What might happen to the organisms in the food web below if the number of phytoplankton and vegetation drastically decreased?

i don't now exactly how to answer this when theres' no food chart however as the vegetation and phytoplankton drastically decrease everything decreases. This is because the producer the bottom of the food chain also known as vegetation is the prime source of energy in every food chain. when there's scarce amounts of producers a domino  affect will occur.  There will be no food for the first consumers to eat leading their population to decrease and the animals that feed off them will decrease then the animals that feed of them will decrease and so on until the organisms of the food web die out.

1) gene flow

- it is described as the transfer of genes from one population to another population so that frequency of gene is increased.

-a zebra migrates to join a different herd of zebras.

2) natural selection

- a mechanism of evolutionary change in which individuals with advantageous traits survive and reproduce whereas the others ( those that can not adapt) eliminate from the population.

-competition for sunlight leads to taller trees.

3) mutation

- permanent change in the nucleotide sequence of dna ( genetic material) that could have harmful or useful consequences.

-the dna of a snake changes to make its venom stronger.

4) genetic drift - it is a random phenomenon ( that occurs by chance such as fire) that results in change in the gene frequency in a small population.    

-a grassfire randomly sweeps through a population of buffalo and kills most of the animals.

explanation:

this is more of a statement than a question. is there answers to go along with this, or is it opinion based?