Problems: 1. Plants and animals need nutrients to grow. We add nitrogen and phosphorus to crops to grow more food, quickly. When runoff flows into an estuary it can accumulate too many nutrients. This is not good for the ecosystem. Algae start to grow, extremely fast! So much so that it blocks the sunlight. Plants can't photosynthesize. Oxygen levels decline and fish suffocate.

2. Changing the natural flow of water harms an estuary. Freshwater is in high demand. People need it for homes, farms, schools, factories, electricity generation, and recreation. When humans alter the natural flow of water, it disrupts the natural system. The timing and the amount of fresh water that enters the estuary alters. This changes the salinity of water for animals that are reproducing. It might be too saline (salty) or not saline enough to support estuary species. Erosion and sedimentation are also influenced by the increase or decrease of freshwater input. This ultimately affects fish reproduction, shellfish survival, bird nesting, and seed distribution.

What are the causes of this problems?​