Rocky shore ecosystems are found in some coastal regions. These ecosystems are inhabited by many different species of algae and animals. Twice each day the sea level rises to the high tide level and decreases to the low tide level in a regular pattern. The organisms in these ecosystems are exposed to very different conditions during the course of each day. Algae are photosynthetic protoctists. Multicellular algae are found in many marine ecosystems. Rock pools are formed in depressions in the shore as the water level lowers when the tide goes out. Fig. 1.1 is a profile of a rocky shore ecosystem showing the positions of some rock pools. The mid-shore zone lies between the lower-shore zone and the upper-shore zone. [Figure 1.1] HWM = high-water mark – the highest point on the shore that is covered by sea water at high tide LWM = low-water mark – the lowest point on the shore that is exposed to the air at low tide Fig. 1.2 shows a large rock pool on the mid-shore zone surrounded by smaller rock pools at low tide. [Figure 1.2] A student investigated the distribution and abundance of large, multicellular algae on a rocky shore in North Wales in the UK at low tide on one day in October 2011. The student completed a preliminary survey of the shore at low tide and made some observations. 1. There were several different species of algae growing in the mid-shore zone. 2. Some algae were exposed to the air at low tide, but most grew in rock pools. 3. The distribution and abundance of the algae differed on the shore between the low-water mark (LWM) and the high-water mark (HWM). 4. Large rock pools appeared to have more species of algae than small rock pools. 5. Large rock pools were deeper than small rock pools. The student decided to investigate the relationship between the depth of rock pools and the species diversity of the algae in the rock pools in the mid-shore zone. The student selected 32 rock pools at random. Approximately half the rock pools were small and half were large. The depth of each rock pool at its deepest point was measured and the number of different species of algae in each rock pool was counted.