Primary production under hypertrophic conditions and its relationship with bacterial production

In shallow hypertrophic lakes where light availability restricts the growth of macrophytes and benthic phytoplankton, pelagic phytoplankton modulates importantly ecosystem production and the energy transfer to heterotrophic bacteria. Diel and seasonal variations in primary production (PP) were studied in the hypertrophic Albufera de Valencia (Spain). Additionally, the relationship between PP and heterotrophic bacterial production (BP) was assessed. PP was extremely high, exceeding most values reported for hypertrophic lakes to date. PP displayed marked diurnal variations defined by the solar radiation curve. Likewise, PP changed importantly across seasons. Minimum PP coincided with maximum water transparency and short water residence times in winter, whereas maximum PP was observed in late spring associated with high chlorophyll a. The spring PP maximum contrasted with the summer maximum often observed in hypertrophic lakes. When compared to spring PP values, summer PP values were lower as a result of strong nitrogen limitation. In contrast to PP, BP remained fairly constant across seasons. Nonetheless, there was a joint diminution during increased water transparency followed by an increase in early spring. Phytoplankton was always the most relevant input to particulate carbon production, but the BP/PP ratio showed clear seasonal variations. The BP/PP ratio was minimum in spring, low in summer and highest in winter. The extracellular dissolved organic carbon released by phytoplankton was sufficient to meet bacterial carbon demand in all experimental dates, suggesting that allochthonous carbon sources play a minor role in sustaining BP, though they cannot be excluded. However, we hypothesize that high availability of dissolved organic carbon might explain the lack of coupling observed between BP and PP.     

Under‐ice metabolism in a shallow lake in a cold and arid climate

相似文献   

Effects of Microscale Water Level Fluctuations and Altered Ultraviolet Radiation on Periphytic Microbiota

Abstract Microscale fluctuations in water level (1–20 mm) are common on a diurnal basis in shallow (<5–10 cm) wetlands, coupled to evapotranspiration losses during the daytime in excess of groundwater resupply. These depth variations alter the intensity of UV irradiance reaching attached periphytic algal and bacterial microbial communities. Effects of alterations of UV irradiance by micro-changes in water level on periphytic microbiota were examined experimentally. Attached microbial communities, grown on glass fiber filters in situ in a natural wetland, were exposed experimentally to near-natural levels of UV irradiance of differing spectral quality. UV intensity was altered by varying the distance of the communities from the light source, changes in UV-attenuating natural dissolved organic matter (DOM), and small changes in water level (2 or 4 mm). Algal productivity and photosynthetic oxygen production were significantly reduced by small enhancements of UV-B radiation, by decreased water levels of only 2 mm, and by reductions in concentrations of DOM. UV-B had only small short-term effects on chlorophyll a, although small increases in water depth and DOM concentration reduced pigment damage. Experimental removal of UV-B during in situ growth indicated that algae could adapt to UV radiation during growth in natural environments. Microbial oxygen consumption and bacterial productivity and biomass were also lowered significantly by UV-B exposure, and damage decreased with small (2 mm) increases in water depth or in DOM concentration. Selective inhibitors of algal photosynthesis and production of released extracellular organic substrates caused a concomitant reduction in bacterial productivity and a significant increase in magnitude of UV-B damage to bacterial biomass. These effects suggested that metabolic interactions between the periphytic autotrophs and heterotrophs altered community responses to UV-B radiation. Microscale water level reductions, common on a diurnal basis in shallow wetlands, and associated increased UV intensity can result in rapid alterations in periphytic metabolism. Received: 27 January 1999; Accepted: 18 May 1999     

Bacterial Growth on Photochemically Transformed Leachates from Aquatic and Terrestrial Primary Producers

