Succession of Protists on Estuarine Aggregates

Abstract Colonization by and succession of bacteria and bacterivorous protists on laboratory-made aggregates were determined over a period of 14 days during winter and spring in 1997. Aggregates were generated from natural water from the limnetic zone of the Elbe Estuary using a tilting tube roller system. Within 1 h after the beginning of the experiments, macroaggregates started to form. Aggregates reached a maximum size of 1 mm with a tendency toward large sizes at the end of the experiment after the 10th day. On the first day, high bacterial densities of more than 10⁹ cells ml⁻¹ were detected within the aggregates. The abundances of flagellates and ciliates within aggregates were also two or three orders of magnitude higher than in the surrounding water. Densities of aggregate associated organisms are comparable to those occuring in sediments. The first protistan colonizers on the aggregates were small heterotrophic flagellates, such as choanoflagellates and small euglenids. Later, beginning on the 4th day, small sarcodines and ciliates became abundant. The most abundant ciliates associated with aggregates were small species of the Hypotrichia, Cyrtophorida, and Hymenostomata. After 9 days, large omnivorous and carnivorous ciliates, such as large members of the Hypotrichia and the Pleurostomatida, occurred. In spring, large heterotrophic flagellates and amebae also appeared at this time. These findings indicated the existence of a succession of protists on newly formed aggregates and a microbial food net within the aggregates based on bacterial production. Additionally, most of the species observed during this study were adapted for living on surfaces. In natural environments they are more common in benthic than in pelagic environments. For them, aggregates are havens in the water column comparable to sediment communities. Received: 7 January 2000; Accepted: 15 May 2000; Online Publication: 28 August 2000     

Seasonal dynamics of micro- and mesoplankton of the Temryuk estuarial pelagic ecosystem of the Sea of Azov

The variability in the composition and dynamics of microplankton (ciliate) and mesoplankton in the Temryuk estuarial pelagic ecosystem has been studied at different stages of succession of the pelagic communities. Invertebrates of 54 taxa, including ciliates of 24 taxa, have been identified. Among the ciliates, Myrionecta rubra, Mesodinium pulex, Halteria grandinella, Strombidium conicoides, S. sp. 1, S. sp. 2, Strobilidium sp., Loxmaniella oviformis, and Tintinnopsis minuta were dominant. The share of mesoplankton in the total biomass of zooplankton reached 70% (1.3 g/m³) and was estimated as the maximal value for mesoplankton of the Sea of Azov. Marine and brackish-water rotifers of the genus Synchaeta and Asplanchna priodonta constituted 55% of the biomass. The most abundant meroplanktonic larvae of Amphibalanus improvisus amounted to 30% of the total mesoplankton biomass. In the seasonal dynamics of microplankton development, two peaks of the biomass were revealed: spring (0.45 g/m³) and more expressed summer (0.9 g/m³); in mesoplankton there was one spring peak (3.7 g/m³). Superdense communities of ciliates and detritophages in some periods of the vegetative season are typical for the hypereutrophic ecosystem and can testify to the catastrophic character of its transformation.     

Environmental changes reflected by palynomorphs in the early Middle Ordovician Hanadir Member of the Qasim Formation, Saudi Arabia

Core samples from the QASIM-801 (QSIM-801) water well in the Qasim Region of central Saudi Arabia were investigated palynologically. The interval studied contains the transgressive succession from the Sajir Member of the Saq Formation to the Hanadir Member of the Qasim Formation, and yields well-preserved and abundant cryptospores, microphytoplankton (acritarchs and chlorophycean algae), and chitinozoans, indicative of a Darriwilian (Llanvirn) age. The lowermost portion of the cored interval from the upper Sajir Member consists of fine-grained sandstones deposited in tidal flat, and shallow-marine settings. It is characterized by abundant and diverse cryptospores (permanent tetrads, dyads, monads), cuticle-like phytoclasts, and a low diversity assemblage of marine palynomorphs. The upper part of the cored interval from the Hanadir Member is composed of hemipelagic to pelagic, organic-rich, graptolitic shales, and micaceous shales, of middle- to outer-shelf depositional settings, which yield abundant marine palynomorphs (acritarchs and chitinozoans). Cryptospore-bearing horizons are interspersed in the succession. Changes in the composition of palynomorph assemblages reflect higher frequency environmental changes within an overall transgressive succession. Four new acritarch species are described in open nomenclature, Tyrannus sp. A, Clypeolus sp. A,?Pulvinosphaeridium sp. A, and?Tinacula sp. A.     

