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Underwater daylight measurements in the Sea of Cortez
Deep-Sea Research, 1970, Vol. 17, pp. 271 to 280. Pergamon Press. Printed in Great Britain.
Underwater daylight measurements in the Sea of Cortez* ELIZABETH M .
KAMPA~"
(Received 29 April 1969) As THE mass of information concerning the spatial and temporal variations in the characteristics of light at depth in the oceans grows, it becomes increasingly obvious that in situ, in tempore measurements of the quality and quantity of such light must be made if its intimate relationship to the behavior of the resident animals is to be assessed. In January, 1968 on a biological expedition to the Gulf of California (Sea of Cortez), sets of observations of ambient light to depths of 300 m were made in two basins. It is suggested that similarities and differences between the results obtained can be attributed to both physical and biological factors. INSTRUMENTS
Two light-sensing instruments were used. These are modifications of the bathyphotometer described by BODEN, KAMPAand SNODGRASS(1960); a detailed account of the electronic circuitry will be presented elsewhere, KAMPA(in press). Surface scalar irradiance meter The collector on this instrument is a translucent plastic sphere provided by the Visibility Laboratory of Scripps Institution; the sensor is an RCA 931-A multiplier phototube. Schott BG12 and GG5 filters (each 1 mm thick) are in the light path at all times. The transmission of this combination is a wide band (320-640 nm) with a peak near 470 nm. A 3-position shutter with a rectangular opening (exposing the entire photocathode), a pinhole (area approximately 0.001 that of the rectangular opening), and an opaque position for monitoring phototube dark current can be changed at will from the control panel in the ship's laboratory. The recorder is an x-time Leeds and Northrup Speedomax. By varying shutter position it is possible to measure incoming radiation throughout the 24-hr cycle and to monitor the dark current of the system. The instrument measures scalar irradiance (JERLOV, 1964). Since the purpose of the surface meter was simply to determine the magnitude of changes in energy reaching the sea surface occasioned by changes in the state of the sky, no attempt was made to calibrate it in absolute energy units. A curve giving recorder deflection against incident flux was obtained by attenuating in known steps the light falling on the photometer from a constant light source and measuring the *This investigation was supported in part by the Public Health Service Research G r a n t NB 05628 from the National Institute of Neurological Diseases and Blindness and in part by the National Science Foundation G r a n t GB-4672 to Prof. Carl L. Hubbs and the National Science Foundation Grant GA-1300 to the Scripps Institution of Oceanography for biological ship time. The National Science Foundation Grant GB-1152 provided funds for the photometer. "t'Seripps Institution of Oceanography, University of California, San Diego, La lolla, California, U.S.A.
271
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EUZAB~ M. KAMPA
deflection of the recorder. The light path was normal to the plane of the photocathode, and the light was attenuated by calibrated neutral density filters and by varying the distance between the light source and the instrument. This meter was mounted on the cat-walk above the bridge, and it operated continuously during the 24-day cruise. Bathy-thermo-irradiance meter
As its name implies, this instrument records depth, temperature and light. It is supported in the water by a polythene-insulated, single-conductor, stranded stainless steel cable along which commands and information are telemetered. The cable is wrapped on an electric winch; signals are transmitted to and from the deck unit via interconnected slip-rings on the winch. The circuit to the deck unit is completed via a sea return from the instrument case to a copper plate trailed over the side of the ship on a lead connected to the deck unit. The light sensor is a 931-A multiplier phototube selected for low dark current and high sensitivity from a group of forty such tubes. In front of the phototube window is a collimating tube with a disk of diffusing opal plastic at its outer end, that acts as a Lambert fiat-plate cosine collector. Thus, the instrument measures irradiance (JERLOV, 1964). The collimator has pinholes at its base and is free-flooding in water. Three positions of the shutter are identical with those in the mast photometer; two other positions of the shutter disk are opaque and occlude the light path when depth and temperature are determined. An 8-position filter holder carrying 7 Bausch and Lomb interference filters with Coming glass filters to absorb the unwanted transmission ' tails ' of the interference filters is in the light path as well. The eighth position