The Inner Harbor’s green water this week raised red flags for Baltimore’s water quality monitors who warned residents of a widespread “pistachio tide.”

The pistachio tide, a name inspired by the bright green color that comes from green sulfur bacteria, is more likely to happen as the weather changes, said Leanna Frick, senior director of advancement with Blue Water Baltimore.

Blue Water Baltimore, a water quality watchdog in the city, explained on its Instagram account that the colorful change and rotten-egg smell were part of a “turnover event” in the harbor that can happen during seasonal changes. Frick said this is the most widespread pistachio tide they have seen, extending slightly outside of the Inner Harbor area.

The organization alerted Baltimoreans that the tide could cause another fish kill. Last week, thousands of fish died in the Baltimore harbor. Maryland regulators said there was no single pollutant responsible, but a combination of factors from runoff and wastewater overflows.

Pistachio tides occur through a process called thermal inversion, when layers of water switch because of cool weather. For example, if a top layer of water cools during a cold night, the water becomes denser and sinks under a warmer layer of water. This churns up bacteria and other bottom-dwellers and brings them to the surface.

Green sulfur microbes are anaerobic, so they usually dwell at the bottom of the harbor where there is less oxygen. When the green sulfur microbes rise to the surface during thermal inversion, they cause a vibrant color and rotten-egg smell.

After the process is over, the cold, dense water and bacteria die-offs leave less oxygen for fish. The fish kills will likely start to occur on Thursday, Frick said. There have already been reports of schools of fish “gulping air,” she added.

Climate change and unabated water pollution can make pistachio tides more frequent. Frick said there have been several of them since August.

Baltimore has made progress improving water quality in the harbor over the last two decades, with city officials holding the first public swim in the harbor in decades last year. However, environmental experts stress that water in the harbor remains volatile depending on the weather, and last month, public works officials said it could take another 20 years to fully resolve sewage overflows that have long plagued the health of its waters.