Phytoplankton growth cycle and Eutrophication

Belgian coastal North Sea waters 

Spring phytoplankton bloom observed from Space in 2003. The April image shows a widespread phytoplankton bloom in the southern North Sea for both coastal and offshore waters. Monitoring data in Belgian coastal waters reveals a also a Chlorophyll peak in April.  

On the next page you will see a graph that shows when the Spring phytoplankton bloom is measured from a ship. The ship goes regularly to a a station located at 25km off Oostende (indicated by a red square in the Figure). In situ Chlorophyll-a measurements range from approximately 5 to 30 mg m-3 in April and decrease to an annual minimum level ~1 mg m-3 in December or January. The satellite  chlorophyll varies from 8 to 24 mg m-3 for this station in April 2003, which is consistent with in situ data.

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Question 1: Why is the concentration of algae so low in the winter months?
 
Question 2: Why is the concentration of algae so low in the summer months?

Question 3: Generally we observe a small bloom at the end of the summer, can you explain why?

Question 4: Can you describe the differences that you observe in these images?