Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
OCEANOLOGICA ACTA 1986- VOL. 9 - W 1 ~ -----!~- Production Chlorophyll Light Mode! Profiles Production Chlorophylle Lumière Modèle Profils Primary production profiles in the ocean: estimation from a chlorophylljlight model Alex W. HERMAN", Trevor PLATTb Department of Fisheries and Oceans, Atlantic Oceanographie Laboratory, Bedford Institute of Oceanography, Dartmouth, Nova Scotia B2Y 4A2, Canada. b Department of Fisheries and Oceans, Marine Ecology Laboratory, Bedford Institute of Oceanography, Dartmouth, Nova Scotia B2Y 4A2, Canada. a Received 11/10/84, in revised form 2/7/85, accepted 3/7/85. ABSTRACT A chlorophylljlight model has been developed to estimate high resolution vertical profiles of primary production, and therefore, integrated water column production in the ocean. The model requires continuous profile measurements of chlorophyll and light, surface light, and two parameters characterizing the relationship between photosynthesis and light. Primary production profiles measured from in situ incubation experiments and simulated in situ experiments using a deck incubation box were used to verify the model by statistically fitting the model equations to profile data from the Scotian shelf. Generated profiles showed that the subsurface production maximum occured in the upper chlorophyll gradient, and that much of the subsurface chlorophyll layer was not photosynthetically active because of the unavailability of light. The model can also be used as a tool in analyzing the changes in production that result from small vertical changes in chlorophyll structure and to a lesser extent, from changes in surface light. The variations in estimates of integrated water column production indicated the necessity of resolving horizontal variability of profiles. Oceanol. Acta, 1986, 9, 1, 31-40. RÉSUMÉ Profils de la production primaire dans l'océan: évaluation à partir d'un modèle simulant la teneur en chlorophylle/intensité lumineuse. On a élaboré un modèle simulant la teneur en chlorophylle/intensité lumineuse en vue d'évaluer les profils verticaux à haute résolution de la production primaire et, ainsi, la production intégrée sur la colonne d'eau dans les océans. Le modèle nécessite la mesure en continu de la teneur en chlorophylle et de l'intensité lumineuse, de l'illumination à la surface et de deux paramètres caractérisant la relation entre la photosynthèse et l'intensité lumineuse. On a utilisé les profils de production primaire mesurés au cours d'expériences d'incubation in situ et d'expériences en milieu naturel simulé réalisées dans des incubateurs à étages, pour vérifier le modèle en adaptant les équations qui le décrivent aux données sur les profils obtenues sur la plate-forme Scotian. Selon les profils obtenus, la production sous la surface est maximale dans le gradient chlorophyllien supérieur, et une bonne partie de la couche de chlorophylle sous la surface n'est pas active du point de vue photosynthétique en raison de l'absence de lumière. Le modèle peut aussi servir à analyser les variations de production provoquées par de faibles variations verticales dans la structure chlorophyllienne et, à un moindre degré, par des variations d'intensité lumineuse à la surface. Étant données les variations de la valeur estimée de la production intégrée sur la colonne d'eau, il est nécessaire de résoudre la variabilité horizontale des profils. Oceanol. Acta, 1986, 9, 1, 31-40. 0399-1784/86/01 31 10/$ 3.00/ ~ Gauthier-Villars 31 ._..., .. ,"' l A. W. HERMAN, T. PLATT INTRODUCTION Simulated production profiles Primary production of the oceanic water column can be estimated by the 14C technique using in situ incubation experiments of deck incubations in natural or artificial light. In a few studies, both methods have been used simultaneously (Gargas, Nielsen, 1976; Jitts et al., 1976; Brown, 1982) using samples form approximately 5-6 depths corresponding to predefined light levels, e. g., 100%, 50%,25%, 10%, 1% of surface light. Comparison of the methods bas generally shown reasonable agreement although there have often been systematic differences; deck incubations have generally yielded higher (approximately 20-40%) production measurements than in situ incubations. Fee ( 1973 a; b) bas developed a model for estimating integrated column production using empirical data relating light and photosynthesis. Samples were drawn from only 4m depth and the assumption that phytoplankton were distributed homogeneously throughout the water column. The problem in water column production estimates is the Jack of vertical resolution of either chlorophyll a or production. Fee ( 1973 b) suggested that continuo us in situ chlorophyll fluorescence measurements in the water column would help to refine future production estimates. We describe here a chlorophylljlight mode) which can be used to: 1) generate the fine scale profile of primary production; and 2) to estimate integrated water column production. In practice, once the photosynthesis-irradiance (P-1) characteristics of the sampling area are reasonably weil established, the method requires only profiles of chlorophyll a and irradiance thereby permitting quasi-instantaneous estimates of water column production to be made. Application of the mode! is tested against high resolution (vertical) direct measurements of primary production on the Scotian Shelf and provides insight into the importance of the vertical structure of pigment biomass on water column productivity. The method of generating simulated production profiles (SIS, simultaned in situ) is based on our standard incubator procedure (Platt et al., 1980) using an incubation box and artificial lights. This method differs from the simulated in situ (SIS) profile (e. g., Harrison et al., 1984) which employs ambient solar light for deck incubations. Our experimental design afforded higher vertical resolution ~ 3 to 5 rn, since the samples were incubated at the same light leve! as that of the