alternating layers of dark-green organic 

 matter and lighter colored sedirent 1 to 

 10 cm (0.4 to 4 inches) deep. Algal mats 

 are known to accelerate rates of sediment 

 accretion on tidal flats by mucilagenous 

 trapping of fine-grained sediments. 



The formation of algal mats is prob- 

 ably restricted to the high intertidal 

 zone because of the reduced activities of 

 grazing and burrowing organisms in these 

 areas. Experimental removal of the 

 surface-grazing periwinkle, Littorina 

 littorea , and the mud snail, Ilyanassa 

 obsoleta , from the mid-intertidal portions 

 of a Barnstable Harbor, Massachusetts, 

 sand flat resulted in the formation of a 

 1 to 2 mm thick algal mat within several 

 weeks. Replacement of the snails in these 

 plots resulted in the quick destruction of 

 the mats (Whitlatch unpublished data). 

 Other organisms such as amphipods and fish 

 are also known to feed on the mats and 

 probably help to control their distribu- 

 tion on tidal flats. 



2.2.2 Macroflora 



Because of the fine-grained and un- 

 stable nature of tidal flat sediments and 

 their regular exposure to salt water at 

 high tide and desiccation at low tide, 

 macroalgae and rooted vegetation are rela- 

 tively uncommon. While these factors may 

 preclude the establishment of stable 

 macrophytic communities on tidal flats, 

 several species of ephemerals (short-lived 

 species) are occasionally found in the New 

 England region. These species (notably 

 Ul va spp. - sea lettuce, and Enteromorpha 

 spp. - green algae) are often associated 

 with protected areas, the upper portions 

 of sand flats, or with eutrophic condi- 

 tions (e.g., sewage outfalls). They 

 appear in early spring, continue to thrive 

 throughout the summer, and rapidly decline 

 during fall and winter. 



In some parts of New England, dense 

 populations of Ul va spp. have been docu- 

 mented. Welsh (1980) reported quantities 

 up to 185 g/m2 and several centimeters 

 thick at the Branford Cove, Connecticut, 

 mud flat. Edwards (S. Edwards; University 

 of Rhode Island, Kingston; June 1980; 

 personal communication) found that more 

 than 75% of this same tidal flat was 

 covered by Ul va during the summer. This 



dense coverage resulted in the establish- 

 ment of anaerobic conditions at the sedi- 

 ment surface and contributed to the reduc- 

 tion of microalgae through shading as well 

 as decreased abundance of meio- and macro- 

 fauna. Others (e.g., Woodin 1974; Watling 

 1975) have also found that dense stands of 

 Ulva can create anaerobic conditions at 

 the sediment-water interface that alter 

 infaunal species abundance and composi- 

 tion. Inhibitory effects of Ulva on tidal 

 flat animial populations may also extend to 

 fish species. In a series of laboratory 

 experiments, Johnson (198G) demonstrated 

 that mortalities of post-larval winter 

 flounder ( Pseudopleuronectes americanus ) 

 were greatly increased in the presence of 

 Ulva . She offered the hypothesis that the 

 increased fish mortality rates were the 

 result of a harmful algal exudate. 



Other species of large plants are 

 commonly transported onto New England 

 tidal flats from adjacent salt marshes 

 (e.g., cordgrass- Spartina spp., rush- 

 Juncus sp.), from eelgrass beds ( Zqstera 

 marina ), and from rocky coastlines (e.g., 

 fucoids, Codium in southern New England). 

 These species are most abundant on flats 

 following storm activity or during the 

 fall when they begin to die and decompose. 

 When very abundant, these plant remains 

 form strand or "wrack" lines on the higher 

 elevations of the flats and provide food 

 and protection for small crustaceans. 

 Most of the biomass of these plants, 

 however, is not used by herbivores but 

 is broken down by microorganisms and 

 by physical and biological fragmenta- 

 tion, becoming part of the tidal flat 

 detritus-based food web (see section 

 2.3). 



2.2.3 Phytoplankton 



Phytoplankton are temporary tidal 

 flat components and are present only when 

 water is covering the flat. Phytoplankton 

 are influenced by nutrient concentration, 

 water temperature and circulation pat- 

 terns, and by grazing; pronounced spatial 

 and temporal variability in species com- 

 position and abundance exist along the 

 New England coastline (see TRIGOK-PARC 

 1974 and Malone 1977 for reviews). Typi- 

 cally, phytoplankton concentrations are 

 reduced during winter because of cold 

 water temperatures and low light levels. 



11 



