Associations of testate rhizopods (Protozoa) in the plankton of a Malaysian estuary and two nearby ponds

Plankton samples were collected from the Chukai-Kemaman estuary in Malaysia in order to determine the distribution of testate rhizopods in relation to salinity. Two nearby freshwater ponds were sampled for comparison.
In the ponds, 35 species of testate rhizopods were found, 13 of which were not found in the estuary. From the estuary 53 species were found, of which 31 were not found in the ponds. Some species appeared to have been lifted into the plankton by the rising tide. At a generic level, the Arcella species were very similar in both ponds and estuary (Sorensen Index 80), the Difflugia species were moderately similar (Sorensen Index 47), while Nebela and Quadrullela were present in the estuarine samples, but absent from the pond samples.
The salinity gradient ranged from sea water down to a conductivity of 27 μS. All the estuarine stations were tidal. The first freshwater testate rhizopods were found in a region where the conductivity was between 3000 and 4000 μS at high tide. Where the conductivity was below 1000 μS at high tide the number of species rapidly increased, and reached a maximum of 25 in the samples from the lowest conductivities. The flowering tree, Hibiscus tiliaceus , appears to be an indicator of the region where the first freshwater testate rhizopods might be encountered upstream from the sea.     

长江口赤潮高发区浮游植物与水动力环境因子的分布特征

报道了2002年春季长江口海域(30°50′～31°50′N,121°50′～123°00′E)的22个大面观察站和一个昼夜连续观察站的水样和网样浮游植物的种类组成、丰度分布与水动力环境因子、营养盐的关系．结果表明,长江口区共有浮游植物5门45属110种．主要赤潮生物优势种为中肋骨条藻(Skeletonema castaturn)和具齿原甲藻(Prorocentrum dentatum)等．浮游植物丰度的昼夜变化白天大于夜间,垂直分布不明显．浮游植物主要生态类型可划分为：沿岸河口低盐半咸水类群、沿岸偏低盐广布性类群、外海高盐暖水性类群等．长江口区浮游植物丰度在1．6×10^3～75．2×10^3个.dm-3．浮游植物的种类组成和丰度分布与长江冲淡水密切相关．在该区域存在三股不同性质的水,即长江河口水、长江冲淡水及外海水(台湾暖流)影响着浮游植物的分布．     

Zooplankton in the Schelde estuary, Belgium and the Netherlands: long-term trends in spring populations

A compilation of available data in between 1967 and 2002 on spring zooplankton abundance was made for the brackish and the freshwater zone of the Schelde estuary. The general picture is a significant increase of 1–2 orders of magnitude in abundance for Rotifera, Copepoda and Branchiopoda (mainly Cladocera) in the freshwater zone, while zooplankton abundance in the brackishwater zone remained more constant. Possible natural and management related causes for this increase in zooplankton abundance are briefly discussed.     

The benthic macrofauna along the estuarine gradient of the Schelde estuary

The intertidal benthic macrofauna of the Schelde estuary (The Netherlands and Belgium) was sampled in late autumn of 1990 at 50 stations along the whole salinity gradient (between Vlissingen and Dendermonde), including the freshwater tidal part. All stations were situated in sheltered areas with a relatively muddy sediment. Species richness, diversity and total biomass of the benthic macrofauna decreased along the salinity gradient from Vlissingen to Dendermonde, while total density showed no clear trend. Especially the oligohaline and freshwater tidal part of the Schelde estuary was characterized by a very impoverished benthic community, composed only of Oligochaeta. No other species (freshwater, marine or brackish) was observed in this part of the estuary. The marine part had a more diverse macrozoobenthos structure than that of the brackish part. Species found only in the marine zone areCerastoderma edule, Tharyx marioni, Eteone longa, Nephtys hombergii andCapitella capitata. In the brackish part of the estuary,Corophium volutator was a typical, dominant species. However, a lot of the dominant species were common in both the marine and brackish part of the Schelde estuary (e.g. Heteromastus filiformis, Pygospio elegans, Nereis diversicolor, Macoma balthica). The observed gradient in species composition and dominance is compared with some other European estuaries. The marine and brackish part of the Schelde estuary is quite similar to other european estuaries. The freshwater tidal part, however, was more impoverished.     