Abstract We measured bacterial growth on phototransformed dissolved organic matter (DOM) leached from eight different primary producers. Leachates (10 mg C liter⁻¹) were exposed to artificial UVA + UVB radiation, or kept in darkness, for 20 h. DOM solutions were subsequently inoculated with lake water bacteria. Photoproduction of dissolved inorganic carbon (DIC), ranging from 3 to 16 μg C liter⁻¹ h⁻¹, and changes in the absorptive characteristics of the DOM were observed for all leachates upon UV irradiation. The effects of irradiation exposure on DOM bioavailability varied greatly, depending on leachate and type of bacterial growth criterion. Bacterial carbon utilization (biomass production plus respiration) over the entire incubation period (120 h) was enhanced by UV radiation of leachate from the terrestrial leaves, relative to carbon utilization in non-irradiated leachates. Conversely, carbon utilization was reduced by radiation of the leachates from aquatic macrophytes. In a separate experiment, the stable C and N isotope composition of bacteria grown on irradiated and non-irradiated DOM was estimated. Bacterial growth on UV-irradiated DOM was enriched in ¹³C relative to the bacteria in the non-irradiated treatments; this result may be explained by selective assimilation of photochemically produced, isotopically enriched labile compounds. Received: 17 February 2000; Accepted: 1 May 2000; Online Publication: 28 August 2000     

Nitrogen effects on the pelagic food web are modified by dissolved organic carbon

Global environmental change has altered the nitrogen (N) cycle and enhanced terrestrial dissolved organic carbon (DOC) loadings to northern boreal lakes. However, it is still unclear how enhanced N availability affects pelagic food web efficiency (FWE) and crustacean zooplankton growth in N limited boreal lakes. Here, we performed in situ mesocosm experiments in six unproductive boreal Swedish lakes, paired across a DOC gradient, with one lake in each pair fertilized with N (2011: reference year; 2012, 2013: impact years). We assessed how zooplankton growth and FWE were affected by changes in pelagic energy mobilization (PEM), food chain length (phytoplankton versus bacterial production based food chain, i.e. PP:BP), and food quality (seston stoichiometry) in response to N fertilization. Although PP, PEM and PP:BP increased in low and medium DOC lakes after N fertilization, consumer growth and FWE were reduced, especially at low DOC—potentially due to reduced phytoplankton food quality [increased C: phosphorus (P); N:P]. At high DOC, N fertilization caused modest increases in PP and PEM, with marginal changes in PP:BP and phytoplankton food quality, which, combined, led to a slight increase in zooplankton growth and FWE. Consequently, at low DOC (<12 mg L^?1), increased N availability lowers FWE due to mismatches in food quality demand and supply, whereas at high DOC this mismatch does not occur, and zooplankton production and FWE may increase. We conclude that the lake DOC level is critical for predicting the effects of enhanced inorganic N availability on pelagic productivity in boreal lakes.     

Experimental evidence that terrestrial carbon subsidies increase CO<Subscript>2</Subscript> flux from lake ecosystems

Subsidies are donor-controlled inputs of nutrients and energy that can affect ecosystem-level processes in a recipient environment. Lake ecosystems receive large inputs of terrestrial carbon (C) in the form of dissolved organic matter (DOM). DOM inputs may energetically subsidize heterotrophic bacteria and determine whether lakes function as sources or sinks of atmospheric CO₂. I experimentally tested this hypothesis using a series of mesocosm experiments in New England lakes. In the first experiment, I observed that CO₂ flux increased by 160% 4 days following a 1,000 m C addition in the form of DOM. However, this response was relatively short lived, as there was no effect of DOM enrichment on CO₂ flux beyond 8 days. In a second experiment, I demonstrated that peak CO₂ flux from mesocosms in two lakes increased linearly over a broad DOM gradient (slope for both lakes=0.02±0.001 mm CO₂·m^–2 day^–1 per m DOC, mean±SE). Concomitant changes in bacterial productivity and dissolved oxygen strengthen the inference that increasing CO₂ flux resulted from the metabolism of DOM. I conducted two additional studies to test whether DOM-correlated attributes were responsible for the observed change in plankton metabolism along the subsidy gradient. First, terrestrial DOM reduced light transmittance, but experimental shading revealed that this was not responsible for the observed patterns of CO₂ flux. Second, organically bound nitrogen (N) and phosphorus (P) accompanied DOM inputs, but experimental nutrient additions (without organic C) caused mesocosms to be satuated with CO₂. Together, these results suggest that C content of terrestrial DOM may be an important subsidy for freshwater bacteria that can influence whether recipient aquatic ecosystems are sources or sinks of atmospheric CO₂.     

