Intertidal Stressors

The intertidal zone where organisms are sometimes submerged, and sometimes not, is the transition between terrestrial and marine environments. This thin strip of ecosystem cirlces the continents worldwide.

Along the continental edges of planet earth exists a unique ecosystem, not completely marine, nor entirely terrestrial. This is the intertidal zone. It is home to a wide variety of organisms: from the smallest periwinkle snails, to predatory seastars that can grow up to a meter wide and have more than 17 arms! As the climate changes, organisms found here are faced with new challenges.

Mmmm… steamed mussels

Even without climate change, temperature already poses a challenge to the organisms that inhabit the intertidal zone. At high tide, animals are covered with ocean water that is roughly the same temperature throughout the year. At low tide, however, these organisms are left high and dry, and are exposed to all sorts of weather conditions. In the winter this could mean snow cover or freezing rain. In the summer the sun can be so hot, animals may bake in their own shells.

Intertidal organisms have an amazing thermal range they can exist in, although most are already living at their maximum limits [1]. With temperatures expected to increase between 1°C to 6°C over the next century [2], intertidal zone animals will face stress levels potentially beyond what they can tolerate.

What's my cue?
As many invertebrates grow, they depend on temperature cues to facilitate their development. But when temperatures fluctuate beyond normal levels, natural balances between animals and their food supplies can get out of sync. This has been observed in Europe where warmer waters are triggering clam larvae development earlier than normal, but the phytoplankton they feed on do not bloom until later. As a result, there are fewer clams surviving to adulthood [1].

Get it while it's hot!

The ochre stars, Pisaster ochraceus, may become an even more voracious intertidal predator with warmer waters due to abrupt climate change.

Warmer temperatures are also linked with more active predators. The Ochre Star has been observed to feed on mussel beds at a higher rate in warmer waters, which may be detrimental to mussel populations unable to move away or reproduce fast enough to maintain their populations [1]. Overactive predators have the ability to completely change what an ecosystem looks like, affecting all the organisms within it.

With warmer global temperatures we also see rising sea levels due to the expansion of sea water and runoff from glaciers. So far, sea level rise has been slow, with an average increase of 1.8mm/year from 1961-2003. Over the next century sea levels are estimated to increase between 0.18 m to 0.59 m. Even if the animals adapt to such rises, the habitats they depend on may not be as fortunate. The decrease in habitable areas for intertidal organisms due to rising waters has been estimated to range between 20-70%, a significant amount of habitat to lose in any given environment [1].

Go north and multiply!

Overall, higher sea surface temperatures may lead to reduced populations of northern species of organisms and increased populations of southern species. On the California coast, a long-term study comparing species distributions in the 1930's with the early 1990's showed that many of the southern species had become more abundant, while the northern species were reduced in number [3].

Grab your kite!

Increased winds and storms will also challenge the organisms in the intertidal zone. Winds along the continents create waves that slam into the coast, and with more wave action there is increased shoreline erosion. This has already been observed in the Arctic where beach habitat is being reduced each year [4]. This will lead to fewer sandy beaches in the intertidal zone that many organisms, including birds and invertebrates, depend on.

Changing wind patterns around the globe will alter circulation in the oceans, though there is little agreement on how this will actually affect currents [1]. Some areas are likely to see more upwelling, bringing up nutrients from the bottom to the surface, while other areas will have less. With changing currents, upwelling waters that many productive intertidal areas depend on for nutrients may be altered.

Melting shells

Gooseneck barnacles, Pollcipes polymerus, in the intertidal are just one of the organisms that depend on their hard carbonate shells for protection from the elements and predators.

One of the least understood ways that global warming will affect marine organisms is in regards to changing pH and carbon dioxide (CO₂) concentrations. More CO₂ in the water means lower than normal pH. When the pH drops too low, many species are unable to produce their protective outer shells. Mussels, sea urchins, and snails have all shown stunted growth and survival when exposed to pH levels just 0.03 to 0.7 units lower than normal [1].

With the IPCC estimating ocean pH to drop by 0.14 to 0.35 units in the next century [2] , it will become increasingly difficult for these animals to survive. Threats to shell growth will impact intertidal communities around the globe: from small zooplankton that disperse larvae to large clams that provide food for marine mammals and people alike. Simply put, oceanic animals unable to form shells will not develop to function in good health in their respective ecosystems.

Intertidal zone animals at the edge of the continents live in challenging environments as it is. Unfortunately, as the planet's oceans continue to change, these creatures and their ecosystems will be even further challenged.

Intertidal lesson plan

A Sunflower seastar, Pycnopodia helianthoides, a predatory sea star on the intertidal.

1. Harley, C.D., et al., The impacts of climate change in coastal marine systems. Ecology Letters, 2006. 9 : p. 228-241.

2. Alley, R., et al., Climate Change 2007: The Physical Science Basis, Summary for Policymakers. 2007, Intergovernmental Panel on Climate Change. p. 18.

3. Sagarin, R.D., et al., Climate-related change in an intertidal community over short and long term scales. Ecological Monographs, 1999. 69 (4): p. 465-490.

4. Schrank, W.E., The ACIA, climate change and fisheries. Marine Policy, 2007. 31 : p. 5-18.