Summer feeding activities of zooplankton in Prydz Bay, Antarctica

Grazing of dominant zooplankton copepods (Calanoides acutus, and Metridia gerlachei), salps (Salpa thompsoni) and microzooplankton was determined during the austral summer of 1998/1999 at the seasonal ice zone of the Prydz Bay region. The objective was to measure the ingestion rates of zooplankton at the seasonal ice zone, so as to evaluate the importance of different groups of zooplankton in their grazing impact on phytoplankton standing stock and primary production. Grazing by copepods was low, and accounted for <1% of phytoplankton standing stocks and 3.8-12.5% of primary production for both species during this study; even the ingestion rates of individuals were at a high level compared with previous reports. S. thompsoni exhibited a relatively high grazing impact on primary production (72%) in the north of our investigation area. The highest grazing impact on phytoplankton was exerted by microzooplankton during this investigation, and accounted for 10-65% of the standing stock of phytoplankton and 34-100% of potential daily primary production. We concluded that microzooplankton was the dominant phytoplankton consumer in this study area. Salps also played an important role in control of phytoplankton where swarming occurred. The grazing of copepods had a relatively small effect on phytoplankton biomass development.     

Dynamics of microbial biomass and activity in five habitats of the Okefenokee Swamp ecosystem

A variety of freshwater marsh and swamp habitats are found interspersed in a mosaic pattern throughout the Okefenokee Swamp, Georgia, USA. We examined spatial and temporal patterns in standing stocks and activity in the microbial community of five habitats within this heterogeneous ecosystem. Standing stock dynamics were studied by measuring microbial biomass (ATP) and bacterial numbers (AODC) in both water and sediments over a 14 month period. Abundance varied temporally, being generally lower in winter months than in spring and summer months. However, a large proportion of the measured variability was not correlated with temporal patterns in temperature or with bulk nutrient levels. Spatial variability was characteristic of the Okefenokee at a variety of large and small scales. Habitat-level heterogeneity was evident when microbial standing stocks and activity (measured as [¹⁴C]lignocellulose mineralization) were compared across the five communities, although abundance differences among sites were restricted to nonwinter months when microbial biomass was high. Spatial variation within habitats was also found; patches of surface sediment with differing microbial activity or abundance were measured at scales from 30 cm to 150 m.     

三门湾浮游动物的季节变动及微型浮游动物摄食影响

2002年8月、11月、2003年2月和5月,在三门湾进行了4个航次生物、化学和水文等专业综合调查。根据采集的浮游动物样品的分析鉴定及海上现场实验结果,对浮游动物的群落组成、生物量、丰度、多样性指数的分布和季节变动及其浮游动物对浮游植物的摄食影响进行研究。结果表明,三门湾浮游动物有67属,89种,16类浮游幼体,主要可划分为4个生态类群：以近岸低盐类群为主,其优势种为中华哲水蚤Calanus sinicus、真刺唇角水蚤Labidocera etwhaeta、捷氏歪水蚤Tortanus derjugini、太平洋纺锤水蚤Acartiapacifica、中华假磷虾Pseudeuphausia sinica和百陶箭虫Sagitta bedoti等。半咸水河口类群、暖水性外海类群和广布种相对较少。浮游动物生物量和丰度的平面分布趋势除了夏季有所差异外,其它季节基本一致。2月份和5月份,浮游动物生物量和丰度,从湾顶向湾口呈逐渐增加趋势;8月份,湾口区生物量最高,而丰度高值区出现在湾顶部;11月份,生物量和丰度的平面分布相对均匀。浮游动物种类多样性指数有明显的季节变化,其动态变化与浮游动物种数和丰度的变化一致。微型浮游动物对浮游植物存在摄食压力,且有季节变化,摄食率的变化在0．18．0．68d^-1,微型浮游动物的摄食率低于相同季节的浮游植物生长率。微型浮游动物对浮游植物摄食压力的变化范围为16．1％-49．1％d^-1,对初级生产力摄食压力的变化在58．3％-83．6％d^-1。11月份,微型浮游动物对浮游植物和初级生产力的摄食压力均出现最高值。     