is left empty to allow for an easy check of filter sequence. The effective half-peak band widths of the filter combinations are 8-12 nm. The spectral region from 408 to 533 nm was examined with the filters in the instrument during the cruise. The pressure sensor is a Bourdon-operated potentiometer, and thermistors sense changes in temperature. Shutter (and hence, function) and filter-changer positions are selected at will from the ship's laboratory. The recorder is an x-time Leeds and Northrup Speedomax. The irradiance meter was calibrated in the manner outlined by BODEN, KAMPAand SNODGRASS (1960), but with more exacting techniques. For each color filter a curve giving recorder deflection against incident flux was constructed by attenuating the light failing on the opal cosine collector from a constant source, as described above for the surface meter. Photomultiplier circuits are notorious for temperature dependence. For this reason the calibration was carried out at a number of temperatures throughout the range encountered in the upper 500 m of the oceans. The instrument was also calibrated for absolute energy levels. A U.S. Bureau of Standards source run at constant voltage, an Eppley thermopile and a Keithley nanovoltmeter were used to determine, for each color filter, the energy in/,W/cm2/nm striking the opal cosine collector of the instrument. The deflections of the recorder of the irradiance meter were then measured for each of the color filters. For any other set of lighting conditions the deflections given by the irradiance meter can be found. It is now possible, using these results, together with the curve showing, for each filter, deflection against incident flux, to compare the unknown with the known energy
Underwater daylightmeasurementsin the Sea of Cortez
273
distributions. In this way a curve giving irradiance (H~) in /zW/cmg/nm against wavelength (~) can be found. This calibration is valid for lights in which the spectral distribution of energy is broad compared with the narrow bands of transmission of the color filters. This is true for the measurements of penetration of light into the sea described here. The depth meter was calibrated with an Ashcroft gauge tester using dead weights to produce a series of known pressures. These were applied to the Bourdon pressure sensor, and the deflections of the recorder of the meter were measured. Using an average value for the relationship between pressure and depth of sea water in meters (SVERDRUP, JOHNSONand FLEMING, 1942) a curve was constructed showing recorder deflection against depth. The temperature meter was calibrated by immersing the thermistors in a series of constant temperature baths in 2.5°C intervals through the range 0-35°C. The deflection of the recorder at each step in the series was measured, and a curve giving recorder deflection against temperature was constructed. METHODS
The submersible instrument was activated on deck before the lowering was begun to allow ample time for the circuits to warm up. All of the systems were monitored during this period, and, when they had stabilized, the instrument was lowered with the shutter in the ' depth position' to 500 m. It was held at that depth until the record showed that the effect of cooling on the dark current of the irradiance meter was past its peak. The instrument was raised with the shutter in the ' open position' and the longest-wavelength color filter in place until a reliable reading could be obtained. At that depth, measurements were made through each of the color filters, and temperature and depth were recorded. The meter was then raised in intervals (50-m from maximum depth to 100 m; 25-m from 100 m to near surface), and all functions were measured. The unit was then lowered again quickly to the previous maximum depth with the shutter in the ' depth position' and raised at constant winch speed with the shutter in the' temperature position '. This procedure provided irradiance spectra at various depths in the water column and a continuous recording of its thermal structure. The winch was located on deck such that with a low wire angle it could remain within the cones of the ship's echo-sounder transducers (Gifft Precision Depth Recorder and Raytheon 14/15 Echo Sounder). Echoes from the irradiance meter appeared dearly on the echo-sounder records and afforded a check on the laboratory calibrations of the depth meter. Hydrographic casts were made immediately after each of the irradiance meter lowerings, and these provided confirmation of the temperature meter's findings as well as information on salinity, oxygen and phosphate content of the pertinent section of the water columns. RESULTS
The results reported here were obtained from the Guaymas Basin Oat. 26°55'N., long. 111°24.5'W.) on 15 January, and from the Farall6n Basin Oat. 25°31'N., long. 109°52'W.) on 24 January (Fig. 1), from the Scripps R.V. Thomas Washington.