Regulation of Bacterial Growth Rates by Dissolved Organic Carbon and Temperature in the Equatorial Pacific Ocean

Abstract The effect of dissolved organic matter (DOM) and temperature on bacterial production was examined in the equatorial Pacific Ocean. Addition of glucose, glucose plus ammonium, or free amino acids stimulated bacterial production ([³H]thymidine incorporation), whereas changes in bacterial abundance were either negligible or much less than changes in bacterial production. The average bacterial growth rate also greatly increased following DOM additions, whereas in contrast, addition of ammonium alone never affected production, bacterial abundance, or growth rates. Since the large glucose effect was not observed in previous studies of cold oceanic waters, several experiments were conducted to examine DOM-temperature interactions. These experiments suggest that bacteria respond more quickly and to a greater extent to DOM additions at higher temperatures, which may explain apparently conflicting results from previous studies. We also examined how temperate affects the kinetic parameters of sugar uptake. Maximum uptake rates (V_max) of glucose and mannose increased with temperature (Q₁₀= 2.4), although the half-saturation constant (K_m) was unaffected; K_m+ S was roughly equal to glucose concentrations (S) measured by a high pressure liquid chromographic technique. Bacterial production and growth rates appear to be limited by DOM in the equatorial Pacific, and thus bacterial production follows primary production over large spatial and temporal scales in this oceanic regime, as has been observed in other aquatic systems. Although temperature may not limit bacterial growth rates in the equatorial Pacific and similar warm waters, it could still affect how bacteria respond to changes in DOM supply and help set steady-state DOM concentrations. Received: 26 July 1995; Revised: 19 January 1996     

Bioclimatic influence on water chemistry and dissolved organic matter in shallow temperate lakes of Andean Patagonia: A gradient approach

相似文献   

Chemical and optical changes in freshwater dissolved organic matter exposed to solar radiation

We studied the chemical and optical changes inthe dissolved organic matter (DOM) from twofreshwater lakes and a Sphagnum bog afterexposure to solar radiation. Stable carbonisotopes and solid-state ¹³C-NMR spectraof DOM were used together with optical andchemical data to interpret results fromexperimental exposures of DOM to sunlight andfrom seasonal observations of two lakes innortheastern Pennsylvania. Solar photochemicaloxidation of humic-rich bog DOM to smaller LMWcompounds and to DIC was inferred from lossesof UV absorbance, optical indices of molecularweight and changes in DOM chemistry. Experimentally, we observed a 1.2 enrichment in ¹³$C and a 47% loss in aromaticC functionality in bog DOM samples exposed tosolar UVR. Similar results were observed inthe surface waters of both lakes. In latesummer hypolimnetic water in humic LakeLacawac, we observed 3 to 4.5enrichments in ¹³C and a 30% increase inaromatic C relative to early spring valuesduring spring mixing. These changes coincidedwith increases in molecular weight and UVabsorbance. Anaerobic conditions of thehypolimnion in Lake Lacawac suggest thatmicrobial metabolism may be turning overallochthonous C introduced during springmixing, as well as autochthonous C. Thismetabolic activity produces HMW DOM during thesummer, which is photochemically labile andisotopically distinct from allochthonous DOM orautochthonous DOM. These results suggest bothphotooxidation of allochthonous DOM in theepilimnion and autotrophic production of DOM bybacteria in the hypolimnion cause seasonaltrends in the UV absorbance of lakes.     

CO2 Saturation and Trophic Shift Induced by Microbial Metabolic Processes in a River-Dominated Ocean Margin (Tropical Shallow Lagoon,Chilika, India)