collection depth. Continuous profiles of irradiance and chlorophyll were sampled with the pump sampler system at about noon. A series of seawater samples was pumped from various irradiance levels as labelled Ll, L2, ... Ln, in Figure 1 a which correspond to an identical series of irradiance levels in the incubation box (Fig. 1 b). The irradiance levels within the box were measured with the same PAR light sensor used on the pump sampler, thus minimizing any potential systematic errors arising from improper matching of irradiance between in situ profile and incubation box. Each irradiance, L1, L2, ... Ln, represents a measured depth, Dl, D2, ... Dn (Fig. 1 a), and the samples in the incubation box represent a profile of measured production. With the number of samples N = 20 taken over a depth range of 0 to 60m, the vertical resolution between samples was ~ 3 m. The first three positions of the incubation box were generally in a light saturation region of the photosynthesis - light curve (1 > 100Wm- 2). The light sources used to irradiate the incubation box were in most cases tungsten-halogen lamps; in earlier experiments, incandescent lamps were used. The dependence of production rates on the spectral characteristics of these lamps is discussed later. Following establishment of the initial irradiance profile with the pumping system at mid-day, the twenty sampies from depths Dl, D2 ... Dn were obtained over a period of ~ 45 mn during which the stability of the profile could be monitored from the temperature, fluorescence, and light measurements. Immediately following the seawater sampling, severa! additional profiles of temperature, chlorophyll, and light were taken (each requiring only ~ 2 min deployment) with the pumping system. Incident solar irradiance was recorded and integrated (h - 1) on the ship's deck. Chlorophyll a concentrations of each of the 20 samples in the incubation box were determined from acetone extractions. INSTRUMENTATION AND METHODS Pumping system A continuous profiling pumping system (Herman et al., 1984) was used for seawater sampling and biological profiling. The system employs a Ramoy progressive cavity pump. An intercomparison of production rate measurements from samples taken with the pump and from Niskin bottle casts showed no statistical difference, implying that no measurable damage to photosynthetic organisms was caused by the pumping system. Sensors mounted on the pump frame were used to and pressure measure temperature ( ± 0.2°C) (±0.2dbar). A quantum light meter (LICOR) cousisting of a flat plat collector with near eosine response measuring irradiance over a 4 decade range from a maximum of~ 500Wm- 2 (integrated over the photosynthetically action radiation (PAR) region of 400-700 nm.) was also mounted on the pump frame. A small portion ( ~ 4 1 min - 1 ) of the pump outflow was diverted to a Turner 111 fluorometer which measured chlorophyll a fluorescence. In situ production profiles Experiments were also performed to measure in situ profiles by inoculating a number of seawater samples with 14C and returning them to their depth of origin for incubation. Three botties (2 light and 1 dark) were attached to a wire at each depth over the full depth range of 3 to 85 rn at 3 rn intervals, between 20 and 50 rn and 5 rn intervals over the remaining depths. There were approximately 20 depths sampled overall. The wire, buoyed at the surface, was deployed at approximately 1100 hrs and recovered after about 4 br of incubation. Chlorophyll and temperature profiles were sampied intermittently near the wire to monitor profile 32 CHLOROPHYLL/LIGHT MODELLING by integrating equation (3) over time t (b) (a) 1\ ln\ o.------- 24 hr UGHT SOURCE Cd (Z) = L1 ,Dl 201- .s:r: 40 1- L2,D2 L3,03 L4,04 x )( x !--=x x x X X X Z ~60 Ca= TEMP. CONTROL FLOW 1---:. 24 hr ff 0 Pv(z, t) dtdz, (4) 0 where Zr, the final depth, is selected to be large enough ( ~ 80 rn) such that increasing it would not increase the integral significantly. 1--- 100 Figure l A representation of in situ wire incubations (a) in which samp/es, duplicate /ight botties plus one dark bottle, were suspended at 20 light depths and (b) the equivalent simulated in situ incubations where samp/es are po.~itioned in the incubator to receive /ight equivalent to in situ leve/s. Non-linear least square analysis Fitting of the mode! equations ( 1 and 2) to directly measured production data required two measured input profiles: light and chlorophyll. Values of a and Pm from previous production studies on the Scotian Shelf (Herman et al., 1981; Herman, Platt, 1983) could be used; however we have no assurance of the constancy of the se parameters from year to year and therefore they were considered free parameters (i e., fixed values, not specified) in the non-linear !east squares analysis. Consider Aj as representative of the dependent variables a ana Pm, theo the optimum values of Aj are obtained by minimizing chi-squared, x2 , with respect to each of the parameters separately: variations. lntegrated hourly solar radiation levels required for calculating integrated daily production profiles were recorded on deck using a LICOR integrating quanta meter. Chlorophyll a concentrations of each of the bottle samples on the wire were determined from acetone extractions. Chlorophyli/Light model Equations Our numerical modelling is based on the construction of production profiles from the relationship between photosynthesis and light. ln our representation, the dependence of primary production P (1) per unit mass of chlorophyll (mgC [mgchl a]- 1 h - 1) on available light is given by the photosynthesis light saturation curve (Jassby, Platt, 1976; Platt, Jassby, 1976; Chalker, 1980). (5) where Pv (Z) represents the calculated value (equation 2) of production and Pe (Z), the experimentally measured in situ value. Chi-squared (x 2 ) can be considered a continuous function of the parameters Aj describing a two-dimensional surface in three-dimensional space which can be searched for the