In 2011, 18 percent of tidal waters had chlorophyll a concentrations that achieved the goal. This is a decrease of 4 percent from 2010.
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Scientists study chlorophyll a to determine the amount of algae present in the Bay.
Algae are the foundation of the food web and are a necessary part of a balanced ecosystem. However, too much algae can block sunlight from reaching underwater grasses, reducing the habitat and oxygen that underwater life need to survive. The range of acceptable chlorophyll a concentrations varies by season and salinity.
The goal is for 100 percent of Chesapeake Bay tidal waters to be below certain threshold concentrations of chlorophyll a that are acceptable to underwater bay grasses.
Long-term Trend (1985-2011)
The area of the Bay meeting (i.e., less than or equal to) its chlorophyll a threshold concentrations has high interannual variability, but has generally shown a decreasing (degrading) trend over the period of record. Goal achievement has averaged 32 percent and ranged from 17 percent to 50 percent of goal achieved.
Short-term Trend (10-year Trend)
Informal trend analyses suggest no significant trend (neither improving nor degrading) over the past 10 years (2002-2011).
Change from Previous Year (2010-2011)
The chlorophyll a score decreased from 22 percent to 18 percent of goal achieved.
Chlorophyll a Variability
Because pollution, weather and water temperature all affect chlorophyll a, levels vary greatly by year, season and location.
High chlorophyll a concentrations are generally a response to increased nutrient inputs to the Bay and are indicative of high algal biomass in the water column. The settling and subsequent decomposition of these algae are largely responsible for the hypoxia and anoxia in the Bay. Therefore, chlorophyll a concentrations are an important indicator of the magnitude of nutrient loading and potential hypoxia and anoxia in the Bay.
Measuring Chlorophyll a
Chlorophyll a is measured in different ways. The status of chlorophyll a is currently assessed according to the limit of 15 micrograms per liter for underwater bay grasses. Although interannual and seasonal variability are high, most areas of the Bay are below this limit for bay grasses, whereas the upper reaches of large tributaries and most upper Bay tributaries are borderline or failing.
The thresholds below have been applied to the entire Bay to provide an assessment of chlorophyll a. The following chlorophyll a thresholds are applied to fixed sampling station data collected over the growing season of interest (March through September, not including June).
Spring (March, April and May) thresholds (micrograms per liter):
Summer (July, August and September) thresholds (micrograms per liter):