The feeding periodicity of striped mullet, Mugil cephalus L., in two Florida habitats

The feeding periodicity of Mugil cephalus (striped mullet) was investigated during the summer and autumn of 1979 using percentage body weight made up of stomach contents as an indicator of feeding intensity. One study site was freshwater, the other was estuarine and both were tidally influenced. Mullet at both sites feed diurnally, with maximum intensity occurring at about 11.00 hours and little or no feeding occurring at night. Fish from the freshwater site have larger standard length-specific intestine lengths and also include more plant material in their diet than do mullet from the estuarine site. At both sites fish longer than 28 cm standard length ( s.l. ) tend to be female; the difference from a 50:50 sex ratio is highly significant in both cases. Females longer than 28 cm s.l. outnumber all other sex and size combinations at the freshwater site, but comprise only 28% of fish collected at the estuarine site.     

Effect of salinity-altering pulsing events on soil organic carbon loss along an intertidal wetland gradient: a laboratory experiment

Salinity changes resulting from storm surge, tides, precipitation, and stormwater run-off are common in coastal wetlands. Soil microbial communities respond quickly to salinity changes, altering the rate of soil organic carbon (SOC) loss and associated biogeochemical processes. This study quantified the impact of salinity-altering pulses on SOC loss, defined as microbial respiration (CO₂ flux) at high and low tide, CH₄ flux, and dissolved OC (DOC) release, in 3 intertidal wetlands (Jacksonville, FL, USA). Intact soil cores from a freshwater tidal, brackish, and salt marsh were exposed to simulated tides and 3 salinity pulsing events during a 53-day laboratory experiment. Soil and water physio-chemical properties, nutrient release, and microbial indicators were measured. Microbial respiration was the dominate pathway of SOC loss (>97 %). Soil hydraulic conductivity was greater in brackish and salt marshes and was critical to overall soil respiration. High tide CO₂ flux was greatest in the freshwater marsh (58 % of SOC loss) and positively correlated with DOC concentration; low tide CO₂ flux was greatest in brackish and salt marshes (62 and 70 % of SOC loss, respectively) and correlated with NH₄ ⁺ and microbial biomass. The freshwater marsh was sensitive to brackish pulses, causing a 112 % increase in respiration, presumably from accelerated sulfate reduction and N-cycling. SOC loss increased in the salt marsh pulsed with freshwater, suggesting freshwater run-off may reduce a salt marsh’s ability to keep-pace with sea level rise. Increased inundation from storm surges could accelerate SOC loss in freshwater marshes, while decreasing SOC loss in brackish and salt marshes.     

Influence of changes in salinity and light intensity on growth of phytoplankton communities from the Schelde river and estuary (Belgium/The Netherlands)

In the Schelde continuum, a succession in the phytoplankton community is observed along the transition from the river to the freshwater tidal reaches of the estuary and from the freshwater to brackish reaches of the estuary. The goal of this study was to experimentally evaluate the contribution of changes in salinity and light climate to this succession. In summer 2000 and in spring 2001, phytoplankton communities from the river, the freshwater tidal reaches and the brackish reaches of the estuary were incubated under high or low light intensities and exposed to a change in salinity. HPLC analysis was used to evaluate the response of different algal groups to changes in light intensity and salinity. When incubated at a light intensity corresponding to the mean underwater light intensity of the freshwater tidal reaches, growth of phytoplankton from the river as well as from freshwater tidal reaches was significantly lower than when incubated at a light intensity corresponding to the mean underwater light intensity of the river. The phytoplankton community from the freshwater tidal reaches did not appear to be better adapted to low light intensities than the phytoplankton community from the river. Although diatoms were expected to be less sensitive to a reduction in light intensity than green algae, the opposite response was observed. Freshwater and brackish water phytoplankton were negatively affected by respectively an increase or decrease in salinity. However, the effect of salinity was not strong enough to explain the disappearance of freshwater and brackish water phytoplankton between a salinity of 0.5 and 10 psu, suggesting that other factors also play a role. In the freshwater phytoplankton communities from the river and the freshwater tidal reaches, green algae and diatoms responded in a similar way to an increase in salinity. In the brackish water phytoplankton community, fucoxanthin displayed a different response to salinity than lutein and chlorophyll a.     