corresponding minimum value of x2 • Problems can be encountered when the hyperspace contains more than one minimum. Therefore the search must be restricted to a region when there is only one known minimum of x2 and this is accomplished by selecting realistic starting values of a and Pm during the search routine. (1) where 1 is the irradiance (PAR-photosynthetically active radiation) in W rn- 2 , a(mgC[Chiar 1 • w- 1 m+ 2 h- 1 ) is the initial slope of the light saturation curve, Pm(mgC[mgchlar 1 h- 1) is the assimilation number, and R(mgC[mgchtar 1 h- 1) is a measure of dark respiration. The magnitude of R is generally ~ 0.1 in our shelf waters and small enough to be ignored in equation (1). The irradiance, 1, is a function of depth and therefore P (1) can also be represented as a vertical production profile. Absolute production profiles in units of (mg C rn- 3 h - 1 ) are obtained by multiplying the production P (1) of equation (1) by the chlorophyll a profile ( units of mg. rn- 3 ) as measured with the pump profiler and is given by Pv(Z) =P(Z). B(Z), (3) The areal primary production per day in the water column (Ca) in units of mgCm- 2 d- 1 can then be obtained by integrating equation (3) over depth INCUBATION BOX l- P(l) =Pm tanh(ai/Pm) -R Pv (z, t) dt. 0 a.. 80 f Dependence of Production on .Spectral Quality The validity of comparing production measurements based on solar and artificial light-incubations was examined here. The differences in spectral quality may give rise to significant differences in production estimates. Examples of the energy spectra of solar, incandescent and tungsten-halogen light are shown in Figure 2. The quantum energy spectra were measured with a quantaspectrometer (QSM-2500, Techtum Instruments). The artificiallight source spectra were measured at ail positions (two shown in Fig. 2) of the incubator box of Figure 1 and the solar spectra were measured at various depths (0-50 rn) in Bedford Basin, Nova Scotia. The (2) where B (Z) is the chlorophyll concentration and Z is the depth in meters. The daily primary production profile, Cd in units of mgcm- 3 d- 1, can be obtained 33 A. W. HERMAN. T. PLATI taeniata (Steeman Nielsen, 1975; Fig. 11), a macrophyte presented to illustrate the magnitude of the effect arising from considerable differences in the action spectrum at larger wavelengths. The production ratio of equation (7) was evaluated using the quantum spectrum, Eart (À.), corresponding to either the incandescent or tungsten-halogen lamps in Figure 2 and the photosynthetic spectrum of Figure 3 a. The ratios for both photosynthetic spectra as a function of integrated PAR quantum energy are plotted in Figure 3 b. High and low values of light energy in Figure 3 b essentially correspond to the front and back positions of the incubator box shown in Figure 1 and from surface to depth in the case of solar energy. The data of Figure 3 b indicate that production measurements using artifical lights are reduced by approximately 10% relative to solar energy in the case of Ph 1 ( Ulva taeniata) and approximately 20-30% in the case of Ph 2 (Pleurochloris). The overall decrease in production is caused by the inability of pigments to utilize efficiently the excess of artifical red light according to its photosynthetic absorption spectra. There is a small but relatively insignificant slope in Figure 3 b, that is, a dependence of the ratio on light intensity. INCANDESCENT 10 0~~~~~--~------~ 7 E 5 TUNGSTEN HA LOG EN c: ~ '!= "' ~ !! Q SOL AR Figure 2 Energy spectra of two artificial light sources, incandescent and tungsten-halogen, and solar radiation. The lower intensity for the artificial light sources correspond to the rear positions of the incubation box; the lower intensity of solar light represents the spectrum at 35 m depth in Bedford Basin. RESULTS A compilation of the statistically derived model parameters, et and Pm, and other associated data are presented in the Table. artificallight sources of Figure 2 exhibited more energy in the red region; however, the tungsten-halogen light had a relatively higher production of blue to red light. The effect of this red light excess on production will depend on the efficiency with which it is used by phytoplankton cells relative to blue light. This is defined by the photosynthetic absorption cross-section or action spectrum, Ph (À.). Production, therefore, can be defined in terms of the product of the photosynthetic crosssection, Ph (À.), and incident energy, E (1..): ~ 1.o (a) l- u ~~.8 Ul~ 'ë ~::J •6 f- w"' J:~ ~~.4 >-Il: U>~ 1 Pr et 0 2=700 f b Ph (À.) E (À.) dl... .2 I (6) a.. ~OL0~~~~-5~0-0~~~~~~~~~~70~0 u=400 WAVELENGTH The fractional change in production induced by artifical light relative to solar light can be represented by: t= ~1.0 u 1.2=700 (Pr) ART/(Pr) SOL= f t= w Ph (À.) >- • lt • l l l •••• "t • • 1 ~ Ph2 T __ Ph 1 __________________ l .8 Ul 0 b .7 1.2=700 f .9 . I ~ u=400 Eart (À.) dl../ (b) 0 oiNCANDESCENT/SOLAR x TUNGSTEN/SOLAR I a.. Ph (À.) Esol (À.) dÀ (7) • • x•• • • x" l • •x. •tl. tl •• x x" JL •• 10 100 1000 LIGHT QUANTA (J.LE m-2 s·l) u=400 The photosynthetic cross-section, Ph (À.), changes with species and is dependent on the light absorbing pigment complex. Two examples are shown in Figure 3 a: Pleurochloris (from Prézelin, 1981; Fig. 3) and Ulva Figure 3 a) The photosynthetic action spectra of two a/gal species with different bluefred energy absorption characteristics; and b) the ratio of the expected photosynthetic rates of samples irradiated with artificial a.~ compared to solar light sources. 