Experiments were conducted to determine whether production of heterotrophic bacterioplankton in a small meso-eutrophic lake was influenced by the dissolved inorganic phosphorus (DIP) supply. DIP may indirectly limit bacterial production by limiting phytoplankton, which in turn may limit the carbon available to bacteria. Direct DIP limitation of bacteria occurs where the availability of DIP for bacteria is insufficient to maintain growth. This work examined direct DIP limitation of bacteria by removing phytoplankton and incubating flasks with or without added P in the dark. Bacterial production was measured via the rate of incorporation of [3lH]thymidine ([3HlTdR) into DNA. Bacterial abundance was followed with epifluorescent direct counts. Rates of [3HjTdR incorporation were significantly greater in flasks with added DIP, and changes in cell abundances generally paralleled increases in [3HjTdR incorporation. Even very small additions of P (0.05 ,uM) were sufficient to stimulate production. DIP addition to whole lakewater also stimulated [3H]TdR incorporation relative to that in zero-addition controls, but there was not a concurrent increase in bacterial cell numbers. The stimulation of [3H]TdR incorporation after DIP addition to whole lakewater was significantly less than the stimulation due to DIP addition to 1-jim-pore-size-filtered lakewater. In this study, addition of DIP caused as much as an eightfold stimulation of [3H]TdR incorporation.

Heterotrophic bacterioplankton production rates have been measured in many aquatic ecosystems over the last two decades, whereas measurements of bacterioplankton respiration rates have been scarce. This paper reports and discusses measurements of carbon-based plankton respiration rates made in a coastal ecosystem over an annual cycle. The coulometric technique was used to measure total inorganic carbon (TCO 2 ) production rates in 0.8-m filtered and unfiltered Massachusetts Bay surface seawater. Bacterioplankton respiration rates, defined as respiration in 0.8-m filtered seawater, varied from 0.01 to 0.15 mol C kg Ϫ1 h Ϫ1 (0.07 Ϯ 0.01 mol C kg Ϫ1 h Ϫ1 ) (mean Ϯ 1 SE) and accounted for a high proportion (median 70%) of respiration in unfiltered seawater. Microplankton (unfiltered seawater) respiration rates varied from 0.04 to 0.24). Bacterioplankton growth efficiencies (BGEs), estimated with concurrent measurements of bacterial production and respiration rates, varied from 0 to 69% (median 22%) and were well correlated with specific production rates (r 2 ϭ 0.67). To assess carbon flow in aquatic ecosystems, BGEs should be measured when possible because of their high variability. Compared with bacterioplankton specific production rates, bacterioplankton specific respiration rates were relatively constant (0.05 Ϯ 0.01 fmol CO 2 cell Ϫ1 h Ϫ1 ). Given the apparent uncertainty in the values of respiratory quotients (Toolan 1996; Robinson and Williams 1999), carbon-based comparisons of microbial respiration and primary production rates will improve evaluations of the role of the coastal ocean in the global carbon cycle.Relative to the plentiful literature on microbial productivity (growth rates of autotrophs and heterotrophs), respiratory activity in marine ecosystems has been understudied (Williams 1984;Biddanda et al. 1994;Jahnke and Craven 1995). Measurement of respiration rates in seawater has challenged oceanographers for decades because of the high precision required to measure small changes of 0.01-1 mol kg Ϫ1 h Ϫ1in O 2 or total inorganic carbon (TCO 2 ) relative to the naturally high background concentrations of O 2 (ϳ200 mol O 2 kg Ϫ1 ) and TCO 2 (ϳ2,000 mol C kg Ϫ1 ). Advances in endpoint detection and automation have increased the precision of the Winkler titration and made O 2 utilization rates in seawater measurable (Pomeroy et al. 1994 and references therein). It is now possible to estimate carbon-based respiration rates with the highly precise coulometric technique, wherein a seawater sample is acidified, and evolved CO 2 reacts with ethanolamine to form hydroxyethylcarbamic acid; the number of moles of OH Ϫ that are generated to titrate the acid is related by the Faraday constant to the product of current and

Massachusetts Water Resources Authority started adding sodium hypochlorite to its raw water (Wachusett Reservoir) in September 1997 to achieve compliance with the requirements of the Surface Water Treatment Rule for unfiltered surface water supplies, mainly the 3‐log C × T (concentration times time) value for Giardia inactivation. There were concerns about the chlorine dosage necessary to achieve pathogen inactivation and the effect of that dosage on the amount of disinfection by‐products (DBPs) formed. Weekly chlorine decay tests were initiated in April 1998 to gather information on chlorine decay so that the necessary parameters to produce an integrated C × T value could be developed (area under the chlorine decay curve). Wachusett Reservoir water quality is also affected by transfer of water from the Quabbin Reservoir, which has lower total organic carbon and UV254 absorbance levels. A model was developed to describe the rate constant as a function of hydroxide concentration (taking both pH and temperature effects into account through the ion product of water), UV254 absorbance, and chlorine dose only. The availability of kinetic parameters allows development of a method for calculating C × T achievement for primary disinfection. Models for DBP formation will be presented in another article. These models together allow for better determination of the necessary chlorine dosage to achieve the required C × T value and minimize DBP formation.