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Fig. 1. Chart of the Gulf of California showing the positions of the Guaymas Basin (15 January 1968) and Farall6n Basin (24 January 1968) stations. The records of light were made between 1230 hr and 1330 hr at both stations from depths well above the depths of the sills between basins. At the Faralltn station it was necessary to manoeuver the ship immediately after the light measurements to retrieve other gear. The temperature profile was obtained, therefore, an hour later on the twenty-fourth than on the fifteenth. A comparison of the temperatures recorded at each of the depth intervals during the light measurements with those at corresponding depths on the continuous trace of temperature showed that no significant change in thermal structure had occurred in the hour of delay. The surface of the Guaymas Basin on 15 January was disturbed only by small sea waves. Sea swell was negligible, the wire angle was maintained near 0 °, and the instrument's progress from depth to depth was evident on the traces of both echo sounders. In the Faralltn Basin, sea waves were negligible, but the sea swell amounted to as much as 2 m, and the effect of the motion of the ship on the light records was apparent, particularly at shoaler depths. For this reason, the instrument was held with the shutter in the ' d e p t h position' for a minute at each level from 150 m upwards to determine the effect of surface swell on the actual depth of the instrument. This effect has been accounted for in the data presented here. On both days the cloud cover was less than 10~o. Visual estimates of sky state were confirmed by the surface scalar irradiance meter which demonstrated a straightline record throughout each set of underwater observations. The level of scalar
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Fig. 2. Curvesshowingattenuationof lightwithdepthin the GuaymasBasinon 15 January 1968. irradiance at the sea surface in the Gulf of California on 24 January was slightly higher (1.2 times) than that on 15 January. The attenuation of irradiance with depth in Guaymas Basin (Fig. 2, Table 1) was orderly, and the water acted as a monochromator (TYLER, 1958). Spectra constructed from the irradiance values at various depths (Fig. 4, solid lines) show that the light was blue-green (~max near 500 rim) at about 50-m depth, and, that below 80 m, the peak of the spectrum shifted toward the blue until the maximum value at a depth of 280 m was ~ 0.00003/zW/cm2/nm at 485 rim. In the FaraU6n Basin (Fig. 3, Table 1) the attenuation of light with depth was less regular. At 50-m depth the peak of the transmittance spectrum was at about 515 nm. Between 50 and 150 m the irradiance level dropped more abruptly than did the level at the same wavelength in the Guaymas Basin. Despite this greater attenuation at the longest wavelength (533 nm) measured, the peak of the spectrum at a depth of 280 m was at 493 nm (Fig. 4), nearly 10 nm farther toward the green portion of the spectrum. At all depths measured the irradiance values were greater in the Guaymas Basin than in the Farall6n, although the level of scalar irradiance at the surface was slightly higher at the Farall6n station. The characteristics of, and the seasonal variations in, the waters of the Gulf of California have been described in detail (RooEN, 1958, 1964; RODEN and GROVES, 1959). The Guaymas Basin is within the area of the Gulf classified by Roden as central. The Farall6n Basin is midway between Roden's central and lower Gulf areas. Temperature, salinity and oxygen determinations from the hydrographic cast off Guaymas on 15 January 1968 (Fig. 5a-c, solid lines) are similar to those measured in the same area in February 1957 (RODEN, 1964). The temperature of the water column below the 30-m surface layer decreased gradually from ~-~ 17.5° to 10.5°C at 300 m.
276
ELIZABETHM. KAMPA
Table 1. Extinction coefficient ka = In Ha, ~ - - In Ha, (z+l) where Ha, z and Ha, ( z + 1 ~ are the irradiance o f wavelength )~ on horizontal surfaces at depths z and (z q- 1) m. Depth interval (m) 48-74
74-100
100-I 47
147-195
195-243
243-280
A(rim) 408 441 456 470 480 498 533 408 441 456 470 480 498 533 408 441 456 470 480 498 533 408 441 456 470 480 498 533 441 456 470 480 498 533 456 470 480 498 533
Extinction coefficients GuayrnasBasin FaralldnBasin 0-081 0'043 0"048 0"046 0"042 0"048 0"072 0"065 0.060 0"067 0'056 0"053 0"061 0"059 0"053 0"064 0-057 0'055 0'050 0.056 0"052 0'055 0.054 0"057 0.050 0.048 0.047 0"059 0"053 0-053 0"058 0"050 0"054 0"056 0"060 0-069 0"061 0"063
0"083 0"075 0"058 0"053 0"050 0.067 0"083 0"053 0'051 0.056 0'050 0.049 0"052 0"061 0"051 0"051 0-052 0"053 0"049 0"049 0"057 0"056 0"055 0'056 0'057 0'052 0'048 0"055 0"064 0"065 0"059 0'052 0"050 0"055 0'054 0.043 0"052 0-049 0"045
In the Farall6n Basin (Fig. 5, a-c, broken lines), the salinity values were somewhat lower throughout the water column than in the Guaymas Basin. Oxygen levels were similar in the two. The major hydrographic difference between the two basins was in thermal structure. The temperature in the surface layer (0-30 m) in the Farall6n water was 0.5°C higher than that in the Guaymas area, and the thermocline was more marked. The temperature fell from 18.6 to 14°C in the layer between 30 and 80 m. Such discrepancies in thermal structure are often reflected by discrepancies in photic structure. Comparison of the light spectra at similar depths (Fig. 4) in the two areas indicates that such was the case in January 1968. The phosphate-depth curves (Fig. 5, d) for both areas would lead one to believe that considerable phytological activity was present in both areas. Phosphate was being depleted rapidly in the 0-30-m layer and again in the 50-75-m layer in both areas. The rate was slightly higher in the FaraU6n Basin. The optical evidence also suggests that this biological activity was greater in the Farall6n Basin. Above the thermocline there, the light at 470 nm had already been
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The observations described here lend credence to the claim that generalizations about light conditions at depth in the sea are dangerous, particularly in situations where further generalizations are made about the photic responses, or lack thereof, of animals to such light. It has been shown that at a single station the light affecting the behavior of animals at depth may vary greatly from day to day (BODENand KAMPA, 1967). In the present study, the differences in light at depth between two neighboring regions in a relatively small body of water show that spatial as well as temporal variations must be considered before inferences can be made as to the photosensitivities of animals in any water column.
