Microbial activity at the sediment-water interface in Halifax Harbor, Canada

Abstract

The sediment-water interface in Halifax Harbor supports a microbial population of 6.95 x 109 cells per g (dry weight). As determined by the standard technique of suspending subsamples in filtered seawater, the uptake of added glutamic acid by this population is 113.5 ng g (dry weight)-1 h-1. An alternate technique was developed to measure the heterotrophic activity of the interface over longer periods of time, using undisturbed cores with the sediment-water interface intact. Under these conditions, the microbes in the water column and the interface increased exponentially in number, with mean doubling times of 9.6 and 4.5 days, respectively. The uptake of glutamic acid by the microbial population of the interface was determined to be 12.7 ng g (dry weight)-' h-1, almost an order of magnitude less than the uptake determined by the previous method. This indicates that substrate diffusion and competition for substrate by the microbes in the water column are important factors when considering the heterotrophic activity of the sediment microbial population. After 48 h of incubation, uptake and respiration ceased, probably due to the exhaustion of labeled substrate. Additional substrate added after 48 h of incubation was taken up at a rate similar to that measured after the first addition. It appears that the microbial population of the interface is able to respond quickly and repeatedly to relatively large nutrient additions. After 10 days of incubation, the number of "viable" cells as determined by autoradiography was much smaller than the increase in numbers as determined by direct counts. Apparently a large part of the viable population is unaffected by nutrient addition.A previous study (13) has identified the sediment-water interface of a coastal marine bay as an area of intense microbial activity. Because of this activity, the interface must be considered as an important part of the sediment-water ecosystem. Unfortunately, few studies have investigated the microbiology of the marine sedimentwater interface (1, 2, 13), and, to my knowledge, only one such study has addressed the problem directly (13). Likewise, the freshwater sedimentwater interface has also not been studied. OlMh (17) examined the sediment-water interface of shallow lakes and found generally higher viable counts at the sediment surface than in the water column or deeper in the sediment but presented no data on microbial activity.To understand the microbial processes at the sediment-water interface and to determine what factors influence these processes, the present study was initiated. Specifically, it was the purpose of this study to characterize the microbiology of the interface and to examine a method to measure the heterotrophic activity of the microbial population of the interface. Since the interface is, by definition, composed of both sediment and water components, I decided to study the interface by keeping all components of the sediment-water system intact. By using techniques that use entire core samples, I attempted to collect data th...