Quantifying local variation in tidal regime using depth-logging fish tags

Understanding the relationships between frequency of tidal inundation and elevation is vital for the design of coastal flood protection schemes, cross-site comparisons of intertidal ecology, reconstructing past sea level from palaeoecological data and predicting the likely ecological development of created intertidal habitats. However, very few studies relate intertidal ecology to measured inundation frequencies. Here we describe a versatile, relatively low-cost method to determine the precise relationship between elevation and inundation frequency at multiple locations over periods of several months. We used compact, autonomous depth-sensing data loggers (designed as fish tags) to measure water depths and high-resolution differential GPS to relate water depth to a terrestrial datum. Regression against records from permanent tide gauges at standard ports was then used to determine annual averages such as mean high water of spring or neap tides (MHWS/MHWN) or relationships between elevation and inundation frequency. Measurements of tides at 23 locations showed that differences in the levels of MHWN were large, even over short distances (up to 28 cm over 1.5 km; 70 cm over 40 km). MHWS was less variable over short distances, but varied substantially between sites further than 10 km apart (up to 90 cm over 40 km). Data from standard ports, therefore, give a rather poor guide to the tidal regime at most locations, particularly for MHWN. However, using our methods, it is straightforward to relate intertidal ecology at multiple locations to actual measurements of the tidal regime at each.