Responses of bacterioplankton and phytoplankton to organic carbon and inorganic nutrient additions in contrasting oceanic ecosystems

Da Caron,Mr Dennett,Rw Sanders,El Lim,Ug Berninger

doi:10.3354/ame022175

Abstract

Experiments were carried out on Georges Bank, a productive coastal region in the northwestern sector of the North Atlantic Ocean, and in the oligotrophic western Sargasso Sea to examine the effects of nutrient (inorganic nitrogen and phosphorus) and organic carbon (glucose) additions on bacterial and phytoplankton growth. Four experiments were conducted in each environment. Phytoplankton growth was monitored over a 36 h period by following changes in the concentration of chlorophyll in unfiltered seawater and in seawater prefiltered through 5 μm screening to reduce grazing pressure. Bacterial production was estimated initially and after 24 h using the 3H-thymidine (TdR) method in unfiltered seawater and in 1 μm filtrate. Phytoplankton biomass increased significantly in response to nutrient additions in all but 1 experiment, whereas chlorophyll concentrations remained unchanged or decreased in all of the unamended (control) treatments or treatments supplemented with glucose. Responses of the phytoplankton community were similar for the <5 μm and unfiltered treatments. Bacterial production increased after 24 h in all of the treatments on Georges Bank, and there was little effect of nutrient or glucose addition in unfiltered seawater relative to unamended controls. However, glucose addition to the <1 μm filtrate caused substantial increases in bacterial production relative to controls and N/P-amended treatments in 2 of the experiments from this environment. Glucose had no stimulatory effect (relative to unamended treatments) in 3 of the 4 Sargasso Sea experiments, and only a marginal effect in the fourth. However, the addition of inorganic nitrogen and phosphorus in the latter ecosystem resulted in higher bacterial production (relative to unamended treatments or glucose addition) in 2 of the experiments with unfiltered seawater, and very large increases in 3 of the experiments with 1 μm filtrate. The magnitude of the changes in bacterial production differed greatly between unfiltered and filtered seawater in both ecosystems, indicating an important role for bacterial grazers in controlling bacterial population growth. The results of this study indicate different nutritional restraints on bacterial production in these contrasting environments.

Highlights

IntroductionAquat Microb Ecol 22: 175–184, 2000 rial growth rates are dictated by available organic carbon (energy for growth) has been challenged in recent years, and nitrogen and phosphorus have been recognized as elements whose scarcity might restrain bacterial production in some aquatic ecosystems (Caron 1994, Kirchman 1994, Elser et al 1995, Rivkin & Anderson 1997)
Conventional wisdom among plankton biologists has evolved during the past decade regarding the factors that limit primary and bacterial production in theAquat Microb Ecol 22: 175–184, 2000 rial growth rates are dictated by available organic carbon has been challenged in recent years, and nitrogen and phosphorus have been recognized as elements whose scarcity might restrain bacterial production in some aquatic ecosystems (Caron 1994, Kirchman 1994, Elser et al 1995, Rivkin & Anderson 1997)
Experiments were carried out on Georges Bank, a productive coastal region in the northwestern sector of the North Atlantic Ocean, and in the oligotrophic western Sargasso Sea to examine the effects of nutrient and organic carbon additions on bacterial and phytoplankton growth

Summary

Introduction

Aquat Microb Ecol 22: 175–184, 2000 rial growth rates are dictated by available organic carbon (energy for growth) has been challenged in recent years, and nitrogen and phosphorus have been recognized as elements whose scarcity might restrain bacterial production in some aquatic ecosystems (Caron 1994, Kirchman 1994, Elser et al 1995, Rivkin & Anderson 1997) Fueled by these insights, there is a growing recognition that the factors limiting productivity of bacteria and phytoplankton in the ocean are likely to change over a variety of spatial and temporal scales. The cellular stoichiometry of bacteria appears to be less flexible than the carbon:nutrient ratios for some plankton (Caron 1990) These results imply that the overall C:N:P ratio of bacterial substrate(s) in nature must be relatively low in order to meet bacterial N and P demand (Goldman et al 1987, Goldman & Dennett 1991). This prediction must be tempered by considerations of bacterial growth efficiency because carbon incorporation dictates nitrogen and phosphorus demand by the growing cells (Coffin et al 1993, del Giorgio & Cole 1998)

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