HYDROBIOLOGICAL BULLETIN 22(2): 127 - 134 (1988)

THE INFLUENCE OF THE SPECTRAL COMPOSITION OF IRRADIANCE ON PRIMARY PRODUCTION IN THE EASTERN SCHELDT (THE NETHERLANDS)

J. STRONKHORST

KEYWORDS : Phytoplankton; 14C incubator filters; spectral attenuation; actionspectrum. ABSTRACT The error in in vivo 14C incubator measurements of primary production in the Eastern Scheldt when neutral density filters were used and the error obtained when no account was taken of the spectral changes in submarine irradiance that occur with increasing depth, were evaluated theoretically. By multiplying the photosynthetic action spectra of two marine algae by calculated irradiance in the euphoric layer using Kd and Kd(;~) respectively, the gross: primary production P[Ed(400- 700)] and P[Ed(;~)] was computed. In the green-brown waters of the Eastern Scheldt estuary the use of neutral density filters was sufficient to simulate the underwater light conditions. In clear waters it can'cause an overestimation of the gross production. INTRODUCTION The primary production of phytoplankton in natural waters can be calculated using data on photosynthetic characteristics, obtained with the in vivo 14C incubator method (FEE, 1973; BI RNBAUM, 1978), the incident irradiance, and the downdwelling irradiance attenuation coefficient Kd for the photosynthetically available radiation (PAR) between 400- 700 nm. In most incubators neutral density filters are used to simulate the mean spectral attenuation of daylight underwater. The spectral composition of the submarine irradiance changes with increasing depth, because of selective extinction at different wavelengths (J E R LOV, 1976). The composition of the irradiance can determine the photosynthetic response of algae (MOREL, 1978; JEFFREY, 1981). An action spectrum of algae represents this response to photosynthesis as a function of wavelength ~. For this reason JITTS (1963) and JITTS eta/. (1976) used blueglass filters instead of neutral filters in their measurements of primary production in clear oceanic waters. In this study the relative error made by taking no account of the spectral influence in in vivo 14C incubator measurements in the Eastern Scheldt (The Netherlands) was estimated. A simple theoretical model was used to relate the action spectrum of an alga to the spectral irradiance Ed(~) and the irradiance Ed(400. 700 nm), respectively. Integrating over the euphoric layer and the spectral range 400- 700 nm results in a calculated integral gross production P. P[Ed(400- 700)] represents the production that is measured with neutral density filters, whereas P[Ed(;~)] represents the.production when the spectral variations are taken into account. A similar approach was used by DRING (1981) in his study on chromatic adaptation of photosynthesis in benthic marine algae.

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The calculations were performed with data from measurements of Kd and Kd(X) carried out in the Eastern Scheldt estuary and, for comparison, in the salt-water Lake Grevelingen in the Dutch Region (Fig. 1). Published data on two action spectra (NEORi etal., 1986) were used. The results showed little difference between P[Ed(400- 700)] and P[Ed(X)] in the turbid water of the Eastern Scheldt.

4k

ROTTERDAM

NORTH SEA REVELINGEN ~

~

~

|

J/

(RHINE)

MEUSE 0EASTERN _4 ~

SCHEL

IIDDELBURG

Fig. 1. Stations in Eastern Scheldt and Lake Grevelingen where optical measurements were carried out. MODEL The model which calculates P[Ed(400-700)] can be expressed as

P[Ed(400-700)] =

z euph 700 f f z=0 X=400

a(X).Ed(0,X).exp(-Kd.z)dz.dX

(1)

where PIEd(400- 700)] is the integral gross primary production ( g C . m - 2 . r l ) , a(X) is the spectral action coefficient at wavelength ~ (gC.quantum--1), Ed(o,X)dX is the downwelling incident irradiance (quanta.m-2.s - 1 ) directly below the water surface in wavelength interval (X,X+dX), z is the depth (m), z euph. is the depth of euphotic layer (m), and Kd is the downwelling irradiance attenuation coefficient ( m - l ) . Kd is expressed as - d In(Ed) Kd =

