Activation of spawning in zebra mussels by algae-, cryptomonad-, and gamete-associated factors

In zebra mussels (Dreissena polymorpha) simultaneous release of gametes and peaks in larval densities at particular locations suggest that spawning is triggered by synchronizing stimuli. Furthermore, spawning tends to occur only after an adequate environmental temperature is reached. To test the hypothesis that phytoplankton and gamete-associated chemicals initiate spawning in zebra mussels and that the responsiveness to such chemicals is affected by ambient temperature, the spawning response of zebra mussels to extracts from algae, a cryptomonad, and a cyanobacterium and to water associated with released gametes was assayed in animals acclimated to 12 ^C and 17 ^C. For animals held at 12 ^C, only serotonin, a known activator of bivalve spawning used as a positive control, stimulated spawning. However, for animals acclimated to 17 ^C, extracts made from a diatom (Phaeodactylum), a brown alga (Fucus), and a cryptomonad (Rhodomonas) stimulated spawning in both sexes; extracts from green algae (Platymonas and Dunaliella) and a cyanobacterium (Oscillatoria) did not cause spawning. Water associated with either released sperm or eggs elicited spawning in both females and males. Positive controls, stimulated with serotonin, spawned at a high (>90%) rate, whereas no negative control spawned. Thus, phytoplankton chemicals and gamete-associated factors may have a role in synchronizing spawning in zebra mussels once adequate ambient temperature is reached.     

Feeding by larval and post-larval ctenophores on microzooplankton

Feeding by the coastal ctenophorc, Mnemiopsis leidyi, on microplanktonwas investigated. Larval ctenophores (tentaculate stage) grewbest and had the highest survival rates when offered a mixtureof ciliates and copepod nauplii. Larvae did not survive whenoffered phytoplankton alone. Clearing of planktonjc ciliatesby post-larval ctenophores was a function of the ciliate speciesand the size of the predator. Removal of small ciliates (<20µm in size) and phytoplankton was negligible. Small post-larvalctenophores (volume <4 cm³) had higher biovolume-specificclearing rates (0.5–1.5 1 cm^–3 day^–1) thandid larger ctenophores fed the same ciliate species. Duringin situ incubations, adult M. leidyi removed ciliates, rotifersand copepod nauplii from natural microplankton assemblages.The data indicate that non-crustacean microzooplanlctoo arean important component of the diet of larval and post-larvallocate cteoophores, particularly when copepod standing stocksare low.     

Seasonal species composition of barnacle larvae (Cirripedia: Thoracica) in Rhode Island waters, 1977-1978

Cirripede larvae can occur year-round in temperate and tropicalwaters, often in significant numbers, yet the species compositionof the ‘Balanus sp.’ component is rardy studied.Weekly plankton samples were analyzed qualitatively for larvalcirripede species and stage over a year (1977–1978) attwo Rhode Island stations. Six species of larvae were foundin Lower Narragansett Bay (30° salinity). Semibalanus balanoidesand Balanus balanus have a single winter brood. S. balanoidesis the predominant winter breeder with a minor release of naupliiin early December and major release in March followed by cypridsin mid-April.B. balanus populations release all larvae in Marchwith cyprids in mid-April. Balanus crenatus is mainly a winterbreeder, but has multiple broods; it does not breed in July-Septemberwhen the water temperature is above 18°C. Balanus venustusis the predominant summer breeder, and larvae were observedfrom May through December (water >8°C). Larvae of Chthamalusfragilis and Balanus eburneus occur in low numbers from May-October.At the Pettaquamscutt River site (12 salinity), Balanus improvisuslarvae predominate and early stage nauplii (I-II) occur in samplesyear round (0–27°C). Two peaks of later stage naupliiand cyprids occur in late spring (May) and early winter (Nov.-Jan.).Continued temperatures bdow 5°C or above 20°C appearto inhibit larval development. Comparison of results with existing literature reveals severalsignificant findings. The bimodal rdease of S. balanoides larvaeis unusual and may be in response to the phytoplankton dynamicsof the year; however, the existence of distinct races of S.balanoides may also be a factor. Larvae of B. venustus predominatein the lower bay during the summer, yet this species is unreportedin past studies. B. improvisus nauplii are more cold tolerantthan previously reported. Comparison of findings with reportedbreeding patterns in Florida indicate significant differencesin temperature responses between northern and southern populations. ¹Contribution No. 188 from EPA Environmental Research Laboratory,South Ferry Rd., Narragansett, RI 02882     