278
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It is impossible to say precisely what occasioned the difference between the lightabsorptive character of the waters in the Guaymas and Farall6n Basins in January 1968. From the hydrographic evidence it could be hypothesized that the phytoplankton growth in the warmer surface waters of the Farall6n Basin was greater than that off Guaymas. Such an assumption is supported by the estimates of phosphate in the surface waters. The photometer used was too blunt an instrument to delineate chlorophyll absorption bands as TYLERand SMITH(1967) have done with the Scripps Spectroradiometer in the surface waters of other parts of the Gulf. Such near-surface measurements could give definitive answers to the questions arising from observations at greater depths. It is necessary now to extend the present observations in time to determine whether the phenomena outlined here were chance occurrences or whether a real and stable difference in the character of underwater daylight in the two basins exists. If such a difference occurs constantly, then depth-controlled collections of sonic-scatterers must be made, and the spectral sensitivities of the vertical migrants must be determined to test the hypothesis of BODENand I~AMPA(1965) that isolated communities adapt to the photic environments they encounter.
279
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Fig. 5. Temperature (a), salinity (b), oxygen (c), and phosphate (d) profiles for the Guaymas (solid lines) and Farall6n (broken lines) stations. Acknowledgements--I am particularly indebted to Professor CARL L. HUBBS for inviting my participation in the MV 68-I cruise. Captain NOEL L. FEm~IS, the officers and the crew of the R.V. Thomas Washington were most cooperative in maintaining positions to reduce wire angles. Mr. RICHAED
PENCE, electronics technician aboard the ship, rendered invaluable assistance with the operation of both the surface and submerged photometers.
280
ELIZABETH M. KAMPA REFERENCES
BODEN B. P. and E. M. KAMPA (1965) An aspect of euphausiid ecology revealed by echosounding in afjord. Crustaceana, 9 (2), 155-173. BODBNB. P. and E. M. KAMPA(1967) The influence of natural light on the vertical migrations of an animal community in the sea. Symp. zool. Soc. Lond. 19, 15-26. BODEN B. P., E. M. KAMPAand J. M. SNOD6RASS(1960) Underwater daylight measurements in the Bay of Biscay. J. mar. biol. Ass. U.K. 39, 227-239. JERLOV N. G. (1964) Optical classification of ocean water. Physical Aspects of Light in the Sea, 45-49. RODEN G. I. (1958) Oceanographic and meteorological aspects of the Gulf of California. Pacific Science XII, (1), 21-45. RODEN G. I. (1964) Oceanographic aspects of the Gulf of California. Mar. Geology of the Gulf of California, 30-58. RODEN G. I. and G. W. GROWS (1959) Recent oceanographic investigations in the Gulf of California. J. mar. Res. 18, (1-3), 10-35. SVERDRtYP H. U., M. W. JOrlNSOtq and R. I-I. FLEMING(1942) The Oceans, Their Physics, Chemistry, and General Biology. Prentice Hall, 1087 pp. TYLER J. E. (1958) Natural water as a monochromator. Limnol. Oceanogr. 4 (1), 102-105. TYLER J. E. and R. C. SMrrn (1967) Spectroradiometric characteristics of natural light under water. J. opt. Soc. Am. 57 (5), 595-601.