Diet,stable isotopes and morphology of Eurasian perch (Perca fluviatilis) in littoral and pelagic habitats in the northern Baltic Proper

We present morphology, stable isotope signals and stomach contents of Eurasian perch (Perca fluviatilis) from littoral and pelagic habitats in a brackish water embayment in the northern Baltic Proper. Studies conducted in freshwater habitats repeatedly state that littoral perch have deeper bodies than the pelagic ones. In this study we observed the opposite; the perch from the pelagic study site had deeper bodies than the littoral ones, indicating that more factors than habitat structure affect the perch morphology. A possible explanation to this discrepancy is the diet choice; the pelagic perch in this study were more benthivorous than freshwater pelagic perch. Our results on stable isotope signals combined with the stomach contents also shed new light on the dietary preferences of perch. Perch is known to be a generalist predatory fish, but our results indicate that perch have individual diet preferences. Based on our results, it seems that at some point in their lives the perch in brackish water choose between the littoral and pelagic habitats and also specialise in a certain diet. This study shows that the perch morphology and diet in the Baltic Sea coast differ among habitats, but the patterns are not similar to those observed in freshwater studies.     

Seasonality in macroinvertebrate community composition across a neglected ecological boundary, the freshwater-estuarine transition zone

The boundary between freshwater and estuarine ecosystems at the extreme head of estuaries has received little attention from benthic ecologists. This short communication addresses this research caveat, presenting preliminary data on the spatial and temporal (seasonal) variation in the composition of benthic macroinvertebrate communities across the transition zone at the head of a small British estuary. Cluster analysis and multidimensional scaling ordination demonstrated that a distinct community occurred at a point just preceding the recognised transition between fresh and brackish waters (salinity <0.2PSU). This transition community contained a mixture of freshwater and estuarine taxa and, despite seasonal migrations at the head of the estuary, community structure remained significantly distinct from that at adjacent sites throughout the year. These preliminary data suggest that the community dynamics across the transition zone show complex patterns and may offer an important research opportunity in aquatic ecology.     

Phytoplankton growth rates in the freshwater tidal reaches of the Schelde estuary (Belgium) estimated using a simple light-limited primary production model

During the course of 1996, phytoplankton was monitored in the turbid, freshwater tidal reaches of the Schelde estuary. Using a simple light-limited primary production model, phytoplankton growth rates were estimated to evaluate whether phytoplankton could attain net positive growth rates and whether growth rates were high enough for a bloom to develop. Two phytoplankton blooms were observed in the freshwater tidal reaches. The first bloom occurred in March and was mainly situated in the most upstream reaches of the freshwater tidal zone, suggesting that it was imported from the tributary river Schelde. The second bloom occurred in July and August. This summer bloom was situated more downstream in the freshwater tidal reaches and appeared to have developed within the estuary. A comparison between phytoplankton growth rates estimated using a simple primary production model and flushing rate of the water indicated that no net increase in phytoplankton biomass was possible in March while phytoplankton could theoretically increase its biomass by 20% per day during summer. Chlorophyllaconcentrations at all times decreased strongly at salinities between 5–10 psu. This decline was ascribed to a combination of salinity stress and light limitation. Phytoplankton biomass and estimated annual net production were much higher in the freshwater tidal zone compared to the brackish reaches of the estuary (salinity > 10 psu) despite mixing depth to euphotic depth ratios being similar. Possible reasons for this high production include high nutrient concentrations, low zooplankton grazing pressure and import of phytoplankton blooms from the tributary rivers.     

Zooplankton in the Schelde estuary (Belgium/The Netherlands). The distribution of Eurytemora affinis: effect of oxygen?

During the 1990s, a spatial shift of the population of Eurytemoraaffinis (Copepoda, Calanoida) from brackish water towards lowersalinities in the Schelde estuary coincided with an increasein oxygen concentration in the freshwater zone. Other ecologicaland hydrodynamic factors potentially influencing the spatialdistribution of the species did not change significantly ineither zone, which suggests that the E. affinis population activelyresponded to the change in oxygen concentration.     

