(Received 15 June 2000; in revised form 5 January 2001; accepted 11 January 2001)

Transcription

1 Journal of Oceanography, Vol. 57, pp. 275 to 284, 2001 Bio-optical Properties of Seawater in the Western Subarctic Gyre and Alaskan Gyre in the Subarctic North Pacific and the Southern Bering Sea during the Summer of 1997 HIROAKI SASAKI 1 *, SEI-ICHI SAITOH 1 and MOTOAKI KISHINO 2 1 Graduate School of Fisheries Sciences, Hokkaido University, 3-1-1, Minato-cho, Hakodate, Hokkaido , Japan 2 RIKEN, 2-1, Hirosawa, Wako, Saitama , Japan (Received 15 June 2000; in revised form 5 January 2001; accepted 11 January 2001) Chlorophyll a concentrations (chl a) and the absorption coefficients of total particulate matter [a p (λ)], phytoplankton [a ph (λ)], detritus [a d (λ)], and colored dissolved organic matter: CDOM [a CDOM (λ)] were measured in seawater samples collected in the subarctic North Pacific and the southern Bering Sea during the summer of We examined the specific spectral properties of absorption for each material, and compared the light fields in the Western subarctic Gyre (area WSG) with those in the Alaskan Gyre (area AG), and the southern Bering Sea (area SB). In the area WSG, the irradiance in the surface layer decreased markedly, indicating high absorption. In the area AG, the radiant energy penetrated deeply, and the chl a and absorption values were low throughout the water column. In the area SB, light absorption was high in the surface layer on the shelf edge and decreased with increasing depth; on the other hand, light absorption was low in the surface layer in the shelf area and increased with increasing depth. Keywords: Bio-optical properties, absorption coefficient, chlorophyll a, Western subarctic Gyre, Alaskan Gyre. 1. Introduction Absorption and scattering decrease the radiant energy of light. The wavelength with the highest radiant energy can be used as an index of water color (Morel, 1978). The clearest ocean waters are deep blue, coastal waters are usually green, and inshore estuarine waters are often brown or red. These differences in color are attributed to variation in the magnitude of absorption and the scattering of sunlight (Yentsch, 1960). The light energy absorbed by phytoplankton is used in photosynthesis or transformed into heat energy. Scattering processes change the incident direction of the light energy. Most of the attenuation of the incident light is caused by absorption, and some is due to scattering. The light absorption properties of an aquatic medium are characterized by the medium s absorption coefficient. The total absorption coefficient [a(λ)] includes the coefficients for total particulate matter (phytoplankton, detritus, and sediment) [a p (λ)], colored dissolved organic matter (CDOM) [a CDOM (λ)], and pure water [a W (λ)]: * Corresponding author. sasaki@salmon.fish.hokudai. ac.jp Copyright The Oceanographic Society of Japan. a(λ) = a p (λ) + a CDOM (λ) + a W (λ). (1) Seawater in the open ocean is usually referred to as Case I water, which is defined as water the optical properties of which are controlled by phytoplankton and detrital material (Morel and Prieur, 1977). Ocean color is largely determined by the spectral changes of the incoming radiation, caused by the water itself, and by substances suspended and dissolved in the water column. The subarctic North Pacific is one of the most productive marine regions in the world. This region has two gyre systems: the Western subarctic Gyre in the west, and the Alaskan Gyre in the east (Favorite et al., 1976, 1977; Harrison et al., 1999). The Western subarctic Gyre (area WSG) is northeast of Japan and southeast of the Kamchatka Peninsula. The average value of chlorophyll a concentration (chl a) within the euphotic zone in summer is 1.0 mgm 3 in the western subarctic North Pacific and spring blooms cause concentrations to reach mgm 3 (Shiomoto et al., 1998). The eastern Alaskan Gyre (area AG) is located in the Gulf of Alaska. Since 1950, many scientists have studied the physical and biological processes that occur in 275

