The second stage of hydrosere is occupied by plants such as

Hydrosere succession is a plant succession that occurs in aquatic atmospheres, such as lakes and ponds, in which a pond community gets converted into a land community. 

Various stages of hydrosere succession are:

1. Phytoplankton stage: Examples include green algae, diatoms, and so on.
2. Submerged stage: Examples include Vallisneria, Hydrilla, and so on.
3. Floating stage: Examples include Azolla, Pistia, and Nymphaea.
4. Swamp stage: An example is Typha.
5. Sedge-meadow stage: Examples include Cyperaceae, and Gramineae.
6. Woodland stage: Examples include Salix, Populus, and Lantana.
7. Forest stage: An example is tropical rainforest.

Hence, the second stage of hydrosere is occupied by plants such as Vallisneria.