66 



BIOLOGICAL REPORT 31 



the system through biodeposition of suspended par- 

 ticulate nitrogen. This is also true for other marsh 

 species, Mercenaria mercenaria, Mya arenaria, 

 and Gemma gemma: however, given the domi- 

 nance of the ribbed mussel in this marsh, it is re- 

 sponsible for most of the total bivalve filtration and 

 biodeposition. If the amount of particulate nitrogen 

 filtered by these mussels was instead exported from 

 the system, a significant loss of nitrogen to coastal 

 waters would result. Because nitrogen limits phy- 

 toplankton productivity in Buzzards Bay, the in- 

 creased nitrogen retention by ribbed mussel filtra- 

 tion may actually serve to reduce fertilization of 

 adjacent bay waters. 



In addition to their aesthetic value, the impor- 

 tance of marshes as storm buffers, habitats, and 

 nursery grounds for numerous species, and histori- 

 cally as a valuable source of salt marsh hay, has 

 long been a basis for defense in their protection. 

 More recently, the role of salt marshes as nutrient 

 buffers for coastal waters is becoming increasingly 

 evident as our understanding of these complex en- 

 vironments continues to grow. This is especially true 

 for areas such as Buzzards Bay where residential 

 development is continually increasing. 



Because marshes exist at the land and sea inter- 

 face, questions arose in the late 1 960's and early 

 1 970's as to whether salt marshes were nitrogen 

 limited, as are many coastal marine systems, or phos- 

 phorus limited, as are many terrestrial systems. Ex- 

 periments undertaken to answer this fundamental 

 question, most notably long-term fertilization experi- 

 ments initiated in 1970 in the Great Sippewissett 

 Salt Marsh, identified nitrogen as the nutrient limit- 

 ing production in the salt marsh environment ( Valiela 

 etal. 1975; Teal 1986). Much attention has been 

 paid in recent years to the role of nitrogen-limited 

 salt marshes in intercepting or buffering nitrogen in- 

 puts from terrestrial sources as they move toward 

 coastal waters. The increased understanding of 

 marsh processes in this regard has contributed to 

 the development of artificial wetland ecosystems 

 (such as Solar Aquatics; Teal and Peterson 1 99 1 ) 

 for the tertiary treatment of nutrient-rich wastewa- 

 ter and septage. These new technologies hold prom- 

 ise for dealing with the often competing objectives 



of utilization and protection in valuable yet ecologi- 

 cally fragile coastal environments like Buzzards Bay. 

 More information is available on salt marsh ecol- 

 ogy in Teal ( 1 986) and Nixon ( 1 982 ). 



4.2.2. Tidal Flats 



Tidal flats are gently sloping unvegetated areas 

 extending seaward of coastal landforms to mean 

 low water (MLW). These flats are typically exposed 

 at low tide, revealing sediments ranging from sands 

 to muds and silts. Tidal flats are generally deposi- 

 tional environments, with the area and duration of 

 exposure dependent on tidal amplitude. They are 

 often associated with other types of coastal envi- 

 ronments such as embayments, salt marshes, spits, 

 and barrier beaches that provide a source of 

 sediment for development of the flat. 



Tidal currents in Buzzards Bay are primarily re- 

 sponsible for the sediment makeup of these flats. 

 Along shorelines exposed to higher currents and 

 wind-driven wave energies, such as along the edge 

 of the bay proper, these flats tend to be made up of 

 coarser, sandier sediments, while those flats in more 

 protected areas, such as in estuaries, behind bar- 

 rier beaches, or within wetlands or salt ponds, gen- 

 erally have finer, siltier sediments. Their association 

 with other types of marine systems is important for 

 providing both a source of strata and a source of 

 allocthanous organic matter to the organisms that 

 inhabit them. 



Because the overlying water column retreats at 

 high tide, only infaunal and epibenthic animals colo- 

 nize tidal flats. At high tide, however, numerous 

 species offish "'commute" to graze on the benthos 

 and epibenthic algae. The infaunal communities in- 

 habiting the tidal flats along Buzzards Bay provide 

 a valuable resource to the aquatic food web and to 

 the many species of waterfowl that feed on these 

 organisms during low tide. Shorebirds, feeding pri- 

 marily on invertebrates such as polychaetes. mol- 

 lusks, and crustaceans, often follow the water's edge 

 as it advances and retreats over the flats, with 

 maximum foraging during low tide when most of 

 the tidal flat is exposed. Many other species utilize 

 the tidal flats, including crabs such as rock crab 