34 CHLOROPHYLL/LIGHT MODELLING PRODUCTION (mgC·m· 3 (5hrr 1) 4.0 8.0 12.0 0 1 1 1 CHLOROPHYLL (mg 10 20 0 1 1 0 0 approxima tel y 35 m. The measured total production, Pv (mgcm- 3 ) (equation 2) for the 5hr incubation period indicated a distinct subsurface layer with a maximum at approximately 25 m. Here we refer to the subsurface layer as that which has a distinct chlorophyll maximum at depth in contrast to the much lower concentration but more uniform chlorophyll in the mixed layer (i.e., 0 to 15 m). The mode! (best-fit) production curve Pv (Z) of equation (2) was obtained by varying the parameters, ex and Pm, in equation (1). Ambient irradiance 1 was not constant over the 5 hr incubation period and therefore P (Z) in equation (2) was evaluated from equation ( 1) by integra ting over the 5 hr interval. This was accomplished by taking the 1130 h irradiance profile as representative of the period between 1100-1200h and subsequently scaling the profile proportionately with fractional changes in the hourly integrated surface irradiance as measured on deck. Hence production in Figure 4 is expressed as mg C rn- 3 /5 hr interval. Curve fitting the model to the measured in situ production data set resulted in best fit values ex= 0.08 (cr~ ±0.006) and Pm= 1.35(cr ~ ±0.16)(Tab. ). Starldard deviations were extracted from the twOdimensional x2 hyperspace plotted as a function of ix and Pm in Figure 5. The x2 contours describe both the minimum, and first and second standard deviation. Daily areal production Ca was evaluated from equation (4), using measured houri y rates of solar irradiance. For this station, we measured Ca= 327 mg C rn- 2 d - 1 • This was slightly lower than production rates measured in our past work which averaged approximately 400mgcm- 2 d- 1• The slight reduction is due partly to a lower than average Pm, which in previous years was in the range 2.0-2.5 (see Herman et al., 1981; Irwin et al., 1977) and partly due to a deeper than average chlorophyll layer and therefore generally Jess light available. 14.0 1 1 m 3) 1 TEMPERATURE ("C) 2 4 6 3;0 1() 12 STATION No. 67 Ca•327mg C·m" 2 ·d" 1 Figure 4 Station 67 showing chlorophyll, temperature, and production profiles samp/ed from the Scotian Shelf The points represents in situ production measurements. The production curve represents a best-fit to the data set. Ch/orophyll concentrations were estimated at the same depths as production indicated by the data points. The 1 % /ight depth is indicated. In situ production profiles (wire experiments) Station 67 Figure 4 illustrates a vertical profile sampled at the outer edge of the Scotian Shelf including measured chlorophyll a ( determined from acetone extractions), temperature, primary production (mgCm- 3 ), and the mode! generated production curve. The chlorophyll maximum was at approxima tel y 30 rn depth; its vertical distribution was skewed at greater depths such that the centre of gravity of the chlorophyll distribution was Table Compilation of data from both in situ wire experiments and simulated in situ deck experiments including best-fit parameters IX, Pm. Mean light represents an average over 24hrs and are meant to provide relative indications on/y. The standard deviations quoted for ex, Pm are given as means of negative and positive errors which actually differas seen in Figure 5. Positive errors were actually ~ 10% higher and negative errors ~ 10% lower than the mean. Station No. Day April-May Incubation Time (hrs) 27 50 58 72 105 126 28 30 1 2 4 4.0 4.0 4.0 4.0 4.0 4.0 5 Mean light W.m- 2 Ca mgcm- 2 d- 1 IX SIMULATED IN SITU INCUBA TI ONS 109 49.5 196 127.5 282 112.9 125 87.6 59.2 52 59.5 123 (±cr) 0.022 0.036 0.035 0.021 0.036 0.033 Pm (±cr) (0.0014) (0.0047) (0.007) (0.003) (0.005) (0.007) 1.60 (0.13) 2.57 (0.45) 1.67 (0.20) 1.11 (0.09) 1.32 (0.13) 1.47 (0.11) Mean=0.032 (0.0055) 1.62 (0.51) Mean= 55 67 98 119 1 2 4 5 5.0 5.0 4.5 5.5 IN SITU WIRE INCUBATIONS 112.9 235 327 87.6 59.2 214 183 59.5 35 (0.007) (0.006) (0.016) (0.010) 0.98 (0.11) 1.35 (0.16) 1.59 (0.36) 3.09 (0.31) Mean=0.086 (0.010) 1.75 (0.24) 0.044 0.081 0.14 0.079 A. W. HERMAN, T. PLATT x2- CONTOUR at STN 67 was situated again on the upper chlorophyll gradient with the production maximum at approximately 35 to 40 m. A real daily production was low with Ca=214mgcm- 2 d- 1 as a result of two factors: 1) a deeper chlorophyll maximum; and 2) a sharp decrease in incident light caused by fog in the latter 2 hr of the wire experiment. Best-fit estimates of a= 0.14 and Pm= 1.59 were obtained. Note that the model was unable to fit satisfactorily production at 5 m, tending to underestimate it. This suggests that Pm is probably not constant with depth but tend to be higher (approximately 2.0-2.5) near the surface ( < 10 rn). The dashed line curve of Figure 6 indicates the potential production if ambient light were doubled. Under these conditions the vertical structure of production layer remains essentially unchanged; however, the curve area bas increased by approximately 50% such that Ca=334mgCm- 1 d- 1 • Doubling ambient light did not proportionately increase production since the production in the surface water < 15 rn was already saturated and did not benefit from increases in ambient light. a • .081 +- .006 •007 .:: .10 ''.c +'.17 Pm•1.35 -.l 5 ~ ct ..J 5·09 Ë ':'E 3: .08 u E "' ;.o1 1.2 1.3 1.4 1.5 Pm (mgC·[mgCHLg)' 1 ·h' 1 ) 1.1 1.6 Figure 5 contours for STN61 corresponding the first (X 1 =3.11) and second (X 1 = 3.65) standard deviations of r:x., Pm. x1 The temperature profile (Fig. 4) showed; a) a deep layer of nutrient-rich slope water at depths > 50 m intruding over the shelf edge; b) an intermediate cold water layer between approximately 20 and SOm; and c) a surface mixed layer down to 15 m. Both the surface and intermediate layers were approximately 3ec and cooler than temperatures measured in previous years (e.g., Herman et al., 1981), a result of a tate discharge of cooler, fresher Gulf of St. Lawrence water flooding the shelf in March. Generally, we find subsurface chlorophyll layers in the intermediate water layer where the nutricline is situated, above high-nutrient slope water. The temperature structure shown in Figure 4 was typical of the entire cruise. Stations 55 and 119 Figure 7 illustrates stations with deep chlorophyll maxima at 40 m and 50 m depth where production was low. The profile of STN 55 showed surface chlorophyll nearly double that of STN 119 at depths < 30m which