Seasonal and vertical distribution of planktonic bacteria and heterotrophic nanoflagellates in the middle Adriatic Sea

Temporal and spatial patterns of bacteria and heterotrophic nanoflagellates (HNF) were studied monthly from January 1997 to December 1998 in the middle Adriatic Sea. Bacterial and HNF relationships with phytoplankton biomass and temperature were analyzed to examine how the relative importance of bottom-up and top-down factors may shift over seasons and locations. For the coastal area, an inconsistent relationship between bacterial abundance and chlorophyll a and a stronger relationship between bacterial abundance and bacterial production suggest that other substrates than those of phytoplankton origin are important for bacteria. The analysis of simultaneous effects of temperature and bacterial production on bacterial abundance showed that the effect of temperature obscured the effects of bacterial production, suggesting that bacterial growth itself is highly temperature-dependent. The relationship between HNF abundance and bacterial abundance was slightly improved by the inclusion of in situ temperature, bacterial production or both parameters, as additional independent variables. About 60% of the variability in HNF abundance can be explained by bacterial abundance, bacterial production and temperature. In the open sea, tight coupling of bacterial abundance with chlorophyll a concentrations implied that phytoplankton-derived substrates have a dominant role in controlling bacterial abundance. During the colder months, bacterial abundance was high enough to support higher HNF abundance than observed, suggesting that predation exerted a minor depressing influence on bacterial abundance during that period. During the spring-summer period, HNF controlled bacterial standing stocks by direct cropping of bacterial production.Communicated by: H.-D. Franke     

Growth and mortality rates of phytoplankton in the northwestern North Pacific estimated by the dilution method and HPLC pigment analysis

Pigment-based growth rates of phytoplankton and mortality rates due to microzooplankton grazing were estimated using a dilution method combined with high-performance liquid chromatography (HPLC) pigment analysis in the northwestern North Pacific in autumn 1998. The dilution experiments were conducted at different hydrographic stations in both colder and warmer water masses. No significant difference was found between the growth rate of the phytoplankton community (0.38–0.70 day⁻¹; estimated by chlorophyll a) at the colder and warmer water stations, while the mortality rate (0.15–0.88 day⁻¹; estimated by chlorophyll a) tended to be higher at warmer water stations. The combination of estimates of daily chlorophyll a production and particulate organic carbon (POC) production enabled us to assess the carbon to chlorophyll a ratio (C/chl a) of “new” organic matter produced by living phytoplankton. The method provided an implicit value of the C/chl a of in situ living phytoplankton. The rate estimates from taxon-specific pigments suggested a possibility that chlorophyll b-containing green algae were grazed preferentially by microzooplankton during their active growth, and the standing stock of green algae was more strictly controlled by micrograzer than other algal groups such as diatoms. This result is one possible explanation for the fact that blooms of green algae have not been reported in the open ocean, in contrast with diatoms.     

Food Web Structure in the Recently Flooded Sep Reservoir as Inferred From Phytoplankton Population Dynamics and Living Microbial Biomass

