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

Abstract

The effect of water depth on bacterial biomass and their ability to synthesise DNA, by measuring their rate of [3H]-thymidine incorporation, was investigated 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 equation y = 2.05e−0.0011x (r=0.9830 for n=7, P<0.001). However, this relationship failed when a layer of phytodetritus was found overlying the surface sediment and [3H]-thymidine incorporation rates increased by 80–339%. In contrast, bacterial numbers varied between 1.09 and 11.96 × 108 cells g−1 (dry weight) and showed no significant relationships with water depth or sediment POC/TN content. Significant exponential relationships were also found between water depth (x) and the POC (y 1)and total nitrogen (TN, y 2) content of surficial sediments according to the following equations: where y l = 719e −0.0003x (r=0.8700 for n=9, P<0.01) and y 2 = 76e−0.0002x(r=0.7582 for n=9 P<0.02). These relationships were irrespective of the presence or absence of an overlying layer of phytodetritus. This suggests that the POC and TN content of these surficial deep sea sediments is directly related to the flux of material through the water column, which significantly impacts bacterial production.Key wordsbacteria numbers[3H]-thymidine incorporationdeep seasurficial sedimentphytodetritusnortheast Atlantic