Limited space for animals - how is that bad?
The building of a bridge on a river seems like an example of habitat fragmentation. There are a number of reasons why this may be detrimental to the animals.


Reduced resources available. I'm not too familiar with dolphins, but all animals need food, shelter and a place to raise their young. A smaller habitat can't support as many animals due to limited resources.
Isolation from other habitats. The bridge may cut off the dolphins on one side of the bridge from the other side, meaning they can't mate with dolphins on the other side or migrate. The fact that dolphins on one side of the bridge can only mate with the other dolphins that share that size causes a decrease in the genetic variability on both sides of the bridge. Decreased genetic variability means an increase in inbreeding depression and reduced ability to adapt to disease and environmental changes.
The edge effect. This means that areas next to the edge of the habitat (i.e. next to the bridge) will be even less suitable than they were before