Phytoplankton dynamics, bacterial standing stocks and living microbial biomass (derived from ATP measurements, 0.7-200 mm size class) were examined in 1996 in the newly flooded (1995) Sep Reservoir ('Massif Central,' France), for evidence of the importance of the microbial food web relative to the traditional food chain. Phosphate concentrations were low, N:P ratios were high, and phosphate losses converted into carbon accounted for <50% of phytoplankton biomass and production, indicating that P was limiting phytoplankton development during the study. The observed low availability of P contrasts with the high release of "directly" assimilable P often reported in newly flooded reservoirs, suggesting that factors determining nutrient dynamics in such ecosystems are complex. The phosphate availability, but also the water column stability, seemed to be among the major factors determining phytoplankton dynamics, as (i) large-size phytoplankton species were prominent during the period of increasing water column stability, whereas small-size species dominated phytoplankton assemblages during the period of decreasing stability, and (ii) a Dinobryon divergens bloom occurred during a period when inorganic P was undetectable, coinciding with the lowest values of bacterial standing stocks. Indication of grazing limitation of bacterial populations by the mixotrophic chrysophyte D. divergens (in late spring) and by other potential grazers (mainly rotifers in summer) seemed to be confirmed by the Model II or functional slopes of the bacterial vs phytoplankton regressions, which were always <0.63. Phytoplankton biomass was not correlated with phosphorus sources and its contribution was remarkably low relative to the living microbial biomass which, in contrast, was positively correlated with total phosphorus in summer. We conclude that planktonic microheterotrophs are strongly implicated in the phosphorus dynamics in the Sep Reservoir, and thus support the idea that an important amount of matter and energy flows through the "microbial loop" and food web, shortly after the flooding of a reservoir.     

Protozooplankton in the Weddell Sea,Antarctica: Abundance and distribution in the ice-edge zone

Summary Protozooplankton were sampled in the iceedge zone of the Weddell Sea during the austral spring of 1983 and the austral autumn of 1986. Protozooplankton biomass was dominated by flagellates and ciliates. Other protozoa and micrometazoa contributed a relatively small fraction to the heterotrophic biomass. During both cruises protozoan biomass, chlorophyll a concentrations, phytoplankton production and bacterial biomass and production were low at ice covered stations. During the spring cruise, protozooplankton, phytoplankton, and bacterioplankton reached high concentrations in a welldeveloped ice edge bloom 100 km north of the receding ice edge. During the autumn cruise, the highest concentrations of biomass were in open water well-separated from the ice edge. Integrated protozoan biomass was <12% of the biomass of phytoplankton during the spring cruise and in the autumn the percentages at some stations were >20%. Bacterial biomass exceeded protozooplankton biomass at ice covered stations but in open water stations during the fall cruise, protozooplankton biomass reached twice that of bacteria in the upper 100m of the water column. The biomass of different protozoan groups was positively correlated with primary production, chlorophyll a concentrations and bacterial production and biomass, suggesting that the protozoan abundances were largely controlled by prey availability and productivity. Population grazing rates calculated from clearance rates in the literature indicated that protozooplankton were capable of consuming significant portions of the daily phyto- and bacterioplankton production.     

Gradients of intertidal primary productivity around the coast of South Africa and their relationships with consumer biomass

The structure of rocky intertidal communities may be influenced by large-scale patterns of productivity. In this study we examine the in situ rates of production by intertidal epilithic microalgae (chlorophyll a production per unit area per month), intertidal nutrient concentrations (nitrates, nitrites, phosphates and silicates), and standing stocks of different functional-form groups of macroalgae around the South African coast, and their relationships to consumer biomass. Clear gradients of in situ intertidal primary production and nutrient concentrations were recorded around the South African coast, values being highest on the west coast, intermediate on the south and lowest on the east coast. Primary production by intertidal epilithic microalgae was correlated with nutrient availability and could also be related to nearshore phytoplankton production. The dominance patterns of different functional forms of macroalgae changed around the coast, with foliose algae prevalent on the west coast and coralline algae on the east coast. However, overall macroalgal standing stocks did not reflect the productivity gradient, being equally high on the east and west coasts, and low in the south. Positive relationships existed between the average biomass of intertidal intertebrate consumers (grazers and filter-feeders) and intertidal productivity, although only the grazers were directly connected to in situ production by epilithic intertidal microalgae. The maximum body size of a widely distributed limpet, Patella granularis, was also positively correlated with level of in situ primary production. The maximal values of biomass attained by intertidal filter-feeders were not related to intertidal primary production, and were relatively constant around the coast. At a local scale, filter-feeder biomass is known to be strongly influenced by wave action. This implies that the local-scale water movements over-ride any effects that large-scale gradients of primary production may have on filter-feeders. The large-scale gradient in intertidal productivity around the coast is thus strongly linked with grazer biomass and individual body size, but any effect it has on filter-feeder biomass seems subsidiary to the local effects of wave action.     

