When a University of Hawaii research ship cruised into port last week, it was more than just the end of the latest monthly scientific expedition for the Hawaii Ocean Time-­series, known as HOT.

It was the program’s 300th research cruise, a milestone for the legendary oceanography project, which has measured the hydrography, chemistry and biology of the water depths at a remote spot in the Pacific Ocean for 30 years.

“It’s a huge benchmark for the program,” said David Karl, the UH-Manoa oceanography professor who founded the project with colleague Roger Lukas in 1988.

Almost every month since then, a couple dozen scientists set out for Station ALOHA, the permanent cruise destination 60 miles north of Oahu at 22 degrees, 45 minutes north latitude, 158 degrees west longitude.

It is there — at what is likely the most frequently sampled ocean location on Earth — that scientists conduct open-ocean experiments and take measurements of the currents, water chemistry, optical properties, the plankton community and more.

In addition to the monthly ship-based observations, HOT program scientists have access to real-time satellite observations, unattended mooring measurements, self-operating instrumented gliders and floats, and a fiber-optic cabled observatory with an internet connection back to Oahu.

The data, collected over three decades, represent an invaluable long-term record of the chemistry and biology found at a typical deep spot in the subtropical North Pacific.

“It’s extremely important to oceanography and science as a whole,” said Tara Clemente, HOT operations manager. “One sample doesn’t mean much if you can’t put it in context. Measuring the trends have the most value.”

Perhaps the program’s most noted achievement is its contribution to the science of climate change. At Station ALOHA, scientists have recorded a steady rise in near-surface ocean carbon dioxide and corresponding ocean acidification — a rise that matches observations of carbon dioxide in Earth’s atmosphere.

Data from the HOT program have directly supported official scientific reports issued by the United Nations’ International Panel on Climate Change.

UH’s Karl said few people realize what’s happening to the ocean. Like the atmosphere, it is changing for the worse under man-made influences. Among other things, he said, the ocean is losing its oxygen, becoming more acidic, growing warmer and losing fish.

“It’s been shown without a shadow of a doubt,” he said.

All of it has been observed in the scientific research linked to Station ALOHA, an acronym that stands for A Long-term Oligotrophic Habitat Assessment. (“Oligotrophic” refers to environments without a lot of nutrients, such as the ocean far from land.)

More than 2,000 scientists from the United States and across the world have taken the five-day cruise north of Oahu, and some 1,500 scientific papers have relied on data collected at Station ALOHA.

Clemente, chief scientist of the project’s 300th cruise, said the latest expedition on the UH research vessel Kilo Moana went well despite high winds and large swells.

“In the winter you constantly battle the weather,” she said. “It can be tricky figuring out what to deploy and what experiments to conduct.”

Among the instruments not deployed was the Wirewalker, a measuring device that measures temperature, salinity, particle abundance and more as it travels up and down the water column on a cable.

In the end, 85 percent of the cruise’s objectives were completed, Clemente said.

The HOT program was started in 1988, along with a similar program in the Atlantic near Bermuda, with funds awarded by the National Science Foundation.

In searching for a site to establish Station ALOHA, Karl and Lukas looked for a spot far enough away from the islands to be free from coastal influences but close enough to Hono­lulu to make relatively brief cruises logistically and financially feasible. The site needed to be deep and representative of the entire ocean.

Karl, who became director of UH’s Center for Microbial Oceanography: Research and Education, said he and Lukas thought the project might have a life of five years or so. But more scientific questions arose and the data kept coming — and the mounting data sets became more valuable with time. National Science Foundation funding kept getting extended.

Over the years more than $100 million in funding from various sources has supported the project, and many more millions of dollars have been pumped into Hawaii’s economy.

Last month the program received word that it secured its major funding from the National Science Foundation for another five years.

Starting next year Karl will step down from the HOT program, and Angelicque White, a UH oceanography associate professor who earned her Oregon State University doctorate from data she collected from the monthly research cruises, will take over as lead investigator.

“Here we are at 30 years,” said Karl, who not only sailed on the first expedition, but the 300th as well. “Each additional year of observations brings us closer to a fundamental understanding of how the ocean functions and its relationships to climate.”