Hydrobiologia (2009) 620:173–179 DOI 10.1007/s10750-008-9627-2
PRIMARY RESEARCH PAPER
Parameterization of surface irradiance and primary ˚ rhus Bay, SW Kattegat, Baltic Sea production in A Lars Chresten Lund-Hansen Æ Helene Munk Sørensen
Received: 12 December 2007 / Revised: 30 September 2008 / Accepted: 6 October 2008 / Published online: 26 October 2008 Ó Springer Science+Business Media B.V. 2008
Abstract The aims of the present study were to develop a parameterization of a one-year-long observed PAR time-series, apply the PAR parameterization in a primary production relation, and compare calculated and observed time-series of primary production. The PAR parameterization was applied in the generally used relation for the primary production (Pd): Pd = a(BI0Z0) ? b with observed photic depth (Z0) and Chl-a concentrations (B). It was tested whether the PAR parameterization in combination with this simple relation for primary production was able to describe the actual measured primary production. The study is based on a one year long time-series of PAR, CTD-casts (n = 45), and primary production ˚ rhus Bay (56°090 N; measurements (n = 24) from A 0 10°20 E), south west Kattegat. Results showed a high and positive correlation between observed and calculated primary production in the bay, as based on the present PAR parameterization combined with the simple primary production relation. The developed Handling editor: Luigi Naselli-Flores L. C. Lund-Hansen (&) Department of Marine Ecology, Institute of Biological Sciences, Aarhus University, Finlandsgade 14, 8200 Aarhus N, Denmark e-mail:
[email protected] H. M. Sørensen ˚ rhus, Danish Ministry of the Environment Centre A Environment, Lyseng Alle´, Dk 8270, Højbjerg, Denmark e-mail:
[email protected]
PAR parameterization, which calculates total daily surface irradiance per day (M photons m-2 d-1), can be applied in any ecological application taking into account that it was developed for the latitude of 56° N. Keywords Irradiance Parameterization Primary production Chl-a Kattegat/Baltic Sea
Introduction Light as photosynthetic available radiation (PAR 400– 700 nm) is together with nutrients, and temperature the major governing parameters regarding photosynthesis in freshwater (Bleiker and Schanz, 1997; Kunz & Diehl, 2003; Robson, 2005) and marine environments (Kirk, 1994; Falkowski & Raven, 1997; Huisman et al., 1999; Macedo & Duarte, 2006). For instance, the surface irradiance (I0) is direct proportional with primary production (Pd) in the Cloern (1991) relation: Pd = a(BZ0I0) ? b, where B is Chl-a concentration, (Z0) depth of photic zone, and a, b constants. This equation has proven to be highly applicable as demonstrated in several lower latitude (20–40° N) US estuaries in terms of a high correlation between observed and measured primary production (r2 * 0.55 - 0.93) (Brush et al., 2002). However, a parameterization of surface irradiance for present latitude (56° N) is needed as surface irradiance strongly depends on latitude and time of
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year (Brock, 1981; Walsby, 1997). New studies point toward development of satellite-based PAR data (Frouin & Murakmai, 2007). There are, however, until now only few actually measured and longer PAR time-series available, and derivation of PAR times-series from other radiation bands has not been successful (Kirk, 1994). The second purpose is to validate the developed irradiance parameterization in combination with the Cloern (1991) relation, in terms of a comparison of calculated and measured time˚ rhus Bay series of primary production in the A (56°090 N, 10°190 E). There is also a lack of timeseries of primary production and Chl-a with a high
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resolution in time. These are highly applicable as ground truth data for validation of algorithms applied in remote sensing of Chl-a and primary production (Behrenfeld & Falkowski, 1997).