Plankton distribution across a slope current-induced front in the southern Bay of Biscay

Relatively warm (12.50–12.75°C) and high-salinity[<35.640 practical salinity units (PSU)] water flowing eastwardwas detected at the shelf-break during a cruise carried Outin the southern Bay of Biscay in Spring 1987. The slope currentinduced the formation of a convergent front separating well-mixedoceanic waters from haline-stratified coastal waters. Very highconcentrations of dissolved oxygen (295 µmol kg^–1)and chlorophyll a(>4.5 mg m^–3) were found at the outeredge of the frontal boundary. Small autotrophic flagellatesdominated the phytoplankton community. Primary production peakedat the boundary region. Estimated phytoplankton growth ratesindicated that active growth was taking place, with lower turnovertimes integrated over the water column at the frontal station(2.5–5 days) than at coastal (1.5–2.8 days) or oceanic(1.5–3.5 days) stations. The lowest doubling times (1–2days) were calculated for surface frontal populations. Accumulationof zooplankton was also observed associated with the convergentphysical structure, although this relationship was less markedthan for phytoplankton. Copepods, mainly Paracalanus parvus,Acartia clausi and Oithona helgolandica, formed the bulk ofthe mesozooplankton biomass. Compatibility between the sizeof phytoplankton cells and copepod size spectra indicate highfood availability for these animals, particularly in the vicinityof the front. The distribution of fish eggs and fish larvaewas also coupled with the slope current-induced front. Sardinelarvae were more abundant at the coastal side of the front,whereas larval stages of blue whiting reached the highest densitiesat off-shelf stations. Larvae of lamellibranch molluscs andbryozoa were restricted to nearshore waters, as the frontalboundary prevented larval dispersion to the open ocean. Theresults presented in this paper suggest that the Iberian slopecurrent and its associated shelf-break frontal structure werecrucial in controlling phytoplankton primary production, activityof grazers, distribution of larvae of fishes and benthic invertebrates,and ultimately in determining the structure of the pelagic foodweb in the southern Bay of Biscay during the seasonal periodof vertical mixing.     

Biogeography, Competition, and Microclimate: The Barnacle Chthamalus fragilis in New England

Geographic limits of species are commonly associated with climaticor physical boundaries, but the mechanisms of exclusion at thelimits of distribution are poorly understood. In some intertidalpopulations, the strengths of interactions with natural enemiesare mediated by microclimate, and determine geographic limits.The northern limit of the barnacle Chthamalus fragilis in NewEngland is the south side of Cape Cod, Massachusetts. Southof the cape, Chthamalus has a refuge from competition in thehigh intertidal, which is too hot for survival of its superiorcompetitor Semibalanus balanoides. North of the cape, the highintertidal is cooler, and Semibalanus survives, so Chthamalushas no refuge. Thus, geographic variation in the strength ofcompetition may determine the geographic limit of Chthamalus.Intolerance of cold by Chthamalus cannot account for the geographiclimit: transplants of Chthamalus 80 km beyond its northern limitsurvived up to 8 yr in the absence of competition with Semibalanus.At the geographic limit of Chthamalus in the Cape Cod Canalthere are two bridges, 5 km apart. On the southern bridge, Chthamalusis abundant and occupies a refuge above Semibalanus. On thenorthern bridge in 2001, only 7 individual Chthamalus were present.Despite the proximity of the bridges, their microclimates arevery different. The southern bridge, where Chthamalus is abundant,is up to 8°C hotter than the northern bridge. This highertemperature creates a refuge in the high intertidal for Chthamalus.On the cooler northern bridge, there is no refuge for Chthamalus.Because of the difference in temperatures of the water massesthat meet in the canal, heat storage in the rock of the bridgepiers causes the temperatures to differ between the bridges.Thus, geographic change in microclimate alters the strengthof competition, and determines the geographic limit.