An effort has been made for the first time in Asia's largest brackish water lagoon, Chilika, to investigate the spatio-temporal variability in primary productivity (PP), bacterial productivity (BP), bacterial abundance (BA), bacterial respiration (BR) and bacterial growth efficiency (BGE) in relation to partial pressure of CO₂ (pCO₂) and CO₂ air–water flux and the resultant trophic switchover. Annually, PP ranged between 24 and 376 µg C L^?1 d^?1 with significantly low values throughout the monsoon (MN), caused by light limitation due to inputs of riverine suspended matter. On the contrary, BP and BR ranged from 11.5 to 186.3 µg C L^?1 d^?1 and from 14.1 to 389.4 µg C L^?1 d^?1, respectively, with exceptionally higher values during MN. A wide spatial and temporal variation in the lagoon trophic status was apparent from BP/PP (0.05–6.4) and PP/BR (0.10–18.2) ratios. The seasonal shift in net pelagic production from autotrophy to heterotrophy due to terrestrial organic matter inputs via rivers, enhanced the bacterial metabolism during the MN, as evident from the high pCO₂ (10,134 µatm) and CO₂ air–water flux (714 mm m^?2 d^?1). Large variability in BGE and BP/PP ratios especially during MN led to high bacteria-mediated carbon fluxes which was evident from significantly high bacterial carbon demand (BCD >100% of PP) during this season. This suggested that the net amount of organic carbon (either dissolved or particulate form) synthesized by primary producers in the lagoon was not sufficient to satisfy the bacterial carbon requirements. Lagoon sustained low to moderate autotrophic–heterotrophic coupling with annual mean BCD of 231% relative to the primary production, which depicted that bacterioplankton are the mainstay of the lagoon biogeochemical cycles and principal players that bring changes in trophic status. Study disclosed that the high CO₂ supersaturation and oxygen undersaturation during MN was attributed to the increased heterotrophic respiration (in excess of PP) fuelled by allochthonous organic matter. On a spatial scale, lagoon sectors such as south sector, central sector and outer channel recorded “net autotrophic,” while the northern sector showed “net heterotrophic” throughout the study period.     

Primary production of aquatic macrophytes and their epiphytes in two shallow lakes (Peipsi and Võrtsjärv) in Estonia

In shallow lakes with large littoral zones, epiphytes and submerged macrophytes can make an important contribution to the total annual primary production. We investigated the primary production (PP) of phytoplankton, submerged macrophytes, and their epiphytes, from June to August 2005, in two large shallow lakes. The production of pelagic and littoral phytoplankton and of the dominant submerged macrophytes in the littoral zone (Potamogeton perfoliatus in Lake Peipsi and P. perfoliatus and Myriopyllum spicatum in Lake Võrtsjärv) and of their epiphytes was measured using a modified ¹⁴C method. The total PP of the submerged macrophyte area was similar in both lakes: 12.4 g C m^?2 day^?1 in Peipsi and 12.0 g C m^?2 day^?1 in Võrtsjärv. In Peipsi, 84.2% of this production was accounted for by macrophytes, while the shares of phytoplankton and epiphytes were low (15.6 and 0.16%, respectively). In Võrtsjärv, macrophytes contributed 58%, phytoplankton 41.9% and epiphytes 0.1% of the PP in the submerged macrophyte area. Epiphyte production in both lakes was very low in comparison with that of phytoplankton and macrophytes: 0.01, 5.04, and 6.97 g C m^?2 day^?1, respectively, in Võrtsjärv, and 0.02, 1.93, and 10.5 g C m^?2 day^?1, respectively, in Peipsi. The PP of the littoral area contributed 10% of the total summer PP of Lake Peipsi sensu stricto and 35.5% of the total summer PP of Lake Võrtsjärv.     

Bacterial and phytoplankton dynamics in a sub-tropical estuary

Heterotrophic bacterial and phytoplankton biomass, production, specific growth rates and growth efficiencies were studied in July 2001 and January 2002 during both spring and neap tides, along a tidal cycle, at three sites in a subtropical estuary. Major freshwater inputs located in the Northern region led to differences in both phytoplankton and bacterioplankton biomass and activity along the estuary. While in the Northern region phytoplankton is light-limited, with mean phytoplankton production (PP) between 1.1 and 1.9 μg C l⁻¹ h⁻¹ and mean specific growth rates (PSG) between 0.14 and 0.16 d⁻¹, the Southern region registered values as high as 24.7 μg C l⁻¹ h⁻¹ for PP and 2.45 d⁻¹ (mean PP between 3.4 and 7.3 μg C l⁻¹ h⁻¹; mean PSG between 0.28 and 0.57 d⁻¹). On the other hand, maximum bacterial production (BP: 63.8 μg C l⁻¹ h⁻¹) and specific growth rate (BSG: 32.26 d⁻¹) were observed in the Northern region (mean BP between 3.4 and 12.8 μg C l⁻¹ h⁻¹; mean BSG between 1.98 and 6.67 day⁻¹). These bacterial activity rates are among the highest recorded rates in estuarine and coastal waters, indicating that this system can be highly heterotrophic, due to high loads of allochthonous carbon (mainly derived from mangrove forest). Our results also showed that, despite that BP rates usually exceeded PP, in the Southern region BP may be partially supported (∼45%) by PP, since a significant regression was observed between BP and PP (r = 0.455, P < 0.001). Handling editor: P. Viaroli     

