) "
:'" .....
46- 00' N
. ..... \
"'j ".
....
IS
/:
13 ."
13·" 14'"
100 fathGm. '-'j ,/
.. ' "\
\
\
Ii. Q
1241· Oo'w
T AtrOOSH IS.
Figure 9. Map of the study area showing sea surface isotherms as determined by airborne measurements.
N o
4-;'- r,erN ~:~--.
-",a-ooON
12~1· 00' W
T AIT005H IS.
100 fOlhOm,
.,
AUGUST II, 1979 Distribution of Chlorophyll Q
(as determined by airborne measurements of Fluorescence Line Height)
II
G
AIRCRAFT FLIGHT PATH
@ SHIP MEASUREMENTS '(3m dopth)
A 'C'
E
H
AIRCRAFT TRANSECT SHOWN IN FIGURE 2
AIRCRAFT TRANSECT SHOWN IN FIGURE/3
CONGREGATIONS OF FISHING VESSELS
1241',oo'w
Figure la, Map of the study area showing FLH isop~eths for the August 11 1979 measurements. Units are FLH x 10 and are related to chlorophyll a as shown in figure lIb.
N ......
'_'~~'~~_~~'r~~~~"r"'_""",~ __ T_"""""""'''''''''''-~~~, "r.·<"'-,-.·.~·,-.~.,_~,~_~~ .. ~ .... ~.r .. "..-"'_"'_~_,...--_,· .. -=.-.",-.--'_"""""-"'~-"-.--:"":-'7_'TT.':9'U',;"}:77""''1.!'~;:;'F;'.,'''''''_'''-0Z:'''~~.
22 -- Seakem
Figure 11 (a):
Figure 11 (b):
Relationship between sea temperature as measured on board the ship and estimates of sea surface temperature from aircraft measurements for Aug. 11, 1979. The solid line indicates a line of perfect agreement; the dashed lines indicate the ±O.SoC accuracy specified by the manufacturer of the ART.
Relationship between chlorophyll a (from samples taken on board the ship) and estimated FLH-for August 11,1979. The solid line indicates the regression line obtained for these stations; open symbols are samples omitted of regression analysis (see text). The numbers correspond to the stations as plotted on Figures 9 and 10. The dashed line indicates the relationship observed durinq the CFOX operations (see Figure 2) and on the July 12 flight (Figure 6b). Triangles represent chlorophyll a concentrations of samples drawn as the aircraft overflew the ship between stations Sand 6. ~=surface bucket sample, ~= 3m sample.
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16~------.-------'--------r----~~
14
u • .9 -..
12 .. .i! ~ .. Q.
E .. ~
I 10 ~
a::
'" 12 14 16
Shipboard Sensor Temperature ("Cl
150 / 10
/
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/ .7 8 011 .. . c:
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.. 012 u c: ., u .. 2! 013 0
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Chlorophyll .Q. (lIg/l l
Oceanography --
24 Seakem
The contoured temperature map (Figure 9) shows eastern Juan de Fuca was relatively cool (lO-lloC). Except for a very warm patch off Neah Bay there was a gradual increase in surface temperatures north along the coast. Very little temperature structure was encountered north of Cape Beale.
The region of very warm water off Neah Bay was not observed on the outward flight at 1225 hrs (see continuous plot of line DE in Figure 12b) or by the ship as it passed through this area around 1400 hrs (Station 7 on Figure 14b). It was however, the dominant feature on the return flight at approximately 1615 hrs and is indicated by the open diamond for Station 7 on Figure ll~. The contours in Figure 9 are drawn on the basis of both flights.
4.2.2.2 Chlorophyll ~
Figure llb compares fluorescence line height (FLH) as estimated from the Calcomp maps and chlorophyll ~ concentration at 3 m depth at stations occupied by the ship. There is good agreement except for Stations 11, 12 and 13. As described earlier in Section 4.2.1., we suspect there were large position errors along the GH. A comparison of a plot of the airborne continuous data for line GH (Figure 13a) to the appropriate segment of the ship fluorescence trace (Figure 14b) shows very close relationships and substantiates the assumption that a position error (GH was actually further north) was responsible for the disagreement at Stations 11, 12 and 13. The open points were not used to establish the regression in Figure llb. Note that the FLH for Station 11 measured on the return flight (south from Amphitrite), indicated by a black diamond on this figure is a much better prediction of chlorophyll than that FLH on leg GH (open diamond). The open diamond for Station 7 represents observed FLH near this station on the return flight. A slight positional error on the outward bound flight-line or in the position of the ship Station 7 could easily cause such discrepancies, especially in the region of strong gradients like that off Neah Bay. The aircraft apparently skirted this region of elevated fluorescence on the first pass through the area at 1225 hrs (Figure 12a) but the ship did not encounter it at all (1400 hrs, Figure 14b).