"When wetravel from south to north, or from a damp region to a dry,we invariably see some species gradually getting rarer and rarer,and finally disappearing; and the change in climate being conspicuous,we are tempted to attribute the whole effect to its direct action.But this is a very false view: we forget that each species,even where it most abounds, is constantly suffering enormousdestruction at some period of its life, from enemies or fromcompetitors for the same place and food; and if these enemiesor competitors be in the least degree favoured by any slightchange of climate, they will increase in numbers, and as eacharea is already fully stocked with inhabitants, the other specieswill decrease."—Charles Darwin, On the Origin of Species,1859, p. 69.

     

Influence of sperm and phytoplankton on spawning in the echinoid Lytechinus variegatus

The cues triggering large-scale broadcast-spawning events in marine invertebrates are not fully understood. Using the sea urchin Lytechinus variegatus, we tested the effectiveness of a variety of potential spawning cues in eliciting a spawning response. In the laboratory, during two consecutive spawning seasons, about 400 isolated sea urchins were exposed to phytoplankton, sperm, or eggs, singly or in combination. The likelihood of spawning, time to spawning, and spawning behavior were recorded for both sexes. Sperm was most successful at inducing spawning. No response to eggs was noted. Phytoplankton alone did not trigger spawning, but when a phytoplankton cue was followed by the addition of sperm, spawning behavior was induced, the time between addition of sperm and spawning was reduced, and the variance among individuals in the time of spawning initiation was reduced. Males spawned sooner in response to cues than females and rarely spawned spontaneously in phytoplankton or control treatments. A semilunar pattern in the sensitivity to spawning cues was noted. During time periods when sea urchins were less ripe, the ratio of spawning males to spawning females increased. Our results indicate that seasonal and lunar cycles, together with the presence of phytoplankton, increase the sensitivity of these sea urchins to spawning cues and the precision of their responses to conspecific sperm.     

Predaceous feeding habits of Limnocalanus macrurus

Limnocalanus macrurus, a large, glacial-relict copepod, hasbeen assumed an omnivore or a herbivore; predaceous habits ofthe species are unknown. The predaceous feeding habits of Limnocalanusfrom Lake Michigan were studied in the laboratory using naturalprey. Predation rates were highest on copepod nauplii. Copepoditesof Diaptomus spp. and Cyclops spp. were preyed upon at lowerrates. Limnocalanus preyed selectively upon nauplii <300µm. Small cyclopoid copepodites (<–750 µm)were also selected over large copepodites. Experiments usingtwo prey types showed that nauplii were selected over all copepodites,and that no selectivity existed for either diaptomid or cyclopoidcopepodites. Predaceous feeding habits began in the fourth copepoditestage of Limnocalanus. Predaceous feeding rates of Limnocalanuschanged seasonally being highest in late spring and autumn andlowest in summer and early winter. Since Limnocalanus also feedson net-phytoplankton, predation rate changes may be relatedto changes in the relative abundance of large phytoplanktonand naupliar prey in nature. Limnocalanus predation may be animportant factor in structuring the zooplankton community. Present address: Great Lakes Research Division, University Michigan,Ann Arbor, MI 48109, USA     

Ctenophore-zooplankton-phytoplankton interactions in Narragansett Bay, Rhode Island, USA, during 1972-1977