Accounting for littoral primary production by periphyton shifts a highly humic boreal lake towards net autotrophy

相似文献   

Diurnal fluctuations of temperature near the bottom of shallow water bodies as affected by solar radiation,bottom color and water circulation

Measurements made in shallow lakes and tanks have shown that temperatures near and at the surface of substrates respond significantly to changes in solar radiation, water circulation and substrate color. Temperature differences between light and dark colored natural and artificial substrates ranged between 0.8 and 4.0 C°. Diurnal surface temperature ranges on a submerged black wooden disk varied 9C° when incoming solar radiation varied 14% or average daily windspeed changed from 0.9 to 3.6 ms^–1. Fluctuations in substrate temperatures were poorly correlated with changes in nearby air or water temperatures.Supported by NRCC Grant A 3403     

Adaptations of microbial communities and dissolved organics to seasonal pressures in a mesotrophic coastal Mediterranean lake

In lake ecosystems, changes in eukaryotic and prokaryotic microbes and the concentration and availability of dissolved organic matter (DOM) produced within or supplied to the system by allochthonous sources are components that characterize complex processes in the microbial loop. We address seasonal changes of microbial communities and DOM in the largest Croatian lake, Vrana. This shallow lake is connected to the Adriatic Sea and is impacted by agricultural activity. Microbial community and DOM structure were driven by several environmental stressors, including drought, seawater intrusion and heavy precipitation events. Bacterial composition of different lifestyles (free-living and particle-associated) differed and only a part of the particle-associated bacteria correlated with microbial eukaryotes. Oscillations of cyanobacterial relative abundance along with chlorophyll a revealed a high primary production season characterized by increased levels of autochthonous DOM that promoted bacterial processes of organic matter degradation. From our results, we infer that in coastal freshwater lakes dependent on precipitation-evaporation balance, prolonged dry season coupled with heavy irrigation impact microbial communities at different trophic levels even if salinity increases only slightly and allochthonous DOM inputs decrease. These pressures, if applied more frequently or at higher concentrations, could have the potential to overturn the trophic state of the lake.     

Coupling dissolved organic carbon,CO2 and productivity in boreal lakes

相似文献   

Primary production of Lake Peipsi-Pihkva

Primary production (PP) in Lake Peipsi-Pihkva, the tripartite border waterbody between Estonia and Russia, was first measured in 1965–1966. Since 1970 there exists a continuous timeseries of monthly PP measurements from May to October. Detailed investigations of the seasonal and daily dynamics as well as the vertical distribution of PP were carried out in 1985–1987. The long-term average values of integral PP (PP_int) in Lakes Peipsi and Pihkva were equal (0.8 g C m^–2 d^–1), although the values per cubic metre (PP_max) differed more than twofold and characterized L. Pihkva as a eutrophic lake and L. Peipsi as a transition type between meso- and eutrophic lakes. The years from 1973 to 1980, 1987 and 1991 were of low productivity, while in 1971, 1983, 1988 and 1990 PP peaks occurred in both lakes. In the seasonal pattern PP_int had peaks in May and July. In June, after the spring bloom, PP as well as the chlorophyll a (Chl) and ATP content were low. The high Chl peak in autumn was probably built up by the degradation products of chlorophyll, as neither PP nor ATP increased. Seasonal changes in integral PP in L. Peipsi could be well described (R ² = 0.91) by an empirical model relating PP_int to PP_max, Secchi depth (S) and total solar radiation (Q). In mixed conditions prevailing in both lakes, PP was inhibited in the surface layer and its maximum was located at a depth of 0.25...0.5 S. The threshold total solar radiation level for the onset of inhibition was between 1200 and 2000 kJ m^–2 h^–1 in May and July, and decreased to < 500 kJ m^–2 h^–1 in October. As a rule, inhibition started in the morning at a higher irradiance than necessary for keeping it up during evening hours. When compared with PP_max, photosynthesis in the surface layer at noon was suppressed by 56% in May, by 45% in July and by 40% in October.     