长江河口潮间带鱼类群落的时空变化

根据2006年3-11月每月在长江口4个潮间带站点的36网次鱼类采样数据,分析了该水域鱼类群落组成的时空变化特征.结果表明：本次调查采获鱼类55种,隶属20科,其中,鲤科鱼类最多,有18种,占总种数的32.7%,虾虎鱼科其次,有8种,占14.5%,石首鱼科4种,占7.3%;调查水域潮间带鱼类群落可分为口内淡水区鱼类群落和河口咸淡水区鱼类群落2组,河口咸淡水区鱼类的物种多样性比口内淡水区略低,其中,口内淡水区以油似鳊和鳊等淡水鱼类为优势种,河口咸水区以斑尾刺虾虎鱼、鲻和棘头梅童等咸淡水鱼类为优势种;二组间鱼类种类组成的平均相异性为80.75%,23种鱼类对平均相异性的累积贡献率>90%,主要贡献来自油鲻、斑尾刺虾虎鱼、棘头梅童鱼、似鳊、红狼牙虾虎鱼、鲫、鳊和长蛇鮈等;春季的研究区鱼类群落丰度与夏、秋两季均存在一定程度的差异,而夏、秋两季间的差异相对较小.就单组鱼类群落丰度而言,水温对其季节变化的影响较盐度大,而盐度对整个河口鱼类群落的影响较水温大.     

Juvenile downstream migration patterns of an anadromous fish,allis shad (Alosa alosa), before and after the population collapse in the Gironde system,France

Diadromous fish have exhibited a dramatic decline since the end of the 20th century. The allis shad (Alosa alosa) population in the Gironde-Garonne-Dordogne (GGD) system, once considered as a reference in Europe, remains low despite a fishing ban in 2008. One hypothesis to explain this decline is that the downstream migration and growth dynamics of young stages have changed due to environmental modifications in the rivers and estuary. We retrospectively analysed juvenile growth and migration patterns using otoliths from adults caught in the GGD system 30 years apart during their spawning migration, in 1987 and 2016. We coupled otolith daily growth increments and laser ablation inductively-coupled plasma mass spectrometry measurements of Sr:Ca, Ba:Ca, and Mn:Ca ratios along the longest growth axis from hatching to an age of 100 days (i.e., during the juvenile stage). A back-calculation allowed us to estimate the size of juveniles at the entrance into the brackish estuary. Based on the geochemistry data, we distinguished four different zones that juveniles encountered during their downstream migration: freshwater, fluvial estuary, brackish estuary, and lower estuary. We identified three migration patterns during the first 100 days of their life: (a) Individuals that reached the lower estuary zone, (b) individuals that reached the brackish estuary zone, and (c) individuals that reached the fluvial estuary zone. On average, juveniles from the 1987 subsample stayed slightly longer in freshwater than juveniles from the 2016 subsample. In addition, juveniles from the 2016 subsample entered the brackish estuary at a smaller size. This result suggests that juveniles from the 2016 subsample might have encountered more difficult conditions during their downstream migration, which we attribute to a longer exposure to the turbid maximum zone. This assumption is supported by the microchemical analyses of the otoliths, which suggests based on wider Mn:Ca peaks that juveniles in 2010s experienced a longer period of physiological stress during their downstream migration than juveniles in 1980s. Finally, juveniles from the 2016 subsample took longer than 100 days to exit the lower estuary than we would have expected from previous studies. Adding a new marker (i.e., Ba:Ca) helped us refine the interpretation of the downstream migration for each individual.     

Prevalence of zoonotic metacercariae in two species of grouper, Epinephelus coioides and Epinephelus bleekeri, and flathead mullet, Mugil cephalus, in Vietnam

Fishborne zoonotic metacercariae have not been reported from brackish water and marine fish from Vietnam waters although these parasites are common in the country's freshwater fish. Both wild-caught and cultured grouper (Epinephelus coioides and Epinephelus bleekeri), and mullet (Mugil cephalus) from brackish and marine waters located in Khanh Hoa province in central coastal Vietnam were examined, and found positive for zoonotic trematode metacercariae. From grouper, Heterophyopsis continua and Procerovum varium were recovered. The prevalence of H. continua ranged from 2.0 to 6.0% and that for P. varium ranged from 11.6 to 15.8%. Mullet were infected with Pygidiopsis summa and H. continua both of these species are new records for Vietnam. The prevalence of P. summa in mullet was generally high, ranging from 17.6 to 75.5%, and was significantly higher than the prevalence of H. continua (2.5 to 32.4%). There were no significant differences in the prevalence of metacercariae between grouper from natural or cultured habitats, indicating that the highest risk of infection occurs in the wild-caught state prior to their placement in culture. Further, there was no difference in metacercarial prevalence between the 2 species of grouper. Infected wild-caught seed were only observed from January to October. Monthly variation in prevalence suggests seasonal variation in mullet infections occurs in this region with the highest transmission taking place from October to December. Basic investigations on the ecology and epidemiology of these intestinal flukes need to be carried out to determine their significance as a public health problem and the aspects of their biology that may be vulnerable to control interventions.     