Materials and methods The database is a one-year record (2002) of surface irradiance (400–700 nm) (PAR) (lM m-2 s-1) at high resolution (10 min), weekly optical and CTD casts, Chl-a concentrations, and bi-weekly measurements of primary production at a fixed position in the
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˚ rhus Bay—56°09.10 N, 10°19.20 E, southwest A Kattegat (Lund-Hansen et al., 1996) (Fig. 1). Surface irradiance (PAR) was measured with a calibrated LICOR (LI-190) sensor placed on a meteorological mast at 15 m above the ground at the department in ˚ rhus. Values of PAR were measured every minute A and the average of 10 measurements was stored using a Campbell Scientific data logger (CR10-X). CTD and optical data were measured at the position in the bay with a vertical resolution of 0.2 m. The CTD was calibrated every 3 months at the manufacturer EIVA (www.eiva.dk). A PAR sensor (LICOR LI-193) mounted on the CTD measured scalar irradiance, and the major part of the optical casts were carried out between 10 am and 1 pm. Diffuse attenuation coefficient Kd(PAR) was determined for each of the 45 optical casts by linear regression between depth and log transformed scalar irradiance. The Lambert– Beer’s law describes the vertical distribution of the irradiance: Iz ¼ I0 eK0 ðPARÞz , where I0 is the surface irradiance, Iz the irradiance at the depth z (m), and Kd(PAR) the diffuse attenuation coefficient (Kirk, 1994). Statistically significant (P \ 0.001) and high correlation coefficients (r2 * 0.987) were obtained in the regressions between Lambert–Beer’s law and observations as shown for a typical sample (Fig. 2). Primary production was measured in terms of P–I curves, which describe the relation between irradiance (I) (lM m-2 s-1) and the incorporation (P) (mg C m-3 h-1) of radioactive labelled 14C during photosynthesis in a laboratory incubator (Falkowski 1997). The P–I curves were obtained for samples from 1 m depth and for the depths of 25, 10, and 2% Ln(light) 0
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Fig. 2 Observed scalar irradiance with depth (r) and line of ˚ rhus Bay regression on February 7, 2002 in A
Table 1 Average (n = 24) and range (maximum–minimum) of a (alpha) (mg C m-3 h-1/lM m-2 s-1), Pmax (mg C m-3 h-1), and normalized to Chl-a concentrations achl-a (mg C Chl-1 -1 h ) for a-1 (lM m-2 s-1)-1, and Pchl-a max (mg C mg Chl-a ˚ rhus Bay 2002 the A Average (n = 24)
Range (max–min)
a
0.05
(0.41–0.005)
Pmax achl-a
6.51 0.017
(33.06–0.45) (0.03–0.008)
Pchl-a max
2.24
(2.54–0.75)
light in the water column. Samples were exposed to six different levels of irradiance (11.3, 24.5, 91.3, 168.6, 365.0, and 638.7 lM m-2 s-1) at in situ temperature for an incubation period of 2 h. The essential parameter in the 14C method is Pmax (mg C m-3 h-1) where the production rate reaches a maximum. The second parameter is a (alpha) which express the production rate (mg C m-3 h-1) per irradiance (lM m-2 s-1). Average and range of Pmax and a as well as Pmax and a (alpha) normalized to Chl-a concentrations achl-a (mg C Chl-a-1 -2 -1 -1 chl-a (lM m s ) and Pmax (mg C mg Chl-a-1 h-1) are summarized for the primary production data-set (Table 1). A sample from below the interface was added in periods of stratification of the water column, i.e. when a low density top layer was present above a denser bottom layer. Measured incorporation of 14C at each of the depths per time (mg C m-3 d-1) was integrated over the depth of the photic zone Z0, which yielded a production of carbon per surface area per time (mg C m-2 d-1). The present methods for measuring primary production are in accordance with Danish National Monitoring Standards (Andersen et al., 2004). Chl-a concentrations were obtained by filtering 2 l of water collected with 5 l Niskin bottles placed on the CTD. Samples were collected at the surface (1 m depth), the bottom (16 m), and at the depths of a Chl-a maximum, if present as judged from the fluorescence profile. The filter (GF 75 Advantec) with residue was transferred to a glass vial with 5 ml 96% ethanol to extract the Chl-a for a minimum of 6 h and a maximum of 20 h. Chl-a concentrations were determined according to the methods by Strickland and Parson (1972). All analyses were carried out at the certified laboratory EUROFINS (www.eurofins.com).