Photoproduction of dissolved inorganic carbon in temperate and tropical lakes – dependence on wavelength band and dissolved organic carbon concentration

We have evaluated photoeffects of UV-B, UV-A and PAR radiation on dissolved organic matter (DOM). Photochemical production of dissolved inorganic carbon (DIC) was measured in sterile lake water from Sweden and Brazil after 6 hours of sun exposure. Tubes were exposed to four solar radiation regimes: Full-radiation, Full-radiation minus UV-B, Full-radiation minus UV-B and UV-A (PAR) and darkness.In both areas, lakes with most DOC (varying between 3 and 40 mg C l^-1) were highly humic, resulting in high UV-B attenuation coefficients (K_d = 5–466 m^-1). Under Full-radiation, photooxidative DIC-production varied from 0.09 to 1.7 mg C l^-1per 6 h, without UV-B from 0.07 to 1.4 mg C l^-1 and with PAR only from 0.02 to 0.7 mg C l^-1. UV-B radiation explains a minor part (17%) of the photoooxidative DIC-production, while UV-A and PAR have larger effects (39% and 44%, respectively). Photooxidation was proportional to DOC-content and DIC-production was positively related to decrease in DOC and to loss of absorbance at 250 nm. There was no significant difference in DOC and radiation normalized DIC-production between Swedish and Brazilian lakes. The UV-B dose during incubations was approximately 3 times higher in Brazil compared to Sweden, while UV-A and PAR doses were similar. We conclude that DOC from tropical and temperate freshwaters do not seem to differ with respect to sensitivity to photooxidation.     

Integrated cultivation of salmonids and seaweeds in open systems

Bacterial abundance and production in a vertical profile in Lake Kariba (17dgS), Zimbabwe, were affected by solar irradiance. At the surface, 1.87 × 10⁹ bacteria 1^–1 were found and abundance peaked at 10 m (2.5 × 10⁹ bacteria l^-1), then decreasing with depth. Bacterial reproduction at the surface(0.145 µg C1^–1 h^–1) was nearly four times less than the production at 10 m although bacterial numbers were only 26% less. Thus, bacterial production per cell was lower at the surface than deeper down, suggesting that bacterial production is inhibited at the surface.Bacterial production in GF/F filtered lake water in Whirl Pack bags showed an exponential decrease down to 3 m depth. The inhibition was well in accordance with light extinction in the UV region. Phosphatase activity was low in light exposed bags compared to dark, indicating photolysis of extracellular enzymes, or phototransformation of recalcitrant DOM, which substitutes enzyme activity. Hypolimnetic enzyme activity was less affected by solar light than epilimnetic.     

Effect of Seawater–Sewage Cross-Transplants on Bacterial Metabolism and Diversity

Bioassays experiments were conducted to determine the metabolic and community composition response of bacteria to transplants between relatively pristine coastal seawater and sewage-impacted seawater. There were four treatments: (1) pristine seawater bacteria?+?pristine seawater (Pb?+?Pw), (2) sewage-impacted bacteria?+?sewage-impacted water (Sb?+?Sw), (3) pristine seawater bacteria?+?sewage-impacted water (Pb?+?Sw), and (4) sewage-impacted bacteria?+?pristine seawater (Sb?+?Pw). Sewage-derived DOC was more labile and readily utilized by bacteria, which favored the growth of high nucleic acid (HNA) bacteria, resulting in high bacterial production (BP, 113?±?4.92 to 130?±?15.8 μg C l^?1?day^?1) and low respiration rate (BR, <67?±?11.3 μg C l^?1?day^?1), as well as high bacterial growth efficiency (BGE, 0.68?±?0.09 to 0.71?±?0.05). In contrast, at the relatively pristine site, bacteria utilized natural marine-derived dissolved organic matter (DOM) at the expense of lowering their growth efficiency (BGE, <0.32?±?0.02) with low BP (<62?±?6.3 μg C l^?1?day^?1) and high BR 133?±?14.2 μg C l^?1?day^?1). Sewage DOM input appeared to alter the partitioning of carbon between respiration and production of bacteria, resulting in a shift toward higher BGE, which would not enhance oxygen consumption. Taxonomic classification based on 454 pyrosequencing reads of the 16S rRNA gene amplicons revealed that changes in bacterial community structure occurred when seawater bacteria were transferred to the eutrophic sewage-impacted water. Sewage DOM fueled the growth of Gammma-proteobacteria and Epsilson-proteobacteria and reduced the bacterial richness, but the changes in the community were not apparent when sewage-impacted bacteria were transferred to pristine seawater.