resulted in higher production near surface. As a result of significant light absorption in surface waters, there was no distinct production layer formed within the chlorophylllayer at approxima tel y 30 to 40 rn depth. The wire experiment of STN 119 showed low production Ca= 183 mg Cm - 1 d - 1 as a result of low ambient light 1Mean=59.5W.m- 2 • The mean light lmean represents an average over a 24-hr period and is only meant to be used as a relative indicator of average light measured from station to station. Production was restricted to depths (15 rn; the best-fit value of Pm = 3.1 (±0.31). Station 98 The chlorophyll maximum layer in the profile of Figure 6 was at approximately 45 m as a result of a deeper intermediate cold water layer. The production layer as PRODUCTION (mg C m· 3 ,(4,5 hr)'1) 4D1 0 1 ao CHLOROPH~LL LO 0 1 œo 1 PRODUCTION (mgC·m·3 , (5hr)' 1) (mg m' 3 ) 2,0 1 0 3.0 1 ep 4;0 1 TEMPERATURE (•c) LO o.--.-.--------------To--~zr-.-4r-~sr- o,________:4'T,O'-------'B';',O'---- CHLOROPHYLL (mg m' 3 ) O'l---,------';"-'r--------'2"f'-,O 20 20 .5 I40 l- a. w 0 ( eo STATION No. 98 1 Ca= 214 mg C· m-zd- 1 STATION No. 55 Ca= 235mg C·nf2 ,d'1 Figure 6 Station 98 wire experiments showing temperature, chlorophyll, and fitted production profiles. The dashed line production curve corresponds to a doubling of solar light intensity. STATION No. 119 Ca= 183mg C·m2 ·d-1 Figure 7 Stations 55 and 119 wire experiments showing chlorophyll and fitted production curves. 36 CHLOROPHYLL/LIGHT MODELLING ~ 28 rn depth, and low surface concentrations ( ~ O. 2 mg rn- 3) at depths < 15 m. Light penetrations was high at this station resulting in a production layer nearly coïncident in depth with the chlorophyll layer. The mean depth of the production layer was located slighly shallower at approximately 25 rn depth. The best-fit value of Pm=2.5 was the highest obtained for the six stations sampled. PRODUCTION (mg C·m· 3- (4hr)"1) o.__ _ _4.:.r.o~- o 4.0 s.o CHLOROPHYLL (mg m·3 ) 0 or-....---=-r--4.:;;.o::..... o 2.0 1.0 1 ( • ·._FITTED )• PRODUCTION Station 72 It had low areal productivity, Ca=125mgcm- 2 d- 1, as a result of low ambient light, Imean=87.6W.m- 2 • A relatively small production peak was observed at 32 rn depth. ) 60 Î STATION No. 27 Co=I09mgC·m" 2 ·d-1 STATION No. 58 Stations 105 and 126 Co= 282mgC·m" 2• d"1 These stations exhibited low areal production due to low ambient light. There were no subsurface production layers formed (see Fig. 10) as a result of low light penetration into deep chlorophyll maxima between ~ 40-50m depth. Figure 8 Stations 27 ans 58 showing chlorophyll and fitted production profiles corresponding to the simulated in situ (SIS) experiments. Each profile shows a distinct production maximum at the upper chlorophyll gradient. Simulated production profiles (Incubation exp.) PRODUCTION (mg G- m· 3 • (4hr)' 1 ) Stations 27 and 58 Samples used in these simulated profiles were taken at 1300 hr and incubated for 4 hr. The chlorophyll maxima were located at approximately 30m (Fig. 8) and the production maxima were found approximately 5 rn shallower and coïncident with the upper chlorophyll gradient. Although the production curve was fitted for the data derived from incubations at a constant irradiance, daily areal production was obtained by integrating the measured hourly production over day and over depth. Areal production at STN 27 was extremely low (Ca= 109mgm- 2 d- 1) due to low ambient light. Chlorophyll concentrations, however, were extremely high (~ 6mgm- 3 at the maximum). j 0 4.0 4.o 8.0 FITTED 0~-----~1.0~--- ----,_. -PROOU~---- 60 STATION No. 105 Co=52mgC·m·2 -d·l STATION No. 126 Co•l23 mg C·m·2 -d" 1 Figure 10 Stations 105 and 126 showing chlorophy/1 and production profiles. PRODUCTION (mg C·m3·(4hr)' 1 ) 1 o ~r---~~-~0_ _ _~2.0~ The chlorophyll layer of STN 50 (shown in Fig. 9) showed a low maximum concentration ( < 1 mg rn- 3 ), at 4.0 e.o CHLOROPHYLL (mg m·3) Stations 50 and 72 0 4.o 0 DISCUSSION 8.0 CHLOROPHYLL (mg m'3 ) h-r--'ii.O,___ 0 1.0 One objective of this investigation was to examine the vertical structure of production relative to that of chlorophyll. One important vertical feature became apparent in our data set, i. e., carbon production did not parallel chlorophyll in the vertical scale. In general only the upper chlorophyll gradient supported high primary production, i.e., STN 67, 68 (wire experiments and STN 27, 58 (SIS experiments). The lower half of the subsurface chlorophylllayer, below the chlorophyll maximum, contributed little to the areal production in nearly ali cases. There are severa! conditions necessary for this to occur. First, the near-surface chlorophyll must be low in concentration ( < 0.5mgm- 3) such that light can penetrate to the subsurface chlorophyll layer. Second, the subsurface chlorophylllayer must be large enough in biomass to absorb essentially ail available light. If we take the example of STN 98 (Fig. 6) it can be seen that light was able to penetrate through low 2.0 -------- ? :J: .... Q. "'0 STATION No. 50 Ca•l96mgC·ITi 2·d'1 STATION No. 72 Ca= 125 mg C· m' 2·d' 1 Figure 9 Stations 50 and 72 showing chlorophyll and fitted production profiles. 