Effects of Daphnia on microzooplankton communities

Intact phytoplankton and microzooplankton communities from eutrophicStar Lake were incubated for 4 days with and without Daphniapulex, Daphnia galeaia mendotae, or a natural assemblage ofDaphnia species. They were sampled at the onset and terminationof the experiment for bacterial, phytoplankton, ciliate, rotifer,copepod and cladoceran densities. The cladocerans had variedeffects on the rotifers, ranging from significant suppressionof most rotifer species (Keratella cochlearis, Polyarthra remata,Keratella crassa) in the D.pulex jars, to the suppression ofone (K.crassa) or no species in the D.galeata mendotae and StarLake Daphnia assemblage jars, respectively. Small ciliates (<30µm, longest dimension), such as Strobilidium sp. and Pseudo-cyclidiumsp., were adversely affected by most of the cladoceran treatments,while several larger ciliates (>81 µm) were unaffectedin all such treatments. Ciliates were not consistently morevulnerable to cladoceran suppression than similarly sized rotifers.The suppression of ciliates and rotifers was attributable toboth direct effects (predation, interference, or both) and indirecteffects (e.g. resource competition) of the cladocerans. ¹Present address: Department of Biology, University of Louisville,Louisville, KY 40292, USA     

Photosynthetic pigments in the ciliate Laboea strobila from Long Island Sound, USA

Laboea sirobila Lohmann, a marine oligotrich ciliate, was presentin Long Island Sound, USA, from March through June 1984. Peakabundance occurred in mid-June, when it was estimated that 10.2x 10⁶ individuals m^–2 were present in the 37-m deep watercolumn. When preserved specimens were examined with epifluorescencemicroscopy, the photosynthetic pigments chlorophyll and phycoerythrinwere observed within chloroplasts inside the ciliate. Thesepigments were also present in Myrionecta (formerly Mesodinium)rubrum Jankowski, a photosynthetic ciliate which co-occurredwith L. strobila during the present study. Vertical profilesshowed both ciliates to be most abundant in the upper few metersof the water column, especially during periods of strong thermalstratification.     

Top-down impact by copepods on ciliate numbers and persistence depends on copepod and ciliate species composition

Experiments with the copepods Acartia clausi, Centropages hamatusand Pseudocalanus sp. were performed to assess the species-specificeffect of these copepods on the development of monospecificalgae (Nephroselmis pyriformis) and ciliate communities (Strombidiumvestitum, Strombidium conicum, Strombidium sp. and Lohmanniellaoviformis). It was hypothesized that potentially switching copepodslike A. clausi will stabilize the algal community by switchingbetween ciliate and algal food, in contrast to copepods withstereotypic filter feeding behaviour (Pseudocalanus sp.). Intreatments with Pseudocalanus sp. and C. hamatus, all ciliatespecies were wiped out in 2 days, resulting in blooms of N.pyriformis. In treatments with A. clausi, two of the ciliatespecies were able to persist, but the combined ciliate and copepodcommunity was not able to control the algal bloom. Ciliatesbecame abundant in control treatments without copepods, butonly S. vestitum and S. conicum seemed able to establish grazingcontrol. Hence, when evaluating the role of ciliates in foodwebs, their actual numbers and species composition should betaken into account. Likewise, the species composition of copepodsmay be crucial; these experiments demonstrate that small filterfeeding copepods may have tremendous impact on ciliate numbers.     