Phytoplankton growth rates in the Ross Sea, Antarctica, determined by independent methods: temporal variations

The development of the seasonal phytoplankton bloom in the Ross Sea was studied during two cruises. The first, conducted in November-December 1994, investigated the initiation and rapid growth of the bloom, whereas the second (December 1995-January 1996) concentrated on the bloom's maximum biomass period and the subsequent decline in biomass. Central to the understanding of the controls of growth and the summer decline of the bloom is a quantitative assessment of the growth rate of phytoplankton. Growth rates were estimated over two time scales with different methods. The first estimated daily growth rates from isotropic incorporation under simulated in situ conditions, including ¹⁴C, ¹⁵N and ³²Si uptake measurements combined with estimates of standing stocks of particulate organic carbon, nitrogen and biogenic silica. The second method used daily to weekly changes in biomass at selected locations, with net growth rates being estimated from changes in standing stocks of phytoplankton. In addition, growth rates were estimated in large-volume experiments under optimal irradiances. Growth rates showed distinct temporal patterns. Early in the growing season, short-term estimates suggested that growth rates of in situ assemblages were less than maximum (relative to the temperature-limited maximum) and were likely reduced due to low irradiance regimes encountered under the ice. Growth rates increased thereafter and appeared to reach their maximum as biomass approached the seasonal peak, but decreased markedly in late December. Differences between the major taxonomic groups present were also noted, especially from the isotopic tracer experiments. The haplophyte Phaeocystic antarctica was dominant in 1994 throughout the growing season, and it exhibited the greatest growth rates (mean 0.41 day^-1) during spring. Diatom standing stocks were low early in the growing season, and growth rates averaged 0.100 day^-1. In summer diatoms were more abundant, but their growth rates remained much lower (mean of 0.08 day^-1) than the potential maximum. Understanding growth rate controls is essential to the development of predictive models of the carbon cycle and food webs in Antarctic waters.      

Temperature is the key factor explaining interannual variability of Daphnia development in spring: a modelling study

Plankton succession during spring/early summer in temperate lakes is characterised by a highly predictable pattern: a phytoplankton bloom is grazed down by zooplankton (Daphnia) inducing a clear-water phase. This sequence of events is commonly understood as a cycle of consumer-resource dynamics, i.e. zooplankton growth is driven by food availability. Here we suggest, using a modelling study based on a size-structured Daphnia population model, that temperature and not food is the dominant factor driving interannual variability of Daphnia population dynamics during spring. Simply forcing this model with a seasonal temperature regime typical for temperate lakes is sufficient for generating the distinctive seasonal trajectory of Daphnia abundances observed in meso-eutrophic temperate lakes. According to a scenario analysis, a forward shift of the vernal temperature increase by 60 days will advance the timing of the Daphnia maximum on average by 54 days, while a forward shift in the start of the spring bloom by 60 days will advance the Daphnia maximum only by less than a third (17 days). Hence, the timing of temperature increase was more important for the timing of Daphnia development than the timing of the onset of algal growth. The effect of temperature is also large compared to the effect of applying different Daphnia mortality rates (0.055 or 0.1 day(-1), 38 days), an almost tenfold variation in phytoplankton carrying capacity (25 days) and a tenfold variation in Daphnia overwintering abundance (3 days). However, the standing stock of Daphnia at its peak was almost exclusively controlled by the phytoplankton carrying capacity of the habitat and seems to be essentially independent of temperature. Hence, whereas food availability determines the standing stock of Daphnia at its spring maximum, temperature appears to be the most important factor driving the timing of the Daphnia maximum and the clear-water phase in spring.     