Plankton dynamics at a station in lower Narragansett Bay, RIare compared for six summer and fall seasons, 1972–1977.In four of these years, initiation of the summer pulse of thectenophore Mnemiopsis leidyi was accompanied by a rapid declinein zooplankton abundance and a summer phytoplankton bloom. Terminationof the phytoplankton bloom coincided with depleted ctenophoreabundance and increased zooplankton biomass in two of the years.Yearly variations in the summer abundance of the diatom Skeletonemacostatum were positively related to the magnitude of the ctenophorepulse. The magnitude of ctenophore population was related tothe zooplankton biomass present at the start of the pulse. Theserelationships, the timing and magnitude of the plankton eventssuggest that M. leidyi regulated summer zooplankton and phytoplanktondynamics. Ctenophores may control phytoplankton blooms indirectlythrough their predation on herbivorous zooplankton and directlyby the nutrient excretion accompanying such grazing. This evidencethat a planktonic carnivore two trophic steps removed from thephytoplankton regulates the latter's dynamics in NarragansettBay is analogous to reported regulation of benthic algal (kelp)dynamics by the sea otter, lobster and various crabs throughtheir predation on herbivorous sea urchins. The factors responsiblefor the seasonal decrease in ctenophores remain unresolved;ctenophore predators on Mnemiopsis are absent in NarragansettBay. Infection by the vermiform larval anemone, Edwardsia lineata,grazing by the butterfish, Peprilus triacanthus, and changesin food availability, temperature and salinity likewise do notexplain this disappearance.     

Grazing on colonial and filamentous, toxic and non-toxic cyanobacteria by the zebra mussel Dreissena polymorpha

Colony forming and toxic cyanobacteria form a problem in surfacewaters of shallow lakes, both for recreation and wildlife. Zebramussels, Dreissena polymorpha, have been employed to help torestore shallow lakes in the Netherlands, dominated by cyanobacteria,to their former clear state. Zebra mussels have been presentin these lakes since they were created in the 19th century bythe excavation of peat and are usually not considered to bean invasive species. Most grazing experiments using Dreissenahave been performed with uni-cellular phytoplankton laboratorystrains and information on grazing of larger phytoplankton taxahardly exists. To gain more insight in to whether D. polymorphais indeed able to decrease cyanobacteria in the phytoplankton,we therefore performed grazing experiments with zebra musselsand two species of cyanobacteria, that greatly differ in shape:colony forming strains of Microcystis aeruginosa and the filamentousspecies Planktothrix agardhii. For both species a toxic anda non-toxic strain was selected. We found that zebra musselscleared toxic Planktothrix at a higher rate than non-toxic Planktothrix,toxic or non-toxic Microcystis. Clearance rates between theother strains were not significantly different. Both phytoplanktonspecies, regardless of toxicity, size and shape, were foundin equal amounts (based on chlorophyll concentrations) in theexcreted products of the mussels (pseudofaeces). The resultsshow that zebra mussels are capable of removing colonial andfilamentous cyanobacteria from the water, regardless of whetherthe cyanobacteria are toxic or not. This implies that the musselsmay be used as a biofilter for the removal of harmful cyanobacterialblooms in shallow (Dutch) lakes where the mussels are alreadypresent and not a nuisance. Providing more suitable substratefor zebra mussel attachment may lead to appropriate mussel densitiescapable of filtering large quantities of cyanobacteria.     

The zebra mussel (Dreissena polymorpha), a new pest in North America: reproductive mechanisms as possible targets of control strategies

Summary

The zebra mussel (Dreissena polymorpha) has spread rapidly in temperate fresh waters of North America since its introduction into the Great Lakes in 1985 or 1986. It attaches to hard substrates, forming layers, occluding water intakes, encrusting and killing native mussels, filtering algae in competition with other planktivores, and possibly interfering with fish spawning. It reproduces prolifically, suggesting that an approach to its control may be by controlling its reproduction. Previous literature suggests that spawning in bivalves is regulated by both environmental and internal chemical cues. A suggested sequence is that phytoplankton chemicals initially trigger spawning; chemicals associated with gametes provide a species-specific pheromonal positive feedback for spawning; and the response to environmental chemicals is mediated internally by serotonin (5-HT). The role of 5-HT in zebra mussels is under investigation. Both males and females can be induced to spawn by either injection or external application of 5-HT. The response can also be activated by 8-hydroxy-2-(di-n-propylamino)-tetralin, an agonist at 5-HT_1A receptors. HPLC analysis has detected 5-HT as the major biogenic amine in both male and female gonads. 5-HT immunocytochemistry demonstrates nerves containing serotonergic fibers innervating gonads of both males and females, with prominent varicosities surrounding the follicles in both sexes. A role of 5-HT in mediating spawning responses in zebra mussels is thus strongly supported. These studies have shown that reproductive behavior of zebra mussels can be modified by outside chemicals, a property that may be exploited for purposes of control.     

