The North Sea is particularly rich in nutrients, which are supplied from the land by the many rivers. Moreover, due to the shallow depth, sunlight can easily penetrate to the bottom almost everywhere, allowing plankton to grow easily. Due to this great abundance of plant material, there is enough food to sustain the many species of fish, animals and other organisms in the North Sea. The North Sea itself has marginal seas that are even more nutrient-rich, such as the Wadden Sea .