Inter-annual variability in phytoplankton and nutrients in the Gulf of
Elat/Aqaba
Abstract
The Gulf of Elat/Aqaba exhibits high inter-annual variability in mixed
layer depth. Observations from the northern Gulf show differences of
hundreds of meters in winter mixing depth, which ranges between 300 m in
years with shallow mixing and up to 700 m in years with deep mixing.
Deep mixing events can occur in two consecutive years or after four
consecutive years of shallow mixing. The mixing depth has an effect on
the concentration of nutrients and chlorophyll (and other tracers) in
the surface and deep water. Using a 3D coupled physical-ecological
model, we study the effect of shallow vs. deep mixing on the processes
controlling the phytoplankton bloom and on nutrient accumulation in the
deep water. We found that years with deep mixing are characterized by
larger spatial variability in surface and integrated chlorophyll
concentration during the mixing season. We also found that horizontal
advection is more important for integrated phytoplankton concentration
in years with deep mixing in the northern Gulf. Even when mixing was
deep and nutrient limitation decreased, light limitation on growth was
enhanced more in the north compared with the south. In addition, we
showed that the nutrient accumulation in the deep water after a year
with deep mixing of the northern Gulf was initially affected mostly by
physical processes (such as advection and vertical mixing), and less
from ecological regeneration and switched gradually to be dominated by
ecological processes alone during the third year of shallow mixing.