37 A. W. HERMAN, T. PLATT surface concentrations (0.25mgm- 3 ) to the subsurface chlorophyll layer and drive maximum production at 35 m. The subsurface chlorophyll layer was situated between approxima tel y 32 and 55 rn depth with à maximum at 43 rn; however, only the chlorophyll subpeak at 35 rn depth was active in carbon production. Similar observations were made for the remaining STNS 67, 27 and 58. · At STN 55 (Fig. 7) slightly higher surface concentrations (> 0.5mgm- 3 ) of chlorophyll resulted in high surface absorption of light and active production there. Despite a high irradiance Omean = 113 W.m - 2 ), there was little penetration to the subsurface chlorophyll layer and as a result there was no observable production peak within the upper vertical chlorophyll gradient at approximately 35 m. At STN 50 (Fig. 9) the production peak was nearly coïncident with the chlorophyll maximum. This was a consequence of 1) low surface chlorophyll concentrations permitting light penetration into the subsurface chlorophyll layer; and 2) a relatively shallow and low concentration subsurface chlorophyll layer. In the remaining stations (105, 119, 126) carbon production was generally low and these profiles had no subsurface production peaks. This most likely resulted from 1) low surface irradiance and 2) deeper subsurface chlorophyll layers. Changes in production were most sensitive to vertical changes in the subsurface chlorophyll layer. This was due to the rapid exponential decay of light with depth which is dependent on principally two components roughly equal in magnitude: a) attenuation due to inorganic material in water column; and b) that due to photosynthetic plant material. Light dependence may be represented by I = Io exp [- (CX;n + rxp 1)Z], where rx;n represents attenuation due to inorganic material and a.P1 represents attenuation due to photosynthetic plant material. If we were to shift, for example, a chlorophylllayer with a maximum at 40m depth to a depth of 20 rn, the chlorophyll layer would gain a considerable increase (exponential) in light intensity which was previously attenuated between 20 to 40m depth by inorganic material. The more intensive light at 20 rn results in greater penetration throughout the chlorophylllayer rather than just the upper chlorophyll gradient. Hence a large increase in production results. Mode! estimates indicate that a 20 rn deepening results in a decrease of approximately 2.5 in areal production. The second objective of this investigation was to test the utility of the chlorophylljlight model in estimating water column carbon production as determined from equation (4). Each measured production data set was curvefitted to the modelled production equation (2) by adjusting the parameters ex and Pm. The mode! equation (4) (not the experimental data set), using the bestfit values of ex and Pm, was used to estimate areal water column production. The most striking feature of these estimates was the wide range of areal production estimates, which we attributed primarily to vertical changes in the subsurface chlorophyll. For example, the wire experiment of STN 67 (Fig. 4) showed a subsurface chlorophyll maximum at approximately 25 rn which supported areal carbon production of Ca= 327 mg Cm - 2 d-t. La ter that afternoon an SIS ex periment STN 72 (Fig. 9) showed a depth increase of chlorophyll maximum now at approxima tel y 35 rn which resulted in a significant decrease in a production peak at that depth and a similar decrease in areal production where Ca= 125mgcm- 2 d- 1 • A similar situation occurred in the wire ex periment of STN 98 (Fig. 6) where a subsurface chlorophyll layer was found 10m deeper in the afternoon profile of STN 105 (Fig. 10). The resulting increase in chlorophyll maximum depth accompanied by a decrease in surface irradiance resulted in a fourfold decrease in areal production. In comparing the wire experiment of STN 55 (Fig. 7) with the SIS experiment of STN 58 (Fig. 8) performed that afternoon, we encountered a decrease of approximately 10 rn in the subsurface chlorophyll layer which resulted in an increase for the SIS estimate of areal production. It was also clear that production is most sensitive to vertical changes and to a lesser extent to absolute surface irradiance. The latter instance was seen in the example of STN 98 (Fig. 6) where the ambient light was arbitrarily doubled in the calculation and resulted in an integrated production (short dashed line) increase of on! y approximately 50%. This comparative!y smaller increase was caused by light saturation in the surface layer, < 20 rn as seen in Figure 6 where rouch of the incident light was not utilized. The curve-fitting analysis of each of the wire and SIS experiments yielded independent estimates of ex and Pm. The mean values of the assimilation number, Pm, for either wire (Pm= 1.62) or SIS (Pm= 1.75) experiments were in close agreement, within approximately 10%. An independent check of the assimilation number was made on the same cruise from an experiment reported in Herman et al. (1984). Fifteen samples were taken from various depths and incubated (triplicate) for 4 hr at light saturation (I ~ 175 W.m- 2 ). The mean assimilation number for 15 samples was Pm=1.3±0.2 indicating agreement with the curve-fitted data. The absolute values of Pm were somewhat lower than previous years' measurements (Herman et al., 1981) where Pm= 2.4. There were also indications of sorne dependence of Pm on depth in STNS 98 (Fig. 6) and 126 (Fig. 10). The mode! equations underestimated Pm in the 5 rn surface layer. More realistic values of Pm= 2-4 would have improved the fit in this depth region. We have evidence from our previous data (Herman et al., 1981) which favors the view that photosynthetic parameters are constant with depth. This depends, however, on the degree of stratification in the water column. The tendency of phytoplankton cells to modulate their photosynthetic apparatus according to ambient light levels (photo-adaptation) is opposed by vertical mixing which redistributes the cells to different light levels. The resulting vertical structure of photosynthetic properties will then depend on the ratio of the characteristic time scale for vertical mixing to that for photoadaption (Lewis et al., 1984a). It is estimated that, in the sea, vertical mixing caused by winds 38 CHLOROPHYLL/LIGHT MODELLING ~ 5 rn s- 1 is strong enough to erode vertical stratification (upper 30m) in the photosynthetic properties of phytoplankton (Lewis et al., 1984b). This situation would prevail on the Scotian Shelf much of the time. We judge, therefore, that the assumption of uniform photosynthesis parameters throughout most of the photic zone as used in our analysis is a reasonable one for most of the stations sampled. We have observed sorne deviations from uniformity