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Results
respect to time, which here is the hours of daylight each day. The distribution of total surface daily irradiance per day (M m-2 d-1) as a function of time of year f(t) is parameterized by means of a minimum difference between observations and parameterization, which yields: pt 2:2 f ðtÞ ¼ 34 sin þ2:3; ð1Þ 365
Time-series of calculated maximum surface irradiance (___) and observed (j) midday surface irradiance (lM m-2 s-1) are shown in Fig. 3A. Maximum surface irradiance was calculated following Walsby (1997), whereas every data point in the observed time-series is the average of three measurements (11:50 pm, 12:00 and 12:10 am). The extended variation in measured maximum surface irradiance attributes to meteorological conditions as cloud cover, rain, and water vapor in the atmosphere (Kirk, 1994). Periods of missing data in the observed irradiance are due to maintenance and repair during winter (see ‘‘Discussion’’). Nevertheless, the clear difference between observed and calculated maximum surface irradiance stresses the difficulty of applying a calculated surface irradiance in ecological models. However, it is the total surface irradiance per day (M m-2 d-1) which a phytoplankton cell is actually exposed to during hours of daylight that is of interest (Brush et al., 2002; Kelly and Doering, 1997). Accordingly, total surface irradiance per day is obtained by integrating observed irradiance with
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Fig. 3 A Observed (j) and modelled (____) maximum (12:00) surface irradiance (lM m-2 s-1), B observed (h) and modelled (____) total surface irradiance (M m-2 d-1) with confidence bands where (j) are days with optical casts in the bay
where (t) is day number. Output from the parameterization (1) is shown as a full thick line with observations of total daily surface irradiance per day as both black and white squares (Fig. 3B). The black squares indicate days (n = 45) of both total daily irradiance and optical casts in the bay, and white squares days (n = 299) of only total daily irradiance. It is seen that days of optical casts are evenly distributed through the year. A total of 71 percent of the observations are enclosed within the ±10.0 (M m-2 d-1) bands shown as two full lines (Fig. 3B). The correlation between parameterization (1) and observation is high (r2 = 0.52; n = 299) and statistically significant (P \ 0.001). Ideally, the number of total irradiance per day should be 365, but is
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only 299 due to periods of maintenance and malfunction during the winter (see ‘‘Discussion’’). The parameterization (1) is applied as input data (I0) (M m-2 d-1) in the Cloern (1991) relation for primary production (mg C m-2 d-1): Pd = a(BZ0I0) ? b, where B is a Chl-a concentration (mg Chl-a m-3), Z0 (m) depth of photic zone, and a, b are constants. Primary production (mg C m-2 d-1) is calculated for each of the 24 days of primary production measurements in the bay. This comprise the actual measurements of surface (1 m) Chl-a concentrations (B), total surface irradiance per day (I0) at that specific day, and depth of photic zone Z0. Depth of photic zone is defined as the depth where there is only 1% irradiance relative to 100% at the surface (Kirk, 1994). This depth is Z0 = -4.61/-Kd(PAR) derived through: Iz ¼ I0 eKd ðPARÞz and 1 ¼ 100 eKd ðPARÞz and ln(1/100) = -Kd(PAR)*z, where Kd(PAR) is diffuse attenuation coefficient (m-1) determined from the optical casts. Average and range of present study parameters are summarized in Tables 1, 2. The correlation between this calculated production and observed production is high and statistically significant (r2 = 0.6, P \ 0.001, n = 24). The coefficients a, b in Pd = a(BZ0I0) ? b are determined by reaching a 1:1 relation with zero intercept between observed and calculated primary production. The obtained relation with coefficients is accordingly: Pd = 0.56(BZ0I0) - 55.8. Observed and calculated primary productions are shown as time-series and the strong resemblance emphasizes the high correlation with an exception at day number 270 where observed production is quite lower (Fig. 4A). It is emphasized ˚ rhus Bay primary production timethat the present A series as well as photosynthesis-irradiance parameters a (mg C m-3 h-1/lM m-2 s-1), Pmax (mg C m-3 h-1), and normalized to Chl-a concentrations achl-a (mg C Chl-a-1 (lM m-2 s-1)-1, and Pchl-a max (mg C Table 2 Average (n = 24) and range (minimum–maximum) of variables Average (n = 24)
Range (min–max)
Kd(PAR)
0.29
(0.24–0.42)
Chl-a
2.9
(0.6–13.0)
16.0
(10.9–18.9)
Z0 P.P.
