60 



BIOLOGICAL REPORT 31 



important to the establishment of these species, as 

 waves prevent the formation of a stable substrate 

 (Redfield 1 972). In the initial formation of a wet- 

 land, a gradation in sediment type exists, from sandy 

 toward the mouth of the wetland to silty toward the 

 head. This gradation reflects the characteristics of 

 the suspended matter, as tidal waters have a lower 

 ability to keep heavier materials like sand in sus- 

 pension, resulting in sand deposition near the mouth 

 and subsequent deposition of finer particles nearer 

 the headwaters. Once the substrate is available at 

 suitable elevation and the plants begin to colonize, 

 the extensive root and rhizome systems of marsh 

 species stabilize the sediments, and the marsh be- 

 comes established. About half of the production of 

 the dominant low marsh species Spartina 

 alterniflora is in belowground production. 



The value of these highly productive intertidal 

 wetlands has long been recognized — as habitat for 

 waterfowl and shellfish, as storm buffers for adja- 

 cent upland, as nursery grounds for various species 

 offish, and as potential buffers for terrestrial nutri- 

 ent inputs to coastal waters. Tidal flushing of salt 

 marshes is also postulated as a mechanism for ex- 

 port of plant detritus to estuarine food webs in 

 embayments like Buzzards Bay. Wilson et al. ( 1 985 ) 

 estimated between 5% and 7% of the organic mat- 

 ter in Buzzards Bay sediments was made up of vas- 

 cular plant remains, with the bulk of the balance of 

 organic matter derived from phytoplankton. They 

 also estimated an export of 3-4 x 1 5 kg particulate 

 organic carbon annually from marshes into the bay. 

 amounting to 25-30% of the total amount of vascu- 

 lar plant debris in the top 1 cm of surface sediment. 



Saltwater marshes in New England, including 

 those in Buzzards Bay, are generally divided into 

 two rather distinctive zones: the low marsh, domi- 

 nated by the salt marsh cordgrass. Spartina 

 alterniflora; and the high marsh, dominated by the 

 salt marsh hay. Spartina patens, and the spike 

 grass, Distichlis spicata. Flooding frequency and 

 duration are the primary determinants to the distri- 

 bution of low and high marsh zones. The low marsh 

 zone is located between mean low water and mean 

 high water, while the high marsh is the region lying 

 between mean high water and spring high 



water. Both the low and high marshes are sufficiently 

 flooded by seawater to inhibit the growth of more 

 freshwater marsh plants such as Typha (cattail) and 

 Phragmites (reed). 



Low marsh is typically flooded on every high 

 tide and is almost exclusively colonized by Spartina 

 alterniflora, occasionally with Limonium nashii 

 (sea lavender) or Salicornia (glassworts) present. 

 Spartina alterniflora exhibits two growth forms, 

 the tall form (up to 1 -2 m in height), which grows 

 1-3 m inland from creeks, and the short form (less 

 than 50 cm), which grows inland from the tall zone. 

 The differences in these morphologies is generally 

 attributed to a combination of nutrient availability, 

 sediment oxidation, and plant-sediment interactions, 

 with the more productive tall form growing in better 

 drained, more oxidized sediments (therefore, plants 

 possess increased ability to uptake nitrogen) with 

 low concentrations of plant growth inhibitors (such 

 as sulfides; Howes et al. 1 986). In response to the 

 anoxic sediments resulting from the high organic mat- 

 ter inputs and frequent inundation, these plants have 

 adapted an aerenchyma system of gas-filled lacu- 

 nae to transport oxygen to their roots and rhizomes, 

 which support aerobic respiration and nutrient 

 uptake (Teal and Kanwisher 1 966; Howes and Teal 

 1 994). The physiological difficulties of plant water 

 uptake and evapotranspiration in saline sediments 

 has been diminished by the evolution of salt glands, 

 which secrete a concentrated salt solution to main- 

 tain osmotic balance while water is being lost dur- 

 ing evapotranspiration. The naturally high levels of 

 primary productivity found in salt marshes are gen- 

 erally attributed to the abundance of Spartina 

 alterniflora. 



The high marsh supports greater plant diversity 

 than the low marsh and is dominated primarily by 

 salt marsh hay and spike grass. Along the upland 

 border where the duration of tidal flooding is least, 

 salt-tolerant plants such as saltmeadow rush (Ji/ncus 

 gerardii), switch grass (Panicum virgatum), 

 chairmaker's rush (Scirpus americanus), salt 

 marsh bulrush {Scirpus robustus), and marsh elder 

 (Iva frutescens) are commonly found. In most of 

 the marshes around Buzzards Bay where the 



