TUE UNDERWATER LIGHT CLIMATE

W. W. Gieskes, R. Heusel, G. Kraay, M. M. Tilzer

The water of the Southern Ocean is only to a minimal extent influenced by terrestrial inputs and consequently has an extremely low content of allochthonous seston and gilvin ("Gelbstoff"), Phytoplankton and other suspended particles, therefore, should play a greater role in influencing the optical properties of the water than in other water bodies. We believe that these optical characteristics, also have important consequences for the energetics of the production process of phytoplankton.

Objectives

i) To describe the optical properties of Antarctic Ocean water both in the visible and in the ultraviolet spectral ranges.

ii) To examine and quantify the impact of phytoplankton and detritus On water transparency.

Work at sea

Our work included measurements both in situ during station work and in the laboratory On collected samples.

i) Underwater spectroradiometry: On 26 stations a total of 32 underwater light profiles were assessed to a maximum depth of 75 m. A Model MER 101 0 underwater spectro-radiometer (Biospherical Instruments, San Dieg0,U.S.A.) was used. This instrument is equipped with a cosine receptor. Underwater irradiance spectra were automatically recorded at 12 wavelengths between 410 and 694 nm every 10 milli-seconds. Ten scans were averaged and stored on-line by computer. At the Same time, incident irradiance was measured by a spherical sensor for integral photosynthetically active radiation (PAR) on deck as a reference for the underwater measurements.

The assessment of each light profile consisted of two casts: downwelling irradiance was recorded with the light sensor facing upwards, and upwelling radiance by turning the instrument by 180 degrees.

Both data acquisition and reduction were perforrned by using a software package provided by the manufacturer. In addition, calculation Programmes were developed On board by one of us.

ii) Underwater UV measurements: Immediately following the spectro- radiometer casts, UV was recorded at 30 stations down to a depth of 27 m by an underwater ultraviolet sensor developed by N.I.O.Z.,Texel,

The Netherlands. This instrument records UV radiation at 4 wavelengths between 340 to 405 nm. The data were stored on a LiCor data logger and thereafter printed and evaluated.

iii) Secchi-depth readings: During a total of 20 stations Secchi-disk readings were performed by using a Secchi disk of 35 cm diameter.

iv) In vivo light absorption spectra of suspended matter filtered onto Whatman GFIC filters were registered by using a LKB Ultraspec spectrophotometer. The spectra were plotted on a printer.

Preliminary results

i) Underwater spectra of photosynthetically available radiation (PAR) and phytoplankton abundance:

At most stations, downwelling underwater radiation was chararacterized by maximum water transparency in the blue spectral range ( 488 nm).

Chlorophyll a concentrations at these stations roughly ranged from 0.1 to 1.0 mg m-3 (chlorophyll measurements by G. Dieckmann and E.-M.

Nöthig) Only at the stations with chlorophyll concentrations well above 1 mg m-3 (Stations 135 - 139) maximum water transparency shifted somewhat towards the green with a rather broad waveband of maximum transparency (488-540 nm).

This shift in the spectral properties of the underwater light can be explained by the spectral absorption and scattering properties of algal suspensions which absorb most of the light in the blue spectral range (see iii, Fig. 19). Overall transparency then was greatly reduced. Red light, by contrast, is rapidly absorbed by the water molecules themselves and showed considerably smaller sensitivity to chlorophyll concentration (Fig 17).

The vertical light attenuation coefficient (Kd) was evaluated for each of the 12 wavelengths where underwater irradiance had been measured and will be used to quantify the impact of phytoplankton on water transparency as a function of wavelength and chlorophyll concentration. Vertical differences in chlorophyll concentrations led to corresponding variations of vertical light attenuation coefficients (Fig. 18).

Upwelling irradiance comprised less than 1 % of downwelling light ( Fig. 17). It is caused by scattering of light by both water molecules and suspended particles. The spectral composition of upwelling radiance was very similar to that of downwelling light. This indicates that scattering from particles (which is largely wavelength-independent) is the major source of upwelling light.

However, it remains to be examined to which extent the ratio of upwelling to downwelling irradiance depends on seston concentration or phytoplankton biomass.

ii) Vertical gradients of underwater PAR and Secchi disk transparency:

By integrating over the spectrum, overall gradients of PAR were determined. The penetration depth of 1 % of surface PAR ( "euphotic depth", Ze,, ) is frequently considered to be in a constant proportion to the Secchi depth ( Zc ), and conversion factors of 2-3 are applied to predict euphotic depth from Secchi readings. We examined this relationship during this cruise. We found that the euphotic depth in fact was non- linearly related to the Secchi depth by

By using this equation, euphotic depths can be predicted from Secchi readings with reasonable accuracy (maximum error 17 %). The ratio of euphotic to Secchi depth increases with rising turbidity from values below 2 in very clear water to over 4 at the maximum phytoplankton biomass observed during our cruise ( Table 4 ).

This non-linear relationship is explained by the fact that Secchi depth is more dependent On light scattering by particies than is vertical light attenuation. Our findings are in agreement with current theory but the non-linear relationship has only rarely been demonstrated empirically.

iii) Absorption of light by particulate matter:

At most stations suspended matter absorbing light clearly did not consist of phytoplankton alone: cultures of Antarctic diatoms dominant in the survey area grown on board absorbed less in the green, blue and ultraviolet part of the spectrum than filtered suspended matter in seawater samples; the additional absorption was due to detrital particles (Fig. 19).