of Pm in the upper few metres of surface water. Similar discrepancies were observed by Harrison et al. (1984) in both surface layer (Om depth) estimates and the 10% light depth. White recognizing these problems, it must be also noted that this surface layer strata and the 10% light depth represent relatively small fractions of the overall water column production resulting in changes of only 10 to 15% in the areal production estima tes. The mean values of IX for either wire (1X=0.086) or SIS (1X=0.032) experiments differed by nearly a factor of 3. However, the mean standard deviation for the wire experiments was nearly an order of magnitude greater than that of the SIS experiments, the latter clearly yielding more consistent measurements. We offer no clear explanation for these deviations; spectral differences in the wire and SIS experiments make little difference to photosynthetic rates according to our estimates in Figure 3. One common difference is the constancy of light used in the SIS experiments as opposed to the more variable ambient source experienced in the wire ex periment. Harrison et al. (1984) compared data from a number of deck incubations experiments using five light depths with production estimates made from our model equations and experimentally determined light saturation parameters IX and Pm. He found good agreement in the comparison with the exception of small systematic differences where the mode! overestimated production in the surface layer and underestimated near the bottom of the euphotic zone at (1% light level, 30-40m depth). Estimates of areal production using either method, however, differed by only 10 to 15% and were not statistically different. We have no basis of comparison since neither of our experiments were deck incubations; however, our model in sorne cases slightly underestimates production in the surface layer. Harrison et al. (1984) also found better agreement between direct measurements and model estimates when derived from the hyperbolic tan equation than when derived from the more detailed exponential functions (Platt et al., 1980) which allow for photo-inhibition at high irradiances. This indicates that the hyperbolic tan function, simpler in form, is suitable for computing primary production. necessity for detailed information on the vertical structure of chlorophyll and light. The spectral quality of the light source did not affect the photosynthetic rates appreciably. The effect of using artifical lights was to reduce photosynthetic rates by approximately 10 to 30% as shown in Figure 3. This means that the areal production estimates from the SIS experiments in Table 1 are probably underestimated and should be increased by approxima tel y 20%- Such an increase, however, is insufficient to alter any of our previous conclusions. In ali this data analysis we have used chlorophyll a concentrations derived from acetone extractions and not relied solely on fluorescence measurements. The implication for future sampling strategies is that fluorescence profiles accompanied by calibrations are adequate while overall accuracy will clearly depend on calibration quality. In most cases we find that fluorescence measurements are preferable to use since they represent a continuous systematic measurement. By taking the mean calibration we may then reference the fluorescence profile to absolute chlorophyll concentrations. CONCLUSIONS The chlorophyllflight model provides: 1) high resolution vertical profiles of primary production; and 2) integrated water column production. In practice, model production estimates require only measurement of: (PAR) profile; 3) surface solar irradiance; and 4) the photosynthetic parameters IX and Pm. Production profiles may then be generated from equation (2) and areal production from equation (4). The photosynthetic parameters IX and Pm must be determined from field measurements of P-I curves as described by (Gallegos, Platt, 1981). Deck incubation using artificial lights yielded the least variable estimates of IX and Pm in this work (Tab. 1). The extent of these measurements in the field and their adequacy in characterizing the environment remain indeterminate. As little as four to five deck incubation experiments (20 samples per incubation box) were sufficient for sorne cruises on the Scotian Shelf; however, three to four times as many experiments were required for Peruvian shelf waters due to high variability of vertical chlorophyll structure (Herman, 1981; Herman, 1984). Considering the wide range of production measurements, it is clear that we must account for spatial and temporal variability of areal pr_oduction estimates in any survey. For example, we have obtained data at the outer edge of the Scotian Shelf using a Batfish vehicle (Herman, Denman, 1979; Herman et al., 1981) where profiles were sampled continuously 0.5 km apart over transect lengths of 40 to 50 km. Analyses of these data (to be published) indicated spatial variability of ± 25% for production over the transect length. However, while moving from the Scotian Shelf to the shelf/slope where a shelf break front was encountered, we found as much as a factor of 2 in production variability along with as much a variability in biomass and depth of chlorophyll maxima. For example, in sorne areas we found that Earlier modelling in the Great Lakes by Fee (1973 a) assumed vertical homogeneity while recognizing the problem of vertical variability in phytoplankton distribution. It is clear from our work that integrated water column production is most sensitive to vertical structure in the biomass profile; even small changes of approximately 5 to 10m in the vertical displacement of the subsurface chlorophyll layers can result in large changes in the areal production. This points to the 39 A. W. HERMAN, T. PLATT subsurface chlorophyll layer in Scotian shelf waters (Herman et al., 1981), Baffin Bay (Herman, 1983) and, to sorne extent, in Peruvian Shelf waters (Herman, 1984). The mode! can be used as a diagnostic