COMPARISON OF UNDERWATER LIGHT FIELD PARAMETERIZATIONS AND THEIR EFFECT ON A 1-DIMENSIONAL BIOGEOCHEMICAL MODEL AT STA-TION ESTOC, NORTH OF THE CANARY ISLANDS

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ABSTRACT Light abundance is a major prerequisite for primary production in pelagic ecosystems. Throughout the last decades a number of analytic descriptions for the radiative transfer of light in oceanic water, in particular of the photosynthetically
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  Extended Abstract Ocean Optics XIV, 10.-13. November 19981 COMPARISON OF UNDERWATER LIGHT FIELD PARAMETERIZATIONS ANDTHEIR EFFECT ON A 1-DIMENSIONAL BIOGEOCHEMICAL MODEL AT STA-TION ESTOC, NORTH OF THE CANARY ISLANDS O. Zielinski *  , A. Oschlies † and R. Reuter  ** Fachbereich Physik, Universität Oldenburg, 26111 Oldenburg, Germany †  Institut für Meereskunde an der Universität Kiel, 24105 Kiel, Germany ABSTRACTLight abundance is a major prerequisite for primary production in pelagic ecosys-tems. Throughout the last decades a number of analytic descriptions for the radiativetransfer of light in oceanic water, in particular of the photosynthetically available radiation(PAR) were developed and validated against experimental data (see for example the com-parisons by M OBLEY ET AL . (1993) and S IMPSON AND D ICKEY (1981)). With the increas-ing number of biogeochemical models for oceanic ecosystems various descriptions of PAR(z) were also applied (e.g. F ASHAM ET AL . (1990), E VANS AND P ARSLOW (1985),D ONEY ET AL . (1996)).This paper compares the effect of different underwater light field parameterizationsat the ESTOC station (29°10’N, 15°30’W) north of Gran Canaria, Canary Islands. It dis-cusses the typical winter and spring situation, derived from experimental data. ThesePAR(z) descriptions are incorporated in a 1-dimensional biogeochemical model of theupper ocean driven by daily forcing fields that are taken from the ECMWF reanalyzes overa five-year period. The turbulence closure scheme of G ASPAR ET AL . (1990) was used tocompute the evolution of the upper ocean physical environment on a fine vertical grid( ∆ z=2 m). The comparison of diagnostic parameters such as the annual primary produc-tion (aPP) gives comparable results independed of the parameterizations chosen (aPP=6.5g C/(m 2 a) ± 15%). Differences in the phytoplankton distribution in the water column aretaken into account by an exponentially weighted detection function for an imaginary sat-ellite, thus simulating the penetration depth of ocean color remote sensors. The largevariation ( ± 45%) of this parameter underlines the important role of a realistic thoughcomputationally effective parameterization of the underwater light field in biogeochemicalmodels.1. INTRODUCTIONThe  European Station for Time-Series in the Ocean Canary Islands (ESTOC) islocated 60 nautical miles north of Gran Canaria and Tenerife (29°10’N, 15°30’W, waterdepth 3600 m). Its surface waters are part of the Canary current, the eastern branch of thesubtropical gyre [T OMCZAK AND G ODFREY (1994)] and hence is characterized by partiallyoligotrophic conditions. The station is equipped with moored traps and current meters,with additional monthly ship-based measurements taken since 1994 as a part of JGOFS[L LINAS ET AL . (1997)]. Since 1997 these time-series measurements are done as a part of the European CANIGO project (Canary Islands Azores Gibraltar Observation). The ex-perimental data presented in this paper were obtained in these two projects.  Extended Abstract Ocean Optics XIV, 10.