A mesocosm tool to optically study phytoplankton dynamics

Abstract

Optical instruments on board satellites have shown to be indispensable in the study of phytoplankton dynamics and related biogeochemical cycles on a global scale (Behrenfeld et al. 2009;Kirk 1994). Satellite observations daily cover the whole surface of the oceans, but they still do not allow the accurate retrieval of the large amount of variables, such as nutrient concentrations and physiological rates, that can be measured in situ. On the other hand, drawbacks of in situ phytoplankton investigations, such as the measurements of primary and secondary production, are the small number of water samples that can be examined and the relatively small areas of the sea that can be covered daily. Mesocosms are experimental tools mimicking natural water bodies that allow manipulation of, for instance, nutrient concentrations and irradiance enabling high-frequency sampling. The aim of this paper is to describe and test an indoor mesocosm facility for the study of phytoplankton dynamics under controlled conditions by using both in situ techniques for algal biomass and physiology estimations, as well as water-leaving radiance measurements. Such a facility would be essential to study the quantitative relations between fluorescence, biomass, and growing conditions to interpret variability in remotely sensed algal fluorescence (Huot et al. 2005), and in examining the effect of nutrient perturbations on phytoplankton dynamics and associated optical properties.A widely used proxy for phytoplankton biomass in both in situ and remote-sensing studies is the chlorophyll a concentration. Chlorophyll a (Chl a) is estimated from remotely sensed water-leaving radiance in a number of ways (Huot et al. 2005;Kirk 1994;Van Der Woerd and Pasterkamp 2008). First, by measuring the water-leaving radiance absorption spectrum that is influenced by the absorption characteristics of the phytoplankton species, phytoplankton biomass, non-phytoplankton seston, and gilvin (yellow substances) (Kirk 1994 AbstractThe accuracy of remote sensing algorithms for phytoplankton biomass and physiology is difficult to test under natural conditions due to rapid changes in physical and biological forcings and the practical inability to manipulate nutrient conditions and phytoplankton composition in the sea. Therefore, an indoor mesocosm was designed to examine the optical properties of phytoplankton under controlled and manipulated conditions of irradiance, temperature, turbulence, and nutrient availability. Equipped with hyperspectral radiometers and bottom irradiance meters, it is shown that under semi-natural environmental conditions biogeochemically relevant species as Emiliania huxleyi and Phaeocystis globosa can be grown with good precision (± 20%) between duplicate mesocosms and between duplicate sensors (<5% deviation). The accuracy of chlorophyll estimates by absorption, using an Integrating Cavity Absorption Meter, and fluorescence using water-leaving radiance was 74% to 80%, respectively, as it was negatively influenced by changes in phytoplankton physi...

Disparities between in situ and optically derived carbon biomass and growth rates of the prymnesiophyte &lt;i&gt;Phaeocystis globosa&lt;/i&gt;

“…A detailed description of the mesocosms is given in Peperzak et al (2011). Temperature during Phaeocystis growth was kept at 15 • C. Irradiance was provided in a semi-sinusoidal light-dark (16 : 8 h) cycle with a maximum surface PAR of 41 W m −2 in mesocosm 1 and 45 W m −2 in mesocosm 2.…”

“…Temperature during Phaeocystis growth was kept at 15 • C. Irradiance was provided in a semi-sinusoidal light-dark (16 : 8 h) cycle with a maximum surface PAR of 41 W m −2 in mesocosm 1 and 45 W m −2 in mesocosm 2. Turbulence of the water was provided by pumping surface water to the bottom of the mesocosm at a turnover rate of 1 h. The water was enriched with macronutrients to 30 µM NO − 3 , 6.3 µM PO 3− 4 , and trace metals and vitamin B1 (Peperzak et al, 2011). On day 8 of the experiment, when cells were in the stationary growth phase, mesocosm 1 received enrichment with the initial nutrient concentrations to examine the effect of alleviation of nitrogen limitation on the physiological and optical properties of Phaeocystis.…”

“…A detailed description of the analyses is provided elsewhere (Peperzak et al, 2011). See Table 1 for a list of measured and derived variables.…”

“…Phaeocystis spectra of a * Chl , together with reflectance data, were used to determine the appropriate wavelengths in algorithms for the estimation of chlorophyll a (c) absorption from reflectance spectra. Details of the ICAM absorption, irradiance and radiance measurements are provided elsewhere (Peperzak et al, 2011).…”

“…Phytoplankton dynamics in the mesocosms were experimentally manipulated under semi-natural conditions of temperature, irradiance and turbulence (Peperzak et al, 2011). The prymnesiophyte Phaeocystis globosa, a key species in marine primary production, was used as test organism (Wassmann et al, 1990;Smith et al, 1991;DiTullio et al, 2000;Vogt et al, 2012).…”