MARINE BACTERIA AND THEIR RÔLE IN THE CYCLE OF LIFE IN THE SEA

III. THE DISTRIBUTION OF BACTERIA IN THE OCEAN WATERS AND MUDS ABOUT CAPE COD

A study has been made of the number of bacteria to be found in ocean waters and muds in the region of Cape Cod, Massachusetts, and their variation with distance from shore, depth of water, depth in the mud, and carbon content of the mud.

The number of bacteria in ocean water not near land and of a depth sufficient to preclude the agitation of the bottom by currents or wave motion was very small. One cubic centimeter portions of the water were often found to contain no bacteria capable of growing on an agar plate. A few samples of water were found to contain comparatively large numbers of bacteria, probably due to the presence of decomposing remains of some small plankton organism.

Sea water in shallow regions where the water was mixed from top to bottom, due to tidal or wave motions, had a consistently higher bacterial content, ranging from 20 to 200 cells per cubic centimeter. Such water had a higher content of mineral nutrients and consequently a richer plankton content. It is to the larger amount of organic matter thus made available to the bacteria together with an actual transport upward of cells by vertical currents that their increased abundance under these conditions is to be attributed.

On open, exposed shores receiving little land drainage, the direct effect of the land on the bacterial content of the sea water appears to be negligible. Along shores receiving considerable land drainage any effect of the proximity of land does not appear to extend beyond a mile from the shore so far as the number of bacteria in the sea is concerned. In harbors receiving sewage drainage, the number of bacteria may rise to very high levels, depending upon the amount of pollution introduced and the degree to which circulation may take place between the harbor water and the outer uncontaminated water.

Bacteria in the bottom deposits were much more numerous than in the overlying water. Muds contained many more bacteria than did sands. The number of bacteria decreased with depth into the mud, the most marked decrease being in the first 2.5 centimeters. Bacterial numbers in the surface layer, compared to those in the lower layers, were much greater than could be accounted for by the difference in organic carbon content. There appeared to be on the surface of the mud a very thin layer containing organic matter in a less advanced stage of decomposition supporting a bacterial growth much richer not only in numbers but also in types of bacteria than in the lower layers. The carbon content of the surface layer was higher than that of the lower layers. The vertical distribution of bacteria in the mud was not related to the carbon content, since the amount of carbon sometimes increased with depth, while the number of bacteria always decreased with depth.

The horizontal distribution of bacteria in the extreme surface layer of mud was independent of its carbon content, but probably depended upon its composition or the degree of its decomposition. The horizontal distribution of bacteria in the mud layers below the surface was directly related to their carbon content.

The number of bacteria in the surface mud on the continental shelf east of Cape Cod decreased regularly with distance from land. This was not the case with the bacteria in the deeper layers of mud, where the number remained fairly constant.

There was a wide variation in the bacterial content of all layers of mud between two sampling dates approximately one month apart. The fluctuations at the surface and at depths of 2.5 and 10 cm. in the mud were in opposite directions in mud from the open ocean off Cape Cod Light but were parallel in mud from Cape Cod Bay.