Size Class Dependent Relationships between Temperature and Phytoplankton Photosynthesis-Irradiance Parameters in the Atlantic Ocean
2018
Over the past decade, a number of methods have been developed to estimate
size-class primary production from either in situ phytoplankton pigment data or
remotely-sensed data. In this context, the first objective of this study was to compare
two methods of estimating size class specific (micro-, nano-, and pico-phytoplankton)
photosynthesis-irradiance (PE) parameters from pigment data. The second objective
was to analyse the relationship between environmental variables (temperature,
nitrate and PAR) and PE parameters in the different size-classes. A large dataset
was used of simultaneous measurements of the PE parameters (n = 1,260) and
phytoplankton pigment markers (n = 2,326), from 3 different institutes. There were no
significant differences in mean PE parameters of the different size classes between
the chemotaxonomic method of Uitz et al. (2008) and the pigment markers and
carbon-to-Chl a ratios method of Sathyendranath et al. (2009). For both methods,
mean maximum photosynthetic rates (PBm
) for micro-phytoplankton were significantly
lower than those for pico-phytoplankton and nano-phytoplankton. The mean light
limited slope (�B) for nano-phytoplankton were significantly higher than for the
other size taxa. For micro-phytoplankton dominated samples identified using the
Sathyendranath et al. (2009) method, both PBm
and �B exhibited a significant, positive
linear relationship with temperature, whereas for pico-phytoplankton the correlation
with temperature was negative. Nano-phytoplankton dominated samples showed a
positive correlation between PBm
and temperature, whereas for �B and the light saturation
parameter (Ek) the correlations were not significant. For the Uitz et al. (2008) method,
only micro-phytoplankton PBm
, pico-phytoplankton �B, nano- and pico-phytoplankton
Ek exhibited significant relationships with temperature. The temperature ranges
occupied by the size classes derived using these methods differed. The Uitz et al.
(2008) method exhibited a wider temperature range compared to those derived
from the Sathyendranath et al. (2009) method. The differences arise from the
classification of mixed populations. Based on these patterns, we therefore recommend
using the Sathyendranath et al. (2009) method to derive micro-phytoplankton PE parameters at sea water temperatures up to 8◦C during monospecific blooms and
the Uitz et al. (2008) method to derive PE parameters of mixed populations over the
temperature range from 8 to 18◦C. Both methods exhibited similar relationships between
pico-phytoplankton PE parameters and temperatures >18◦C.
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