The availability of light is one of the most important factors determining the way in which photosynthesis proceeds. Wave bands of solar radiation in the range 400‑700 nm are called photosynthetically active radiation (PAR), given their ability to stimulate the process of photosynthesis. Given that relevant data remain of only limited availability, and that PAR shows relationships at regional and local levels, there is a continued need for independent research in this sphere, not least in relation to areas considered to be of high natural value.
The main goal of the work detailed here has thus been to characterise the photosynthetically active radiation incoming in the Middle Biebrza Basin of NE Poland, on the basis of a 9‑year (2013‑2021) measurement series obtained at the Kopytkowo site (53°35’30.8” N, 22°53’32.4” E). Analysis was carried out for both the instantaneous values obtained, and their 24‑hour totals. In addition to the basic statistical characteristics, attention was also paid to radiation on cloudless days, as well as to the relationship between PAR values noted at the Earth’s surface and at the top of the atmosphere. Given that empirical data of the above kind are bound to remain limited, proposals have been made as regards simple parameterisations of PAR radiation suitable for use in environmental studies.
The research was carried out in NE Poland, on the edge of Biebrza National Park, in what is known as the Middle Basin of the River Biebrza (Fig. 1). The area can be characterised as having natural, almost unchanged and uninhabited wetlands overgrown by swamp vegetation. The measurement site is located around 350 m north of the small village of Kopytkowo, which itself has only a handful of permanent residents.
Date were collected using a PQS1 PAR Quantum Sensor from the firm Kipp & Zonen, as mounted at a height 2.7 m a.g.l. (Fig. 2). The sensor is part of an eddy‑covariance system measuring the exchange of greenhouse gases between the ecosystem and the atmosphere. Data were sampled every 10 seconds and then stored in a (CR500) datalogger as 5‑minute averages. The results are shown as photosynthetic photon flux density (PPFD), the units beings micromoles per square metre per second (μmol∙m⁻²∙s⁻¹). Collection was noted in relation to Central European Time (CET).
Long‑term measurements of photosynthetically active radiation in the Middle Biebrza Basin reveal a clear annual cycle. The highest values occur in June, when PAR reaches the level of approximately 2000 μmol∙m⁻²∙s⁻¹. However, in the summer it is possible for the so‑called mirror effect to operate, ensuring that instantaneous PAR values may sometimes be higher than those recorded in the circumstances of a cloudless sky. It is in the winter season that PAR values are lowest, even on sunny days, reaching only about 400 μmol∙m⁻²∙s⁻¹ (Fig. 3).
The ratio of the measured PAR to its value at the top of the atmosphere (the PAR clearness index, kTPAR) in conditions of a cloudless sky takes the form of a hyperbolic function of the Sun’s height, hS (eq. 3) changing form 82‑83% for high Sun (h S = 60°) to 65‑75% for hS = 30° and dropping below 55% for hS = 10° (Fig. 4).
The average photosynthetic photon flux density along the Middle Biebrza Basin at particular hours is determined by day‑length during the year (Fig. 5). The mean values represent the influence of Sun height and atmospheric transparency. Results with the highest values were recorded in the hours 11:00‑12:00. During the summer solstice in June, PPFD values reach 1440 μmol∙m⁻²∙s⁻¹, while in December they may be as low as some 180 μmol∙m⁻²∙s⁻¹. In May and July, a slight decrease in PAR values is to be observed in the afternoon hours, most likely due to the development of convective cloud cover, but this effect is generally weak. The same months also recorded the highest and lowest mean 24‑hour totals: June – 64.8 mol∙m⁻²∙day⁻¹, December – 0.6 mol∙m⁻²∙day⁻¹ (Table 2). Similarly, the highest mean values for 24‑hour PAR totals, above 50 mol∙m⁻²∙day⁻¹, are observed in the first part of June, even as the lowest – of below 5 mol∙m⁻²∙day⁻¹ – characterise December (Fig. 6).
Due to the limited availability of PAR data, we proposed three simple ways in which it might be parametrised. Where total solar radiation, Kd, data are available, PPFD can be expressed (Eq. 4‑7) as a function of Kd, and the easy‑to‑calculate Sun height, hS, and/or clearness index, kT = Kd/K0 , where K0 is solar radiation at the top of the atmosphere (Eq. 1). Although Eq. 7 gives the best results in the light of statistical measures, in many simple applications it may suffice to use Equation 4, or even directly the PAR/Kd ratio, which in our case is 0.44 when PAR is expressed in W∙m⁻², and 2.0 for PPFD.
When data on total solar radiation are not available, it is relatively easy to estimate the PPFD on clear days (Eq. 8, Table 3). This can be developed further through the application of a factor dependent on the diurnal air‑temperature amplitude, ΔT (Eq. 9‑10). Although in the case of individual days the results of such parametrisation are subject to quite a high level of uncertainty (Fig. 9), they allow for the value of PAR radiation to be estimated on a monthly or multi‑day scale. They can therefore prove useful as changes in photobiological conditions over a longer period are analysed (Fig. 10).
Krzysztof Fortuniak [firstname.lastname@example.org], Uniwersytet Łódzki, Instytut Klimatologii i Hydrologii
Jan Górowski [email@example.com], Uniwersytet Łódzki, Szkoła Doktorska Nauk Ścisłych i Przyrodniczych
Mariusz Siedlecki [firstname.lastname@example.org], Uniwersytet Łódzki, Instytut Klimatologii i Hydrologii
Włodzimierz Pawlak [email@example.com], Uniwersytet Łódzki, Instytut Klimatologii i Hydrologii
APA: Fortuniak, K., Górowski, J., Siedlecki, M., & Pawlak, W. (2023). Cechy charakterystyczne i parametryzacje dochodzącego promieniowania fotosyntetycznie czynnego w środkowym basenie Biebrzy. Przegląd Geograficzny, 95(3), 291-311. https://doi.org/10.7163/PrzG.2023.3.5
MLA: Fortuniak, Krzysztof, et al. "Cechy charakterystyczne i parametryzacje dochodzącego promieniowania fotosyntetycznie czynnego w środkowym basenie Biebrzy". Przegląd Geograficzny, vol. 95, no. 3, 2023, pp. 291-311. https://doi.org/10.7163/PrzG.2023.3.5
Chicago: Fortuniak, Krzysztof, Górowski, Jan, Siedlecki, Mariusz, and Pawlak, Włodzimierz. "Cechy charakterystyczne i parametryzacje dochodzącego promieniowania fotosyntetycznie czynnego w środkowym basenie Biebrzy". Przegląd Geograficzny 95, no. 3 (2023): 291-311. https://doi.org/10.7163/PrzG.2023.3.5
Harvard: Fortuniak, K., Górowski, J., Siedlecki, M., & Pawlak, W. 2023. "Cechy charakterystyczne i parametryzacje dochodzącego promieniowania fotosyntetycznie czynnego w środkowym basenie Biebrzy". Przegląd Geograficzny, vol. 95, no. 3, pp. 291-311. https://doi.org/10.7163/PrzG.2023.3.5