347.6 -1
(10.2–1607.6)
Units are: Kd(PAR) (m ), Chl-a (mg Chl-a m-3), Z0 (m), P.P. ˚ rhus Bay 2002 (mg C m-2 d-1) for the A
mg Chl-a-1 h-1) (Table 1) are comparable and similar to those obtained in analogous marine environments compiled by Forget et al. (2007). There is a pronounced seasonal pattern in the primary production with a strong spring bloom in mid-March and some minor blooms during summer and autumn (Fig. 4A). There is a clear correlation between primary production and Chl-a concentrations in the water column shown by a comparison of production time-series and the Chl-a isopleth (Fig. 4A, B). The isopleth (Fig. 4B) was obtained by a calibration of the CTD fluorometer signal into a Chl-a concentration. Nevertheless, this seasonal pattern in primary production and Chl-a concentrations is typical for the bay (Lund-Hansen, 2004, 2006). The measured daily average production equals 347.6 (mg C m-2 d-1) with a total yearly production of 126.9 (g C m-2 y-1).
Discussion A parameterization of total surface irradiance per day (M m-2 d-1) at 56°090 N was developed and sustained by a high (r2 = 0.52; n = 299) and statistically significant (P \ 0.001) correlation between observations and parameterization (Fig. 3B). The high numbers of observations as well as the high degree of statistical significance counterbalance the slightly low squared correlation coefficient. Periods of missing PAR data occurred mainly during winter time where primary production is low and the possible error is thought to be limited. The present parameterization applied a sine (p) function (1) similar to the cosine (2p) relation by Kremer & Nixon (1978). The two relations are nearly identical in terms of the mathematical expressions though there is quite a difference in the irradiance (I0) output data between the two relations. For instance, total surface irradiance per day (I0) at spring equinox (day number 79) equals 26.0 (M m-2 d-1) in the Kremer & Nixon (1978) relation but only 14.6 (M m-2 d-1) in the present study. Applying these radiance values in the present relation: Pd = 0.55 (BZ0I0) - 56 and in the Keller (1988) relation: Pd = 0.59(BZ0I0) ? 199 for identical values of Chla (5 mg Chl-a m-3) and Z0 (Z0 = 15 m), gives Pd figures of 1349.5 and 557.4 (mg C m-2 d-1) in the Keller (1988) and present case, respectively. The Kremer & Nixon (1978) parameterization applied here
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Fig. 4 A Observed (h) and parameterized (j) primary production (P.P.) (mg C m-2 d-1) at the ˚ rhus Bay in position in A 2002, B Chl-a (mg Chla m-3) distribution through the year where vertical lines show measurement points of fluorescence, which was converted into a Chl-a concentration
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in the Keller (1988) relation was developed near Narragansett Bay (41°300 N), about 1700 km south of ˚ rhus Bay (56°090 N). The difference the present A (1349.5 - 557.4 = 792.1 mg C m-2 d-1) in primary production of 862.6 (mg C m-2 d-1) between the Keller (1988) and the present relation strongly emphasizes the importance of latitude as only total daily surface irradiance per day, and constants a, b varied between the two relations. The correlation (r2 = 0.60) between measured and calculated primary production in the present study was slightly lower than the average correlation coefficient of (r2 = 0.75; n = 18) in the review by Brush et al. (2002). However, it is strongly appealing as only Chl-a concentrations, depth of photic zone, and total daily surface irradiance are required for a prediction of the primary production. Though it is emphasized that the coefficients a, b in Pd = a(BZ0I0) ? b can only be determined relative to time-series of observed primary production. The relation: Pd = a(BZ0I0) ? b is based on a valid assumption of a positive correlation between Chl-a and primary production, whereby a comparison of
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measured and calculated primary production is not totally independent. Taking this into account it is encouraging that a complex parameter as primary production can be predicted with this simple relation in a marine environment where advection adds to the complexity in the bay (Lund-Hansen et al., 1996, 2006).
Conclusions Observed surface PAR irradiance showed a strong day-to-day variation. A parameterization of total daily surface irradiance per day was developed. Application of the parameterization for the primary ˚ rhus Bay showed a strong correlation production in A between observed and calculated primary production. It was emphasized that a primary production model must be developed on the basis of irradiance data obtained in the latitudinal region where the parameterization will be applied. The developed irradiance parameterization can be applied in any kind of ecological model but was here validated for primary production data.
Hydrobiologia (2009) 620:173–179 Acknowledgments This was a part of the research programme on Subsurface Blooms financially supported by the Danish Natural Science Foundation contract number SNF ˚ rhus is greatly acknowledged for 1424-28808. The County of A access to CTD, optical, and primary production data.
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