CHAPTER 1 

 GENERAL FEATURES OF TIDAL FLATS 



1.1 INTRODUCTION 



Intertidal sand and mud flats are 

 soft to semi-soft substrata, shallow-water 

 habitats situated between the low and high 

 tidal limits. Tidal flats are found where 

 sediment accumulates and are, therefore, 

 associated with coastal embayments, behind 

 spits and barrier beaches, and along the 

 margins of estuaries. The occurrence and 

 extent of tidal flats varies according to 

 local coastline morphology and tidal 

 amplitude. These habitats are sometimes 

 bordered landward by salt marshes and sea- 

 ward by tidal channels and/or subtidal 

 eel grass ( Zostera marina ) beds (Figure 1). 

 Tidal flats are common features of the New 

 England coastline, especially in Maine, 

 New Hampshire, and parts of Massachusetts 

 where increased tidal amplitude exposes 

 more of the tidal flats at low tide. For 

 example, tidal flats represent about 48% 

 of the intertidal habitats of Maine (Fefer 

 and Schettig 1980). 



Tidal flats are not static, closed 

 ecological habitats, but are physically 

 and biologically linked to other coastal 

 marine systems. It is generally recog- 

 nized, for example, that organisms inhab- 

 iting tidal flats rely heavily upon 

 organic materials (e.g., plankton, detri- 

 tus) imported from adjacent coastal, estu- 

 arine, riverine, and salt marsh habitats. 

 In addition, many species of estuarine and 

 coastal fishes migrate over tidal flats 

 with the incoming tide to feed on the 

 organisms found on and in the sediments. 



1.2 THE NEW ENGLAND COASTAL ENVIRONMENT 



Climatic conditions of the New Eng- 

 land coastal region exhibit pronounced 

 seasonal temperature fluctuations, a char- 

 acteristic of temperate environments. 

 Extremes in seawater temperatures, warmest 

 in August through September and coolest in 



December to March, are among the greatest 

 in the world (Sanders 1968). The region 

 is commonly divided, for convenience, into 

 two areas: the Gulf of Maine extending 

 from Cape Cod, Massachusetts, to the Bay 

 of Fundy, Nova Scotia, Canada, and the 

 areas south of Cape Cod ranging to western 

 Connecticut including Long Island Sound 

 (Figure 2). This division is based largely 

 on differences in annual water temperature 

 variation in the two regions. Waters in 

 the Gulf of Maine are continually well- 

 mixed by tidal, current, and wind action 

 (Brown and Beardsley 1978) and in the sum- 

 mer do not become as warm as the waters 

 south of Cape Cod. On the south side of 

 Cape Cod, the influence of the Gulf Stream 

 coupled with a shallower coastal plain 

 produces more abrupt increases in summer 

 temperatures. The net effect is that the 

 annual range of seawater temperatures 

 along the coast of New England is closely 

 related to latitude (Figure 3). For 

 instance, in the northern portion of the 

 Gulf of Maine there is a 10°C (50°F) 

 annual temperature range while in portions 

 of Long Island Sound the annual range is 

 about 20°C (68°F). 



Cape Cod is a transition zone rather 

 than a discrete physical barrier separat- 

 ing warm and cool New England coastal 

 water masses. Water associated with embay- 

 ment and estuarine environments is gener- 

 ally shallow and is more likely to be 

 influenced by atmospheric and terrestrial 

 conditions than deeper water areas. Spring 

 runoff from rivers, thermal warming of mud 

 and sand flats with subsequent heat 

 transfer to shallow waters, and low flush- 

 ing rates of water in some estuarine 

 habitats all contribute to warmer water 

 temperatures. Warm water embayments north 

 of Cape Cod do occur (e.g., Barnstable 

 Harbor, Massachusetts; upper reaches of 

 some estuaries in New Hampshire and 

 Maine), but in autumn shallow water 

 habitats respond quickly to the cooler 



