land runoff arc much more difficult to measure or 

 estimate. 



Pollutants have reached all parts of the Chesa- 

 peake Bay system (L. E. Cronin, pers. comm.). 



BAY AND TRIBUTARIES 



Those parts of the Bay system that are perma- 

 nently under water exhibit a marked salinity 

 gradient from fresh water in the more landward 

 locations of the estuary to approximately 30 o/oo 

 salinity at the mouth of the Bay. At any given loca- 

 tion in this partially mixed estuary, salinity is high- 

 est in the summer and fall when river runoff is low, 

 and lowest in the winter and spring when rainfall 

 and runoff are high. Pritchard (1968) provides a 

 general description of water movement in the Bay. 

 Storms can produce abrupt changes in salinity- 

 distribution. 



The salinity regime affects the distribution of 

 species from plankton to benthos to fishes. An eco- 

 logical structure appropriate to any salinity region 

 within the Bay is illustrated in figure 2. However, 

 species composition inside the boxes varies with 

 salinity and season; for example, net zooplankton 

 species are dominated by copepods: Eurytemora in 

 fresh and brackish water, and Acartia in more 

 saline water. Net phytoplankton dominants vary 

 from blue-green algae in fresh water to diatoms in 

 more saline waters. The distribution of fish species 

 (eggs, larvae, juveniles, and adults) depends upon 

 the salinity regime as well as the time of year. 

 Species of benthic infauna and epifauna vary with 

 salinity; oysters and clams are found in the middle 

 salinity regions of the Bay (Lippson 1973). 



A second gradient in the Bay system is that of 

 depth. Community structure changes from emer- 

 gent wetlands to shallows to deeper waters. 



The upper Bay derives most of its carbon 

 from allochthonous sources; particulate organic 

 carbon is transported into the system by the Sus- 

 quehanna River. Only about 10 percent of new car- 

 bon is derived from primary production in situ; in 

 contrast, most new carbon in the middle Bay is 

 fixed by phytoplankton and relatively little is im- 

 ported from upstream (Biggs and Flemer 1972). 

 Total annual carbon inputs to the whole Bay from 

 river transport, marshes, seagrasses, and phyto- 

 plankton production are estimated roughly in 

 appendix B. Phytoplankton carbon production 

 appears to be the most important, followed by that 

 of marshes and seagrasses, and then by river trans- 



port. Inputs from land runoff have not been esti- 

 mated. 



Primary producers in emergent wetlands are 

 marsh plants, epiphytes, and benthic algae. Most of 

 this carbon enters the food web by the detrital 

 pathway. Shallow regions receive carbon from 

 three sources: transport of detrital material from 

 marshes and rivers; production by seagrasses; and 

 production by phytoplankton. In waters too deep 

 for seagrass growth, phytoplankton production in 

 situ and transportation from upstream are the 

 carbon sources. 



Much of the biological activity in the Bay oc- 

 curs in the shoal or shallow waters that are most di- 

 rectly influenced by runoff from the land. Sedi- 

 ment-trap areas may remove sediments, nutrients, 

 and toxic materials from the water column in shal- 

 low waters, preventing much of that material from 

 reaching deeper waters. 



PLANKTON 



Figure 2 was originally planned to distinguish 

 shallow and deeper water communities in the Bay. 

 However, for plankton and some nekton, the dis- 

 tinction is not clear. Plankton species have not 

 been found to vary from shallow to deeper water 

 (Heinle, pers. comm.), although they do vary with 

 salinity (Lippson 1973). The following discussion 

 of the plankton community applies to all Bay 

 waters (fig. 4). 



In Chesapeake Bay, phytoplankton standing 

 stock is apparently limited by availability of P in 

 the spring and inorganic N in the summer (Taft and 

 Taylor 1976). In the winter, biomass is limited by 

 light or temperature (Taft, pers. comm.). Sediment- 

 water interactions in oxygenated and anoxic waters, 

 as well as regeneration by organisms, determine 

 abundance and chemical form of available P and N 

 in the system. 



Primary production rates may be determined 

 by nutrient regeneration rates. While total nutrients 

 place an upper limit on standing stock during the 

 summer, turnover rates may be as rapid as every 2 

 days (Heinle, pers. comm.). It is possible that sum- 

 mer primary production is limited only by the 

 physiological capabilities of plant cells (Taft, pers. 

 comm.). Nannoplankton (plant cells less than 10 

 microns in diameter) contribute at least two-thirds 

 of total primary production on an annual basis 

 (Van Valkenburg and Flemer 1974). 