tool in analyzing vertical production characteristics and trends. For example, we were able to compare the large contribution of a surface spring bloom on the Scotian Shelf as opposed to the Iesser contribution of summer stratified subsurface layer. These differences may be qualitatively self-evident but are often difficult to quantify. In the Eastern Canadian Arctic, we were able to quantify Iow areal production as a result of low irradiance at these high latitudes; however, we also characterized the persistence of surface production as a result of extended daylight. the integrated chlorophyll was reduced by a factor of 2 and yet we encountered a peak in production (30% increase) there simply because the subsurface chlorophyll maximum was shallower there than anywhere else in the tow. By contrast, data we have anlyzed for the Eastern tropical Pacifie showed variability of less than 10% for integrated chlorophyll and production. Here we found a reproducible series of profiles. We have also determined a simple method (to be published) which may be used as a rapid indicator of spatial and temporal variability. Since integrated production is directly dependent on integrated light the 1 % Iight depth can be used as this indicator. ln certain areas of the Scotian shelf we found a inverse Iinear relationship between integrated production and the 1% Iight depth such that we were able to use the 1 % light depth as a direct measure of areal production. A similar pattern was found in the Eastern tropical Pacifie where the 1 % light depth varied Iess than ± 5% about the mean. In data from the Peruvian coast, we found the relationship to be non-Iinear but consistent from tow to tow. There is considerable information derived from vertical profiles of primary production. We have found a strong coïncidence between the subsurface production layer and copepod layers which were situated above the l Acknowledgements We wish to thank Dr. Robert Fournier and Dr. Glen Harrison for their critical review of the manuscript. We also wish to thank the staff of the Metrology (AOL) and Biological Oceanography Divisions (MEL) and the crew and officers of the Lady Hammond for their able assistance during field experiments and the collection of data. REFERENCES Herman A.W., Sameoto D.D., Longhurst A.R., 1981. Vertical and horizontal distribution patterns of copepods near the shelf break south of Nova Scotia, Can. J. Fish. Aquat. Sei., 38, 1065-1076. Herman A.W., Mitchell M.R., Young S.W., 1984. A continuous pump sampler system for profiling copepods and chlorophyll in the upper oceanic Iayers, Deep-Sea Res., 31, 439-450. Irwin B., Harrison W.G., Denman K.L., Platt T., 1977. Phytoplankton and nutrient measurements at the edge of the continental shelf off Nova Scotia between April 28 and May 11, 1977, Fish. Mar. Serv. Data Rep., 62, 90 p. Jassby A.D., Platt T., 1976. Mathematical formulation of the relationship between photosynthesis and light for phytoplankton, Limnol. Oceanogr., 21, 540-547. Jitts H.R., Morel A., Saijo Y., 1976. The relation of oceanic primary production to available photosynthetic irradiance, Aust. J. Mar. Freshwater Res., 27, 441-454. Lewis M.R., Cullen J.J., Platt T., 1984a. Relationship between vertical mixing and photoadaption of phytoplankton: similarity criteria, Mar. Eeol. Progr. Ser., 15, 141-149.· Lewis M.R., Horne E.P.W., Cullen J.J., Oakey N.S., Platt T., 1984b. Turbulent kinetic energy dissipation may control phytoplankton photosynthesis in the upper ocean, Nature (under review). Platt T., Jassby A.D., 1976. The relationship between photosynthesis and light for natural assemblages of coastal marine phytoplankton, J. Phyeol., 12, 421-430. Platt T., Gallegos C.L., Harrison W.G., 1980. Photoinhibition of photosynthesis in 11atural assemblages in marine phytoplankton, J. Mar. Res., 38, 687-701. Prézelin B.B., 1981. Light reaction-s in photosynthesis, in: Physiological bases of phytoplankton ecology, edited by T. Platt, Can. Bull. Fish. Aquat. Sei., 210, 346 p. Steemann-Nielsen E.S., 1975. Marine photosynthesis: with special emphasis on ecological aspects, Elsevier Oeeanogr. Ser., 13, 141 p. Brown P.C., 1982. Phytoplankton production measured in situ and undersimulated in situ conditions in the southern Benguela upwelling region, Fish. Bull. S. Afr., 16, 31-37. Chalker B.E., 1980. Modelling light saturation curves for photosynthesis, an experimental fonction, J. Theor. Biol., 84, 205-215. Fee E.J., 1973 a. A numerical mode) for determining integral primary production and its application to Lake Michigan, J. Fish. Res. Board Can., 30, 1447-1468. Gallegos C.L., Platt T., 1981. Photosynthesis measurements on natural populations of phytoplankton: numerical analysis. Physiological bases of phytoplankton ecology, edited by T. Platt, Can. Bull. Fish. Aquat. Sei., 210. 346 p., 103-112. Gargas E., Nielsen C.S., 1976. An incubator method for estimating the actual daily plankton algae primary production, Wat. Res., 10, 860. Harrison W.G., Platt T., Irwin 8., Harris L., 1984. The utility of light saturation models for estimating marine primary productivity in the field: a comparison with conventional "simulated'" in situ methods (in press). Herman A.W., 1981. Spatial and tempral variability of chlorophyll distributions and geostrophic current estimates on the Peru shelf at 9"S, J. Mar. Res., 40, 185-207. Herman A. W., 1983. Vertical distribution patterns of copepods, chlorophyll and production in northeastern Baffin Bay, Limnol. Oeeanogr., 28, 709-719. Herman A. W., 1984. Vertical copepod aggregations and interactions with chlorophyll and production on the Peru shelf, Cont. Shelf Res. (in press). Herman A.W., Denman K.L, 1979. Intrusions and vertical mixing at the shelf/slope water front south of Nova Stotia, Can. J. Fish. Aquat. Sei., 36, 1445-1453. Herman A.W., Platt T., 1983. Numerical modelling of die! carbon production and zooplankton grazing on the Scotian shelf based on observational data, Ecol. Model., 18, 55-72. 40 -~ ~ 1 1 ' r 1~ 'i