killing the cells. Hence, trampling or 

 driving on the marsh, especially when 

 water is present, results in the death of 

 the Spartina stems. 



Oxygen supply . Another problem that 

 S. al term' flora has in relation to its 

 habitat is that sediments around its roots 

 are typically anoxic. Its roots, while 

 able to exist without oxygen for short 

 periods, must have oxygen for their 

 respiration for long-term survival. This 

 oxygen passes through air spaces that are 

 continuous with the stomata on the leaves, 

 through aerenchyma (air passages) in 

 leaves and stems, through the hollow 

 central space in the rhizomes, to the 

 central air space in the roots. Air moves 

 by diffusion through these spaces with 

 sufficient ease to supply the demands of 

 the underground parts of the plants. It 

 used to be thought that this flux was 

 sufficient to supply oxygen to the 

 sediment immediately surrounding the roots 

 as well (Teal and Kanwisher 1966). More 

 recent work suggests that while this may 

 be true in drained sediments where the 

 roots are surrounded by gases (e.g., at 

 low tide on creek banks), it is not the 

 case when the roots are surrounded by 

 water in saturated muds. Howes et al. 

 (1981) have suggested that some other 

 oxidant may be coming from the roots. An 

 oxidant of some type is expected because 

 the roots are often surrounded by a layer 

 of oxidized iron, and the soil redox 

 potential around productive Spartina is 

 higher than that around poorly growing 

 Spartina which, in turn, is higher than 

 that in sediments without plant cover. 



There is little doubt that the 

 internal gas spaces transmit oxygen to the 

 roots for their own respiration. Gleason 

 and Zieman (1981) showed that the oxygen 

 concentration in the underground plant 

 parts declined during high tide in the 

 dark when oxygen could not be replenished 

 by either diffusion from the air or by 

 photosynthesis. They suggested that this 

 internal oxygen store helps to set the 

 lower limit at which the plants can grow 

 in the intertidal zone. The approximate 

 mid-tide lower limit of S. al ternif lora 

 and the inability of S. patens to 

 successfully invade the regularly flooded 

 parts of the salt marsh could be explained 

 in this manner. 



In highly reduced, waterlogged soils, 

 the air spaces are not sufficient to 

 maintain oxic metabolism in S. 

 al ternif lora roots. This may be a reason 

 for small plant stature in such sites. 

 Mendelssohn et al. (1981) showed that in 

 the more oxidized marsh sediments, 

 Spartina roots function aerobically most 

 of the time. When muds become anoxic, the 

 roots produce malate as the product of 

 their metabolism. Malate is non- toxic and 

 can be accumulated in the roots without 

 damage, but this metabolic pathway 

 produces no net energy for the plant. In 

 highly reduced sediments, the roots 

 develop alcohol dehydrogenase and produce 

 ethanol. Though ethanol is toxic, its 

 production does yield energy for root 

 growth and maintenance but at lower levels 

 than oxygen-supported respiration. The 

 ethanol produced apparently diffuses 

 through the sediments readily enough so 

 that it may not affect the roots. In the 

 most waterlogged conditions where this 

 diffusion is reduced, the ethanol toxicity 

 may contribute to the stunted condition of 

 Spartina . 



Oxygen supply to the roots is also 

 intimately connected with the nitrogen and 

 sulfur metabolism of Spartina . It is, 

 therefore, connected with the cycles of 

 these elements in the marsh system and 

 with resistance of Spartina to soil toxins 

 (Mendelssohn et al. 1982). These points 

 will be addressed in Section 5.3. 



3.2 SALT MARSH ALGAE 



Both macro- and microscopic algae 

 live on the surface of sediments in the 

 salt marsh and are attached to vegetation 

 and other marsh organisms (Figure 9). 

 Ascophyl Turn nodosum (knotted wrack) and 

 Fuc us vesiculosus (rockweed) grow at the 

 lower edge of the S. al term' flora zone and 

 sometimes form fairly dense mats. The 

 macroscopic green algae Enteromorpha 

 (hollow green weeds) and Ul va (sea 

 lettuce) can be abundant, especially early 

 in summer. Cod i urn f ragi le (green fleece) 

 grows on suitable substrates such as 

 oyster shells. Some of these macroalgae 

 are very abundant at times. In early 

 summer, before much growth of marsh 

 grasses at Great Sippewissett Salt Marsh, 

 Ascophyl Turn nodosum may appear to have a 



14 



