24 Papers from the Marine Biological Laboratory at Tortugas. 



After 131 hours a faint cloud, scum, and bubbles developed in the culture; very faint 



nitrite and faint ammonia reactions were given. 

 After 146 hours a faint cloud and scum, no nitrite or nitrate, and very slight ammonia 



reactions were given. 



Twenty cultures were made from samples of water taken well out to sea 

 from Port Royal, and the process of denitrification followed through with 

 each. All gave very similar and consistent results, but the rate of denitri- 

 fication decreased rapidly with the temperature at which the cultures were 

 grown; thus at an average temperature of 27° C. the first trace of the nitrite 

 reaction appeared after about 40 hours, and denitrification was complete 

 after about 100 hours. 



The results of precisely similar experiments that I made with samples of 

 water taken from the English Channel near Plymouth in the autumn of 

 1909 showed that there the process of denitrification was very much slower, 

 and was never complete at the room temperature (17° C). The first trace 

 of the formation of a nitrite in cultures in the modified Gran's medium, as 

 detected by the metaphenylene diamine reaction, occurred about the fifth 

 day, and a large proportion of the nitrite and nitrate always remained, even 

 in the oldest cultures. In similar cultures incubated at 30° C, denitrification 

 was complete by the eighth day at earliest, but uniformly consistent results 

 were not obtained, as in some of the cultures complete denitrification was 

 never obtained, even after several months. 



It would thus appear that even under similar temperature conditions 

 the marine bacteria in the seas off Jamaica are much more active in causing 

 denitrification than those found in the English Channel, and since the rate 

 of denitrification is a function of the temperature, it follows all the more that 

 the destruction of nitrates by bacterial agency in the seas around Jamaica 

 must be far in excess of that occurring in the cooler waters of the English 

 Channel. 



THE INVESTIGATION OF SAMPLES OF SEA-WATER TAKEN AROUND THE 



DRY TORTUGAS. 



The Dry Tortugas consist of a group of eight small keys, the largest of 

 which (Loggerhead Key) is only about three-quarters of a mile long by one- 

 eighth of a mile wide. They are situated about 150 miles from the main- 

 land of Florida, and form the extreme western end of the chain of the 

 Florida Keys. The lOO-fathom line lies some 30 miles to the south and 

 southwest of the islands, and then trends round in a northwest direction; 

 beyond the lOO-fathom line the depth increases with moderate rapidity 

 until depths of from 1,000 to 1,400 fathoms are reached. To the east, 

 northeast, and north as far as the coast of Florida, the water is shallow, 

 the soundings showing from 20 to 30 fathoms in most places. Beyond the 

 loo-fathom line to the southward the influence of the Gulf Stream begins 

 to make itself felt, though the region of maximum current velocity here lies 

 nearer the coast of Cuba. The Tortugas Keys are of purely coral forma- 

 tion, consisting entirely of broken shell and coral sand, with no soil. The 



