The effect of water depth on bacterial numbers, thymidine incorporation rates and C : N ratios in northeast Atlantic surficial sediments

The effect of water depth on bacterial biomass and their ability to synthes ise DNA, by measuring their rate of [H-3]- thymidine incorporation, was inv estigated in the northeast Atlantic at three sites of varying water depth ( 1100-3580 m) and sediment characteristics. Thymidine incorporation rates (y ) in surficial sediments varied between 0.028 and 1.44 pmol h(-1) g(-1) and showed an exponential relationship with depth (x) according to the equatio n y = 2.05e(-00011x) (r=0.9830 for n=7. P<0.001). However, this relationshi p failed when a layer of phytodetritus was found overlying the surface sedi ment and [H-3]-thymidine incorporation rates increased by 80-339%. In contr ast, bacterial numbers varied between 1.09 and 11.96 x 10(8) cells g(-1) (d ry weight) and showed no significant relationships with water depth or sedi ment POC/TN content. Significant exponential relationships were also found between water depth (x) and the POC (y(1)) and total nitrogen (TN, y(2)) co ntent of surficial sediments according to the following equations: where y( 1)=719e(-0.0003x) (r=0.8700 for n=9, P<0.01) and y(2)=76e(-0.0002x) (r=0.75 83 for n=9 P<0.02). These relationships were irrespective of the presence o r absence of an overlying layer of phytodetritus. This suggests that the PO C and TN content of these surficial deep sea sediments is directly related to the flux of material through the water column, which significantly impac ts bacterial production.