(MOREL and SMITH, 1982) dz

128

(2)

where Ed is the downwelling irradiance, preferably measured between 350- 700 nm with a quantum sensor (TYLER, 1966). For wavelength X the spectral attenuation coefficient Kd(X) can be calculated with equation (2) when Ed(X) is known. When Kd(X) is used in equation (1) the spectral weighted gross production P[Ed(X)] can be calculated. The effect of the spectral attenuation on the gross production is expressed by the ratio P[Ed(400,700)] D=

(3) P[Ed(X)]

Ed(o,X) in equation (1) can be written as a product of the maximum downwelling irrediance Ed(o)max and the relative downwelling irradiance Ed*(o,X). Ed(o)ma x is the maximum downwelling irradiance above the watersurface minus the reflection at the water surface which is thought to be wavelength independent (PREISENDORFER, 1976). Therefore reflection does not influence the value of D. Furthermore a(X) in equation (1) can be expressed as product of the maximum action coefficient amax and the relative action coefficient a*(;~). Therefore, the calculation of D is simplified by omitting amax and Ed(o)ma x from numerator and denominator. Data on a*(;~), Ed*(o,~) and Kd(~,) in each 25 nm waveband were used. For all calculations the maximum numeric error in the double integral did not exceed 1 % . DATA For the action spectra considered in this study, a*(X) is expressed as a fraction of the maximum photosynthetic response of Chaetocerosgracilis at 440 nm and of Rhodomonas D3 for 565 nm (Table 1), The action spectrum of the brown diatom Chaetoceros is thought to be representative of the important marine phytoplankton groups of diatoms and dinoflagellates (NEORI etal., 1986) which dominate the phytoplankton population in the Eastern Scheldt, The cryptomonad Rhodomonas was present in the Eastern Scheldt less frequently. The latter

Table 1. Relative action coefficient a* (X) after NEORI et al. (1986) and relative spectral downwelling irradiance Ed* (o,X) recorded on 1984 - 03 - 28 at Middelburg. Wavelength (nm)

a*(X) Chaetocero$

400 425 450 475 500 525 550 575 600 625 650 675 700

0.75 0.90 0.96 0.63 0.45 0.45 0.19 0.06 O.15 0.25 0.21 0.71 0.07

Ed*(o,X) Rhodornonas

0.84 0.84 0.81 0.81 0,63 0.75 0,95 0.95 0.40 0,32 0.37 0.82 0.18

0.44 0.60 0.70 0.75 0.75 0.79 0.83 0.86 0.90 0.92 0.95 0.97 1.00

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species showed an aberrant response with a maximum in the blue, green and red part of the spectrum. A specj;rum of downwelling incident quantum irradiance was measured to calculate Ed*(o,~). The spectrum (Table 1) was recorded at Middetburg on 1984- 03- 28, with a Techtum QSM 2500 spectroradiometer, under a cloudless sky and a solar altitude of 35 o. On 1984- 03- 15, 16, 20 measurements were performed to obtain data on Kd and Kd(;~) in the Eastern Scheldt and in Lake Grevelingen. Ed was measured with a Licor-192 $8 quantum cosine collector, Ed(X) with the spectroradiometer described above. The measurements were performed around noon under a cloudless, slightly hazy sky. RESULTS AND DISCUSSIONS During the measurements in the Eastern Scheldt the seston concentration varied strongly with location and tide, whereas the chlorophyll a concentration was low (Table 2). Assuming a specific attenuation of chlorophyll pigment of 0.016 m--1 (mg chlorophyll.m--3) - 1 (SMITH and BAKER, 1978) the maximum attenuation in the Eastern Scheldt caused by chlorophyll was 0.04 m - l . It can therefore be assumed that in this case transmission of daylight under water was not greatly affected by the amount of algae present. This is also true for Lake Grevelingen. The maximum transmission in the Eastern Scheldt (Fig. 2) was in the greenbrown part of the spectrum. The Eastern Scheldt showed less attenuation between 400- 450 nm than JERLOV's optical watertype 9 (JERLOV, 1976) because the latter was measured in the Baltic Sea, with a high concentration of yellow substance. Lake Grevelingen (Fig. 2) showed good agreement with JERLOV's water type 5 (green coastal waters). In common with MOREL and HOJERSLEV (1979) the best agreement between Kd(;~) and Kd is found when = 500 nm (Fig. 3).