These particles could not be identified under the microscope; they were amorphous, in a size range between 1 and 50 prn.

iv) The contribution of phytoplankton and detritus to light absorption cannot be quantified precisely until the concentration of the algal pigments has been measured. This will be done at home in the laboratory by using HPLC ( high performance liquid chromatography ). However, comparison with absorption spectra of samples free of detritus (judged by microscopic observation ) suggests a low but significant contribution of the detrital particles to overall light absorption, especially at wavelengths below 500 nm, all the way down to 200 nm.

The absorption of particulate matter on the filters at 16 wavelengths between 340 and 693 nm was closely related to the attenuation of light in the water at these wavelengths. This suggests that autochthonous particulate matter, both phytoplankton and detritus, has a greater influence on underwater light penetration in the Weddell Sea than has dissolved organic matter.

iv) The penetration of near-ultraviolet light into the water column:

At the stations with very clear water ( Secchi disk visibility over 40 m ) , 10% and more of ultraviolet radiation ( 340 nm wavelength ) measured above the water surface penetrated to 25 m; extrapolations indicate that

1% of surface UV is present at depths of up to 57 m. At these stations, all of which were located in the Southern part of the study area, attenuation of UV was usually highest in the upper part of the water column ( Fig. 20).

Here the vertical attenuation coefficient Kd was often twice as high as Kd at greater depths.

The higher absorption at 340, 365, 380, and 405 nm in the near-surface water layer was correlated with the presence of relatively high concentrations of pariiculate matter absorbing light effectively at these wavelengths (see section iii). Difference spectra of absorption by particulate matter on filters sampled at less than 0.5 m and 10 m depth are typical of detritus (cf. Fig. 19). The near-surface detritus layer was not only found near the vessel but also at the edge of ice floes distant from the ship ( samples taken by Niskin samplers by Larsson, Sehlstedt and Ljungek and at one occasion by one of the divers).

The origin of the surface detrital layer, which was encountered in the study area wherever sea-ice was present, may have been heterotrophs consuming ice algae, the degradation products being flushed into the Open water from underneath the ice. The enhanced levels of ammonia often obsewed at the very surface in leads and polynyas supports this view. Another possibility is that the surface detritus is older refractory material that accumulated at the surface over prolonged periods of time.

Attenuation of ultraviolet light by sea-ice was highest under ice with a Snow Cover. The clear ice of nilas, however, obstructed UV penetration much less.

Table 4: Euphotic depths ( depth of 1 % of surface irradiance), Secchi depths,where measured, and ratios of euphotic depth to Secchi depth. Also given are predictions of euphotic depth from Secchi depth by using eq. 1 and errors of these predictions.

Station Zeu Zc Zeu/Zs Zen pred error %

Station 117, 2 November 1988

downwell ing light upwelling light 10'

wavelength (nm) Station 135, 12 November 1988

downwelling light upwelling light

Fig.17. Underwater spectra at 2.5-meter depth intervals at the station with the highest (top panels) and with the lowest water transparency observed during the cruise (bottom panels). Left panels: downwelling, right panels: upwelling radiance. The spacing between the spectra reflects the degree of vertical light attenuation. The topmost spectral curves represent values recorded above the water surface. The spectral scans of Station 11 7 show typical properties of very pure water and are almost solely controlled by the optical properties of the water molecules. By contrast, the spectra of Station 135 are strongly influenced by phytoplankton and detritus both of which absorb blue light more efficiently than light of greater wavelengths.

Station 135

Kd 441 m-l

I l

o

10 102 103

Ed n mol m-2 s-1 nm-1

Fig. 18. Profiles of chlorophyll concentration ( left panel ) and semi-logarithmic plots of downwelling radiance at the 4 wavelengths indicated, as well as the vertical attenuation coefficient of light at 441 nrn which corresponds with the blue in situ absorption maxirnurn of chlorophyll.

Note the difference in depth-scales in both panels. The chlorophyll concentration maxirnum at a depth of 60 m is clearly reflected in the corresponding maxirnum in the attenuation coefficient for blue light.

Chlorophyll analyses by G.Dieckmann and E.-M. Nöthig

Y

E

U (L) U C

.Q

0 0

cn n

0

wavelenath nm

wavelength nm

difference

I 2 k

spectrum

0.08

waveleng t h n m

Fig 19.In vitro absorption spectra of suspended matter collected onto glass- fiber filters at 0.5 m and 10 m at Station 126. Dominante of diatoms at 10 m is reflected in the absorption spectrum. The difference spectrum is similar to the spectrum of detrital material alone which implies that at 0.5 m depth more detritus was present than at 10 m depth. Station 126 is representative for most stations in the survey area.

Station 117

0.2; ^{I I} 1 I I I

5 10 15 20 25 30

depth ( m )

0.02. I I 1 1 I 1

5 10 15 20 25 30

depth ( m )

Fig 20. Penetration of ultraviolet light into the water at a station with very clear water (Secchi depth nearly 50

m ,

Station 1 17, See Fig. 17), and at open-water Station 136, located at the northern edge of the survey area (Secchi depth: 12 m ).

2.2.3 SEA ICE PROPERTIES

Im Dokument with contributions of the participants (Seite 79-89)