Waterweed grows near the surface of the water in lakes and ponds. it often grows in dense clumps. it has been observed that the chloroplasts in this plant move around inside the cell. how might this be an advantage and a disadvantage to the plant?

This might be an advantage because the Chloroplast can orient themselves so as to obtain maximum exposure to sunlight for the process of photosynthesis. It can also be a disadvantage because useful energy would be required to relocate the chloroplasts which could have been used elsewhere. 

Explanation; 
Chloroplasts are organelles where the process of photosynthesis, the process in which green plants make their own food, takes place. For maximum photosynthesis to take place the chloroplasts require maximum exposure to sunlight, therefore, the chloroplasts in the waterweed are able to achieve this, since they are moving around the cell, which is advantageous to the plant. However, this movement would cause unequal distribution of light as not all cells will be exposed to maximum sunlight and again the movement of the chloroplasts within the cell requires energy that would have otherwise been used by the cell for other activities.