The annual cycles of phytoplankton biomass

Terrestrial plants are powerful climate sentinels because their annual cycles of growth, reproduction and senescence are finely tuned to the annual climate cycle having a period of one year. Consistency in the seasonal phasing of terrestrial plant activity provides a relatively low-noise background from which phenological shifts can be detected and attributed to climate change. Here, we ask whether phytoplankton biomass also fluctuates over a consistent annual cycle in lake, estuarine–coastal and ocean ecosystems and whether there is a characteristic phenology of phytoplankton as a consistent phase and amplitude of variability. We compiled 125 time series of phytoplankton biomass (chlorophyll a concentration) from temperate and subtropical zones and used wavelet analysis to extract their dominant periods of variability and the recurrence strength at those periods. Fewer than half (48%) of the series had a dominant 12-month period of variability, commonly expressed as the canonical spring-bloom pattern. About 20 per cent had a dominant six-month period of variability, commonly expressed as the spring and autumn or winter and summer blooms of temperate lakes and oceans. These annual patterns varied in recurrence strength across sites, and did not persist over the full series duration at some sites. About a third of the series had no component of variability at either the six- or 12-month period, reflecting a series of irregular pulses of biomass. These findings show that there is high variability of annual phytoplankton cycles across ecosystems, and that climate-driven annual cycles can be obscured by other drivers of population variability, including human disturbance, aperiodic weather events and strong trophic coupling between phytoplankton and their consumers. Regulation of phytoplankton biomass by multiple processes operating at multiple time scales adds complexity to the challenge of detecting climate-driven trends in aquatic ecosystems where the noise to signal ratio is high.     

Primary productivity and trophic status of Lakes Sorell and Crescent,Tasmania

The primary productivity of two turbid, shallow lakes on the Tasmanian Central Plateau was determined by the C¹⁴ technique from half-light day incubations in situ. Graphical integration of depth-rate curves gave estimates of areal day rates of production and of annual rates.The 2 lakes are closely adjacent and very similar physically and chemically, but have very different phytoplankton populations. Lake Crescent has ten times the standing crop biomass of Lake Sorell but its greater turbidity restricts light penetration, and production per unit of surface per day and per year is only 2.6 times that of Sorell.With day rates of 25-(44)-93 mgCm^–2 and annual production of 16.9 gCm^–2 Lake Sorell could be regarded as oligotrophic. Consideration of standing crop biomass and morphometry however indicates oligo-mesotrophy. Lake Crescent with day rates of 35-(115)-250 mgCm^–2 and annual production of 45 gCm^–2 is moderately eutrophic.Incubations in constant light demonstrated considerable variation in production rates in different parts of Lake Crescent.     

Copepod abundance in the western Irish Sea: relationship to physical regime, phytoplankton production and standing stock

The distinct patterns of stratification in the North Channeland stratified region of the western Irish Sea influence theseasonal abundance of phytoplankton. The 3–4 month productionseason in the stratified region was characterized by productionand biomass peaks in the spring (up to 2378 mg C m² day^–1and 178.4 mg chlorophyll m^–2) and autumn (up to 1280 mgC m^–2 day^–1 and 101.9 mg chlorophyll m^–2).Phytoplankton in the North Channel exhibited a short, late productionseason with a single summer (June/July) peak in production (4483mg Cm^–2 day^–1) and biomass (–160.6 mg chlorophyllm^–2). These differences have little influence on copepoddynamics. Both regions supported recurrent annual cycles ofcopepod abundance with similar seasonal maxima (182.8–241.810³ind. m^–2) and dominant species (Pseudocalanus elongatusand Acartia clausi). Specific rates of population increase inthe spring were 0.071 and 0.048 day¹ for the North Channel andstratified region, respectively. Increased copepod abundancein the stratified region coincided with the spring bloom, andwas significantly correlated with chlorophyll standing stock.Increased copepod abundance preceded the summer production peakin the North Channel. This increase was not correlated withchlorophyll standing crop, suggesting that a food resource otherthan phytoplankton may be responsible for the onset of copepodproduction prior to the spring bloom. Hetero-trophic microplanktonas an alternative food source, and advection of copepods fromthe stratified region, are proposed as possible explanationsfor copepod abundance increasing in advance of the summer peakin primary production.     

