Factors Regulating the Relationship Between Total and Size-Fractionated Chlorophyll-a in Coastal Waters of the Red Sea

Abstract

Phytoplankton biomass and size structure are recognized as key ecological indicators. With the aim to quantify the relationship between these two ecological indicators in tropical waters and understand controlling factors, we analyzed the total chlorophyll-a concentration, a measure of phytoplankton biomass, and its partitioning into three size classes of phytoplankton, using a series of observations collected at coastal sites in the central Red Sea. Over a period of 4 years, measurements of flow cytometry, size-fractionated chlorophyll-a concentration, and physical-chemical variables were collected near Thuwal in Saudi Arabia. We fitted a three-component model to the size-fractionated chlorophyll-a data to quantify the relationship between total chlorophyll and that in three size classes of phytoplankton [pico- (<2 μm), nano- (2–20 μm) and micro-phytoplankton (>20 μm)]. The model has an advantage over other more empirical methods in that its parameters are interpretable, expressed as the maximum chlorophyll-a concentration of small phytoplankton (pico- and combined pico-nanophytoplankton, Cpm and Cp,nm, respectively) and the fractional contribution of these two size classes to total chlorophyll-a as it tends to zero (Dp and Dp,n). Residuals between the model and the data (model minus data) were compared with a range of other environmental variables available in the dataset. Residuals in pico- and combined pico-nanophytoplankton fractions of total chlorophyll-a were significantly correlated with water temperature (positively) and picoeukaryote cell number (negatively). We conducted a running fit of the model with increasing temperature and found a negative relationship between temperature and parameters Cpm and Cp,nm and a positive relationship between temperature and parameters Dp and Dp,n. By harnessing the relative red fluorescence of the flow cytometric data, we show that picoeukaryotes, which are higher in cell number in winter (cold) than summer (warm), contain higher chlorophyll per cell than other picophytoplankton and are slightly larger in size, possibly explaining the temperature shift in model parameters, though further evidence is needed to substantiate this finding. Our results emphasize the importance of knowing the water temperature and taxonomic composition of phytoplankton within each size class when understanding their relative contribution to total chlorophyll. Furthermore, our results have implications for the development of algorithms for inferring size-fractionated chlorophyll from satellite data, and for how the partitioning of total chlorophyll into the three size classes may change in a future ocean.

The Influence of Temperature and Community Structure on Light Absorption by Phytoplankton in the North Atlantic

“…Temperature may also covary with the spectral quality of light in the mixed layer, which has also been shown to influence phytoplankton community structure [2]. The influence of temperature on model parameters has not only been seen in the North Atlantic study of Brewin et al [46], but also in other studies, using different datasets and methods, and in different regions, from polar to tropical waters [86,87]. Future work is needed to understand mechanistically how temperature influences model parameters.…”

Phytoplankton Biomass and the Hydrodynamic Regime in NEOM, Red Sea

“…The sea surrounding NEOM, similarly to the wider Red Sea, is optically complex, attributed to greater coloured dissolved organic matter absorption per unit Chl-a concentration [48]. As a result, standard algorithms systematically overestimate Chl-a when compared with in situ observations and region-specific algorithms may provide better results [49,50]. Therefore, our Chl-a dataset was regionally-tuned for the Red Sea by using the OC4-RG algorithm.…”

Phytoplankton Phenology in the Coastal Zone of Cyprus, Based on Remote Sensing and In Situ Observations

“…The composition of phytoplankton communities can be estimated using phytoplankton accessory pigments as biomarkers. Seven major diagnostic pigments (DP) that are associated with phytoplankton size classes have been used [11,48], under the following assumptions: (1) microphytoplankton (>20 µm) comprises diatoms and dinoflagellates, which are characterized by fucoxanthin and peridinin, ( 2) nanophytoplankton (2-20 µm) is composed of cryptophytes, chromophytes and nanoflagellates (alloxanthin, 19 hex-and 19 butanoyloxyfucoxanthin), and (3) green flagellates, prochlorophytes and cyanobacteria (zeaxanthin and TChlb) make up picophytoplankton (<2 µm) (Table 1) [41,49,50]. The equations described by Uitz et al [48] have been used to derive the relative proportions of the phytoplankton size classes (Equations ( 1)-( 3)), as well as the total Chl-a (TChl-a) concentration associated with each size class (Equations ( 5)-( 7)):…”