Hawaiʻ shoreline

The question of the differences between rocky shores in Hawaiʻi versus those in California or Maine has led to a novel methodology to classify shorelines worldwide, developed by a team of scientists at the University of Hawaiʻi at Mānoa and Texas A&M University-Corpus Christi.

For biologists, the intertidal zone is much more than a gradient from wet to dry. It’s not enough to say that something is located in the middle of the shoreline, or high along the shoreline, or even in the “barnacle zone.” The mid-shore of Hawaiʻi is washed by waves most of the time, while the mid-shore of Maine is alternately under water or in air for six continuous hours. That exposure has a dramatic effect on the types of animals and plants that can be found there.

“It’s all about stress,” says lead author Chris Bird, assistant professor of biology at Texas A&M University-Corpus Christi, who was a postdoctoral scholar at the UH Mānoa Hawaiʻi Institute of Marine Biology (HIMB) during the project. “Life is adapted to typical conditions. Rare occurrences are stressful. Common and rare occurrences for shoreline species are substantially affected by whether that shore is either tide- or wave-dominated, and by where on the shore an organism lives.”

Working with co-authors assistant researcher Erik Franklin and researcher Robert Toonen of HIMB and professor Celia Smith in the UH Mānoa Department of Botany, Bird has proposed a new approach to defining the zones that make up intertidal habitat worldwide. Their study was recently published in the scientific journal PeerJ.

Their new wave and tide model of zonation (wave:tide) can be applied to any shoreline worldwide. By examining the relative contribution of waves and tides to shoreline water height, the model predicts three primary categories for intertidal shores—tide-dominated, wave-dominated and co-dominated (when wave height is equal to tidal range). Within these categories, a tidally influenced differentiation between emergent and submergent zones helps to further refine the description.

Worldwide, the majority of offshore oceanic islands are predicted to be wave-dominated and the majority of continental shores are predicted to be tide-dominated on average, according to the new model. Wave-dominated shores are most prevalent in the Southern Ocean where winds blow around the globe with few land barriers. Hawaiʻi’s shores are consistently washed with waves regardless of the tide, and are considered to be wave-dominated.

To the extent that waves and tides define shoreline habitat, changes in wave heights and tidal patterns such as those that could result from climate change could have major impacts on local biology. “For instance, northern California is co-dominated and could easily be pushed to wave-domination if average wave heights increase,” Bird said. “This would result in ‘new’ habitat and potentially unforeseen changes in shoreline organism community structure and composition.”

For more on this study, read the UH Mānoa news release.