Linkages between bacterioplankton community composition, heterotrophic carbon cycling and environmental conditions in a highly dynamic coastal ecosystem

We used mesocosm experiments to study the bacterioplankton community in a highly dynamic coastal ecosystem during four contrasting periods of the seasonal cycle: winter mixing, spring phytoplankton bloom, summer stratification and autumn upwelling. A correlation approach was used in order to measure the degree of coupling between the dynamics of major bacterial groups, heterotrophic carbon cycling and environmental factors. We used catalysed reporter deposition-fluorescence in situ hybridization to follow changes in the relative abundance of the most abundant groups of bacteria (Alphaproteobacteria, Gammaproteobacteria and Bacteroidetes). Bacterial carbon flux-related variables included bacterial standing stock, bacterial production and microbial respiration. The environmental factors included both, biotic variables such as chlorophyll-a concentration, primary production, phytoplankton extracellular release, and abiotic variables such as the concentration of dissolved inorganic and organic nutrients. Rapid shifts in the dominant bacterial groups occurred associated to environmental changes and bacterial bulk functions. An alternation between Alphaproteobacteria and Bacteroidetes was observed associated to different phytoplankton growth phases. The dominance of the group Bacteroidetes was related to high bacterial biomass and production. We found a significant, non-spurious, linkage between the relative abundances of major bacterial groups and bacterial carbon cycling. Our results suggest that bacteria belonging to these major groups could actually share a function in planktonic ecosystems.     

Microbial production,enzyme activity,and carbon turnover in surface sediments of the Hudson River estuary

The detrital food web is a major nexus of energy flow in nearly all aquatic ecosystems. Energy enters this nexus by microbial assimilation of detrital carbon. To link microbiological variables with ecosystem process, it is necessary to understand the regulatory hierarchy that controls the distribution of microbial biomass and activity. Toward that goal, we investigated variability in microbial abundance and activities within the tidal freshwater estuary of the Hudson River. Surface sediments were collected from four contrasting sites: a mid-channel shoal, two types of wetlands, and a tributary confluence. These samples, collected in June to August 1992, were sorted into two to four size fractions, depending on the particle size distribution at each site. Each fraction was analyzed for bacterial biomass (by acridine orange direct counting), bacterial production (by ³H-thymidine incorporation into DNA), fungal biomass (by ergosterol extraction), fungal production (by biomass accrual), and the potential activities of seven extracellular enzymes involved in the degradation of detrital structural molecules. Decomposition rates for particulate organic carbon (POC) were estimated from a statistical model relating mass loss rates to endocellulase activity. Within samples, bacterial biomass and productivity were negatively correlated with particle size: Standing stocks and rates in the <63-m class were roughly twofold greater than in the >4-mm class. Conversely, fungal biomass was positively correlated with particle size, with standing stocks in the largest size class more than 1OX greater than in the smallest. Extracellular enzyme activities also differed significantly among size classes, with high carbohydrase activities associated with the largest particles, while oxidative activities predominated in the smallest size classes. Among sites, the mid-channel sediments had the lowest POC standing stock (2% of sediment dry mass) and longest turnover time (approximately 1.7 years), with bacterial productivity approximately equal to fungal (56 vs. 46 g C per gram POC per day, respectively). In the Typha wetland, POC standing stock was high (10%); turnover time was about 0.3 years; and 90% of the microbial productivity was fungal (670 vs. 84 g C per gram POC per day). The other two sites, a Trapa wetland and a tributary confluence, showed intermediate values for microbial productivity and POC turnover. Differences among sites were described by regression models that related the distribution of microbial biomass (r ² = 0.98) and productivity (r ² = 0.81) to particle size and carbon quality. These factors also determined POC decomposition rates. Net microbial production efficiency (production rate/decomposition rate) averaged 10.6%, suggesting that the sediments were exporting large quantities of unassimilated dissolved organic carbon into the water column. Our results suggest that studies of carbon processing in large systems, like the Hudson River estuary, can be facilitated by regression models that relate microbial dynamics to more readily measured parameters. Correspondence to: R.L. Sinsabaugh     