-13. November 19982 C RUISES AND METHODS Recently, the investigation of the carbon and particle fluxes in case 1 waters byoptical means has met increasing interest [D ICKEY (1991)]. Especially in situ instrumentsprovide a fast and sensitive method for determination of optically active substances likechlorophyll a , Gelbstoff (colored dissolved organic matter) or apparent optical parameterslike the underwater light field. During two cruises north of the Canary Islands a new bio-optical in situ probing system [B ARTH ET AL . (1997a)] was applied. The system includes afluorometer [H EUERMANN ET AL . (1995)], a transmissometer [B ARTH ET AL . (1997b)] anda radiometer with multi-wavelength detection capability. Instruments and methods aredescribed in the cruise reports [W EFER AND M ÜLLER (1998), N EUER AND R EUTER (1999)]and in the above-mentioned publications and are not subject of this paper. RV Meteorcruise M37/2b took place in January and RV Victor Hensen cruise VH397/2 in April1997. Both cruises included a transect along 29°N, from the coastal region near the Afri-can Shelf (10°W), through ESTOC towards 18°W, north of La Palma (Fig. 1). 30°29°28°27°30°29°28°27° 3899436136121188661931 205 18°17° 16° 15° 14° 12°13° NNW 50030002000200 Las PalmasSanta CruzArrecife 350015001000 ESTOC   17° 16°15°14°13°12° W 18° Fig. 1: Shared cruise tracks of M37/2b (01/97) and VH397/2 (04/97) along 29°N. Bothcruises were incorporated in the monthly ESTOC (29°10’N, 15°30’W) sampling scheme.S EASONAL VARIATIONS ALONG 29°NThe typical winter situation at ESTOC (L LINAS ET AL . (1997)) is well representedduring M37/2b (01/97) by a deep mixed layer down to 116 m depth. A homogeneouschlorophyll a concentration is observed in the upper 90 m of  ∼ 0.25 mg chl a m -3 and asmall maximum of 0.30 mg chl a m -3 at 108 m depth. 1 Water masses are identically strati-fied along the whole transect at 29° N. That means, no upwelling occurred at the Africanshelf. The mean 10% and 1% depth of the photosynthetically available radiation (PAR,defined from 400 to 700 nm) are 63 and 124 m (Tab. 1).   1 Extracted chlorophyll a data by kind permission of Dr. O. Llinas, ICCM, Gran Canaria.  Extended Abstract Ocean Optics XIV, 10.-13. November 19983 During the spring cruise (VH397/2 - 04/97) we encountered upwelling conditionsclose to the shelf with chlorophyll concentrations reaching 1.1 mg chl a m -3 . At theESTOC site, however, a shallow mixed layer of 45 m could be observed. Chlorophyll a profiles showed very low surface concentrations above a pronounced deep chlorophyllmaximum at about 94 m with 0.63 mg chl a m -3 (Fig. 2 and 3). Gelbstoff and particulatematerial other than phytoplankton are at too low concentrations, to significantly influencethe underwater light field. Therefore we classify ESTOC as a case 1 water region [M ORELAND P RIEUR (1977)], with Jerlov oceanic water type IA (K d (475nm)=0.0285 m -1 ±8% inthe upper 50 m) [J ERLOV (1976)]. cruiseM37/2b (01/97)VH397/2 (04/97)time period 06/01/ - 22/01/9725/04/ - 03/05/97 mixed layer depth [m] 11645 depth of chl  a abundance [m] nearly homogeneous 0-90small maximum at 108maximum at 94 maximum chl  a concentration[mg chl  a m -3 ] 0.300.63 mean 10% depth [m] 6347 mean 1% depth [m] 12490Table 1: Parameter values for ESTOC derived from the two cruises. Mixed layer depth isdefined by a ∆ T=0.2°C criterion. Chlorophyll a concentrations are derived from in situ chl a prompt fluorescence measurements, calibrated by extracted chl a concentrations. Allvalues are calculated from stations ±30 nautical miles from ESTOC (total of 3 stations).Fig. 2: Temperature profiles at ESTOC inthe upper 200 m for both cruises.Fig. 3: Chlorophyll a profiles at ESTOC inthe upper 200 m for both cruises.  Extended Abstract Ocean Optics XIV, 10.-13. November 19984 2. DIFFERENT PARAMETERIZATIONS OF PAR(Z)Light is a crucial parameter since it limits growth in pelagic ecosystems. Therefore,different biogeochemical models use a variety of parameterizations of the underwater lightfield. Most of these parameterizations concentrate their efforts on the spectrally integratedphotosynthetically available radiation (PAR), using broad band photosynthesis-to-irradiance (P vs. E) relations as growth functions. Others separate the light field into directand diffuse light and/or make use of wavelength-dependent P vs. E-curves (K YEWALYANGAET AL .