CHAPTER 2 

 PRODUCERS, DECOMPOSERS, AND ENERGY FLOW 



2.1 INTRODUCTION 



Estuaries and coastal embayments are 

 well -recognized for their high primary and 

 secondary productivity. High production 

 by New England tidal flats is reflected in 

 their abundant and diverse populations of 

 invertebrates (Chapter 3) and vertebrates 

 (Chapters 4 and 5) that utilize the habi- 

 tat as nursery grounds and feeding sites. 

 In addition, many New England tidal flats 

 support large populations of commercially 

 and recreational ly important shellfish and 

 baitworms. The high productivity of tidal 

 flats is attributed, in part, to the 

 diverse variety of primary food types 

 (e.g., benthic microalgae, phytoplankton, 

 imported particulate organic materials - 

 "detritus") that are available to the 

 organisms of the flat. 



2.2 PRODUCERS 



2.2.1 Microalgae 



New England tidal flats support a 

 large and diverse microflora. These assem- 

 blages typically appear as brownish or 

 greenish films or mats on the sediment 

 surface and tend to be dominated by ben- 

 thic diatoms, euglenoids, dinof lagellates, 

 and blue-green algae. 



The depth of microalgal distributions 

 in tidal flat sediments is affected by the 

 ability of light to penetrate the sedi- 

 ments. Fenchel and Straarup (1971) found 

 that the photic zone (depth of light pene- 

 tration) of fine sands was about half the 

 thickness of that found in coarse sand. 

 Although the majority of microalgae are 

 concentrated in the upper several centime- 

 ters of the sediment, pigmented cells are 

 commonly found below the photic zone. When 

 exposed to light, these cells actively 

 photosynthesize and it has been hypothe- 

 sized that they provide a reservoir of 



potential benthic primary producers if the 

 upper several centimeters of the sediment 

 are eroded by wave action (Van der Eijk 

 1979). 



By virtue of their location, benthic 

 microalgal species composition, abundance, 

 and spatial distribution patterns are 

 strongly influenced by near-surface phy- 

 sical, chemical, and biological processes. 

 These groups of organisms exhibit pro- 

 nounced spatial and temporal variation in 

 abundance. Exposed tidal flats generally 

 have lower abundances of microalgae than 

 protected flats. Marshall et al. (1971) 

 noted that benthic microflora were most 

 abundant from May to August in several 

 southern New England shallow estuaries 

 probably as a result of temperature and 

 illumination cycles. While summer peaks 

 in abundance are typical throughout New 

 England, Watling (L. Watling; University 

 of Maine, Walpole; February 1981 ; personal 

 communication) has observed dense surface 

 films of diatoms on a tidal flat in Maine 

 during winter, possibly a consequence of 

 decreased grazing activities by benthic 

 invertebrates at this time of the year. 



Most of the academic study of the 

 benthic microflora of tidal flats has been 

 concentrated on the diatoms. Diatoms are 

 ordinarily divided by specialists into two 

 categories: the episammic (non-motile) 

 and epipelic (motile) forms. Most studies 

 have concentrated on the epipelic form 

 since the method commonly used to collect 

 diatoms (e.g., Eaton and Moss 1966) 

 depends on the movement of microalgae into 

 layers of fine netting placed on the sedi- 

 ment surface. 



The benthic epipelic diatom tidal 

 flat communities of New England are domi- 

 nated by pennate forms such as Navicula , 

 Hantzschia , and Nitzchia (Moull and Mason 

 1957; Connor 1980). ^These forms can 

 migrate vertically through sediments by 