2 the mixed layer at Station Papa (Stn. P; also known as Ocean Station Papa (OSP) ( N, W)) using in situ measurements and by modeling (e.g. Emerson, 1987; McClain et al., 1996). This area is well known as a high nutrient, low chlorophyll (HNLC) region. The average chl a concentration within the euphotic zone in summer is about 0.4 mgm 3 (Shiomoto et al., 1998). Chl a concentration remains low throughout the year, and there is no spring bloom (Parsons and Lalli, 1988), presumably due to grazing by microzooplankton (Frost, 1991; Boyd and Harrison, 1999) and iron limitation (Martin et al., 1989; Boyd et al., 1998). The Bering Sea has a large continental shelf and a deep basin. Both nutrients and light limit primary production on the shelf (National Research Council, 1996). In the southern region, along the continental shelf where the depth is about 200 m, tidal mixing and eddies formed by the Bering slope current transport nutrients into the euphotic zone, so that primary production is very high (e.g. Springer et al., 1996). In order to understand the behavior of incident light in the water column it is important to understand the light absorption properties of seawater. The spectral characteristics of phytoplankton absorption have been the subject of many studies seeking to estimate primary production and evaluate pigment algorithms for the remote sensing of phytoplankton. The light environment is an important factor in these studies, but there have been few studies of optical properties in the AG (e.g. Muller and Lange, 1989), and very few studies of the optical properties in the WSG. We carried out bio-optical measurements in the eastern and western subarctic North Pacific, and in the southern Bering Sea. The primary objectives of this study were to examine the specific optical properties of seawater and each material comprising the water body in the subarctic North Pacific, and to verify differences in the optical properties of each region. 2. Materials and Methods 2.1 Data collection Data were collected during the KH-97-2 cruise on the R/V Hakuho Maru which took place from 9 July to 8 September 1997, in the subarctic North Pacific and southern Bering Sea (Fig. 1). Optical measurements were conducted using a spectroradiometer and water samples were collected. Measurements were made daily at morning, noon, and evening for 2 days at Stn. 4 ( N, E) and Stn. P ( N, W). In the southern Bering Sea the depth of the bottom was 970 m at Stn. 10 ( N, W) on the shelf edge, and 76 m at Stn. 11 ( N, W) in the shelf area. Hydrocasts were performed using a rosette sampler/ Conductivity-Temperature-Depth (CTD) apparatus equipped with 12-liter Niskin bottles. After optical measurements were made with spectroradiometers, water samples were collected from 8 to 13 depths in the upper 150 m, to measure the absorption coefficient, while only four depths in the surface layer were used for CDOM absorption. The interval between samples was 5 m near the euphotic depth and 10 m at other depths. Water samples (0.2 liters) for chl a measurement were filtered through 25-mm Whatman GF/F glass-fibre filters (pore size 0.7 µm). The samples were analyzed fluorometrically on board ship using a Turner-design fluorometer with DMF (N,N-dimethylformamide) extraction (Suzuki and Ishimaru, 1990). 2.2 Optical measurements Downward spectral irradiance [E d (λ), µwcm 2 nm 1 ] and upward spectral radiance [L u (λ), µwcm 2 nm 1 sr 1 ] were measured using multiwavelength environmental radiometers (MER-2040/2041, Biospherical Instruments Inc.) and profiling reflectance radiometers (PRR-600/610, Biospherical Instruments Fig. 1. Locations of stations on the KH-97-2 cruise in the subarctic North Pacific. 276 H. Sasaki et al.