The relationship between airborne FLH measurements and extracted chlorophyll ~ from 3 m depth is therefore quite good over all parts of the
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survey area. It was however different from that observed for the July cruise and the Canada-France Optics Experiment (CFOX) cruises in late July and August (shown in Figure llb by dashed line).
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26 -- Seakem ~; ...
200
-to
~ ~
1: "" '0; :x: .. c :J .. u c .. u .. ~ 0
'" ii: CD CD CD 10 0
0 10 20 30 40 50 D Distance (km) E
Figure 12a. Continuous record of airborne Fluorescence
u • .. .. '" ~ .. Q.
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Figure 12b.
Line Height measurements for line DE, August 11 1979 (see figures 9 and 10).
16
14
ij~ 12
~ 10 CD CD 0) 10
0 10 20 30 40 50
D Distance (km) E
Continuous airborne temperature record for line DE, August 11, 1979 (see figures 9 and 10) .
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.. c
:.:::i .. u c .. u .. .. .. o .a ....
Distance (km)
Figure 13a. Continuous record of airborne Fluorescence Line Height measurements for line GH, August 11, 1979 (see figures 9 and 10).
16
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12 t-a: <t
10 ® ® ®
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Distance ( km)
Figure 13b. Continuous airborne temperature record for line GH August 11, 1979 (see figures 9 and 10).
27
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c (") ~ ~ ;: Q ~ ~
~ "t:J ;::0-~
Figure 14.(a): Strip chart of shipboard measurements of temperature as the ship passed through the study area on August 11, 1979. The numbers along the axis show the locations of ship stations ..
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Figure 14(b): Strip chart of shipboard measurements of in vivo fluorescence as the ship passed through the study area. -rh-e--numbers along the axis show the location of the ship stations.
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Seakem 29
5. DISCUSSION
5.1 Oceanography of the Area as Determined by Remote Sensing
The aircraft overflight of the ships of opportunity cruises on the Rivtow Viking allows the expansion of their data into two dimensions. The contour maps of FLH and temperature for the July 11th and August 12th overflights (Figures 4, 5, 9, 10) indicate the presence of strong gradients and structures in the study area.
In general, eastern Juan de Fuca appears homogeneous and well-mixed, with colder surface waters. The structures present in western Juan de Fuca are smaller in scale than features off the west coast of the island and were not adequately covered in either of the two flights reported here. Western Juan de Fuca and the area to the northwest were areas of strong gradients and increased horizontal variability particularly during the August flight.
The existence and scale of surface water structures can be determined from satellite data also. Figure 15 shows a thermal IR image of the lower west coast of Canada taken on July 14, 1979. Unfortunately, monitoring the rapid change or movement of these features requires observations relatively closely spaced in time. Due to mechanical problems with this satellite (TIROS-N) only one other image of the area is available for this time period, and it is grossly affected by cloud.
5.2 Problems
A number of significant problems are encountered in this type of program. Optical and infra-red remote sensing depend upon the ability to 'see' the water and thus fog, rain, smoke or haze will affect the measurements. Measurements are best made at high or constant sun angle and under a clear or uniformly cloudy sky. At present our state of development permits partial correction for patchy cloud but some spurious variability remains.
Aircraft measurements of FLH must be made in conjunction with at least some sea truth data for extracted chlorophyll determinations for each flight, and over as large an area as possible in order to relate FLH to chloro-
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30 __ Seakem
Figure 15. TIROS-N satellite thermal infra-red image (band 2) for July 14,1979
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phyll~. This is largely due to the fact that FLH is an ~ vivo fluorescence measurement and is therefore subject to the same problems as other in vivo fluorescence measurements of chlorophyll~. The fluorescence yield (fl.uorescence/chlorophyll ~) may vary widely, and ;s known to depend on such factors as species composition and nutritional status (see for example Kiefer, 1973}.
A second problem, and one which relates more specifically to our own measurements with the I.O.S. spectrometer, is that the relationship between FLH and in vivo fluorescence as measured on board the ship appears to have changed from one cruise to another. This may be a result of differences in stratification, something to do with light penetration, or differences in
instrument calibration. We are currently investigating this problem. A second indicator of chlorophyll (the ratio of reflectance at 560
sin to reflectance at 440 nm) which is not reported here is also under investigation. When this data is more fully analysed, it may provide a chlorophyll indicator that is not related to ~ vivo fluorescence and hence may not be subject to problems encountered when interpreting fluorescence (or FLH) data. Maps of the distribution of chlorophyll as indicated by the reflectance ratio
will be reported later in the study. The size of structure that is detectable in this study ;s largely
determined by the selection of the flight pattern. For this project we attempted to choose patterns to optimize coverage of the area, with the limitations of weather (especially fog}, day length, aircraft range and navigational requirements. There are, as a result, significant holes within the pattern where there is insufficient data to estimate chlorophyll or temperature. This is especially true in Juan de Fuca, where the scale of structures is smaller than off the west coast of Vancouver Island.