Underwater daylight measurements in the Sea of Cortez* ELIZABETH M .
KAMPA~"
(Received 29 April 1969) As THE mass of information concerning the spatial and temporal variations in the characteristics of light at depth in the oceans grows, it becomes increasingly obvious that in situ, in tempore measurements of the quality and quantity of such light must be made if its intimate relationship to the behavior of the resident animals is to be assessed. In January, 1968 on a biological expedition to the Gulf of California (Sea of Cortez), sets of observations of ambient light to depths of 300 m were made in two basins. It is suggested that similarities and differences between the results obtained can be attributed to both physical and biological factors. INSTRUMENTS
Two light-sensing instruments were used. These are modifications of the bathyphotometer described by BODEN, KAMPAand SNODGRASS(1960); a detailed account of the electronic circuitry will be presented elsewhere, KAMPA(in press). Surface scalar irradiance meter The collector on this instrument is a translucent plastic sphere provided by the Visibility Laboratory of Scripps Institution; the sensor is an RCA 931-A multiplier phototube. Schott BG12 and GG5 filters (each 1 mm thick) are in the light path at all times. The transmission of this combination is a wide band (320-640 nm) with a peak near 470 nm. A 3-position shutter with a rectangular opening (exposing the entire photocathode), a pinhole (area approximately 0.001 that of the rectangular opening), and an opaque position for monitoring phototube dark current can be changed at will from the control panel in the ship's laboratory. The recorder is an x-time Leeds and Northrup Speedomax. By varying shutter position it is possible to measure incoming radiation throughout the 24-hr cycle and to monitor the dark current of the system. The instrument measures scalar irradiance (JERLOV, 1964). Since the purpose of the surface meter was simply to determine the magnitude of changes in energy reaching the sea surface occasioned by changes in the state of the sky, no attempt was made to calibrate it in absolute energy units. A curve giving recorder deflection against incident flux was obtained by attenuating in known steps the light falling on the photometer from a constant light source and measuring the *This investigation was supported in part by the Public Health Service Research G r a n t NB 05628 from the National Institute of Neurological Diseases and Blindness and in part by the National Science Foundation G r a n t GB-4672 to Prof. Carl L. Hubbs and the National Science Foundation Grant GA-1300 to the Scripps Institution of Oceanography for biological ship time. The National Science Foundation Grant GB-1152 provided funds for the photometer. "t'Seripps Institution of Oceanography, University of California, San Diego, La lolla, California, U.S.A.
271
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EUZAB~ M. KAMPA
deflection of the recorder. The light path was normal to the plane of the photocathode, and the light was attenuated by calibrated neutral density filters and by varying the distance between the light source and the instrument. This meter was mounted on the cat-walk above the bridge, and it operated continuously during the 24-day cruise. Bathy-thermo-irradiance meter
As its name implies, this instrument records depth, temperature and light. It is supported in the water by a polythene-insulated, single-conductor, stranded stainless steel cable along which commands and information are telemetered. The cable is wrapped on an electric winch; signals are transmitted to and from the deck unit via interconnected slip-rings on the winch. The circuit to the deck unit is completed via a sea return from the instrument case to a copper plate trailed over the side of the ship on a lead connected to the deck unit. The light sensor is a 931-A multiplier phototube selected for low dark current and high sensitivity from a group of forty such tubes. In front of the phototube window is a collimating tube with a disk of diffusing opal plastic at its outer end, that acts as a Lambert fiat-plate cosine collector. Thus, the instrument measures irradiance (JERLOV, 1964). The collimator has pinholes at its base and is free-flooding in water. Three positions of the shutter are identical with those in the mast photometer; two other positions of the shutter disk are opaque and occlude the light path when depth and temperature are determined. An 8-position filter holder carrying 7 Bausch and Lomb interference filters with Coming glass filters to absorb the unwanted transmission ' tails ' of the interference filters is in the light path as well. The eighth position is left empty to allow for an easy check of filter sequence. The effective half-peak band widths of