3 Inc.), the MER-2040 and PRR-600 being underwater instruments. The incident solar spectral irradiance [E s (λ), µwcm 2 nm 1 ] was measured on the upper deck using the MER-2041 and PRR-610 instruments. The MER-2040 and MER-2041 sensors have 13 channels (central wavelengths: 412, 443, 465, 490, 510, 520, 555, 565, 625, 665, 670, and 683 nm with 10 nm bandwidths; the additional channels are for photosynthetically available radiation (PAR) for E d (λ) and at 710 nm for L u (λ)). The PRR-600 and PRR-610 sensors have seven channels (central wavelength: 412, 443, 490, 520, 565 and 670 nm with 10 nm bandwidths; the additional channels are PAR for E d (λ) and 683 nm for L u (λ)). We made measurements from the sea surface to depths of about 60 to 125 m, depending on the surface conditions and the depth at the station. 2.3 Absorption coefficient We measured the absorption coefficients of total particulate matter [a p (λ)], phytoplankton [a ph (λ)], detritus [a d (λ)], and CDOM [a CDOM (λ)]. The present study is based on the assumption that detritus comprises all particles other than phytoplankton. The coefficients a p (λ), a d (λ), and a ph (λ) were determined by the opal-glass method (Shibata, 1958) and the 100% methanol (MeOH) extraction method (Kishino et al., 1985). We then calculated the specific absorption coefficient of the phytoplankton per unit chl a [a* ph (λ)]. The particulate absorption samples ( liters) were filtered through 25-mm Whatman GF/F glass-fibre filters under mild vacuum. After filtration, the samples were either analyzed on board ship immediately or stored in a freezer ( 80 C) for a few days until analysis. These filters were used to measure the optical density [OD(λ)] of total particulate matter from 350 to 750 nm at 1 nm intervals using a multipurpose spectrophotometer (MPS-2000, Shimadzu Co.). All spectra were set to 0.0 at 750 nm, to minimize the difference between sample and reference filters. The optical density measured on the filter was corrected for the path length amplification factor. Equation (2) was used to calculate a p (λ) from the calculated optical density of the suspension [OD s (λ)], which was determined using Eq. (3) (Cleveland and Weidemann, 1993): a p ( ) ODs λ ( λ)= v/ s ( )= [ ( )]+ ( ) ( 2) 2 [ ] () ODs λ OD λ OD λ 3 where is the factor used to convert base 10 to a natural logarithm. The coefficients v and s are the volume filtered and the clearance area of the filter, respectively. The filters were then soaked in approximately 10 ml of MeOH for 60 to 90 min and rinsed with filtered seawater (Kishino et al., 1985). After MeOH extraction, a d (λ) was calculated from the OD(λ) of detritus using Eqs. (2) and (3). The coefficient a ph (λ) was obtained as follows: a ph (λ) = a p (λ) a d (λ). (4) The coefficient was converted into a chlorophyll-specific absorption coefficient [a* ph (λ), m 2 (mgchla) 1 ] as follows: ( ) a a ph λ ph( λ)=. 5 chl a For the seawater samples filtered through the 47-mm nuclepore filter (pore size 0.2 µm), the OD(λ) of CDOM was scanned from 300 to 800 nm with the MPS-2000 using a 15-cm cell. We used Milli-Q water as the reference water. Because of the scattering correction, all spectra were normalized by subtracting the mean value from 650 to 700 nm. The coefficient a CDOM (λ) was calculated as follows: a CDOM λ ( ) OD λ ( )= l ( ) ( 6) where l is the cell path length. The absorption coefficient of pure water [a W (λ)] is a constant and the value reported by Pope and Fry (1997) was used. We could therefore derive a(λ) as the sum of total absorption (Eq. (1)). 3. Results and Discussion 3.1 Spectral characteristics of the underwater light field The light energy at long wavelengths (between 565 and 683 nm) decreased sharply in the surface layer at all stations, especially between the sea surface and m depth, and was not measured in deeper water because of the finite signal resolution of the sensor (Fig. 2). The difference in the wavelength of maximum penetration at short wavelengths (λ < 555 nm) was found at each station. The maximum penetration of irradiance in the surface layer (until around 35 m) at Stn. 4 occurred at wavelengths between 490 and 555 nm (Fig. 2). Irradiance decreased markedly in the upper 25 m. Moreover, maximum penetration shifted gradually to shorter wavelengths (around 490 nm) with increasing depth, indicating that the clarity of the seawater increased gradually with depth. At Stn. 4, the chl a levels were high down to 25 m, but they were highly variable (range: 0.62 to 3.01 mgm 3 ; mean ± SD: 1.58 ± 0.80 mgm 3 ) (Fig. 3). The Secchi- Bio-optical Properties of Seawater 277

