This is a very important question. If we look at
an aquatic (water) food web, we see that the
phytoplankton are essential.
They take energy from the sun and matter from
water and carbon dioxide to make sugar and oxygen.
They can use the energy to make the other parts
of their bodies too.
To the consumers on the next level of the food
web, the phytoplankton are just food. When
zooplankton eat the phytoplankton, there may be
parts of the phytoplankton that they can’t digest,
so the zooplankton don’t get the energy locked in
those chemical bonds. The zooplankton had to
spend energy to catch and digest the
phytoplankton, so that has to be subtracted from
the energy “profit.”
Where does this energy go? Some wastes
might be broken down by decomposers, but they only
get a portion of the energy too. The rest is
lost as heat. It doesn’t disappear, but it
spreads out and is not useful to the other species
in the food web.
A general rule of thumb is that only about
10% of the energy that the phytoplankton
originally took in will make it to the next
level in the food web.
How many kilograms of phytoplankton will it
take to produce 1 kg of fish that eat zooplankton
that eat phytoplankton?
Aquatic ecologists study questions like this. You
may want to pursue a career in that.
Thanks for asking.