Re: What does plankton eat?

Hi, Ray.

     There are actually many different types of organisms that make up 
what we generally refer to as plankton.  The type of organism usually 
brought to mind when we use the term plankton is a group of microscopic 
plants more appropriately called phytoplankton.  The diet of phytoplankton 
depends on size and metabolism.  They all need to get some form of carbon 
and some form of nitrogen from their environment to survive.  Some 
phytoplankton are producers.  This means that they use CO2 to produce the 
sugars and starches they need to survive (a process I'm sure you have 
heard of - photosynthesis).  So, you might say that these plants "eat" 
CO2.  They also need to have some trace minerals, phosphate, and sulfates  
(collectively called nutrients) to survive.  These nutrients and their 
nitrogen sources (such as ammonia or nitrate) are absorbed from the water 
around the phytoplankton.  When very high levels of nutrients suddenly 
flow into the water, you may get what is called a bloom of the 
phytoplankton.  The producers also need some form of energy to drive their 
photosynthesis.  Most phytoplankton get this energy from sunlight, 
although some in the sediment can use chemical energy to drive their 
metabolism.

   There are also phytoplankton that cannot use CO2.  These are called 
consumers.  They absorb their nutrients, nitrogenous compounds and the 
sugars/starches from their environment.  They therefore typically grow in 
dense populations where there is more food, such as near particles 
containing lots of producers that excrete sugars or near bacteria that the 
phytoplankton can break open with enzymes that they excrete.  

   There are also animals that are considered plankton, called 
zooplankton.  Very small zooplankton feed on bacteria (technically called 
bacteriplankton).  As we move to larger sizes, we see zooplankton feeding 
on progressively larger phytoplankton.  

Have fun!