The annual cycle of planktonic ciliates in nearshore waters at Signy Island, Antarctica

The abundance and biomass of marine planktonic ciliates in BorgeBay, Signy Island, were determined at monthly intervals betweenApril 1990 and June 1991. At least 24 different ciliate taxawere recorded from samples preserved in Lugol's iodine, includingthe tintinnids Codonellopsis balechi, Cymalocylis convallaria,Laackmaniella naviculaefera and Salpingella sp., and the aloricatetaxa Didinium sp. and Mesodinium rubrum. Ciliate abundance andbiomass exhibited a clear seasonal cycle with high values duringthe austral summer and low values in the austral winter. Abundanceranged from 0.3 10³l^–1 in September to 2.3 10³l^–1in January, while biomass ranged from 0.5 µg C l^–1in October to 12.6 µg C l^–1 in December. Small ciliatesdominated abundance throughout the year, and biomass duringwinter. Larger ciliates contributed most to biomass during summer.Aloricate ciliates were common throughout the year, while tintinnidscontributed substantially to abundance and biomass only duringsummer. Salpingella sp. was the commonest tintinnid, but C.convallariacontributed most to tintinnid biomass. The seasonal patternof ciliate abundance and biomass matched that of chlorophylla concentration and bacterial biomass, suggesting tight trophiccoupling between ciliates and other components of the pelagicmicrobial community. ¹Present address: Scott Polar Research Institute, Universityof Cambridge, Lensfield Road, Cambridge CB2 1ER, UK     

Laboratory and field observations on the scuticociliate Histiobalantium from the pelagic zone of Lake Constance, FRG

Histiobalantium sp. was found regularly in the pelagic zoneof Lake Constance, FRG, over five annual cycles. Maxima of upto 6400 cells l^–1 were recorded in late summer, with similarnumbers in the 0–8 and 8–20 m depth intervals. Onan annual average, the population accounted for 10–17%of the total biomass of planktonic ciliates. In the laboratory,Histiobalantium grew well on a diet of the cryptophyte Rhodomonassp. Maximum growth rates obtained in batch cultures were 0.21and 0.33 day^–11 at 9 and 18°C, respectively. In situexperiments using diffusion chambers yielded positive growthrates in autumn and winter. The highest values recorded at theambient temperatures 5, 14 and 17°C were 0.17, 0.32 and0.40 day^–1, respectively. Comparing these results withthe different seasonal distributions and higher measured growthrates of other ciliates from Lake Constance, we conclude thatHistiobalantium is a superior competitor at relatively low algalfood concentrations. ²Present address: Fisheries & Oceans Canada, 4160 MarineDrive, West Vancouver, BC, V7V 1N6, Canada     

Indirectly fluorescently labelled flagellates (IFLF): a tool to estimate the predation on free-living heterotrophic flagellates

A procedure was developed to estimate the direct grazing impacton free-living heterotrophic nanoflagellates (HNF). Culturedflagellates were labelled by feeding on brightly fluorescingbacteria (FLB) and then offered as indirectly fluorescentlylabelled flagellates (IFLF) to potential predators of HNF. Thenumber of FLB in the predators' food vacuoles could be convertedinto IFLF uptake and consumption of HNF. This new techniquewas used to study the HNF-ciliate relationship in the pelagiczone of Lake Constance. Three groups of ciliates were detectedas HNF grazers: small representatives of the genus Strobilidium.a small Haltena-like ciliate (probably Halteria grandinella)and a Codonella sp. Tintinnidium sp. group The ingestion ofHNF by these groups of ciliates ranged between 3 and 15, 3 and39, and 3 and 7 HNF ciliate^–1 h^–1; respectively.The IFLF method allows the direct determination of ingestedflagellate prey in the food vacuoles of their predators. Becauseindigenous living prey organisms were used, tracer discriminationcan be reduced.     

