Hawaii’s Water Temperature

Nov 1, 2018 | General Information

Table of Contents
The water temperatures on beaches in Maui, Oahu, Kauai and the Big Island attract visitors any time of year. Ocean temperatures do differ by a few degrees throughout the Hawaiian Islands, but don’t vary much from month to month. The water temperature fluctuates by only 5 °F over the year off Honolulu. At Hilo, the water temperature stays even steadier, varying just 3 °Fannually.The Pacific Ocean bordering both cities reaches its highest temperature in September and October. By March, however, Hilo’s water has dropped to its coldest temperature of the year at 71 °F on average. By comparison, the water during winter at Honolulu’s beaches typically maintains a steady 76 °F from January through to April.

All of this good news about Hawaii’s water temperature is somewhat dampened by the observation that Hawaii’s climate is very slowly changing. In the last century, air temperatures have increased between one-half and one degree (F). During this period the surface of the ocean around Hawaii has warmed by several degrees from the ocean surface to at least 600 feet.

Hawaii’s marine ecosystem also is threatened by increased ocean acidity. Greenhouse gases are changing the temperatures of the world’s oceans as more carbon dioxide reacts with water to form more carbonic acid. Consequently warming and acidification in the oceans around Hawaii has damaged coral reefs, shellfish and other organisms. It’s a long-term process, occurring over centuries, but an increase of 25 percent in the ocean’s acidity over the past three centuries should be a cause for concern for those who depend on, and cherish, Hawaii’s marine ecosystem.

The causes and their effects on water temperature and coral health vary from island to island as a result of the fact that Hawaii’s islands have such a diverse collection of micro-environments, above and below water, each with its own effects on plants and animals.But around each island, water temperature determines which marine organisms will thrive and which will diminish in numbers and size. For each organism there is a “thermal death point” as well as a temperature or range of temperatures that produce optimal abundance.

Increased water temperature is an important consideration when toxic substances are present in water. Many toxic substances exhibit increased toxicity when temperatures rise. Higher temperatures also diminish the solubility of dissolved oxygen and thus decrease its availability. At the same time that higher water temperatures may be decreasing the availability of oxygen, elevated water temperatures tend to increase the oxygen demand of fish as well as their respiration.

Our goal in this article is to raise aware of the critical importance of water temperature for the health of Hawaii’s coral and marine life. Much research has been done by the State of Hawaii, federal research agencies and university research centers on the impact of temperature changes on ocean fish generally and Hawaii’s topical fish specifically. But we still know too little about the impacts on various fish species discussed in our article on Hawaii’s Tropical Fish and a long list of others.However, we do know from all of this research that marine protected areas, like those discussed in our article on Hawaii’s MLCDs, are the best ways to protect and preserve vulnerable marine ecosystems.