(1992)). S IMPSON AND D ICKEY (1981) make a comparison of the first group of PAR(z) parameterizations, identifying a) simple exponential, b) bimodal and c) multibandapproaches. This paper compares different PAR(z) parameterizations following this clas-sification. To evaluate their influence on primary production and other seasonal processesthey are incorporated into an 1-d biogechemical model at station ESTOC. PAR(0) is theradiation just below the surface and is taken as 45% of the total solar irradiance above sealevel, though B AKER AND F ROUIN (1987) show a dependence of this ratio on solar eleva-tion, aerosols, water vapor and ozone.Here, we compare the following PAR(z) parameterizations:a) Exponential functions:a1) D ONEY ET AL . (1996) used in their 1-d biogeochemical model for BATS 2 a simpleempirical description introduced by   M OREL (1988): PARzPARchlz ()()exp(.) .428 = ⋅ − ⋅ ⋅ 00121 0 ,with chl [mg chl a m -3 ] as the mean chlorophyll concentration over the euphoticlayer and  z [m] the depth. This parameterization results in a constant attenuationcoefficient kzPARz zPARz PAR ()()() = − ⋅ dd1over the whole water column.a2) In their analytical computation of daily mean growth rates, E VANS AND P ARSLOW (1985) assumed an exponential decay of irradiance by the water attenuation coef-ficient k  w =0.04 m -1 and the specific chl a attenuation k  chl =0.019 m 2  /(mg chl): PARzPARkzkchlzz wchl z ()()exp(')' = ⋅ − −      ∫  0 0 d.b) The bimodal parameterization proposed by P AULSON AND S IMPSON (1977) incorporatesthe high attenuation of larger wavelengths within the first 10 m (D ERA AND G ORDON (1968)). This results in increased k  PAR (  z ) values, by using a double-exponential function: PARzPARR z R zkchlzz chl z ()()exp()exp(')' = ⋅−     + −−−      ∫  01 120 ζ ζ  d.For Jerlov type IA waters they found  R =0.62, ζ 1 =0.60 m and ζ 2 =20 m. The influ-ence of chl a is, analog to a2), included in the shorter wavelength function.   2 Bermuda Atlantic Time series Study  Extended Abstract Ocean Optics XIV, 10.-13. November 19985 c) The multiband description of M OREL (1988) subdivided the 400-700 nm region in  N  =61bands, as an accurate though computationally expensive approach: PARzPARakzchlzz iiwichl zi N  ichl ()()exp(')' = ⋅ − − ∫ ∑ = 0 01  χ  η dwith k  iw , χ ichl and η ichl empirically resolved and given in tabular form. Theweighting coefficients a i were taken from S MITH AND B AKER (1981).To compare these four descriptions we introduce two simulated chlorophyll a dis-tributions that are assumed typical for the ESTOC station in winter and spring. A ho-mogenously distributed phytoplankton population of 70 m depth with a chl a concentra-tion of 0.25 mg chl a m -3 (Fig. 4a), and secondly a Gaussian deep chl a maximum at 100 mdepth with a 0.6 mg chl a m -3 peak concentration (Fig. 5a). The parameterizations a) - c)are compared as logarithmic profiles of E d in Fig. 4b) and 5b) respectively. Table 2 sum-marizes the results from the comparison for both situations. simulated chl  a distribution winter situationspring situation deep chl  a maximum [m] homogenous from 0-70100 max. chl  a concentration[mg chl  a m -3 ] 0.250.6 PAR(z) parameterization a1a2bca1a2bc mean 10% depth [m] 4753254949592758 mean 1% depth [m] 93105671049710973126 total PAR(0-150m) [µE/(m 2 s)] 196622856901616205225257531799Table 2: Results from the comparison of light field parameterizations a) - c).PAR(0)=1000 µE/(m 2 s), representing approx. 535 W/m 2 solar radiation above sealevel.Fig. 4a: Simulated chlorophyll a distribu-tion for the winter situation at ESTOC.Fig. 4b: Comparison of PAR(z) fromparameterizations a) - c).
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