the filter combinations are 8-12 nm. The spectral region from 408 to 533 nm was examined with the filters in the instrument during the cruise. The pressure sensor is a Bourdon-operated potentiometer, and thermistors sense changes in temperature. Shutter (and hence, function) and filter-changer positions are selected at will from the ship's laboratory. The recorder is an x-time Leeds and Northrup Speedomax. The irradiance meter was calibrated in the manner outlined by BODEN, KAMPAand SNODGRASS (1960), but with more exacting techniques. For each color filter a curve giving recorder deflection against incident flux was constructed by attenuating the light failing on the opal cosine collector from a constant source, as described above for the surface meter. Photomultiplier circuits are notorious for temperature dependence. For this reason the calibration was carried out at a number of temperatures throughout the range encountered in the upper 500 m of the oceans. The instrument was also calibrated for absolute energy levels. A U.S. Bureau of Standards source run at constant voltage, an Eppley thermopile and a Keithley nanovoltmeter were used to determine, for each color filter, the energy in/,W/cm2/nm striking the opal cosine collector of the instrument. The deflections of the recorder of the irradiance meter were then measured for each of the color filters. For any other set of lighting conditions the deflections given by the irradiance meter can be found. It is now possible, using these results, together with the curve showing, for each filter, deflection against incident flux, to compare the unknown with the known energy
Underwater daylightmeasurementsin the Sea of Cortez
273
distributions. In this way a curve giving irradiance (H~) in /zW/cmg/nm against wavelength (~) can be found. This calibration is valid for lights in which the spectral distribution of energy is broad compared with the narrow bands of transmission of the color filters. This is true for the measurements of penetration of light into the sea described here. The depth meter was calibrated with an Ashcroft gauge tester using dead weights to produce a series of known pressures. These were applied to the Bourdon pressure sensor, and the deflections of the recorder of the meter were measured. Using an average value for the relationship between pressure and depth of sea water in meters (SVERDRUP, JOHNSONand FLEMING, 1942) a curve was constructed showing recorder deflection against depth. The temperature meter was calibrated by immersing the thermistors in a series of constant temperature baths in 2.5°C intervals through the range 0-35°C. The deflection of the recorder at each step in the series was measured, and a curve giving recorder deflection against temperature was constructed. METHODS
The submersible instrument was activated on deck before the lowering was begun to allow ample time for the circuits to warm up. All of the systems were monitored during this period, and, when they had stabilized, the instrument was lowered with the shutter in the ' depth position' to 500 m. It was held at that depth until the record showed that the effect of cooling on the dark current of the irradiance meter was past its peak. The instrument was raised with the shutter in the ' open position' and the longest-wavelength color filter in place until a reliable reading could be obtained. At that depth, measurements were made through each of the color filters, and temperature and depth were recorded. The meter was then raised in intervals (50-m from maximum depth to 100 m; 25-m from 100 m to near surface), and all functions were measured. The unit was then lowered again quickly to the previous maximum depth with the shutter in the ' depth position' and raised at constant winch speed with the shutter in the' temperature position '. This procedure provided irradiance spectra at various depths in the water column and a continuous recording of its thermal structure. The winch was located on deck such that with a low wire angle it could remain within the cones of the ship's echo-sounder transducers (Gifft Precision Depth Recorder and Raytheon 14/15 Echo Sounder). Echoes from the irradiance meter appeared dearly on the echo-sounder records and afforded a check on the laboratory calibrations of the depth meter. Hydrographic casts were made immediately after each of the irradiance meter lowerings, and these provided confirmation of the temperature meter's findings as well as information on salinity, oxygen and phosphate content of the pertinent section of the water columns. RESULTS
The results reported here were obtained from the Guaymas Basin Oat. 26°55'N., long. 111°24.5'W.) on 15 January, and from the Farall6n Basin Oat. 25°31'N., long. 109°52'W.) on 24 January (Fig. 1), from the Scripps R.V. Thomas Washington.
