It is unclear why phytoplankton do not follow “Jenzen’s
rule” in the same way as terrestrial organisms (Addo-
Bediako et al., 2000; Deutsch et al., 2008; Gaston et al.,
2009). One speculation might be because the highest temperature–
latitude relationship differs between the land
and the ocean. The maximal environmental temperatures
are relatively invariant with latitude on the land so that environmental
temperature ranges broaden at high latitudes
(Addo-Bediako et al., 2000). However, this is not the case
for the ocean, which has large heat storage, and the temperature
variations are less abrupt than on the land.
Another factor might be also related with the high mortality
rate of phytoplankton in the ocean, in which environmental
temperature extremes are only correlated with, but
do not directly determine, the thermal limits of phytoplankton.
We can see from Fig. 2 that the maximal growth
temperatures seem to decrease more rapidly than minimal
temperatures with increasing latitude, leading to the plausible
pattern that thermal breadth of marine phytoplankton decreases with latitude (Fig. 2F). However, the random
effects and errors cannot be overlooked (Table I). This
pattern needs to be corroborated by more data and
requires a reasonable explanation if it really exists.