One thing that has often proved true for the Chesapeake Bay is that, as often than not, good news is the prelude to bad news.

The apparent comeback of a fish species is followed by a spawning collapse. Improved water quality is followed by the smothering impacts of a hurricane. A treasured stream is compromised by development.

So it makes me nervous to report that underwater grasses — a critically important part of the Bay ecosystem — are not only at record levels, but have expanded in each of the last five years, the longest period of sustained expansion since annual surveys began in 1984.

Such trends make me worry. Doesn’t that mean we’re overdue for a big tropical storm, with torrential rains that vanquish the comeback under a flood of nutrients and sediment?

To be sure, there are a lot of caveats to the underwater grass comeback. Grasses in the high-salinity regions cover about a third less area than in the mid-1990s, in part because the Bay is warming and the primary species found there — eelgrass — doesn’t like hot conditions.

And much of the recent acreage record is built on a huge expanse of widgeon grass in the Mid Bay which, in the past, has rapidly disappeared when conditions turn bad.

But a paper published in the Proceedings of the National Academy of Sciences, which I reported on last month, shows that the long-term Bay grass comebacks transcends year-to-year weather conditions because of improved water quality stemming from the region’s nutrient reduction efforts.

The really good news, though, is that not only are grasses coming back, but the beds are large and increasingly dense — both factors that help them better withstand poor conditions. 

Further, those large beds increasingly seem to be influencing their own recovery. Big beds of dense grasses help improve surrounding water quality, allowing for their own further expansion. Many also produce copious amounts of seeds that disperse near and far, further accelerating and expanding their coverage.

The ability of something like a grass bed to improve local water quality is known as a “positive feedback,” and it’s hard to overstate how important that is for the Bay’s recovery.

Computer models can predict how Bay water quality will respond to reduced nutrient pollution. But it is difficult for them to fully account for positive feedbacks that stem from the recovery of grass beds or other habitats, such as oyster reefs, because they are harder to understand and predict.

Positive feedbacks, if they kick in, can help improve conditions more than would otherwise be predicted. That’s important because it’s increasingly difficult to see when the region will achieve its nutrient reduction goals — an effort that now seems likely to stretch well beyond 2025.

But the grass bed recovery shows that we might get a better Bay than we anticipate even as that work goes on. That doesn’t mean the next tropical storm isn’t going to cause a setback or that next year’s headline won’t be highlighting a decrease in grass acreage if this year’s weather gets weird.

Even if that happens, it seems increasingly likely that, at least with grasses, bad news may only be the prelude to good news.