274
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Fig. 1. Chart of the Gulf of California showing the positions of the Guaymas Basin (15 January 1968) and Farall6n Basin (24 January 1968) stations. The records of light were made between 1230 hr and 1330 hr at both stations from depths well above the depths of the sills between basins. At the Faralltn station it was necessary to manoeuver the ship immediately after the light measurements to retrieve other gear. The temperature profile was obtained, therefore, an hour later on the twenty-fourth than on the fifteenth. A comparison of the temperatures recorded at each of the depth intervals during the light measurements with those at corresponding depths on the continuous trace of temperature showed that no significant change in thermal structure had occurred in the hour of delay. The surface of the Guaymas Basin on 15 January was disturbed only by small sea waves. Sea swell was negligible, the wire angle was maintained near 0 °, and the instrument's progress from depth to depth was evident on the traces of both echo sounders. In the Faralltn Basin, sea waves were negligible, but the sea swell amounted to as much as 2 m, and the effect of the motion of the ship on the light records was apparent, particularly at shoaler depths. For this reason, the instrument was held with the shutter in the ' d e p t h position' for a minute at each level from 150 m upwards to determine the effect of surface swell on the actual depth of the instrument. This effect has been accounted for in the data presented here. On both days the cloud cover was less than 10~o. Visual estimates of sky state were confirmed by the surface scalar irradiance meter which demonstrated a straightline record throughout each set of underwater observations. The level of scalar
Underwater daylightmeasurementsin the Sea of Cortez 10-6
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Fig. 2. Curvesshowingattenuationof lightwithdepthin the GuaymasBasinon 15 January 1968. irradiance at the sea surface in the Gulf of California on 24 January was slightly higher (1.2 times) than that on 15 January. The attenuation of irradiance with depth in Guaymas Basin (Fig. 2, Table 1) was orderly, and the water acted as a monochromator (TYLER, 1958). Spectra constructed from the irradiance values at various depths (Fig. 4, solid lines) show that the light was blue-green (~max near 500 rim) at about 50-m depth, and, that below 80 m, the peak of the spectrum shifted toward the blue until the maximum value at a depth of 280 m was ~ 0.00003/zW/cm2/nm at 485 rim. In the FaraU6n Basin (Fig. 3, Table 1) the attenuation of light with depth was less regular. At 50-m depth the peak of the transmittance spectrum was at about 515 nm. Between 50 and 150 m the irradiance level dropped more abruptly than did the level at the same wavelength in the Guaymas Basin. Despite this greater attenuation at the longest wavelength (533 nm) measured, the peak of the spectrum at a depth of 280 m was at 493 nm (Fig. 4), nearly 10 nm farther toward the green portion of the spectrum. At all depths measured the irradiance values were greater in the Guaymas Basin than in the Farall6n, although the level of scalar irradiance at the surface was slightly higher at the Farall6n station. The characteristics of, and the seasonal variations in, the waters of the Gulf of California have been described in detail (RooEN, 1958, 1964; RODEN and GROVES, 1959). The Guaymas Basin is within the area of the Gulf classified by Roden as central. The Farall6n Basin is midway between Roden's central and lower Gulf areas. Temperature, salinity and oxygen determinations from the hydrographic cast off Guaymas on 15 January 1968 (Fig. 5a-c, solid lines) are similar to those measured in the same area in February 1957 (RODEN, 1964). The temperature of the water column below the 30-m surface layer decreased gradually from ~-~ 17.5° to 10.5°C at 300 m.
276
ELIZABETHM. KAMPA
Table 1. Extinction coefficient ka = In Ha, ~ - - In Ha, (z+l) where Ha, z and Ha, ( z + 1 ~ are the irradiance o f wavelength )~ on horizontal surfaces at depths z and (z q- 1) m. Depth interval (m) 48-74
74-100
100-I 47
147-195
195-243
243-280
A(rim) 408 441 456 470 480 498 533 408 441 456 470 480 498 533 408 441 456 470 480 498 533 408 441 456 470 480 498 533 441 456 470 480 498 533 456 470 480 498 533
Extinction coefficients GuayrnasBasin FaralldnBasin 0-081 0'043 0"048 0"046 0"042 0"048 0"072 0"065 0.060 0"067 0'056 0"053 0"061 0"059 0"053 0"064 0-057 0'055 0'050 0.056 0"052 0'055 0.054 0"057 0.050 0.048 0.047 0"059 0"053 0-053 0"058 0"050 0"054 0"056 0"060 0-069 0"061 0"063
0"083 0"075 0"058 0"053 0"050 0.067 0"083 0"053 0'051 0.056 0'050 0.049 0"052 0"061 0"051 0"051 0-052 0"053 0"049 0"049 0"057 0"056 0"055 0'056 0'057 0'052 0'048 0"055 0"064 0"065 0"059 0'052 0"050 0"055 0'054 0.043 0"052 0-049 0"045
In the Farall6n Basin (Fig. 5, a-c, broken lines), the salinity values were somewhat lower throughout the water column than in the Guaymas Basin. Oxygen levels were similar in the two. The major hydrographic difference between the two basins was in thermal structure. The temperature in the surface layer (0-30 m) in the Farall6n water was 0.5°C higher than that in the Guaymas area, and the thermocline was more marked. The temperature fell from 18.6 to 14°C in the layer between 30 and 80 m. Such discrepancies in thermal structure are often reflected by discrepancies in photic structure. Comparison of the light spectra at similar depths (Fig. 4) in the two areas indicates that such was the case in January 1968. The phosphate-depth curves (Fig. 5, d) for both areas would lead one to believe that considerable phytological activity was present in both areas. Phosphate was being depleted rapidly in the 0-30-m layer and again in the 50-75-m layer in both areas. The rate was slightly higher in the FaraU6n Basin. The optical evidence also suggests that this biological activity was greater in the Farall6n Basin. Above the thermocline there, the light at 470 nm had already been
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The observations described here lend credence to the claim that generalizations about light conditions at depth in the sea are dangerous, particularly in situations where further generalizations are made about the photic responses, or lack thereof, of animals to such light. It has been shown that at a single station the light affecting the behavior of animals at depth may vary greatly from day to day (BODENand KAMPA, 1967). In the present study, the differences in light at depth between two neighboring regions in a relatively small body of water show that spatial as well as temporal variations must be considered before inferences can be made as to the photosensitivities of animals in any water column.