Small-scale distribution of macroplankton and micronekton in the Ligurian Sea (Mediterranean Sea) as observed from the manned submersible Cyana

A series of eight submersible dives (the MIGRAGEL I cruise)was made during late April 1986 using the French submersibleCyana to investigate macrozooptankton in the upper 400–700m of the water column. Paired day and night dives were madeat stations 3, 6, 13 and 23 nautical miles off Cape Ferrat,near Villefranche-sur-Mer, France; the distances represent differentareas in the frontal system of the Ligurian Sea. Detailed day/nightvertical distribution data are shown for the most abundant species;these include the narcomedusa Solmissus albescens, teleost fishCyclothone spp., small appendiculanans (primarily Oikopleuraalbicans), large appendicularians (an undescribed oikopleurid),diphyid siphonophores (mostly Chelophyes appendiculata) andan abundant lobate ctenophore. Salps, pyrosomes, amphipods (Phroniumasedentaria), pteropods (Cavolinia inflexa), macroscopic ‘star-like’protozoa and marine snow are also briefly discussed. The coastalzone was dominated by small appendicularians in the upper layers,with other filter feeders including large appendicularians indeeper water—these just above a non-migratory populationof carnivorous Cyclothone. The carnivorous medusa Solmissusalbescens moved throughout the upper 600 m in the course ofits diel vertical migration. Offshore, carnivores were dominantthroughout the water column, with numerous diphyid siphonophoresin the upper layers, and Cyclothone, lobate ctenophores andmacroprotozoa abundant in deeper water. Solmissus was also present,and was more numerous offshore than in the coastal zone.     

Comparative growth rates and yields of ciliates and heterotrophic dinoflagellates

Growth rates, ingestion rates and grazer yields (grazer volumeproduced/prey volume consumed) were measured for six protozoanspecies (ciliates: Favella sp., Strombidinopsis acuminatum,Uronema sp.; heterotrophic dinoflagellates: Amphidinium sp.,Gymnodinium sp., Noctiluca scintillans) in laboratory batchculture experiments. Comparative growth data indicate that theprymnesiophyte Isochrysis galbana, the prasinophyte Mantoniellasquamata, two cryptophyte species and several autotrophic dinoflagellatespecies were suitable foods for these grazers. When grown onoptimized diets at 13C, maximum ciliate growth rates (range0.77–1.01 day^–1 uniformly exceeded maximum heterotrophicdioflagellate growth rates (range 0.41–0.48 day^–1).A compilation of published data demonstrates that this growthrate difference persists across a range of ciliate and dinoflagellatetaxa and cell sizes. Comparison of volume-specific ingestionrates and yields for the six species studied here showed thatthere was no single explanation for this growth rate disparity.Heterotrophic dinoflagellates exhibited both low ingestion ratesand, in one case, low yields; ciliates were able to achievehigher growth rates via either higher ingestion rates or higheryields, depending on ciliate species. Volume yield increasedover time throughout the exponential growth phase in nearlyall experiments, suggesting variation in response to changingfood concentrations or long-term acclimation to culture conditions.Higher maximum ciliate growth rates mean that these grazershave the potential to exercise tighter control over incipientblooms of their prey than do heterotrophic dinoflagellates.     

Effects of rotifers and ciliates on the growth and survival of Daphnia

Daphnia can suppress ciliates and rotifers through predationand interference competition, but it is not known whether thisproduces any direct benefit to Daphnia. We conducted survivorshipand cohort lifetable experiments to determine whether Daphniacan utilize ciliates and rotifers as food. Three species ofoligotrich ciliates (Halteria grandinella, Strobilidium gyransand Strobilidiumvelox) and one rotifer (Keratella cochlearis)were used. Lifetable experiments were conducted with a basallevel of algae (Cryptomonas sp.), plus either ciliates or rotifers,while survivorship experiments had only the rotifers or ciliates.Densities of 30 H.grandinella ml^–1, 50 S.gyrans ml^–1and 15 S.velox ml^–1 enhanced Daphnia pulex's populationgrowth rate 35–50% over controls with only algae. TenS.gyrans ml^–1 did not produce a significant change inDaphnia's growth rate. Densities of 100 and 300 K.cochlearis^–1 increased Daphnia population growth rates by II and10%, respectively. Both 10 and 50 S.gyrans ml^–1 enhancedDaphnia's survivorship compared to starved controls, but neither100 nor 300 K.cochlearis l^–1 enhanced its survivorship.The amount of enhancement of Daphnia growth rates by rotifersand ciliates is roughly proportional to the death rates imposedby Daphnia. The death rate imposed by Daphnia on rotifers isa function of both algal density and Daphnia size. Per unitbiomass, neither ciliates nor Keratella appear to be as nutritiousfor Daphnia as is Cryptomonas.     

