EOSC 315 Notes - Instructor: Michael Lipsen - The Ocean Ecosystem PDF

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Instructor: Michael Lipsen...

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NOTES Discussion of phytoplankton blooms. Nutrient cycle During the winter months, there is a deepening of the mixed layer due to increased storms and wind currents which mix the upper layer of ocean water. There is also a weakening of the stratification or thermocline as the mixed layer temperature decreases and density increases to about the same as the deeper layers. Increased mixing and a deeper mixed layer bring up more abundant nutrients from the depths. This time around, the phytoplankton has more access to nutrients. Light access and blooms In winter, shorter days and the distribution of phytoplankton over a greater depth below the critical depth cause these autotrophs to be light limited. Come spring when the days are longer and the warmer season heats up the ocean surface, the mixed layer shallows and there is a strengthening of the thermocline which concentrates the phytoplankton population and nutrients accumulated over the winter in the mixed layer. These conditions help produce a spring bloom. Since the increase in phytoplankton biomass means increased food supply for the primary consumers, also called zooplankton, the biomass increase of the latter also follows sometime after the phytoplankton bloom. However, their increase is small compared with the autotrophs due to the loss of energy up the trophic level. Although light is not a limiting factor, with the bloom the phytoplankton uses up the available nutrients in the mixed layer but are not replenished. This is due to the stronger thermocline when summer temperatures are higher. The spring bloom then crashes and the phytoplankton dies and sinks down the water column to the depths. Come fall when there are more storms and stronger winds, with the cooler temperature and light still available, mixing of the waters may bring up nutrients from below the mixed layer. These conditions are sufficient to result in a small early fall bloom before the winter cycle repeats itself....