First observations of predation by New Zealand Greenshell mussels (Perna canaliculus) on zooplankton

Observations made overseas of predation by blue mussels and zebra mussels on mesozooplankton (>200 μm) have raised concern within New Zealand that the Greenshell mussel, Perna canaliculus, which is cultured in large tonnages throughout hundreds of marine farms within the New Zealand coastal zone, could exert ecologically detrimental effects by preying on zooplankton. We conducted experiments at Clova Bay, Pelorus Sound in May 2002 to determine the rates that P. canaliculus ingests prey, up to and including the mesozooplankton size range. Single mussels from farms were incubated with seawater enriched with zooplankton (>60 μm) in gently circulated 15-l pails. Depletion of chlorophyll-a (chl-a), ciliate microzooplankton, and nauplii, copepodites, and adults of copepods was determined over 5 h, relative to controls with no mussels. Two experiments were made over consecutive days. Gut contents of these experimental mussels, and of mussels examined soon after collection from a farm, were described.Gut contents of experimental and of freshly collected mussels (standard shell length ∼90 mm) had numerous copepod parts, whole copepods and larval bivalves present. Experimental mussels cleared chl-a and ciliates from 59- to 137-l individual⁻¹ day⁻¹, respectively, averaged across the two experiments. Faster ciliate than chl-a clearance was probably caused by the high proportion (56%) of phytoplankton below the retention size for P. canaliculus (ca. 5 μm) and by faster ciliate grazing in controls than treatments. The average clearance rates of adult, copepodite, and naupliar copepod stages by mussels were 20, 31, and 49 l individual⁻¹ day⁻¹, respectively. The clearance rates of each copepod stage were not significantly different between the two experiments. Clearance of nauplii was significantly greater than of adults and copepodites, while adult and copepodite clearance rates were nearly significantly different. The mean lengths of the adult, copepodite, and naupliar copepods were 430, 265, and 165 μm, respectively. The decreasing clearance rates with increasing size and development of prey (from ciliates, through naupliar, copepodite to adult copepods), suggested that prey escape ability, related to body size and/or morphology, affected capture rates. Mussel faecal samples indicated complete digestion of the gut contents. Pseudofaecal samples showed very low rejection rates of mesozooplankton by mussels. The results are considered in context of current biophysical modelling studies of impacts of large mussel farms in New Zealand. Designs of future experiments to improve accuracy of estimates of mesozooplankton clearance rates by P. canaliculus are considered.     

Sexual maturity in young Dreissena polymorpha from Lake Como (N. Italy)

Summary

The reproductive biology of the zebra mussel Dreissena polymorpha was studied during 1999 in a natural population living in Lake Como (N. Italy), with particular reference to the differences in behavior between young and adult mussels. Histological analysis of gonads was used to study the gametogenic cycles and to evaluate when the young reached sexual maturity. Signs of active gametogenesis were detected in ≥4 mm shell length mussels. Mature gametes and the first spawning events were observed in ≥5 mm males and in ≥6 mm females. The gonad feature of ≥7 mm mussels was similar to that of the adults. Size frequency distributions revealed that the new generation (0⁺) first spawned in the year following birth, but only after winter. The length of larval time seemed to vary greatly depending on the spawning period, in addition to the environmental conditions. The first settled mussels ≥500 μm in shell length were observed 3–4 months after the first depositions. Water temperature seemed to be the most important factor which regulated sexual phenomena of both young and adult mussels. Its role in the onset of the spawning events was confirmed, but it is important to point out that temperature is not enough to trigger reproduction since gamete release is impossible without sufficient energy, especially for young mussels.     