4 Fig. 2. Spectral distribution of relative underwater downward spectral irradiance [E d (λ)] at each depth (0, 2, 5, 7, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, and 100 m) relative to the distribution just below the sea surface at four stations. disk and euphotic zone depths therefore both varied markedly ( m and m, respectively). It is thought that the vertical structure of the chl a distributions in the surface layer varied over short periods, affecting the light field. At Stn. P, radiant energy at nm penetrated deeply without decreasing markedly down to a depth of 100 m (Fig. 2). The euphotic zone was deeper than that of Stn. 4 (71 84 m). Although the chl a levels at Stn. P were higher around the euphotic zone than near the surface, the chl a levels within the water column were very low (Fig. 3). Radiant energy at long wavelengths (λ > 555 nm) penetrated to 30 m, and shoulders appeared around 683 nm at m, which might have been due to natural fluorescence by phytoplankton. At Stn. 10 the chl a level was relatively high in the surface layer (2.67 mgm 3 at 15 m) (Fig. 3) and the euphotic zone was the shallowest of the four stations (21 m). The ocean color at this station was green rather than blue. The wavelength of maximum penetration was 555 nm (a green wavelength) and the radiant energy at short wavelengths (λ < 555 nm) diminished sharply in the top 30 m. This was because the wavelength of maximum penetration shifts from blue to green when chl a increases (Kishino et al., 1984, 1986; Sathyendranath and Platt, 1988). At Stn. 11 the chl a level was low ( mgm 3 ). At this station a marked thermocline was found near 20 m and the water column was thermally stratified. The radiant energy near 490 nm penetrated markedly and the wavelength of maximum penetration was unchanged because of the low chl a levels, comparable to those of Stn. P. However, light attenuation was greater in the deeper layer than in the surface layer. 278 H. Sasaki et al.

5 Fig. 3. Vertical profiles of chlorophyll a concentration (chl a) at four stations. Values for Stns. 4 and P are mean ± SD for six casts at each station. Fig. 4. Spectral distribution of the total absorption coefficient [a(λ)] at the sea surface (left) and at selected depths (right). 3.2 Relative contribution of the absorption coefficient At Stns. 4 and 10 the absorption at the sea surface exhibited a minimum value around 560 nm (Fig. 4). At a depth of 20 m at Stn. 4, absorption decreased at the shorter wavelengths (λ < 490 nm) and the minimum values extended at the wavelength range ( nm). The absorption is evident in the high value at a depth of 10 m at Stn. 10. The low absorption at Stns. P and 11 is represented by the short wavelength. This corresponds to the observation that the water near the surface was clear with low attenuation (Fig. 2). With increasing depth and then decreasing chl a (Figs. 2 and 3), it appears that the minimum value of the absorption shifted to shorter wavelengths (around 490 nm). Therefore, it is considered that the shift in the wavelength of maximum penetration (Fig. 2) is attributed to that of the minimum absorption. However, at Stn. 11 the absorption at 30 m increased at shorter wavelengths. The salts present in seawater appear to have no significant effect on absorption in the visible region (Smith and Baker, 1981). The absorption coefficients of pure water and phytoplankton account for the absorption at longer wavelengths (λ > 600 nm), and pure water absorption predominated for more than 90% of these wavelengths (Fig. 5). It is evident that pure water absorbs very little light of wavelengths below 600 nm. The mean value of a W (λ) from 400 to 700 nm ( ã W ) was dominant at each station, ranging from 59.9 to 84.6% (Table 1). CDOM and particulate matter contributed primarily to the absorp- Bio-optical Properties of Seawater 279

6 Fig. 5. Relative contribution of the absorption coefficients of pure water [a W (λ)], CDOM [a CDOM (λ)], phytoplankton [a ph (λ)], and detritus [a d (λ)] to the total absorption coefficient [a(λ)] at the sea surface at four stations. Table 1. Relative contribution of absorption coefficients of pure water ( ã W ), CDOM ( ã CDOM ), phytoplankton ( ã ph ) and detritus ( ã d ) to total absorption coefficient ( ã ) integrated over the range of PAR ( nm) for samples collected at the surface. Chl a data at Stns. 4 and P show mean ± SD (6 casts). Owing to CDOM absorption samples at Stns. 4 (1 cast) and P (4 casts), only the absorption values at Stn. P show mean ± SD. Station chl a ã W ã CDOM ã ph ã d ã (mgm 3 ) (%) (%) (%) (%) (m 1 ) Stn ± Stn. P 0.23 ± ± ± ± ± ± Stn Stn tion at short wavelengths (Fig. 5). The values of a CDOM (λ) and a d (λ) dominated more than that of a W (λ) towards short wavelengths. The value of a ph (λ) dominated at blue-green wavelengths (around nm) at Stns. 4 and P. The light absorption by total particulate matter [a p (λ) = a ph (λ) + a d (λ)] ( %) was approximately equal to that of CDOM ( %) at wavelengths between 440 and 490 nm at Stns. 4 and 10. The value of a CDOM (λ) at Stns. P and 11 was larger than a p (λ) at short wavelengths. The mean values of a p (λ) and a CDOM (λ) from 400 to 700 nm ( ã p and ã CDOM ) at Stn. 4 were 15.3 and 16.2%, respectively (Table 1). Similarly, the values of ã p and ã CDOM at Stn. 10 were 20.5 and 19.6%, respectively. 280 H. Sasaki et al.