Enclosure experiment on the control mechanism of planktonic bacterial standing stock

In order to understand the control mechanisms of a large, stable bacterial standing stock, enclosure experiments were conducted in a eutrophic lake, where both bacterial productivity and grazing pressure were very high. Total bacterial number in the different enclosures ranged from 1.2 to 2.7×10⁷ cells mL⁻¹ throughout the experiment. The average bacterial cell production rate estimated from a grazer eliminating experiment was 6.3×10⁵ cells mL⁻¹ h⁻¹. Difference in the bacterial cell production rate between shaded and unshaded enclosures was not apparent. Bacteria showed a reduction in standing stock of only about 25–30% even after the supply of light was cut to 1%. Bacteria in the shaded enclosures then recovered their production rate in the first 12 days of perturbation. Grazing pressure in the shaded enclosures was not less than that for the control. Thus, it was considered a control mechanism of bacterial stable standing stock that the bacteria shifted their organic substrate from extracellular dissolved organic carbon freshly released from phytoplankton to that already stocked in the water column, though it is not known whether the dominant bacteria were the same.     

Variation in number of vertebrae and gill rakers of sockeye salmon,Oncorhynchus nerka,in North America

Synopsis The latitudinal, regional, and annual variation in number of vertebrae and number of gill rakers present in sockeye salmon, Oncorhynchus nerka, stocks in North America was examined. Stocks in more northern areas had higher numbers of vertebrae and gill rakers than did those in more southern ones. Significant annual variability in the frequencies of these meristic characters within stocks was observed. When stocks were grouped into three regions (southern, central, and northern), heterogeneity in vertebral and gill raker frequencies was greater among regions than among stocks within the regions. Similarly, heterogeneity was greater among stocks than among sampling years within stocks. Differences in vertebral and gill raker frequencies are only useful for stock identification of sockeye salmon on a broad regional basis.     

Cladoceran and rotifer grazing on bacteria and phytoplankton in two shallow eutrophic lakes: in situ measurement with fluorescent microspheres

Metazooplankton grazing on bacteria and on the phytoplanktonof various sizes was estimated in shallow eutrophic lakes Kaiavereand Võrtsjärv (Estonia) by in situ feeding experimentswith fluorescent microspheres (diameters 0.5 µm for bacteriaand 3, 6 and 24 µm for phytoplankton). Zooplankton communitycomposition, abundance and food density were important factorsdetermining grazing rates in these lakes. Cladocerans and rotifersfiltering rates (FR) and ingestion rates (IR) on bacteria andphytoplankton were several times higher in Lake Kaiavere wherebacterivorous rotifers and Daphnia contributed more to zooplanktonassemblage. While cladocerans were generally the main phytoplanktonconsumers, both lakes differed with respect to the groups ofbacterivores. Based on consumption of fluorescent microspheres,the metazooplankton grazing rates were relatively low and hadlow impact on production and standing stock of bacteria andingestible phytoplankton (<30 µm). On average, 0.5and 0.1% of standing stock of bacteria and 2.6 and 1.0% of standingstock of ingestible phytoplankton was grazed daily by metazooplanktonin lakes Kaiavere and Võrtsjärv, respectively. Thatcorresponded to daily grazing of 4.1% of the bacterial productionand 0.43% of the total primary production (PP) by metazooplanktonin Lake Kaiavere compared with 4.3 and 0.06% in Lake Võrtsjärv,respectively. The results suggest that the majority of consumptionof the bacterial and phytoplankton PP is most likely channelledthrough the microbial loop.     