Unravelling the life-history patterns and habitat preferences of the Japanese eel (Anguilla japonica) in the Pearl River,China

Eels have fascinated biologists for centuries due to their amazing long-distance migrations between freshwater habitats and very distant ocean spawning areas. The migratory life histories of the Japanese eel, Anguilla japonica, in the waters of south China are not very clear despite its ecological importance, and the need for fishery regulation and management. In this study, strontium (Sr) and calcium (Ca) microchemical profiles of the otoliths of silver eels were measured by X-ray electron probe microanalysis based on data collected from different habitats (including freshwater and brackish habitats), in the large subtropical Pearl River. The corresponding habitat preference characteristics were further analysed using redundancy analysis (RDA). A total of 195 Japanese eels were collected over 6 years. The collected individuals ranged from 180 to 771 mm in total length and from 8 to 612 g in body weight. Two-dimensional pictures of the Sr:Ca concentrations in otoliths revealed that the A. japonica in the Pearl River are almost entirely river eels, spending the majority of their lives in fresh water without exposure to salt water, while the catadromous migration time has delayed about 1 month in the Pearl River estuary in the past 20 years. RDA analysis further indicated that juveniles and adults preferred water with high salinity and high tide levels. Youth preferred habitats with high river fractals. Our findings contribute to a growing body of evidence showing that the eels are extremely scarce currently and conservation measures against them are imminent, including the protection of brackish and freshwater areas where they live in south China.     

Covariance Analysis of Chlorophyll Distribution in the Sundays River Estuary,Eastern Cape

Summary

An analysis of covariance performed on chlorophyll a distribution data from the Sundays River estuary identified five persistent water masses with significantly different chlorophyll a contents. These corresponded to different hydrodynamic regions within the estuary. The relationship between salinity and chlorophyll a was used to identify a transition zone between the low-salinity upper estuary and the brackish riverine inflow. Chlorophyll a concentrations ranged from <6 μg 1^?1 near the mouth to >100 μg 1^?1 in the middle and upper reaches. High variance of chlorophyll a data in this region was the result of temporal chlorophyll a fluctuations in two time scales. The first related to the daily ebb/tide and the second to the lunar spring/neap tidal cycle. Unlike its importance in larger and deeper estuaries, light played only a minor role in the horizontal distribution of chlorophyll a. The presence of a chlorophyll a maximum in the turbid low salinity region is explained in terms of the relatively constant photic depth/mixed depth ratio along the estuary and the high residence time of water in this region.     

Coastal zone use and migratory behaviour of the southern population of Mugil liza in Brazil

We analysed the ratios Sr:Ca and Ba:Ca in the otoliths of 55 adults of the southern population of Mugil liza in Brazil (Paraná to Rio Grande do Sul) to investigate its coastal zone use and migratory behaviour. All individual M. liza analysed had Sr:Ca and Ba:Ca values indicating that their birth was in the marine environment, which is consistent with the southern population migration to spawn in the ocean,. Juveniles exhibited at least three coastal use and recruitment strategies (contingents): the majority (89%) of M. liza juveniles migrated toward brackish water. They entered the estuary before completing the first year of life (64%) or after (25%) their first year of life. The remaining 11% did not appear to enter brackish or freshwater water as a nursery or at any point in their life cycle. Some adults returned to the estuary after spawning in the ocean but others (of both sexes) never returned to the estuary after spawning, remaining in the marine environment. The pattern of juvenile habitat use in the Brazilian southern population of M. liza seems to be recurrent throughout the extent of its distribution as a consequence of the reproductive spawning aggregation behaviour, which mixes all contingents (with marine or estuarine preferences).     

Diet of otters (Lutra lutra) in closely associated freshwater, brackish and marine habitats in south-west Portugal

The diet of otters was studied in closely associated freshwater, brackish and marine habitats, from spraints collected on the Portuguese south-west coast over an 18-month period. In areas where marine prey was the only available resource, diet was dominated by blennies, wrasses and gobies, but other prey was taken in areas near coastal lagoons, marshes and estuaries. Eels and amphibians were considered the typical prey of freshwater habitats, and grey mullet the typical prey of brackish water habitats. If the inland habitats near the coast were large enough, otters preferred to forage there rather than in the sea. In one area where otters alternatively used marine and inland habitats, the former were used most extensively in autumn and winter, and the latter in spring and summer. It is suggested that otters prefer to forage inland rather than in the sea.     

