WRRCTMR No.25 Algae an an Indicator of Fresh Water Coastal Out flow in Hawaii

Abstract

Zonation of algal communities in the intertidal areas of the Hawaiian islands has been correlated with zones of reduced salinity caused by submarine fresh-water springs. Laboratory salinity tolerance experiments showed that the green seaweed Ulva (known as "sea lettuce") tolerated brackish water of half of normal ocean salinity, whereas, Acanthophora sp. did not. A field survey of salinity and algal species zonation at a coastal spring on the island of Oahu between Diamond Head and Black Point verified the laboratory findings and showed UIva fasciata, Peyssoneila sp., and Gelidium sp. predominating in the brackish areas and Acanthophora sp. and Sargassum sp. in the areas of higher salinity. An algal-salinity survey between Kawaihae and Kona on the island of Hawaii showed that, along most of the coastline, algal communities were non-existent or too sparse for use as a salinity indicator. Spectral measurements and photographic experiments showed that two salinity indicators, Ulva sp. (low salinity) and Sargassum sp. (high salinity) could be mapped by color infrared photography from aircraft. Wratten #12 (yellow) and Kodak CC50C-2 (cyan) filters with Type 8843 Ektachrome IR film were used to enhance the color differences of these species from each other and from their backgrounds. For limited, accessible areas, established communities of marine algae can be mapped directly to derive a map of time-averaged salinity anomalies. For large or inaccessible areas, aerial infrared photography with appropriate filters is recommended.