Between tide and wave marks: a unifying model of physical zonation on littoral shores

PeerJ. 2013 Sep 19:1:e154. doi: 10.7717/peerj.154. eCollection 2013.

Abstract

The effects of tides on littoral marine habitats are so ubiquitous that shorelines are commonly described as 'intertidal', whereas waves are considered a secondary factor that simply modifies the intertidal habitat. However mean significant wave height exceeds tidal range at many locations worldwide. Here we construct a simple sinusoidal model of coastal water level based on both tidal range and wave height. From the patterns of emergence and submergence predicted by the model, we derive four vertical shoreline benchmarks which bracket up to three novel, spatially distinct, and physically defined zones. The (1) emergent tidal zone is characterized by tidally driven emergence in air; the (2) wave zone is characterized by constant (not periodic) wave wash; and the (3) submergent tidal zone is characterized by tidally driven submergence. The decoupling of tidally driven emergence and submergence made possible by wave action is a critical prediction of the model. On wave-dominated shores (wave height ≫ tidal range), all three zones are predicted to exist separately, but on tide-dominated shores (tidal range ≫ wave height) the wave zone is absent and the emergent and submergent tidal zones overlap substantially, forming the traditional "intertidal zone". We conclude by incorporating time and space in the model to illustrate variability in the physical conditions and zonation on littoral shores. The wave:tide physical zonation model is a unifying framework that can facilitate our understanding of physical conditions on littoral shores whether tropical or temperate, marine or lentic.

Keywords: Desiccation; Disturbance; Emersion; Hawaiʻi; Immersion; Intertidal; Intertidal environment; Predictive model; Stress gradient; Temperature.

Grants and funding

This research was funded in part by a graduate research grant from the University of Hawaiʻi Ecology, Evolution, and Conservation Biology (EECB) program (to CEB), a project development award from the University of Hawaiʻi Sea Grant College Program (CMS & RJT), the Hawaiʻi Coral Reef Initiative (RJT & CEB), NOAA Office of National Marine Sanctuaries, MOA 2005-008/6882 (to RJT), and the National Science Foundation Grant OCE-1260169 (RJT). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.