7 Fig. 6. Spectra of absorption coefficients of phytoplankton [a ph (λ)] and detritus [a d (λ)] at selected depths at four stations. Bio-optical Properties of Seawater 281

8 The value of ã p at Stn. P (5.9 ± 0.4%) was slightly higher than that of Stn. 11 (3.7%). However, the value of ã CDOM at Stn. 11 (14.7%) was significantly higher than that of Stn. P (9.5 ± 3.2%). Kishino (1994) has shown that the value of ã CDOM is approximately 3.0% in the clearest water, and increases to over 30.0% in turbid waters at the sea surface in various water types around Japan. The values of ã CDOM were thus relatively high at Stns. P and 11 in the clear water. In particular, the higher value of ã CDOM at Stn. 11, near the coast on the shelf, may be due to the terrestrial materials from the coast region. 3.3 Spectral characteristics of the absorption coefficients of each particulate component Phytoplankton absorption [a ph (λ)] showed two peaks: near 440 nm (blue absorption maximum: m 1 ) and near 675 nm (red absorption maximum: m 1 ) near the sea surface (0 10 m) at Stn. 4 (Fig. 6). Light absorption by detritus was high at short wavelengths and decreased toward long wavelengths. The clarity of seawater increased with depth, so the detrital absorption was low. At Stn. P the variation in a ph (λ) was very small from the surface to 80 m (blue absorption: m 1 and red absorption: m 1 ). However, the shape of the peak in the blue part of the spectrum varied with depth. One peak around 430 nm was high, and the other peak around 470 nm varied and decreased rapidly towards wavelengths above 490 nm. The spectra might have been affected by the absorption of other pigments (chlorophylls or carotenoids). The spectrum of a d (λ) at 70 and 80 m can be obviously seen in the phycobilin peak (around 500 nm), which was not removed by methanol extraction. This peak was also found at 90 m (not shown). At Stn. 10 phytoplankton absorption in the blue part of the spectrum was greater at 15 m (0.117 m 1 ) than at the surface. The change in the spectral distribution with depth did not differ significantly from that of Stn. 4. However, the shape of the peak of a ph (λ) at short wavelengths differed from that of Stn. 4. At Stn. 10 there was another peak around 470 nm. This peak might have been due to chlorophyll b, which is known to have an absorption band in this region (Bidigare et al., 1990). At Stn. 11 the value of a ph (λ) was higher at 15 m than at other depths. The blue part of the spectrum of a d (λ) Fig. 7. Spectra of chlorophyll-specific absorption coefficient of phytoplankton [a* ph (λ)] at the sea surface (above) and selected depths (below) of Stns. 4 and P. Values for both stations are mean ± SD. 282 H. Sasaki et al.