Effects of environmental factors on ichthyoplankton communities in the Miramichi estuary, Gulf of St Lawrence

Ichthyoplankton in 20 taxa (17 identified to species, threeto genus) representing 14 families were collected in 10 surveysof the Miramichi estuary between May and September 1992. Thetaxonomic composition was typical of other estuaries in theGulf of St Lawrence and Gulf of Maine. Larvae of three anadromousspecies [rainbow smelt (Osmerus mordax), alewife (Alosa pseudoharengus)and blueback herring (Alosa aestivalis)] were several ordersof magnitude more abundant than any other taxon. The upper estuary(Miramichi River and its tributaries) probably serves as a nurseryground for larvae of these species and others such as Atlantictomcod (Microgadus tomcod). The species composition of the lowerestuary (Miramichi Bay) was dominated by typically marine formsand probably serves as a nursery ground for winter flounder(Pleuronectes americanus), smooth flounder (Pleuronectes putnami).sculpin (Myoxocephalus sp ) and sand lance (Ammodytes sp) Ofthe environmental factors investigated, salinity was the mostuseful predictor of larval distribution in the estuary.     

Diatom-Specific Highly Branched Isoprenoids as Biomarkers in Antarctic Consumers

The structure, functioning and dynamics of polar marine ecosystems are strongly influenced by the extent of sea ice. Ice algae and pelagic phytoplankton represent the primary sources of nutrition for higher trophic-level organisms in seasonally ice-covered areas, but their relative contributions to polar marine consumers remain largely unexplored. Here, we investigated the potential of diatom-specific lipid markers and highly branched isoprenoids (HBIs) for estimating the importance of these two carbon pools in an Antarctic pelagic ecosystem. Using GC-MS analysis, we studied HBI biomarkers in key marine species over three years in Adélie Land, Antarctica: euphausiids (ice krill Euphausia crystallorophias and Antarctic krill E. superba), fish (bald notothens Pagothenia borchgrevinki and Antarctic silverfish Pleuragramma antarcticum) and seabirds (Adélie penguins Pygoscelis adeliae, snow petrels Pagodroma nivea and cape petrels Daption capense). This study provides the first evidence of the incorporation of HBI lipids in Antarctic pelagic consumers. Specifically, a di-unsaturated HBI (diene) of sea ice origin was more abundant in ice-associated species than in pelagic species, whereas a tri-unsaturated HBI (triene) of phytoplanktonic origin was more abundant in pelagic species than in ice-associated species. Moreover, the relative abundances of diene and triene in seabird tissues and eggs were higher during a year of good sea ice conditions than in a year of poor ice conditions. In turn, the higher contribution of ice algal derived organic matter to the diet of seabirds was related to earlier breeding and higher breeding success. HBI biomarkers are a promising tool for estimating the contribution of organic matter derived from ice algae in pelagic consumers from Antarctica.     

Predation by ciliates on a metalimnetic Cryptomonas population: feeding rates, impact and effects of vertical migration

