Bacterial activity in Lough Neagh (N. Ireland) measured by the 3H-Thymidine incorporation method

Abstract

Bacterial activity in the water column and sediments of Lough Neagh, a large (387 km2) but relatively shallow (mean depth, 8.9 m) eutrophic lake, was investigated for two years by the tritiated thyrnidine incorporation method.

83-99% of planktonic bacteria were 3µm in diameter were a net substrate source, supplying 35% of the requirement of free bacteria. The depth distribution of bacterioplankton activity showed diurnal changes during spring and summer, influenced by light. Seasonal variation of bacterioplankton activity (16-460 pmol r 1 h-1) was remarkably uniform, both horizontally and vertically, at different sites. Substrate supply from cyanobacteria, temperature and grazing were each important controlling factors that contributed to large differences between 1989 and 1990.

The horizontal (km scale) and vertical (mm scale) variation in bacterial activity was more pronounced in sediments than in the water column. Sites included an erosive inshore area (5 m water depth), the main offshore basin (13m) and a deeper, depositional site (15 m). Whereas the thymidine method is in routine use in planktonic studies, recovery of labelled DNA from sediments is still problematic, because of the risk of unspecific binding. Therefore, three techniques were evaluated using different extraction pathways. In the erosive areas, bacterial activity was very variable, being affected by frequent settling and resuspension events. In contrast, depositional sites had considerable bioturbation, with associated intensive bacterial activity being maintained 30 cm below the sediment surface. On a whole lake basis, 40% of the yearly bacterial activity took place in the water body, 20% in the inshore sediments, 25% in the main lake basin sediments and 15% in the sheltered deeper sites. The implications for sediment/water interactions and links to other trophic levels are discussed.

Date of Award	Aug 1993
Original language	English
Supervisor	Roger Marchant (Supervisor)