AQUATIC MICROBIAL ECOLOGY Vol. 20: 245-260,1999 Published December 30 Aquat Microb Ecol 1 Bacterioplankton intra-annual variability: importance of hydrography and competition Johan Wikner1g2f*, he~agstrorn~ 'Department of Microbiology, Umed University, 901 87, UmeB, Sweden 'Umed Marine Sciences Centre, Umed University, Norrbyn, 910 20, Hornefors, Sweden 3Department of Marine Science, Kalmar University, Box 905,398 29, Kalmar, Sweden ABSTRACT: Field data from a 1.5 yr intensive study of 1 coastal (0 to 20 m) and 2 offshore stations (0 to 100 m) in the northern Baltic were analysed. Specific interest was paid to the difference in the spatiotemporal variation of bacterioplankton and its controlling factors. Less than 31 % of the annual bacterial biomass production (Pb)occurred in the photic zone during the productive season at the off- shore stations. This suggested an uncouphg between Pb and phytoplankton carbon fixation, which was further supported by the lack of a significant correlation between these variables in the photlc zone. The basin w~thhigh allochthono'us load~ngand long residence time showed hlgh P, relative to autochthonous carbon fixation and low vanance of P, and bacterial abundance (N,), suggesting an important contribution of terrestrial dissolved organic carbon to the carbon and energy supply. Bacter- ial per capita growth rate (r,) was highest dunng spring, whlle P, was highest during summer at all sta- tions. The seasonal variation in Pbwas mainly explained by variation in the r,, rather than in N,.A pos- itive correlation of N, with temperature, and a negatlve correlation wirn salinlry, suggesrea tnar >OL70 of the seasonal variation in Nb was a consequence of the formation of a stratified photic zone with a higher carrying capacity. Temperature limitation of r, only occurred in the stratified photic zone, sug- gesting that other growth factors were sufficient during this period. A density Lirmtation of the maxi- mum r, was observed at all stations during autumn and winter in both depth layers, suggesting competition to be of periodic importance. Bacterioplankton with a low r (intrinsic growth rate) and high K (carrying capacity) strategy dominated when sedimenting particles were a major resource in the aphotic zone, while the opposite strategy dominated during winter at low cell densities, when dissolved substrates were the major resource. KEY WORDS: Estuarine. Pelagic . Bacteria. Growth . Abundance . Seasonal. Temperature . Salinity. Control . Competition INTRODUCTION tion between bacterial and phytoplankton variables within studied ecosystems is in contrast to, but does not Intra-annual temperature control of bacterioplank- contradict, the observation that these variables corre- ton biomass and its production has been derived from late in inter-ecosystem comparisons (Cole et al. 1988, several field studies based on observed correlations White et al. 1991). However, it suggests that phyto- (Findlay et al. 1991, White et al. 1991, Ducklow & plankton production and biomass may not be directly Shiah 1993, Hoch & Kirchman 1993, Turner & Borkman responsible for controlling bacterioplankton variability 1993). In contrast to this evidence, correlations be- on the intra-annual and intra-ecosystem scale. This tween phytoplankton carbon fixation and bacterial may be especially important in estuarine ecosystems, biomass production are lacking. Neither has a clear where supply of allochthonous dissolved organic car- relationship between bacterial abundance and chloro- bon (DOC) may uncouple bacterial biomass productiv- phyll a been demonstrated. The lack of a clear correla- ity from phytoplankton carbon fixation. Lower spa- tiotemporal association between bacterial variables 'E-mail: [email protected] and phytoplankton carbon fixation on the seasonal 0 Inter-Research 1999 246 Aquat Microb Ecol20: 245-260, 1999 The control of P, is expected to be a combination of factors affecting either rb or Nb. Limitation of substrate or temperature would primarily affect rb, while, for SWEDEN example, predator or parasite activity, via its effect on Nb, also would influence the control of Pb (Thingstad & Sakshaug 1990). Evidence for the occurrence of resource competition in natural waters has been inferred from inter-ecosys- tem comparisons (Billen et al. 1990), but has not been extensively elucidated on the intra-annual scale in the current Literature. Other biological factors that may FINLAND control the intra-annual variation in bacterial abun- dance include bacterivores, who may annually graze the bacterioplankton production (Wikner & Hagstrom 1991), and bacteriophages (Proctor & Fuhrman 1990). The relative importance of bacterivores and bacterio- phages for the intra-annual variation in bacterial abun- dance is, however, unclear and is expected to vary with season (Psenner & Somrnaruga 1992). In this study a field data set covering 18 mo and 0 to 100 m depths from 3 sites with different water ex- change and freshwater discharge was investigated. We hypothesised that an environment with a high sup- Fig. 1. Sampling stations