ZoBell . 96 — Marine Microbiology 



Henrici and McCoy (1938) found that bacterial counts from littoral 

 stations occupied by aquatic plants are much higher than profundal ones 

 where there is little or no vegetation. In general, a correspondence was 

 observed between the abundance of bacteria in lake bottoms and the pri- 

 mary productivity of lakes. Primary productivity is principally due to 

 photosynthetic organisms, most of which are phytoplankton. The most 

 intense bacterial activity in the sea observed by ZoBell and Feltham 

 (1942) was in surface layers of sediments in mud flats where photosyn- 

 thetic organisms are active. 



The effect of vegetation on the abundance of bacteria in bottom de- 

 posits is shown by the investigations of Henrici (1939) in Lake Alexander 

 in Minnesota. He found the following numbers of bacteria per ml. at 

 different stations described according to the predominating vegetation: 



Station i, Phytoplankton of open lake 93,45° 



Station 2, Dense mat of Ceratophyllum, Naias, Elodea, and Potamogeton 278,600 



Station j, Bottom carpeted with Char a 126,100,000 



Station 4, Potamogeton, Elodea, Myriophyllum, etc. 150,000,000 



Station 5, Transition between Stations i and 4 1,103,400 



From these and similar observations, Henrici (1939) concluded that 

 "probably the most important factor determining the shoreward distribu- 

 tion of bacteria is the occurrence of aquatic plants. Where these are 

 abundant, bacteria are numerous. Mere shallowness of the water is not 

 an important factor. It is probable that the increase in bacteria in the 

 vicinity of aquatic plants is due to larger amounts of organic matter de- 

 rived from these plants." 



Large numbers of bacteria are ingested by mud-dwelling animals, but it 

 is diihcult to estimate what effects predatory animals may have on the 

 bacterial population of bottom deposits. Although bacteria may consti- 

 tute an appreciable proportion of the food of mud-eating animals (Mac- 

 GiNiTiE, 1935; Baier, 1935), the presence of such animals may in turn 

 enhance the multiplication of bacteria. As a rule the largest bacterial 

 populations are found in bottom deposits richest in fauna. It is problem- 

 atical whether this is a cause or an effect, or whether environments which 

 promote the growth of animals are also beneficial to bacteria. But it is 

 clearly indicated that in spite of their ingestion of bacteria, bacterivorous 

 animals do not restrict the bacterial population of bottom deposits. 



In seeking to appraise quantitatively the importance of bacteria as an 

 ecological factor. Mare (1942) estimated from plate counts that there are 

 from 0.3 to 2 mgm. of living bacteria per gram of marine mud (dry basis). 

 All other organisms, both plant and animal, amounted to about 1 7 mgm. 

 per gram of mud. In a square meter of mud to the stated depth she esti- 

 mated the relative weights of organisms as f oUows : 



Large macrobenthos in topmost 10 cm. 75- 00 grams 



Small macrobenthos in topmost 5 cm. 2>2 ■ 0° grams 



Meiobenthos in topmost o. 5 cm. 1. 15 grams 



Microbenthic fauna in topmq^t 0.5 cm. 0.02 grams 



Bottom diatoms in topmost 0.5 cm. 0.05 grams 



Bacteria in topmost 0.5 cm. 0.36 grams 



All groups of organisms except the bacteria were enumerated by direct 

 counts. The bacteria were enumerated by plate counts employing Reus- 

 zer's (1933) medium. Since plate counts detect only i to 10 per cent of 

 living bacteria, the estimated weight of bacteria should probably be mul- 

 tiplied by a factor of between lo and loo in order to give comparable data. 