Table 2. Kd, seston and chlorophyll concentration during the measurements on 1984-03- 15, 16, 20 in the Eastern Scheldt and Lake Grevelingen. Location

Kd (m-- 1)

Seston+ (g.m--3)

Eastern Scheldt 1 2 3

0.74 0.79 1.17

4

1.42

10

1.71

5 6 Lake Grevelingen

1.66 2.38 0.27

22 31 1

2.59 2.69 0.44

5 5 9

Chlorophyll a++ (rag.m--3) 1.50 1.82 2.19

+ Suspendedmatter on a Whatman GF/C filter. ~+ According to Lorenzen fluorimetric method. The D values calculated for both action spectra are presented in Table 3. For the combination of Chaetoceros in Lake Grevelingen P[Ed(400- 700)] was 4 1 % higher than P[Ed(~)], mainly because in the intervals 400- 475 nm and 660- 680 nm Ed is higher than Ed(;~), and these intervals coincide with peaks in the action spectrum of Chaetoceror For Chaetoceros in the E~s(ern Scheldt P[Ed(400- 700)] was significant (P(0,0025) higher than P[Ed(~)], which is caused by an overestimation in the intervals 400 - 475 nm. However, the difference is small. For Rhodomonas in the Eastern Scheldt there was no significant 130

difference becausethe overestimationof P[Ed(400-700)] between400- 475 nm and 625- 675 nm is compensatedby an overestimationof P[Ed(X)] between500- 600 nm. A comparison betweenan in $itu experimentand an incubator experiment madeby 3,2 3,0 2,8

2,6

2.4 -t 2.2 2,O

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~

6

_

5

~8

1,6

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..~8, /

/

0,2 0,0 400

I

I

I

I

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i

450

500

550

600

650

700

--~ wavelength ~ (nm) Fig. 2. Kd(;~) in the EasternScheldt (curve 1- 6), LakeGrevelingen (curve 7), Jerlov's optical water type 5 (curve 8) and water type 9 (curve9). 131

STEEMANN N I EL SEN (1974) for a coastal (green)water also indicated that the difference in the gross production is small. In the clear waters of Lake Grevelingen D is higher than in the more turbid waters of the Eastern Scheldt. Data on Kd(X) of J E R LOV's optical coastal watertypes (J E R LOV, 1976) were =

2.5

2,0

1,5 Q O u3 "o ,.s

1.o

O.5

I

I

0.5

I

I

I

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1.0 1.5 Kcl (400-700 nm)

I

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2.0

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2.5

Fig. 3. Kd (400- 700 nm) vr Kd (500 nm); n = 7, R2 = 0,99.

used to study the relation between D and turbidity. It seemed that for both action spectra (Table 4) D decreased with decreasing transmittance of the water type and the associated shift of maximum transmittance from blue, in the clearest waters (type 1), over green to brown in the most turbid waters (type 9). The lowest D value was found for Rhodomonas in most turbid waters of type 9, because at the peak of the action spectrum between 525- 575 nm, Kd is considerably higher than Kd(;~). For Chaetoceros in the clearest coastal waters (type 1), P[Ed(400-700)] was 1.7 times P[Ed(;~)], because Ed values are higher than Ed(;~) in the intervals 400- 500 nm and 600- 700 nm and these intervals coincide with peaks in the action spectrum. This confirms the need to use colour filters in in vivo incubator measurements to approximate the spectral attenuation characteristics of clear (blue) waters, as done by JITTS (1963).