Deep algal maxima are frequently overlayed by dense populationsof ciliates, rotifers and crustaceans. This has been interpretedas evidence of heavy predation on the algae, although the impactof this predation has never been determined experimentally.We determined the vertical and seasonal distribution of thealga Cryptomonas phaseolus and its most relevant predators,the ciliates Coleps sp. and Prorodon sp., forming metalimneticmaxima in Lake Cisó. On several dates, in situ feedingrates of ciliates were determined by three independent methods:(i) epifluorescence counts of ingested algal cells togetherwith estimates of the food turnover time of the ciliates; (ii)in situ incubations with radioactively labeled algae: (iii)HPLC determination of alloxanthine content in the predator sizefraction. Feeding rates varied between 0.07 and 0.64 Cryptomonasciliate^–1 h^–1. We then calculated integrated predationon the algae. using the functional response of the ciliatesand the vertical distribution of each population. We found thateven though the ciliates were always food saturated, their predationimpact on Cryptomonas was not very large: as an average, 5–25%of the biomass of Cryptomonas was removed daily by the ciliates.Finally, we studied the effects of the diel vertical movementsof these populations on predation impact. By migrating intothe sulfide-rich hypolimnion during the night, Cryptomonas couldreduce its predation losses by 38%. Thus, the algae were protectedfrom predation during several hours of each diel cycle and maintaineda very large biomass throughout stratification, although thisresulted in a very slow growth. Slow growth, coupled to largebiomass, seems to be a general feature of metalimnetic accumulationsof organisms.     

Fine scale spatial correlations between planktonic ciliates and dinoflagellates

Fine scale spatial distributions of planktonic ciliates whichprey on dinoflagellates were investigated in a small estuary.The horizontal distributions of Favella sp., Balanion sp. andStrobilidium sp. were positively correlated with the patchydistribution of dinoflagellates. The vertical distribution ofFavella was positively corrdated with the distribution of dinoflagellates.Throughout the diel cycle, the vertical distribution of Faveilawas similar to the vertical distribution of dinoflagellates.We speculate that behavioral responses are responsible for theassociation of diliates with their prey in the water column.Vertically coincident ciliate and algal populations should respondsimilarly to turbulent mixing and water displacement causedby wind stress, bathymetry, or frontal convergences. This wouldresult in horizontal patches in which the ciliates and algaeremain associated. This fine scale spatial coupling betweenciliates and their prey should result in higher ciliate growthrates and greater impact of ciliate grazing on phytoplanktonpopulations than would be predicted from average ciliate oralgal densities. Coincident patches of algae and ciliates mayalso provide higher food concentrations for larger grazers whichcan use both resources. ^{Contribution No. 5532, W.H.O.I.}     

Microbial Community Structure and Dynamics in the Largest Natural French Lake (Lake Bourget)

We investigated the dynamics and diversity of heterotrophic bacteria, autotrophic and heterotrophic flagellates, and ciliates from March to July 2002 in the surface waters (0–50 m) of Lake Bourget. The heterotrophic bacteria consisted mainly of “small” cocci, but filaments (>2 μm), commonly considered to be grazing-resistant forms under increased nanoflagellate grazing, were also detected. These elongated cells mainly belonged to the Cytophaga-Flavobacterium (CF) cluster, and were most abundant during spring and early summer, when mixotrophic or heterotrophic flagellates were the main bacterial predators. The CF group strongly dominated fluorescent in situ hybridization–detected cells from March to June, whereas clear changes were observed in early summer when Beta-proteobacteria and Alpha-proteobacteria increased concomitantly with maximal protist grazing pressures. The analysis of protist community structure revealed that the flagellates consisted mainly of cryptomonad forms. The dynamics of Cryptomonas sp. and Dinobryon sp. suggested the potential importance of mixotrophs as consumers of bacteria. This point was verified by an experimental approach based on fluorescent microbeads to assess the potential grazing impact of all protist taxa in the epilimnion. From the results, three distinct periods in the functioning of the epilimnetic microbial loop were identified. In early spring, mixotrophic and heterotrophic flagellates constituted the main bacterivores, and were regulated by the availability of their resources mainly during April (phase 1). Once the “clear water phase” was established, the predation pressure of metazooplankton represented a strong top-down force on all microbial compartments. During this period only mixotrophic flagellates occasionally exerted a significant bacterivory pressure (phase 2). Finally, the early summer was characterized by the highest protozoan grazing impact and by a rapid shift in the carbon pathway transfer, with a fast change-over of the main predators contribution, i.e., mixotrophic, heterotrophic flagellates and ciliates in bacterial mortality. The high abundance of ciliates during this period was consistent with the high densities of resources (heterotrophic nanoflagellates, algae, bacteria) in deep layers containing the most chlorophyll. Bacteria, as ciliates, responded clearly to increasing phytoplankton abundance, and although bacterial grazing impact could vary largely, bacterial abundance seemed to be primarily bottom-up regulated (phase 3).     

