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 ...
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- (<2µm),
nano- (2–20µm) and micro-phytoplankton (>20µm)]. 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, C
m
p
and C
m
p,n
, 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 C
m
p
and C
m
p,n
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
Brewin et al. Total and Size-Fractionated Chlorophyll-a in the Red Sea
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