Seagrasses are in a group of plants that moved from the sea to the land and then returned to the sea. The water in which they abide can become cloudy because of sediments and/or phytoplankton. This is why seagrasses need relatively clear water. They co-habit the water column with fish of varying size and (in this image, macroalgae. These are the golden colored plants in the background. They can be bad for seagrasses because macroalgae roll along like tumble weeds in cowboy movies. During times when the tumble weed macroalgae stop tumbling they sit on the seagrasses and deprive them of light. Given enough time to sit they block light, introduce toxic conditions and kill the seagrass bed. (see image below)

After doing damage to the seagrass bed other species can come in and start to grow. This is called succession. Below is Caulerpa mexicana with Caulerpa prolifera. It has spread from the jetties to the seagrass meadows. What next? Dogs and cats living together?

The image below shows what a seagrass bed looks like when drift macroalgae suffocate the seagrass. No more pretty co-existing. Just plain old dead.