Cyanide from seeds breaking down in a pond
Loquat seeds contain Amygdaline, which is also found in apricot kernels. Amygdaline belongs to the group of cyanogenic glycosides. 

In these compounds, a potentially cyanide releasing aglycon, such as mandelic nitrile, is bound to a carbohydrate (= a "sugar"). These compounds are quite abundant in nature; other known examples are Prunasin from the japanese bush cherry or Sambunigrin from elderberry seeds.

In some cases, cyanide is released by enzymatic cleavage of the glycoside, that is when the plant material is digested by animals. Consequently, some of the plants (or at least some parts of them) are known to be toxic. In other cases, the glycosides do degrade in water.

If hydrogen cyanide is released to the pond, I'd assume it stays there - $\ce{HCN}$ is highly soluble in water.

However, I wouldn't be very concerned. Seeds aren't like paper bags full of toxic material. 


It will likely take a while until the seeds break up to release Amygdaline.
To my knowledge, Amygdaline is rather stable in water, hydrogen cyanide isn't released until a sugar-splitting enzyme or acid acts on the molecule.