In the Gulf of Rothnla, northern ply of allochthonous DOC and long residence time Baltic. Station specifications are given in the 'Methods' would show lower seasonal variability and clearer decoupling from phytoplankton if allochthonous DOC was a carbon and energy source of importance. Nat- scale would thus be expected where allochthonous ural patterns of bacterial community biomass produc- DOC is an important source of materials and energy. tivity and abundance were further analysed with the In view of the importance of bacterioplankton in the aim of finding controlling factors. Samples were com- carbon cycle (Cole et al. 1988, Hagstrom et al. 1988), pared on the scale of individual sampling bottles (i.e. interpretation of natural patterns should constitute an 5 l scale). We present different associations suggesting important step towards an understanding of the con- controlling or limiting factors for the bacterial intrinsic trol of bacterioplankton. Studies on the ecosystem rate of growth and abundance. level in this context have the advantage of including the multitude of trophic interactions and controlling factors occurring in natural environments (Pace 1998). METHODS Patterns from ecosystem studies may indicate factors of importance for the control of bacterioplankton at the Stations and hydrographic measurements. The sam- seasonal scale. At the same time the complexity of nat- pled stations were located in Bothnian Bay (northern ural environments imposes careful interpretation as basin, 64" 42.50' N, 22" 04.00' E; Stn A13, Helsinki different environmental factors may be indirectly cor- Commission standard nomenclature), the Bothnian related, w~thouta causal relationship. Associations Sea (southern basin, 62" 35.20' N, 19" 58.32' E; Stn. Cl) may further be limited to certain seasons or environ- and the Ore estuary (northern Bothnian Sea, 63" 30.50' mental conditions and thus obscured in data sets N, 19" 48.00' E; Stn B3) (Fig. 1). The sites differ in mor- aggregated on larger scales. phometric, hydrographic and geochemical characteris- A first step towards an understanding of the control of tics (Wulff & Rahm 1990).The Bothnian Bay has double bacterial biomass production (Pb)is to seek associations the river supply of DOC and a longer residence time between the intrinsic rate of increase of each bacterial than the Bothnian Sea (Table 1).The Ore estuary has a cell (i.e.bacterial specific growth rate), rb,and the bac- more than 10-fold higher supply of riverine DOC, but a tenal dens~ty,Nh, and potential controlling factors in much shorter residence time, primarily through ex- field data sets. Ph is according to ecological theory (Be- change with the main basin of the Bothnian Sea (Fors- gon et al. 1988) the product of rh and Nh by the equation: gren & Jansson 1993). Wikner & Hagstrom: Bacterioplankton ~ntra-=~nnualvariability 247 Table 1. Morphometric and hydrographic charactenstics of the sampling sites. et al. 1977). At least 10 fields or 300 D,,,,: mean depth of the basins; W the volume of the basins; Q,,: the annual cells were counted per sample, freshwater discharge; TR: the residence time of the basins; Qnoc: the annual DOC dscharge (for a water column of mean depth). Morphometric and fresh- at +SE 15%. water discharge values are from HELCOM (1990) and Forsgren & Jansson The standard (1992).Discharge of DOC is based on data from Pettersson (1992) sampling bottles representing the nat- ural variability was estimated to aver- Location D.,,, QDCSC ageBacterial 25 U/o (data secondary not shown). production. 1Bothnian Bay Bacterial growth was estimated with Ore estuary 10 1.2 0.027 208 the thymidine incorporation technique Bothnian Sea 68 4308 105 2.8 7.7 essentially according. to Fuhrman & Azam (1982) on samples collected at 0, 4, 8, 14, 20, 40, 60, 80 and 100 m Sampling was performed every second week during depth. [3H-methyl]-thymidinewas added to triplicate April to September and approximately monthly during (partly duplicate) 5 m1 samples and controls at a final the rest of the year. Samples were taken at discrete concentration of 24 nM, determined to give a saturat- depths with 5 1 Niskin-type bottles with Teflon seals ing concentration of thymidine. Controls were pre- (Hydro-Bios).The vertical distribution of samples was treated with 0.25 m1 5 M NaOH for 10 min. Samples 0, 4, 8, 14, 20, 40, 60, 80 and 100 m (20 m maximum and controls were incubated 1 h at the in situ tempera- depth at the coastal station) on most sampling occa- ture in the euphotic zone and aphotic zone, respec- sions. tively. Incubations were terminated by adding 0.25 m1 Temperature, salinity and pH. Temperature and 5 M NaOH and then precipitated with ice-cold 10% salinity were measured continuously through the TCA. The precipitate was collected on 0.2 pm polycar- water column with a calibrated CTD-probe UCM 40 bonate filters (MSI) and washed repeatedly with 5% MkII (Simtronix Environmental Monitoring a/s, Nor- trichloroacetic acid, with and without the lid of the way). multi-filter unit. Dried filters were counted in the scin- Light irradiance.