Much greater resolution is possible, however. A tighter grid could be flown, particularly using some sort of electronic navigation aid, but the area of coverage would be diminished due to the limited range of this aircraft. In addition, the integration mode can be changed to reduce the "footprint ll size
without significantly altering the signal-to-noise ratio. As noted earlier, navigation off the outer coast and down the centre
of Juan de Fuca posed a significant problem. Original plans called for use of an Inertial Navigation System, but this instrument was out of service most of
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32 Seakem
the summer. An attempt to use it in October, resulted in overloaded circuitry and forced abortion of the flight. Another smaller and more reliable system is required for future progress.
5.3 Observation of Commercial Fishing Vessels
During earlier remote sensing operations and during both the July and August missions, large congregations of fishing vessels (trollers) were observed. These congregations were usually associated with temperature or water colour structure (not necessartly" chlorophyll concentrati on). In July (Fi gures 4 and 5) boats were clustered along a narrow band of high chlorophyll water which occurred in conjunction with a half degree temperature discontinuity (not shown in Figure 5, half degree isotherms are not plotted). In August (Figures 9 and 10) the congregation was between two patches of high FLH, nearer the rather abrupt edge of one and in slightly colder water. Our conversations with fishermen indicate that there is a strong correlation betlt/een fish abundance and the oceanographic phenomena that manifest themselves as water co10~r. This bears further investigation.
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REFERENCES:
Arvesen, J.C., J.P. Millard and E.C. Weaver, 1973. Remote Sensing of Chlorophyll and Temperature in Mari:ne and Fresb Waters. Astronauti" ca Acta. 18:229-23q
Borstad, G., G. Louttit and D. Gale, 1979. Ship of Opportunity Study Project Outline and Cruise Reports RV001 and RV002. Unpubl. Report by Seakem Oceanography Ltd. for Dept. Fisheries and Oceans, DSS Contract 1SB78-00255.
Borstad, G., G. Louttit and D. Gale, 1979. Ship of OpportunHy Cruise Report RV003. Unpubl.Report by Seakem Oceanography Ltd. to Dept. Fisheries and Oceans. DSS Contract 1SB78-00255.
Borstad, G., G. Louttit and D. Gale 1979. Ship of Opportunity Cruise Report RV004/4a. Unpubl. Report by Seakem Oceanography Ltd. to Dept. Fisheries and Oceans. DSS Contract 15B78-00255.
Borstad, G., and J Gower. Analysis of British Columbia Coastal Water using the I.O.S spectrometer. (in prep.).
Clarke, G.L., G.C. Ewing and C.J. Lorenzen. P170. Spectra of backscattered light from the sea obtained from ai·rcraft as a measure of chlorophyll concentrati on. Science, 167: 1119-1121.
Curran, R.J. 1972. Ocean Colour Determination through a scattering atmosphere. Applied Optics. 11:1857-1866.
Gower, J.F.R., and R.A. Neville, 1977. A method for the remote measurement of The Vertical Distribution of Phytoplankton in Seawater. 4th Canadian Symposium on Remote Sensing, Quebec City, May 1977.
Kiefer, D.A. 1973 a. Flourescence Properties of Natural Phytoplankton Populations. Mar. Biol. 22:263-269.
Miller, J.R., K.S. Gordon, and D. Kamykuwski, 1916. Airborne Watercolour Measurements off the Nova Scotia Coast. Can. J. Rem. Sensa 2: 42-47.
Morel, A. 1978. In water and remote sensing of ocean colour. In Passive remote sensing of the ~; IUCRM Colloquim, Sidney, B.C. July 1978. Morel, A. and L. Prieur. 1977. AnalYSis of Variations in Ocean Colour. Limnol. Oceanog. 24:709-722.
Neville, R.A., and J.F.R. Gower, 1977. lankton via Chlorophyll a Fluorescence.
Passive Remote Sensing of Phytop-J. Geophys. Res. 82:3487-3493.
33
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34 Seakem
Walker~ G.A.H., V.L. Buchholz, D. Camp, B. Isherwood, and J.F.R. Gower. 1975. A Silicon Diode Array Spectrometer for Ocean Colour Monitoring. Can. J. Remote Sense 1: 26-30.
Walker, G.A.H., V.L. Buchholz, D. Camp, B. Isherwood, J. Glaspey, R. Coutts, A. Condal, and J.F.R. Gower. 1974. A Compact Multichannel Spectrometer for Field Use. Rev. Sci. Instrum. 45:1349-1352.
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