MARINE Al&JE OF BEAUFORT, N. C. 397 



the inner jetties at Fort Macon by means of oyster tongs and diving, a day being chosen 

 when the water was about 15 cm. below the usual low tides. This showed algse occur- 

 ring abundantly to a depth of about 75 cm. below the usual level, then becoming scarcer 

 and ceasing about i .4 m. below this level, none being found as low as i .7 m. In October, 

 1906, one of the jetties at Fort Macon, being undermined by the current, sank to a depth 

 of about 6 m. When the rocks of this jetty were dredged up the following July they 

 were entirely bare of algae, although in the previous autumn they had borne numerous 

 plants of Fucus, Sargassum, Dictyota, Hypnea, and other species occurring in this 

 locality. 



The lower limit of the algae in this region is undoubtedly determined by the turbidity 

 of the water and the consequent great diminution of the light penetrating to even 

 moderate depths. It has been shown that the light reaching a depth of 90 cm. has an 

 intensity of not more than 15 per cent of that of full sunlight, and that from 60 cm. to 

 1.2 m. there is a great decrease in the strength of the light. It will be observed that it 

 is just at these depths that the algae become scarcer and finally cease. 



This turbidity, however, besides affecting the amount of light, probably itself plays 

 a part in limiting the depth to which the algae may grow, since these will receive sedi- 

 ment from all the water above them, and so will receive more deposits the greater the 

 depth of the water covering them. 



It is worthy of note that, while the algae in the harbor grow to a depth of only 1.4 m., 

 those on the coral reef grow to a depth of 25.5 m. This is undoubtedly due to the 

 greater clearness of the water over this reef. All of the plants of Brongniartella, Dasya, 

 Grinnellia, and Nitophyllum gathered from this reef were exceedingly pale in color, 

 being much paler than plants of Dasya and Grinnellia growing in the harbor at the same 

 time or than plants of Brongniartella and Nitophyllum observed in summer. This pale 

 color may have been due to the weak light occurring at that depth or to a combination 

 of this and other factors, but we do not yet know enough about the color of algae to 

 venture an explanation. 



Except in the spring, the upper limit of the great majority of algae in this region is 

 determined by the height of the usual lowest low tides. Lyngbya confervoides, Hydro- 

 coleum, several undeterminable species of Myxophyceae, mats composed of minute plants 

 of Enteromorpha, Ulva, Chaetomorpha, and Cladophora, and plants of Fucus, Gelidium, 

 Gymnogongrus, and Actinococcus occur between tide lines; but, except for these 

 species and occasional plants growing in shaded or otherwise especially favorable loca- 

 tions, all algae occurring here in summer are strictly limited to the zone below low tide. 

 This is undoubtedly due to the intense insolation and heat to which the exposed plants 

 are subjected, the air temperature sometimes rising to 36 C. At the time of the spring 

 tides, when the range of tide is greatest, low tide occurs here about noon, so that all the 

 algae above low water are exposed to the sun during the hottest part of the day. Plants 

 of Gracilaria, Hypnea, Chondria, Herposiphonia, and Nitophyllum have frequently been 

 observed with a part or all of their thallus exposed by successive very low tides, and in 

 every case they had been killed to the level of the water. Dictyota, Padina, and Rosen- 

 vingea seemed slightly more resistant, since plants that had been similarly exposed 

 appeared uninjured in some cases, but at other times they too were killed to the water 

 level. While a single very low tide, caused by the wind, may kill the exposed parts of 

 the most tender species, it has little effect on the range of the algae, but the successive 



