Record rainfall helped tick up levels of two widespread pollutants in the Chesapeake Bay watershed in 2018, and worried scientists say bigger storms could be the new norm, according to a recent report.

The biennial report by the Chesapeake Bay Foundation said the health of the bay decreased one point to a score of 33, earning it the equivalent of a D-plus in its scoring system, mostly because of heavy rain that swept in nitrogen and phosphorous from runoff. Water clarity also took a hit. The foundation has marked progress from scores as low as 20 in the 1970s. A score of 100 would indicate pristine. The foundation’s goal is a passing score of about 70.

“Overall, the result is a substantial drop in the scores relative to 2016,” the report said. “Winter and early spring water flows down the Susquehanna, the largest river feeding the bay, were higher than average in both years [2017 and 2018]. And record rainfall during summer 2018 caused flooding and more pollution to enter the region’s waterways.”

Though nitrogen and phosphorus do occur naturally, too much coming from a variety of human activities means nutrient pollution in waterways.

The report also states that climate change models suggest more frequent and severe storms in the future.

William Baker, the foundation’s president, said the Trump administration’s environmental rollbacks also pose a challenge.

But Beth McGee, the foundation’s director of science and agricultural policy, said all the news was not bad.

“The good news is that scientists are pointing to evidence of the bay’s increased resiliency and ability to withstand and recover from these severe weather events," McGee said in a statement. “And this resiliency is a direct result of the pollution reductions achieved to date.”

The Chesapeake Bay Foundation has been issuing its state of the bay report since 1998. The report is compiled by scientists and is based on data collected on 13 indicators, each scored from 1 to 100. Taken together, they create an index score.

Though the bay is in Maryland and Virginia, its watershed — all the waterways and land that divert flow into it — spans six states, starting in Cooperstown, N.Y. The Susquehanna River is one of its biggest sources and flows the entire length of Pennsylvania.

In July and August, heavy rains and flood waters flowed into the Chesapeake Bay from Pennsylvania. The record rain carried tons of sediment and debris over the Conowingo Dam, which regulates flow from the Susquehanna coming out of Pennsylvania five miles upstream.

Rain from July 22 to 27 was so intense that the river ran three feet above flood stage, forcing Exelon, the power company that operates the dam, to open 20 flood gates. Flows at the dam exceeded 300,000 cubic feet per second, a rate not seen since Tropical Storm Lee in 2011.

Though storms might have caused pollutants to flow, there were some positives over the last two years as levels of underwater grasses and oxygen, key for fish and other aquatic life such as crabs, both improved. The level of blue crabs has also been one of the success stories in the bay, earning a B in the most recent report. The bay has been under a federal cleanup program since 1983.