Natural variability in size and condition at settlement of 3 species of marine invertebrates

Experimental studies have demonstrated that for many marineinvertebrate species, variability in larval condition or qualityat settlement may have important effects on post-settlement,early juvenile performance. Relatively few studies, however,explicitly examine natural variability in larval condition atsettlement. This study examines natural variability in larvalattributes (size and lipid index) at settlement for terminal-stagelarvae of intertidal mussels (Mytilus sp.) and barnacles (Pollicipespolymerus and Chthamalus dalli) from southern California. Despitesignificant differences among cohorts in larval attributes,for all 3 species a greater percentage of the variance in larvallength (80–100%) and lipids (58–83%) occurred amongindividuals within a cohort, rather than among cohorts. Forall 3 species, coefficients of variation within a cohort forlength were much smaller (3–8%) than those for lipid index(30–93%), suggesting that lipid storage is a much moreplastic attribute than size for larvae. For mussels, settlementintensity and larval attributes were decoupled, such that averagelarval condition of a cohort was not related to the number oflarvae that settled. At the cohort level, Mytilus and Pollicipessettling together across 3 dates showed similar trends of decreasinglipid index over time, suggesting that environmental conditionsmay influence co-occurring planktonic larvae similarly acrossspecies. This work highlights the need for further experimentsin the field on the effects of larval history on recruitmentsuccess in natural populations, and further studies to determinewhat factors influence larval attributes for planktonic larvaein the field.     

Variations in the Reproductive Cycle of Dreissena Polymorpha in Europe, Russia, and North America

SYNOPSIS. The reproductive cycle of the zebra mussel {Dreissenapolymorpha) is highly variable throughout its range in Europe,Russia², and North America. The environmental factors influencingthis variation are poorly understood, but successful reproductionis occurring in areas where it was initially believed that adultzebra mussels could not survive (i.e., southern United States).The differences in mussel reproduction occurring from site-to-sitemake it difficult to predict timing of specific events, suchas the start of larval production, that are important in initiatingcontainment or control procedures. For example, the amount oftime required for a fertilized egg to develop into a juvenilemussel can be as short as 8 days, or as long as 240 days. Releaseof gametes by adults can be a highly synchronized event, focusedover a 1–2 week period, or it can be completely non-synchronized,occurring throughout the year. Zebra mussels in some localitiesstart spawning at water temperatures of 12–13°C, butdo not start until water temperatures reaches 22°C at othersites. While some of this variability in reproductive behaviorstems from mussel adaptation to local conditions, part is dueto difficulties in sampling these events. It is difficult todetermine reproductive success of a specific population becauseof the problems in separating locally produced larvae from larvaedrifting in from other areas. Further research is needed notonly on the relationship between reproduction and environmentat the community level, but also on the variability in responseof individual mussels.     

Interactions between red tide microalgae and herbivorous zooplankton: the noxious effects of Gyrodinium corsicum (Dinophyceae) on Acartia grani (Copepoda: Calanoida)

Gyrodinium corsicum is a dinoflagellate responsible for recurrentwater discolourations during winter in Alfacs Bay (Ebre Riverdelta, NW Mediterranean). Since first detected in 1994, episodicmortality of mussels and fish was attributed to this organism,although no direct evidence was obtained. In order to establishthe direct role of the dinoflagellate in the mortality of marinefauna, we have studied the effects of G.corsicum on a potentialpredator, the co-occurring planktonic copepod Acartia grani.Female A.grani were exposed to different concentrations of intactcells, and <5 µm and <0.2 µm filtrates ofthe dinoflagellate. At concentrations of     

Feeding efficiency of the larval ctenophore Mnemiopsis leidyi A. Agassiz (Ctenophora, Lobata)

Larval stages of the ctenophore Mnemiopsis leidyi rely on metazoanprey, such as Acartia tonsa nauplii and copepodites, to supporthigh growth rates. However, M. leidyi larvae <0.5 mm (totallength) had low retention efficiencies (REs) (proportion ofencountered prey actually ingested), 5.78 ± 2.6% (mean± SE), of nauplii and were often damaged by their encounters.REs of nauplii rapidly increased, 38.94 ± 3.73%, as larvaegrew to a size of     

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.     