Protist distribution in the Western Fram Strait in summer 2010 based on 454‐pyrosequencing of 18S rDNA

In this study, we present the first comprehensive analyses of the diversity and distribution of marine protist (micro‐, nano‐, and picoeukaryotes) in the Western Fram Strait, using 454‐pyrosequencing and high‐pressure liquid chromatography (HPLC) at five stations in summer 2010. Three stations (T1; T5; T7) were influenced by Polar Water, characterized by cold water with lower salinity (<33) and different extents of ice concentrations. Atlantic Water influenced the other two stations (T6; T9). While T6 was located in the mixed water zone characterized by cold water with intermediate salinity (~33) and high ice concentrations, T9 was located in warm water with high salinity (~35) and no ice‐coverage at all. General trends in community structure according to prevailing environmental settings, observed with both methods, coincided well. At two stations, T1 and T7, characterized by lower ice concentrations, diatoms (Fragilariopsis sp., Porosira sp., Thalassiosira spp.) dominated the protist community. The third station (T5) was ice‐covered, but has been ice‐free for ~4 weeks prior to sampling. At this station, dinoflagellates (Dinophyceae 1, Woloszynskia sp. and Gyrodinium sp.) were dominant, reflecting a post‐bloom situation. At station T6 and T9, the protist communities consisted mainly of picoeukaryotes, e.g., Micromonas spp. Based on our results, 454‐pyrosequencing has proven to be an adequate tool to provide comprehensive information on the composition of protist communities. Furthermore, this study suggests that a snap‐shot of a few, but well‐chosen samples can provide an overview of community structure patterns and succession in a dynamic marine environment.     

Quantifying marine snow as a food choice for zooplankton using stable silicon isotope tracers

Aggregates of biogenic origin >0.5 mm, known as marine snow,represent a concentrated potential source of food for zooplankton.Little is known, however, about whether aggregates are commonlygrazed by zooplankton in the field. While previous laboratorystudies have shown that the euphausiid Euphausia pacifica, andthe copepod, Calanus pacificus, common crustacean zooplankters,consume marine snow if it is the only food source available,it is not known if euphausiids will select marine snow in thepresence of edible dispersed cells, as readily occurs in nature.To examine this question, we offered E. pacifica the diatom,Nitzschia angularis in aggregated and dispersed form as preysimultaneously. Aggregates and dispersed food contained identicalcell types so that differing prey quality, taste or motilitywould not be a factor. A new method was developed to track foodsources by labeling the frustules of aggregated cells with differentnaturally occurring, but rare, stable isotopes of silicon, ³⁰Siand ²⁹Si. Food selection was then estimated by measuring theisotopic composition of silica within fecal pellets producedby animals feeding on mixtures of the two labeled foods. Resultsindicate that E. pacifica consumed both aggregates and dispersedcells, even when more cells were made available in dispersedform than in aggregated form. This suggests that aggregatesmay indeed be a food source in the field, even when dispersedcells are relatively abundant. The method of labeling diatomcells with stable isotopes of silica may prove useful for futuregrazing experiments to distinguish identical cell types.     

A new species of pelagic amphipod of the genus Lestrigonus (Crustacea: Amphipoda: Hyperiidea: Hyperiidae) from eastern Australia

A new species of pelagic amphipod, Lestrigonus ducrayi sp. nov.,from eastern Australia, is described and illustrated. It isvery similar to Lestrigonus bengalensis Giles, 1887, but isreadily distinguished by a relatively shorter telson (aboutone-third peduncle uropod 3) and, in mature males, pereonites1–5 are fused as in females. The two species often occurtogether in the same plankton sample; the new species beingmore abundant inshore, although both species seem to preferinshore waters. A key to the species of Lestrigonus is provided.