278
ELIZABETH /V[. KAMPA lO ~
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It is impossible to say precisely what occasioned the difference between the lightabsorptive character of the waters in the Guaymas and Farall6n Basins in January 1968. From the hydrographic evidence it could be hypothesized that the phytoplankton growth in the warmer surface waters of the Farall6n Basin was greater than that off Guaymas. Such an assumption is supported by the estimates of phosphate in the surface waters. The photometer used was too blunt an instrument to delineate chlorophyll absorption bands as TYLERand SMITH(1967) have done with the Scripps Spectroradiometer in the surface waters of other parts of the Gulf. Such near-surface measurements could give definitive answers to the questions arising from observations at greater depths. It is necessary now to extend the present observations in time to determine whether the phenomena outlined here were chance occurrences or whether a real and stable difference in the character of underwater daylight in the two basins exists. If such a difference occurs constantly, then depth-controlled collections of sonic-scatterers must be made, and the spectral sensitivities of the vertical migrants must be determined to test the hypothesis of BODENand I~AMPA(1965) that isolated communities adapt to the photic environments they encounter.
279
Underwater daylight measurements in the Sea of Cortez
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Fig. 5. Temperature (a), salinity (b), oxygen (c), and phosphate (d) profiles for the Guaymas (solid lines) and Farall6n (broken lines) stations. Acknowledgements--I am particularly indebted to Professor CARL L. HUBBS for inviting my participation in the MV 68-I cruise. Captain NOEL L. FEm~IS, the officers and the crew of the R.V. Thomas Washington were most cooperative in maintaining positions to reduce wire angles. Mr. RICHAED
PENCE, electronics technician aboard the ship, rendered invaluable assistance with the operation of both the surface and submerged photometers.
280
ELIZABETH M. KAMPA REFERENCES
BODEN B. P. and E. M. KAMPA (1965) An aspect of euphausiid ecology revealed by echosounding in afjord. Crustaceana, 9 (2), 155-173. BODBNB. P. and E. M. KAMPA(1967) The influence of natural light on the vertical migrations of an animal community in the sea. Symp. zool. Soc. Lond. 19, 15-26. BODEN B. P., E. M. KAMPAand J. M. SNOD6RASS(1960) Underwater daylight measurements in the Bay of Biscay. J. mar. biol. Ass. U.K. 39, 227-239. JERLOV N. G. (1964) Optical classification of ocean water. Physical Aspects of Light in the Sea, 45-49. RODEN G. I. (1958) Oceanographic and meteorological aspects of the Gulf of California. Pacific Science XII, (1), 21-45. RODEN G. I. (1964) Oceanographic aspects of the Gulf of California. Mar. Geology of the Gulf of California, 30-58. RODEN G. I. and G. W. GROWS (1959) Recent oceanographic investigations in the Gulf of California. J. mar. Res. 18, (1-3), 10-35. SVERDRtYP H. U., M. W. JOrlNSOtq and R. I-I. FLEMING(1942) The Oceans, Their Physics, Chemistry, and General Biology. Prentice Hall, 1087 pp. TYLER J. E. (1958) Natural water as a monochromator. Limnol. Oceanogr. 4 (1), 102-105. TYLER J. E. and R. C. SMrrn (1967) Spectroradiometric characteristics of natural light under water. J. opt. Soc. Am. 57 (5), 595-601.