INTERTIDAL COMMUNITY STUDIES: where the ocean meets the land

When we think of marine biology or zoology our minds more often than not wander to the large, charismatic animals – the white sharks, the humpback whales, the giant squids.

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What we commonly fail to recognize, and fully comprehend, is the wealth of sea life that is much less exposed and celebrated in mainstream thought. The marine environment as a whole is built upon a vast biomass of invertebrate organisms – that is the group of animals that lack an internal skeleton, the absence of these creatures would inevitably lead to the total collapse of the oceanic systems. These often slimy, shelled, plated or spiny creatures fill niches from the inhospitable depths of the ocean floor to the vast shorelines that separate the sea from the land.

It is therefore paramount that the assemblages of these organisms is dedicated an appropriate amount of time and resources to effectively monitor and assess their health, as it may potentially provide insights into the health of the ecosystem at a higher level. One particular area where invertebrate organisms are highly prevalent is the intertidal zone – that is the region that is sometimes exposed to harsh conditions of the terrestrial world and also to the conditions of the ocean. It is a buffer zone between sea and land, water and dirt, and is often a harsh environment to survive in due to the unrelenting power of the breaking waves that pummel the shore.

Mossel Bay and neighbouring Dana Bay are similar in that they both have shorelines that are a mixture of vast sandy beaches and rocky, tide pool filled stretches. These rocky areas are home to a large assortment of invertebrate species that have adapted to the extreme and frequent changes of conditions that are associated with intertidal zones. While Dana Bay is relatively undisturbed from anthropogenic influences, Mossel Bay has a large volume of human activity, including a commercial harbour and large shipping traffic. One of the interests of Oceans Research is to compare the assemblages in these two bays to potentially determine the effects that humans are having on the health of the ecosystems.

The intertidal study that is currently being undertaken in Mossel and Dana Bays by Oceans Research aims to gain an estimate of biodiversity and overall individual abundance of invertebrate species within the intertidal zone. We have 6 sites distributed in rocky intertidal zones around Mossel Bay: these include The Point, Diaz Beach, De Bakke, Kleinbrak River, Reebok and Terginet. These sites vary in their exposure, with the former three sites relatively sheltered from wave action, while the latter three are under constant impact from the crashing waves. We also have 2 sites in Dana Bay that are relatively exposed to the environmental conditions and are used as a control for anthropogenic effects.

Our sampling design is a transect method – whereby 1 square meter quadrats are surveyed every 5m from the high tide mark (where the water is at the highest) to the low tide mark (where the water is too deep to continue sampling). Each quadrat is surveyed for the presence of invertebrate species with the total number of individuals of each species also recorded. In this way we are able to sample not only the total biodiversity and species abundance, but also the distribution of species along the length of the intertidal zone, which may provide insights into the relationships between species – such as competition and predation.

Through this method of surveying, we have observed and identified over 60 species of invertebrates from numerous phyla such as Molluscs, Arthropods, Sipunculids, Annelids and Cnidarians. Through consistent sampling of these sites we are trying to answer numerous questions about the invertebrate communities that reside along the shorelines. For example, how does the assemblage at each site change over time? Are certain species more prevalent at the site during the summer rather than the winter? Are there extinction events that lead to a repopulation of the area, and if so, in what order do the species re-establish themselves? These questions – and many more – are able to be investigated through studies such as these. The answers of which may provide pivotal information pertaining to the overall health of the ecosystem as well as the interactions between species.