People boating and fishing on the Chesapeake last year kept noticing something they don’t often see — clear water.

Citizens, the news media and resource agency staff all took note of it. Water was so clear in places it fueled explosive growth of ecologically valuable underwater grass beds — so much so that it spurred complaints from some boaters that grasses were tangling their vessels’ props, said Mark Trice, of the Maryland Department of Natural Resources Tidewater Ecosystem Assessment Division.

But was the Bay actually significantly clearer? It turns out to be surprisingly hard to make blanket statements about water clarity. It can vary widely from place to place, and because of tides, currents, wind and runoff, even a single location can have wide clarity swings in a matter of days.

But Trice and other scientists recently analyzed their data for last fall and concluded that in many — but not all — areas, clarity was indeed notably better than in the past few years. “We all pretty much got the same results, that we were seeing clearer water in certain places,” said Katie Kirk, who oversees the National Oceanic and Atmospheric Administration’s fleet of 10 continuous monitoring buoys in the Bay. “It wasn’t consistent throughout the whole Bay. The entire Bay did not get clearer.”

Indeed, Kirk reviewed turbidity data — a measure of particles in the water — collected by the Chesapeake Bay Interpretive Buoy System last November and found statistically significant reduced turbidity at five of them.

But two buoys had worse conditions — ironically at opposite ends of the Bay — one off Jamestown, VA, and the other at the mouth of the Susquehanna at the top of the Bay. Three others had no significant trend.

Trice analyzed data collected by several dispersed monitoring sites last fall, and Ron Vogel, a NOAA scientist, reviewed satellite data collected for the same time period. All came to the same conclusion — water clarity did appear to be much better in places, particularly along the Eastern Shore and parts of the Mid Bay compared with the previous several years.

Trice reviewed other data collected during the year and pointed to a number of factors that likely contributed, but cautioned, “whenever you are dealing with environmental data, it is almost never just one thing.” For one thing, river flows into the Bay were normal or below normal for most months. That means rain washed fewer nutrients off the land and into the Bay, so the algae blooms that feed on them were sparser. Plus, lower flows meant there was less sediment to cloud the water.

Less freshwater flowing into the Bay also means higher salinity levels. Higher salinity water tends to be clearer because it is often farther removed from runoff sources  and because sediment particles tend to settle out of it more readily. In addition, data show that last year was a bit less windy than normal. Wind can cause waves and stir sediment up into the water; calmer conditions help to keep sediment on the bottom. The difference was not huge, Trice said, “but anytime you have predominantly lower wind condition, you are having less overall sediment resuspension.”

Other factors may have come into play at certain times, or places.

For instance, last October brought something of a perfect storm, Trice said, at least from a water clarity perspective. Scientists went out to capture the impact of a predicted nor’easter, but the storm largely fizzled out before it reached the Bay. Still, it produced enough wind for just a short period of time to push fresh surface water out of Eastern Shore rivers, or mixed them with saltier bottom waters, causing them to become saltier, and clearer — evidenced by monitoring results after the event.

In some places, expanding underwater grass beds may have helped. Baywide estimates for 2015 are not available, but preliminary figures suggest it may have been a banner year for submerged aquatic vegetation. Grasses not only provide habitat for aquatic life, but help remove nutrients and sediment from the water, thereby improving clarity. “We saw grass where we hadn’t seen it before out in the field,” said Brooke Landry, a DNR biologist and chair of the Bay Program SAV Workgroup. “But we also saw areas that were not clear, so it was definitely site-specific.”

Finally, field biologists last year reported seeing an unusual abundance of tunicates, or sea squirts, which are also powerful water filterers. Trice said an increase in the creatures was not seen in all areas with improved water clarity, but that tunicates and filter feeders can have localized effects on clarity.

Overall, Trice said it is likely that most of the improvements in clarity were driven by weather — less rain to drive nutrients off the land and into the water, and less wind, with other factors possibly playing a supporting role in places.

But the clear water offered a glimpse of what the Bay could be. “Although this event was weather-driven in many aspects,” Trice said, “it provides insights into the future as we begin to see improvements in Bay water quality associated with significant reductions in nutrients and sediments entering the Bay.”