Response of phytoplankton and bacteria to nutrients and zooplankton: a mesocosm experiment

Although both nutrient inputs and zooplankton grazing are importantto phytoplankton and bacteria in lakes, controversy surroundsthe relative importance of grazing pressure for these two groupsof organisms. For phytoplankton, the controversy revolves aroundwhether zooplankton grazers, especially large cladocerans likeDaphnia, can effectively reduce phytoplankton populations regardlessof nutrient conditions. For bacteria, little is known aboutthe balance between possible direct and indirect effects ofboth nutrients and zooplankton grazing. However, there is evidencethat bacteria may affect phytoplankton responses to nutrientsor zooplankton grazing through direct or apparent competition.We performed a mesocosm experiment to evaluate the relativeimportance of the effects of nutrients and zooplankton grazingfor phytoplankton and bacteria, and to determine whether bacteriamediate phytoplankton responses to these factors. The factorialdesign crossed two zooplankton treatments (unsieved and sieved)with four nutrient treatments (0, 0.5, 1.0 and 2.0 µgphosphorus (P) l^–1 day^–1 together with nitrogen(N) at a N:P ratio of 20:1 by weight). Weekly sieving with 300µm mesh reduced the average size of crustacean zooplanktonin the mesocosms, decreased the numbers and biomass of Daphnia,and increased the biomass of adult copepods. Nutrient enrichmentcaused significant increases in phytoplankton chlorophyll a(4–5x), bacterial abundance and production (1.3x and 1.6x,respectively), Daphnia (3x) and total zooplankton biomass (2x).Although both total phytoplankton chlorophyll a and chlorophylla in the <35 µm size fraction were significantly lowerin unsieved mesocosms than in sieved mesocosms, sieving hadno significant effect on bacterial abundance or production.There was no statistical interaction between nutrient and zooplanktontreatments for total phytoplankton biomass or bacterial abundance,although there were marginally significant interactions forphytoplankton biomass <35 µm and bacterial production.Our results do not support the hypothesis that large cladoceransbecome less effective grazers with enrichment; rather, the differencebetween phytoplankton biomass in sieved versus unsieved zooplanktontreatments increased across the gradient of nutrient additions.Furthermore, there was no evidence that bacteria buffered phytoplanktonresponses to enrichment by either sequestering P or affectingthe growth of zooplankton.     

Distribution of planktonic ciliates in highly coloured subtropical lakes: comparison with clearwater ciliate communities and the contribution of myxotrophic taxa to total autotrophic biomass

SUMMARY 1. The planktonic ciliate communities of eleven organically coloured north and central Florida lakes representing a variety of trophic conditions were examined during 1979–80. The total abundance and biomass of ciliates were not significantly different from comparable clearwater lakes and only minor taxonomic replacements were noted at the order level.
2. Timing of population peaks of oligotrophic lakes was dissimilar to clearwater lakes of the same trophic state, but seasonality in meso-trophic and eutrophic lakes resembled patterns described for comparable clearwater lakes.
3. Various ciliate components were strongly correlated with chlorophyll a concentrations, but only moderately correlated to dominant phytoplankton groups. No significant correlations were found between ciliate components and bacterial abundance.
4. Myxotrophic taxa numerically dominated oligotrophic systems, particularly during midsummer, and accounted for a large percentage of the total ciliate biomass. Estimates of the ciliate contribution to total autotrophic biomass indicate that these zoochlorellae-bearing protozoa may account for much of the autotrophic biomass during midsummer periods in coloured lakes, and thus may lead to an overestimation of phytoplankton standing crops available to zooplankton grazers if chlorophyll a is used as a surrogate measure of algal biomass.