CHAPTER 2. THE FLORA OF INTERTIDAL FLATS 



2.1 MICROFLORA 



To the untrained eye, an intertidal mud or 

 sand flat usually appears barren and almost totally 

 uninhabited by plants. Microalgae, however, are 

 extremely abundant in and on the sediments of 

 intertidal flats. These small plants can be best 

 viewed and identified under a microscope, but 

 they are frequently obvious to the naked eye as a 

 conspicuous discoloration of the sediment surface. 

 Benthic diatom films appear as brownish stains on 

 the sediments, while green microalgae occasionally 

 tint the sediments with bright greens. 



Several different groups of microalgae are 

 important on intertidal flats: benthic diatoms, 

 benthic dinoflagellates, filamentous greens, and 

 blue-green algae. Of these, the diatoms are usually 

 the most numerous with pennate forms such as 

 Navicula and Nitzschia predominant. Benthic dia- 

 toms such as Navicula occasionally form dense 

 multilayered sheets on intertidal flats (Pamatmat 

 1968). Mats of blue-green algae are often obvious 

 features of protected flats (Brenner et al. 1976, 

 Polimeni 1976). These mats, composed of several 

 blue-greens (Lyngbya, Microcoleus, and Phormi- 

 dium in North Carolina) and also some diatoms, 

 are located in the high intertidal zone. During low 

 tides, they often dry into a hard black or blue- 

 green crust that resembles asphalt. Although these 

 blue-green mats in the high intertidal and the dia- 

 tom mats in the lower intertidal are perhaps the 

 most prominent examples of intertidal microalgae, 

 even where they are not conspicuous, benthic 

 microalgae are usually important primary produc- 

 ers on intertidal flats. 



Benthic microalgae are of two types — mobile 

 and attached. The attached forms tend to be rela- 

 tively small and to adhere to the surfaces of sedi- 

 ment particles (Meadows and Anderson 1968). In 

 some areas, mobile forms migrate vertically in the 

 sediments. For instance, Pomeroy (1959) demon- 

 strated vertical migration among the microalgae 

 on an intertidal mud flat in Georgia. When the 

 tide was out the algae lived on the sediment sur- 

 face, whereas at high tide the densest concentra- 



tion of benthic microalgae was lower in the 

 sediments. In other systems, such as on the inter- 

 tidal sand flats of False Bay, Washington (Pamat- 

 mat 1968), the algae apparently do not migrate 

 vertically. Living algae can often be found to 

 depths of 10cm, but the majority is usually loca- 

 ted within the top centimeter of sediments. In 

 turbulent areas and in places undergoing erosion, 

 the biomass of benthic microalgae is much lower 

 than on relatively protected flats. 



2.2 MACROPHYTES 



Although an intertidal flat contains, by defini- 

 tion, no marsh plants or seagrasses, macrophytic 

 algae are often a conspicuous element of this 

 environment. Attached to shell debris, pebbles, 

 and other small fragments of hard substrate, large 

 numbers of macrophytic marine algae can fre- 

 quently be found. In North Carolina, which lies at 

 the boundary of two major biogeographic prov- 

 inces, the macrophytic algae of intertidal flats 

 undergo an almost total turnover during the year. 

 In winter (from around November through March) 

 various species of Ectocarpus, a filamentous 

 brown alga, are extremely common on intertidal 

 sand and mud flats in areas of relatively high sal- 

 inity. This alga is replaced in spring by equally 

 massive amounts of the filamentous greens, En fer- 

 omorpha and Cladophora. These greens are abun- 

 dant from around February through June on 

 North Carolina flats. In summer (April —July) the 

 leafy green alga, Ulva, is dominant in this environ- 

 ment. Only in the fall do macrophytic algae fail 

 to be prominent in the intertidal zones of North 

 Carolina's sand and mud flats. Although most of 

 these macrophytes are attached to hard substrates, 

 floating mats of actively growing and healthy 

 Enteromorpha and other green filamentous algae 

 are also common on many intertidal flats. These 

 mats can occasionally be.quite extensive, covering 

 up to 50% or more of the total area of some inter- 

 tidal flats during summer months. 



2.3 BACTERIA AND FUNGI 



Fungi and especially bacteria are extremely 