9 increased with depth, especially around the thermocline (between 15 and 25 m), and was higher than the peak of a ph (λ) at short wavelengths (λ < 440 nm). The high irradiance attenuation with increasing depth (Fig. 2) was presumably due to the high values of a d (λ). However, since the bottom was shallow at this station, the increase in absorption might be due to the resuspension of suspended sediment from the sea bottom. Equation (5) incorporates the specific absorption as a function of chl a. Unlike a ph (λ), a* ph (λ), which represents the absorption cross section of phytoplankton per mass unit of chl a, is independent of chl a and depends only on algal cell size and intracellular pigment concentration (e.g. Morel and Bricaud, 1981; Sathyendranath et al., 1987; Hoepffner and Sathyendranath, 1992; Bricaud et al., 1995; Dupouy et al., 1997). At the surface of Stns. 4 and P, the mean values of a* ph (λ) in the blue part of the spectrum (440 nm) were ± and ± m 2 (mgchla) 1, respectively (Fig. 7). The variability in phytoplankton absorption in the blue part of the spectrum is due mainly to carotenoids. At 25 m depth at Stn. 4, the chl a level was highly variable (Fig. 3), a* ph (440) value was ± m 2 (mgchla) 1. In contrast, at 70 m depth at Stn. P, the chl a level did not vary greatly (0.47 ± 0.02 mgm 3 ), a* ph (440) value was ± m 2 (mgchla) 1, indicating higher variations of deviation in comparison with 25 m depth at Stn. 4. These variations between stations and between depths in the shape of the absorption spectra of the phytoplankton may indicate that the phytoplankton communities were composed of different species, or of the same species with different pigment compositions due to physiological acclimation to a varying environment (e.g. nutrients, temperature and radiant energy). Therefore, in order to understand the optical characteristics of phytoplankton, it is necessary to consider pigmentation and species of phytoplankton communities. In a study to be conducted in the near future, in addition to the pigment data obtained using HPLC measurement, which was gathered by another research group who participated to the same research cruise KH-97-2 (Hayashi et al., 2001), we shall continue to discuss the absorption characteristics of phytoplankton in more detail with respect to the pigmentation and phytoplankton species. Acknowledgements We should like to thank the officers and crew of R/V Hakuho Maru for their help and cooperation in the field observations. We also thank the scientists and technicians who participated in the KH-97-2 cruise for their help and support on board. The National Space Development Agency of Japan (NASDA) supplied the optical instruments used in this study. We also appreciate the help of J. R. Bower, who improved an earlier version of the manuscript. Part of this study was also supported through the program of Arctic research using the IARC (International Arctic Research Center)-NASDA Information System (INIS) and satellite data funded by NASDA, organized by the Earth Science and Technology Organization (ESTO). References Bidigare, R. R., M. E. Ondrusek, J. H. Morrow and D. A. Kiefer (1990): In vivo absorption properties of algal pigments. Proc. SPIE Int. Soc. Opt. Eng., Ocean Optics X, 1302, Boyd, P. W. and P. J. Harrison (1999): Phytoplankton dynamics in the NE subarctic Pacific. Deep-Sea Res. II, 46, Boyd, P. W., J. A. Berges and P. J. Harrison (1998): In vitro iron enrichments at iron-rich and -poor sites in the NE subarctic Pacific. J. Exp. Mar. Biol. Ecol., 227, Bricaud, A., M. Babin, A. Morel and H. Claustre (1995): Variability in the chlorophyll-specific absorption coefficients of natural phytoplankton: Analysis and parameterization. J. Geophys. Res., 100, Cleveland, J. S. and A. D. Weidemann (1993): Quantifying absorption by aquatic particles: A multiple scattering correction for glass fiber filters. Limnol. Oceanogr., 38, Dupouy, C., J. Neveux and J. M. André (1997): Spectral absorption coefficient of photosynthetically active pigments in the equatorial Pacific Ocean (165 E 150 W). Deep-Sea Res. II, 44, Emerson, S. (1987): Seasonal oxygen cycles and biological new production in surface