The overall Bay Health score (53%) in 2015 was one of the highest recorded. Only 1992 and 2002 scored as high or higher. Both 1992 and 2002 were major drought years that also followed either drought or regular flow years. 2015 was not a major drought year, nor were the previous years. Since rainfall leads to increased nutrient and sediment runoff from land, it is not surprising that drought leads to improvements in Bay Health. Overall, the streamflow was below normal for 2015, but nowhere near the drought conditions in 1992 and 2002. So, the high score for 2015 was particularly noteworthy. Bay Health is also increasingly influenced by high summer temperatures. For example, high water temperatures in 2005 and 2010 led to declines in aquatic grasses. Dissolved oxygen levels are influenced by high water temperatures as well. In 2015, summer water temperatures in Chesapeake Bay were relatively mild, even though 2015 was overall the hottest year on record for
air temperatures.

Bay health improvements were widespread

Chlorophyll a improvements were widespread, extending from the Upper Bay to the mouth of Chesapeake Bay. Improvements in major tributaries like the Potomac, Rappahannock, James, and Choptank Rivers were also seen.

Water clarity improvements were also widespread, but more pronounced in the central and lower Bay.

Aquatic grass improvements were primarily associated with the freshwater species in the Upper Bay and at the head of tributaries. In addition, widgeon grass expanded in the mid-range salinity areas of the Bay.

The causes of these improvements could be related to the successive years of moderate weather, the continuing sewage treatment upgrades, widespread use of winter cover crops, and reductions in atmospheric nitrogen deposition. However, the exact degree to which each of these factors contributed individually or cumulatively to the improvements is unknown.

Lower phosphorus scores in 2015 are unexplained

Despite overall improvements in Bay Health and improvements in almost every other Bay Health indicator, the score for total phosphorus worsened to 70% in 2015, down from 79% the previous year. This indicator is a measure of total phosphorus concentrations in the water at over 130 stations throughout the tidal waters of Chesapeake Bay. Although modeled estimates showed a decrease in phosphorus loadings from the watershed, phosphorus concentrations in the Bay increased, causing the scores to go down. This disconnect is difficult to explain—lower inputs to the Bay should result in less phosphorus in the water. And, while scores decreased from 2014-2015, phosphorus scores are showing a long-term improving trend from 1986-2015. Further research, like the study underway at UMCES on the Conowingo Dam, is needed to answer this question.