Assessing Chlorophyll a Criteria Attainment 
Phytoplankton are actively growing and consuming nutrients throughout the surface 
mixed layer of the water column. The pycnocline region below the this layer, as well 
as other depth strata below the pycnocline, rarely contain sufficient light for active 
photosynthesis. Therefore, there is little or no autotrophic growth below the surface 
mixed layer, although phytoplankton accumulate within and below the pycnocline 
due to the physical processes of sinking and estuarine circulation. Given that the 
chlorophyll a concentrations throughout the water column will be expressed at the 
surface at some point during the natural cycling of phytoplankton and for the 
sampling reasons described above, the chlorophyll a criteria are applied to surface 
waters only. 
Chlorophyll a samples used in determining attainment of numerical chlorophyll a 
criteria should be collected at 0.5 to 1 meter below the surface. The majority of 
historical and current chlorophyll a data are collected from a discrete surface depth. 
The potential for assessing broad areas of the estuary via high-speed vessels and 
flow-through technologies or remote sensing can only be tapped if the criteria apply 
only to surface chlorophyll a distributions. In general, chlorophyll a concentrations 
are highest in the surface layer of the water column. 
The formulation and ultimately the assessment of numerical chlorophyll a criteria 
should be based upon seasonal dynamics and concentrations of chlorophyll a in the 
Chesapeake Bay and its tidal tributaries. Spring and summer were chosen for these 
purposes because chlorophyll a concentrations attain annual peaks during these 
months in the estuary’s various salinity regimes. Any site-specific numerical 
impairment-based chlorophyll a criteria should be applied as salinity regime-based 
spring (March through May) and summer (July through September) seasonal mean 
concentrations. 
In spring, river inputs with high dissolved inorganic nitrogen dominate, dissolved 
inorganic nitrogen is abundant, phytoplankton are primarily limited by the avail¬ 
ability of phosphorus, and bottom waters are oxygenated. By contrast, under summer 
conditions, recycling of nitrogen and phosphorus is the dominant supply, both 
dissolved inorganic nitrogen and dissolved inorganic phosphorus are low, phyto¬ 
plankton are primarily limited by the availability of nitrogen and deep bottom waters 
are anoxic. The ecological implications of chlorophyll a concentrations in spring and 
summer are vital to physical and chemical processes such as hypoxia and anoxia, 
nutrient recycling and light attenuation, and biological processes such as the avail¬ 
ability of sufficient and appropriate food for filter and suspension-feeders. 
After years of monitoring the Chesapeake Bay and its tidal tributaries, characterizing 
phytoplankton dynamics and analyzing these data, Bay scientists have found that 
June is indeed a ‘transition’ month from spring to summer. During certain years, 
June behaves more like spring in the types and quantity of phytoplankton that are 
present, while in other years, the flora reflect the summer patterns of composition 
and densities. This means that in attempts to measure ‘spring’ and ‘summer’ 
chapter vi • Recommended Implementation Procedures 
