Explain why fertilizer use by farmers can have negative effects on aquatic ecosystems?

Fertilizer can lead to algal bloom in aquatic ecosystems, which means that its usage may lead to oversaturation of algae in the seas and oceans. Although algae are good for these kinds of waters in limited amounts, when there are too much of them it can stifle all other life, thus having negative effects on aquatic ecosystems. 

Homologous chromosomes with heterozygous alleles d) heterozygous chromosomes with homozygous alleles

Ithink the answer is b
not sure