Reproductive isolation in Chara aspera populations

Possible reproductive isolation between freshwater and brackish water populations of the dioecious charophyte Chara aspera was studied by means of cross-fertilization experiments and AFLP (Amplified Fragment Length Polymorphism). Three Swedish freshwater populations and three (German and Swedish) Baltic Sea populations of C. aspera were sampled. Cross-fertilization experiments were performed in a full combination setup of all populations and with two different salinities (0 and 10 PSU). Both freshwater and brackish water females formed about 70% more gametangia at 0 than at 10 PSU. Male individuals collected from freshwater had higher fertility than brackish water males at both salinities. 57% of all gametangia of females from freshwater developed into oospores compared to only 8% of gametangia of brackish water females. 42% of all oospores were fertilized in crosses between habitats (freshwater–brackish water) compared to 36% in crosses within habitats, the difference was not significant.Oospore and bulbil germination was investigated using propagules from freshwater and brackish water populations and incubation salinities of 0, 5, 10 and 20 PSU. None of the oospores collected from brackish water germinated. Germination of oospores and bulbils from freshwater was higher at 0 and 5 PSU than at higher salinities. Only around 40% of bulbils from brackish water germinated at 20 PSU compared to around 70% at the other three salinities. Germination of all bulbils was delayed at 20 PSU compared to other salinities.Genetic similarities (Jaccard indices of AFLP data) were higher within than between populations, but comparisons within habitat (freshwater–freshwater and brackish water–brackish water) were not different from comparisons between habitats.Our results did not identify any reproductive isolation between freshwater and brackish water populations, but indicate low gene flow between the two habitats. Oospore and bulbil germination success were highest at salinities corresponding to the conditions of their original habitat, suggesting genetic adaptation to their environmental conditions and indicating that propagules transported from freshwater to brackish water or vice versa will hardly develop into fertile plants. Additionally, brackish water plants perform poorer in all aspects of sexual reproduction than freshwater plants. Possibly, successful dispersal of oospores is not subjected to high selective pressure within the Baltic Sea where new sites easily can be colonized by means of vegetative reproduction. We assume that these adaptations will favour speciation within C. aspera and support the idea of the geologically young Baltic Sea as a “cradle of plant evolution”.     

Loss of Estuarine Bacteria by Viral Infection and Predation in Microcosm Conditions

The bacterioplankton density in Ria de Aveiro, a shallow estuarine ecosystem, varied in the broad range of 1.9-10.6 109 cells L-1. The range of values was about 2 times higher in brackish water than in marine water. At high tide bacterial abundance was 2-3 times lower than at low tide. The overall variation in virioplankton was in the range of 2.4-25.0 1010 particles L-1. Brackish water was about 2 times richer in viral particles than the marine water. Near low tide the virioplankton was 2-3 times higher that at high tide. Viral density followed the pattern of bacterial abundance (it explained 40% of virioplankton variation). The viruses to bacterium ratio varied, throughout tidal cycles, by a factor of about 10 establishing the range 4.7-55.6 (average 17.6). This ratio was rather similar in the two estuarine zones. We compared the effects of infection and predation on the control of bacterioplankton size in the two zones of the estuary. The approach to this question was conducted in experimental microcosms, set up in six combinations of plankton variables affecting the presence/absence of predators, virus-to-bacterium ratio (10-fold increase), virus-to-bacterium distance (2.2-fold increase), and bacterial growth rate. The results showed that predation was similar, in a percent basis, in marine (69%) and brackish water (73%). Viral infection was, however, higher in brackish water (59%) than in the marine water (36%). We conclude that the bacterioplankton along the salinity gradient evolves under biological pressures that are in different balance in the marine and brackish water zones. The effect of viral lysis on bacterial communities with enhanced growth (after yeast extract addition) was masked even when the initial ratio was 10-fold greater than in the natural samples. The high density of the virioplankton did not preclude the large and rapid increase in bacterial density. We suggest that the dynamics of the equilibrium between bacteria and viruses in the environment is driven to higher numerical levels during periods of intensive bacterial growth. On the contrary, at low bacterial growth rates the temporarily increased virus-to-bacterium ratio may drive the equilibrium to its lowest levels.