Interannual variability of the initiation of the phytoplankton growing period in two French coastal ecosystems

Abstract. Decadal time series of chlorophyll a concentrations sampled at high
and low frequencies are explored to study climate-induced impacts on the
processes inducing interannual variations in the initiation of the
phytoplankton growing period (IPGP) in early spring. We specifically detail
the IPGP in two contrasting coastal temperate ecosystems under the influence
of rivers highly rich in nutrients: the Bay of Brest and the Bay of Vilaine.
In both coastal ecosystems, we observed a large interannual variation in
the IPGP influenced by sea temperature, river inputs, light availability
(modulated by solar radiation and water turbidity), and turbulent mixing
generated by tidal currents, wind stress, and river runoff. We show that the
IPGP is delayed by around 30 d in 2019 in comparison with 2010. In situ observations
and a one-dimensional vertical model coupling hydrodynamics,
biogeochemistry, and sediment dynamics show that the IPGP generally does not
depend on one specific environmental factor but on the interaction between
several environmental factors. In these two bays, we demonstrate that the IPGP
is mainly caused by sea surface temperature and available light conditions,
mostly controlled by the turbidity of the system before first blooms. While
both bays are hydrodynamically contrasted, the processes that modulate the IPGP
are similar. In both bays, the IPGP can be delayed by cold spells and flood
events at the end of winter, provided that these extreme events last several
days.
