CHAPTER 2. PHYSICAL ENVIRONMENT 



This chapter characterizes the 

 physical environment of the South Atlantic 

 Bight. The physical variables most 

 important to rubble communities include 

 the temperature and salinity of the water, 

 and the amount of available light and 

 nutrients. Water movement in the form of 

 currents, tides, and waves is also 

 important, particularly because of the way 

 it affects sediment transport along the 

 beach. In this chapter we consider each 

 of these physical variables in turn. 



2.1 TEMPERATURE AND SALINITY 



The American Atlantic Temperate 

 Region extends from Cape Cod, MA, to 

 southern Florida (Gosner 1979). Cape 

 Hatteras is a natural bi ogeographic 

 boundary along the east coast dividing 

 this region into the Virginian province in 

 the north and the Carolinian province in 

 the south. The southern boundary of the 

 Carolinian province is Cape Canaveral. 

 Thus, the Carolinian province coincides 

 with the area treated in this profile. 



Cape Hatteras and Cape Canaveral mark 

 significant temperature transition zones. 

 In the northern portion of the Carolinian 

 province (North Carolina) water 

 temperatures can exceed 30 °C in summer 

 and drop to °C in Winter (Sutherland and 

 Karlson 1977; W. Kirby-Smith, Duke 

 University Marine Laboratory; pers. 

 comm.). As latitude decreases, winter 

 temperatures especially are gradually 

 ameliorated. In central Florida, water 

 temperatures range from 30 °C in summer to 

 14 °C in winter (Mook 1980). 



In this profile we restrict our 

 attention to the outer coast and to the 

 sounds and estuaries where the salinity 

 generally remains above 20 ppt. The flora 



and fauna of these regions are basically 

 marine. 



2.2 LIGHT AND TURBIDITY 



In the South Atlantic Bight, tides, 

 waves, and wind-generated turbulence re- 

 suspend bottom sediments in the shallow 

 waters on the Continental Shelf. This 

 sediment resuspension, combined with high 

 estuarine and nearshore phytopl ankton 

 productivity, produces turbid inshore 

 waters that drastically reduce light 

 penetration. Reduced light penetration 

 appears to be the major factor restricting 

 plant growth to the shallow portions of 

 most jetties. Figure 3 shows the 

 relationship between irradiance and 

 wavelength taken on the same day for 

 different depths at an inshore site in 

 Bogue Sound, NC (the turning basin at 

 Morehead City), and a site approximately 6 

 km offshore from Bogue Sound. As can be 

 seen, light decreases dramatically with 

 depth in nearshore waters, where rubble 

 structures are generally located. 



2.3 NUTRIENTS 



Nutrient conditions surrounding 

 jetties are important because they affect 

 the growth of the seaweeds attached to the 

 jetties and the growth of phytoplankton in 

 the overlying waters. These phytoplankton 

 are consumed by benthic filter feeders on 

 the jetties. On jetties subject to strong 

 wave or tidal action, plants may be 

 minimally affected by low nutrients 

 because new water is constantly flowing by 

 and because turbulence interrupts the 

 formation of diffusion barriers around the 

 seaweed thallus. Nitrogen is most 



