Nutrient addition incubation experiments over an annual cycle in coastal Newport Beach, CA

Phytoplankton communities in coastal California are influenced by changing
physical and chemical conditions throughout the year. To identify the
conditions that limit phytoplankton growth in coastal Southern California
over an annual cycle, 15 monthly nutrient addition incubation experiments
were conducted off of Newport Beach from August 2017 through September
2018. Treatments included control (no addition), nitrate, ammonium,
phosphate, silicate, and a simulated upwelling treatment that received all
of the listed nutrients in combination.  Results showed that the
availability of light and N were the two main growth limiting factors of
the bulk phytoplankton community. The January, February, March, and April
2018 experiments are examples of relief from light limitation; whereas,
the remaining experiments were all limited by N. In several of the N
limited experiments, the effect of light limitation was sometimes apparent
at low levels. These included experiments in August and September of both
years, as well as the May 2018 experiment. Light limitation is likely the
effect of winter mixing, which brings nutrients into the surface mixed
layer, but limits cells for light as they are mixed away from the surface.
N limitation is likely the result of greater stratification in the warmer
summer months, where a shallower mixed layer allows enough sunlight for
cells to grow and deplete N to growth-limiting levels.