Phytoplankton

The name "phytoplankton" refers to the group of microscopic plants that float with ocean currents in the photic zone, the upper layer of the ocean in which light penetrates and allows photosynthesis to occur. Phytoplankton levels in the ocean vary with the supply of nutrients, and can experience rapid blooms of growth during favorable conditions. Phytoplankton growth can increase during upwelling events; upwelling occurs when along-shore winds cause water to move offshore due to Ekman transport. Cold, deep, nutrient-rich water rises to the surface to fill the void created by the displaced surface water. In the case of Northern Hemisphere waters off the coast of California, coastal surface waters are moved offshore by the dominant north-westerly winds.

Nutrients that control phytoplankton growth are nitrogen, phosphorus, and silicate, along with the micronutrient iron. In different environments each of these nutrients can be "limiting", as their limited availability will halt phytoplankton growth while other nutrients are still abundant. In the waters of Monterey Bay, iron, nitrogen, and occasionally phosphorus are limiting at different times of the year.

Many of the phytoplankton found in coastal California waters fall into two groups, diatoms and dinoflagellates. Each are adapted to thrive under specific conditions, and they bloom at different times of year. Diatoms, which have a unique cell wall composed of silica, often become dominant during upwelling. During calm, warm periods when water becomes stratified, dinoflagellates become dominant. Dinoflagellates are propelled by one or more flagella, which gives them the ability to swim up and down in the water column and utilize nutrients in deeper layers when the surface waters become depleted.

Left: Asterionella, diatom. Right: Ceratium, dinoflagellate