wise released largely by plants, and (2) dissolved 

 inorganic compounds. Bacteria that can grow on 

 dissolved inorganics are termed chemo-autotrophs 

 because they produce particulate organic matter 

 without directly utilizing the sun's energy in ph(^- 

 tosynthcsis. Instead, they utilize energy in the 

 chemical bonds of certain inorganic compounds, 

 notably hydrogen sulfide. Bacteria on marine soft 

 sediments exhibit a broad range of functional 

 strategies from 100% chemo-autotrophs to 100% 

 utilizers of solar energy. Despite the importance 

 of bacteria as chemo-autotrophs in oceans and the 

 abundance of high-energy sulfur compounds in 

 intertidal mud flats, this sort of production of 

 particulate food is apparently not very significant 

 on tidal flats, not even on mud flats. Primary pro- 

 duction based upon photosynthetic pathways far 

 outweighs the contribution frc^n chemo- 

 autotrophy in such a well-lighted environment. 



Although most of the detritus upon which de- 

 composers are operating is produced in other 

 estuarine habitats, much of this detritus ultimately 

 does reach the intertidal flat. Newly sloughed-off 

 plant material usually rafts away from the imme- 

 diate vicinity of its production (Odum and de la 

 Cruz 1967), and, because it floats on the water 

 surface, much of it is deposited by the wind and 

 tides in the intertidal zone, especially along the 

 most recent high-tide line. Here numerous animals 

 fragment it, process it, and gradually work it into 

 the sediments so that the detrital content of inter- 

 tidal flats can be quite substantial (Odum 1970a). 

 Food levels for detritivores can thus be high on 

 intertidal flats, especially on mud flats (as shown 

 by Ferguson and Murdoch 1975 for a North Caro- 

 lina estuary). Decomposition (mineralization) of 

 this detritus, which is derived from other habitats, 

 helps to fuel the substantial rates of productivity 

 tj,y benthic microalgae on the intertidal flats and 

 even by phytoplankton in the overlying water col- 

 Lunn. Rublee and Dornseif ( 1978) counted bacteria 

 in sediments taken directly from an intertidal 

 marsh in North Carolina and found that bacterial 

 abundances declined significantly with depth in 

 the sediments, suggesting that food levels for 

 detritivores are far higher in surface sediments. 



2.4 PHYTOPLANKTON 



areas. In North Carolina's estuaries, various dia- 

 toms, especially Skeletonema, dominate (Carpen- 

 ter 1971, Williams 1973). Winters are charac- 

 terized by low levels of phytoplankton probably 

 because of low light levels and low temperatures. 

 Coastal waters are therefore quite clear in winters, 

 except when clouded by silt in the runoff after 

 heavy rains. Phytoplankton concentrations usu- 

 ally peak in spring and remain almost as high 

 during summer, substantially increasing the turbi- 

 dity of coastal waters. Intertidal flats contribute 

 significantly to total phytoplankton production 

 in estuarine systems because at high tide when the 

 Hood waters spread out across the flats the total 

 area (and volume) of the euphotic zone (the zone 

 where light levels are sufficient for net photosyn- 

 thesis) is greatly increased, often by a factor of 

 two or more (Figure 5). The degree to which the 

 intertidal flats enhance phytoplankton production 

 by increasing the euphotic zone at high tide can 

 probably be approximated by the proportion of 

 the estuarine bottom that is intertidal. However, 

 no studies have yet been done to measure this 

 effect quantitatively. Despite the relatively high 

 levels of nutrients in estuaries, phytoplankton 

 production is limited in North Carolina's estuaries 

 by nitrogen concentrations (Williams 1966). 



2.5 MEASURMENTS OF PRIMARY 

 PRODUCTIVITY 



In a review of the data on the rate of produc- 

 tion of plant matter (primary productivity) on 

 intertidal flats, one must necessarily consider each 

 of the major types of plant on the flats, namely 

 (1) benthic macrophytes, (2) benthic microalgae, 

 and (3) phytoplankton. Because the intertidal 

 flats receive organic input from other wetland 

 habitats, the productivity of other major elements 

 of the whole estuarine system must also be des- 

 cribed here. Fragments, large and small, of impor- 

 tant producers, such as marsh plants (e.g., Spar- 

 tina, Juncus) and seagrasses (e.g., Zostera, Halo- 

 dule, Ruppia), are found abundantly on and in 

 the sediments of intertidal flats. The intertidal 

 flats are not a closed system ecologically, but 

 rather rely upon organic inputs from other wet- 

 land habitats as well as their own in situ produc- 

 tion. 



At high tide, when the intertidal flats are 

 covered by fkjod waters, phytoplankton have the 

 opportunity to grow and reproduce in intertidal 



The plants of the salt marsh have received a 

 neat deal of attention as a result of several studies 



13 