waters of the subarctic Pacific Ocean. J. Geophys. Res., 92, Favorite, F., A. J. Dodimead and K. Nasu (1976): Oceanography of the subarctic Pacific region, Bull. Int. North Pac. Fish. Comm., 33, Favorite, F., T. Laevastu and R. R. Straty (1977): Oceanography of the northeastern Pacific ocean and eastern Bering sea, and relations to various living marine resources. Northwest and Alaska Fisheries Center Processed Report, 280 pp. Frost, B. (1991): The role of grazing in nutrient-rich areas of the open sea. Limnol. Oceanogr., 36, Harrison, P. J., P. W. Boyd, D. E. Varela, S. Takeda, A. Shiomoto and T. Odate (1999): Comparison of factors controlling phytoplankton productivity in the NE and NW subarctic Pacific gyres. Prog. Oceanogr., 43, Hayashi, M., K. Furuya and H. Hattori (2001): Spatial heterogeneity in distributions of chlorophyll a derivatives in the subarctic North Pacific during summer. J. Oceanogr., 57, this issue, Hoepffner, N. and S. Sathyendranath (1992): Bio-optical characteristics of coastal waters: Absorption spectra of phytoplankton and pigment distribution in the western North Atlantic. Limnol. Oceanogr., 37, Kishino, M. (1994): Interrelationships between light and phytoplankton in the sea. p In Ocean Optics, Oxford Univ, New York. Bio-optical Properties of Seawater 283

10 Kishino, M., C. R. Booth and N. Okami (1984): Underwater radiant energy absorbed by phytoplankton, detritus, dissolved organic matter, and pure water. Limnol. Oceanogr., 29, Kishino, M., M. Takahashi, N. Okami and S. Ichimura (1985): Estimation of the spectral absorption coefficients of phytoplankton in the sea. Bull. Mar. Sci., 37, Kishino, M., N. Okami, M. Takahashi and S. Ichimura (1986): Light utilization efficiency and quantum yield of phytoplankton in a thermally stratified sea. Limnol. Oceanogr., 31, Martin, J. H., R. M. Gordon, S. Fitzwater and W. W. Broenkow (1989): Phytoplankton/iron studies in the Gulf of Alaska. Deep-Sea Res., 36, McClain, C. R., K. Arrigo, K. Tai and D. Turk (1996): Observations and simulations of Physical and biological processes at ocean weather station P, J. Geophys. Res., 101, Morel, A. (1978): Available, usable, and stored radiant energy in relation to marine photosynthesis. Deep-Sea Res., 25, Morel, A. and A. Bricaud (1981): Theoretical results concerning light absorption in a discrete medium, and application to specific absorption of phytoplankton. Deep-Sea Res., 28, Morel, A. and L. Prieur (1977): Analysis of variations in ocean color. Limnol. Oceanogr., 22, Muller, J. L. and R. E. Lange (1989): Bio-optical provinces of Northeast Pacific Ocean: A provisional analysis. Limnol. Oceanogr., 34, National Research Council (1996): The Bering Sea Ecosystem. National Academy Press, Washington, D.C., 307 pp. Parsons, T. R. and C. M. Lalli (1988): Comparative oceanic ecology of the plankton communities of the subarctic Atlantic and Pacific Oceans. Oceanogr. Mar. Biol., 26, Pope, R. M. and E. S. Fry (1997): Absorption spectrum ( nm) of pure water. 2. Integrating cavity measurements. Appl. Opt., 36, Sathyendranath, S. and T. Platt (1988): The spectral irradiance field at the surface and in the interior of the ocean: A model for applications in oceanography and remote sensing. J. Geophys. Res., 93, Sathyendranath, S., L. Lazzara and L. Prieur (1987): Variations in the spectral values of specific absorption of phytoplankton. Limnol. Oceanogr., 32, Shibata, K. (1958): Spectrophotometry of intact biological materials. J. Biochem., 45, Shiomoto, A., Y. Ishida, M. Tamaki and Y. Yamanaka (1998): Primary production and chlorophyll a in the northwestern Pacific Ocean in summer. J. Geophys. Res., 103, Smith, R. C. and K. S. Baker (1981): Optical properties of clearest natural waters ( nm). Appl. Opt., 20, Springer, A. M., C. P. McRoy and M. V. Flint (1996): The Bering Sea Green Belt: shelf-edge processes and ecosystem production. Fish. Oceanogr., 5, Suzuki, R. and T. Ishimaru (1990): An improved method for the determination of phytoplankton chlorophyll using N, N-Dimethylformamide. J. Oceanogr. Soc. Japan, 46, Yentsch, C. S. (1960): The influence of phytoplankton pigments on the colour of sea water. Deep-Sea Res., 7, H. Sasaki et al.