132

Table 3, D values of the Eastern Scheldt (mean and standard deviation of 6 stations) and Lake Grevel ingen. Water wpe

Chaeroceros

Rhodomona$

Eastern Scheldt

1.12 + 0.06

1.03 +__0.05

Lake Grevelingen

1.41

1.20

Table 4. D-values of Jerlov's optical water types. Water type

Chaecocero$

Rhodornonas

1

1.70

1.46

3

1.58

1.35

5

1.37

1.19

7

1.23

1.06

9

1,05

0.88

CONCLUSIONS In turbid green-brown coastal waters like the Eastern Scheldt the difference in P[Ed(400- 700)] and P[Ed(~,)] was very small and the use of neutral density filters in a 14C incubator w i l l therefore be sufficient to simulate the underwater light conditions. In clear coastal waters the use o f neutral filters can lead to the integral gross production, being overestimated considerably. ACKNOWLEDGEMENTS I wish to thank my colleagues J.P.G. van de Kamer, J.C.H. Peeters and D. Spitzer for their comments. REFERENCES BIRNBAUM, E.L., 1978. Estimating in situ algal production rates with the help of light measurements and experimentally measured production rates. Hydrobiol. Bull., 12: 127- 133. DRING, M.J., 1981. Chromatic adaption of photosynthesis in benthic marine algae : An examination of its ecological significance using a theoretical model. Limnol. Oceanogr., 26:271 -284. FEE, E.J., 1973. A numerical model for determining integral primary production and its application to Lake Michigan. J. Fish Res. Bd Canada, 30: 1447- 1468. JEFFREY, S.W., 1981. Responsesto light in aquatic plants. In : O.L. Langa, P.S. Nobel, C.B. Osmond and H. Ziegler, Eds., Physiological Plant Ecology I. SpringeroVerlag,Berlin; pp. 249-271. JERLOV, N.G,, 1976. Marine Optics. Elsevier, Amsterdam, 231 pp. JITTS, H.R., 1963. The simulated in situ measurement of oceanic primary production. Aust. J. mar. Freshwat. Res., 14: 139- 147. JITTS, H.R., A. MOREL and Y. SAIJO, 1976. The relation of oceanic primary production to available photosynthetic irradiance. Aust. J. Mar. Freshwater Res., 27:441 -454. MOREL, A., 1978, Available, usable and stored radiant energy in relation to marine photosynthesis. Deep Sea Re=., 25: 673- 688. MOREL, A. and N.K, HOJERSLEV, 1979. COnversion of quasi-monochromatic downward irradiance into downward quanta irradiance (370-700 nm). PrOces-Verbauxno. 15, XVll General assembly at Canberra: p. 119. 133

MOREL. A, and R.C. SMITH, 1982. Terminology and units in optical oceanography. Marine Geodesy. 5: 335- 349. NEORI, A., M. VERNET, O. HOLM-HANSEN and F.T. HAXO, 1986. Relationship between action spectra for chlorophyll-a fluorescence and photosynthetic 02 evolution in algae. J. Plankton Res., 8: 537 * 548, PREISENDORFER, R.W., 1976. Hydrologic Optics. U.S. Dep. of Commerce, National Oceanic and Atmospheric Administration, Environmental Research Laboratories. STEEMANN NIELSEN, E., 1974. Light and Primary Production. In : N.G, Jerlov and E. Steemann Nielsen, Eds., Optical aspects of oceanography. Academic Press, London; pp. 361 - 388. SMITH, R.C. and K.S. BAKER, 1978. The bio-optical state of ocean waters and remote sensing. Limnol. Oceanogr., 23: 247- 259. TYLER, J.E., 1966. Report on the second meeting of the joint group, of experts on photosynthetic radiant energy. UNESCO Techn. Pap, Mar. Sci., 2 : 1 - 11.

Address of the author : Ministry of Transport and Public Works, Tidal Waters Division, Grenadierweg 31, 4338 PG Middelburg, The Netherlands.

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