Positive effects of UV radiation on a calanoid copepod in a transparent lake: do competition, predation or food availability play a role?

Zooplankton tolerant to ultraviolet radiation (UVR) could beindirectly affected by UVR through interactions with UV-sensitivespecies in the same ecosystem. In Lake Giles, Pennsylvania,USA, the calanoid copepod Leptodiaptomus minutus is more UVRtolerant than the cohabiting species Daphnia catawba and Cyclopsscutifer. We asked whether L. minutus is affected by UV-inducedmortality of a food competitor (D. catawba) or a predator ofits nauplii (C. scutifer). We conducted two in situ enclosureexperiments with six treatments: L. minutus alone, L . minutus+ Daphnia and L. minutus + Cyclops in the presence and absenceof UVR. There were few differences in survival among treatmentsin Experiment 1, which had enhanced food and a cumulative UVR(320 nm) dose of 9.3 kJ m^–2. In Experiment 2, which hadambient food and a UVR (320 nm) dose of 20.0 kJ m^–2, L.minutus survival and reproduction were higher in the +UVR comparedto –UVR, regardless of competitors or predators. Chlorophylla (Chl a) in Experiment 2 was higher in the +UVR than –UVR.While interactions between zooplankton species of differingUVR tolerances are potentially important, these results insteaddemonstrate that the beneficial UVR effect on L. minutus isindependent of concurrent detrimental UVR effects on competitorsand predators. Further research on the phytoplankton communityis necessary to determine whether UVR alleviates bacterial competition,increases nutrient availability or affects phytoplankton byother mechanisms.     

Copepod grazing on a phytoplankton community containing the toxic dinoflagellate Dinophysis acuminata

Copepod grazing on the toxic dinofiagellate Dinophysis acuminatafrom the west coast of France (La Rochelle) was studied witha concentrated (40–70 µm) phytoplankton assemblagedominated by Leptocylindrus danicus, D.acuminata, Ceratium fususand Ceratium furca. Copepod nauplii were also present. Threeto five copepods/copepodites (Acartia clausi, Isias clavipesand Centropages typicus) were incubated together with the phytoplankton.Dinophysis acuminata was grazed upon by all copepod species.However, to some extent, I.clavipes and C.typicus avoided itas food. Dinophysis acuminata cells represented for them only5–10% of total ingested carbon during the first 24 h,and almost all individuals survived and thrived well. In contrast,A.clausi did not avoid D.acuminata. which represented 30% ofingested carbon in 1 day. Acartia clausi then had a lower survivalthan the two other copepod species. However, the survival ofA.clausi was high in control incubations, where a plankton communitywithout D.acuminata was used as food. It is concluded that theokadaic acid of D acuminata is potentially toxic to some grazers,and/or might function as an allelopathic grazer repellent.     

Diel patterns of zooplankton grazing in a shallow lake

A diel survey of in situ species-specific zooplankton clearancerates (with radioactively labelled Chlorella cells) was conductedin the shallow lake of Cr?teil characterized by small-sizedplanktonic forms (algae<10µm and zooplankton <1.3mm). Experiments were performed every 4h at two depths (1 and4m). Power functions relating individual filtering rates tobody length were established for the three most abundant cladoceransand for calanoids (nauplii being included in this feeding group),for each depth and time. No filtering penodicity was observedin Ceriodaphnia spp., adults and copepodites of Eudiaptomusgracilis and Eurytemora velox, and nauplii. On the contrary,clear nocturnal filtering peaks were obtained for Daphnia spp.and for Diaphanosoma brachyurum, these being more pronouncedfor the larger individuals at 1 m deep. The observed diel periodicitycannot be explained by variations in physico-chemical parametersor food concentration alone. The ecological significance ofthis phenomenon in the polymictic lake of Cr?teil is discussedin the light of previously published data and the hypothesesaccounting for it.