You may know that northern ocean is not so blue even if the sky gives its color to the ocean. It's rather dark green reflecting the color of spruce leaves.
But the ocean on the west side of Oshima Peninsula is not so fertile because the short rivers can not provide enough nutrition and minerals.
The fallen leaves can not be dissolved enough in the short journey in the river. Because of that, the seabed of the ocean on the west side of the peninsula can not raise seaweed enough and only calcium compiles on the rocks under the water.