The Birthwell appears to be the ocean surrounded by a beach covered in shell.
The Birthwell's saltiness is controlled by the coral reefs built at the mouth of the sea, separating it from the ocean. When the weather is warm, the creatures in the sea flourish, building the reefs until they choke the entrance to the bay. Then the sea becomes saltier without the fresher water of the ocean to leaven it, and the creatures in the water die. By the end of winter, the coral has died back enough to allow more creatures in, and the cycle restarts.
During the cool age, the Birthwell remained mostly open, supporting a medium range amount of life but not enough to clog the sea's entrance. There was a state of equilibrium.