三峡水库神农溪2014年春季浮游藻类演替成因分析

摘要：【目的】研究三峡水库神农溪库湾春季水华期间浮游藻类演替及其成因分析。【方法】2014年3–5月在神农溪库湾布置了6个断面(SN01–SN06),在神农溪汇入长江干流河口附近水域设置1个断面CJBD,对浮游藻类、相关环境因子及水动力因子进行了同步监测,据此分析了水体层化结构及水动力特性。【结果】神农溪在监测时段内共检测到浮游藻类6门38种(属);库湾浮游藻类生物量时间上差异显著(ANOVA,P<0.05)。春季浮游藻类群落结构具有明显的演替规律,3月份暴发大面积的硅藻水华（藻密度>100×105 cells/L),小环藻(Cyclotella spp.)为优势藻种;4月在SN02–SN06暴发以小球藻(Chlorella spp.)为主要优势种、衣藻(Chlamydomonas spp.)为次优势种的绿藻水华(藻密度>100×105 cells/L),5月份受水位大幅消落影响,浮游藻类生物量降低且无明显优势藻种。【结论】在具备充足的营养盐的水体中,水体层化结构与水动力特性对浮游藻类演替影响重大。三峡水库水位处于快速消落阶段时,流速成为抑制神农溪库湾藻类生长的主要因素。     

GRAZING AND GROWTH OF THE MIXOTROPHIC CHRYSOMONAD POTERIOOCHROMONAS MALHAMENSIS (CHRYSOPHYCEAE) FEEDING ON ALGAE

The growth and grazing characteristics of Poterioochromonas malhamensis (Pringsheim) Peterfi (= Ochromonas malhamensis Pringsheim) (ca. 8 μm) feeding on phytoplankton, including the cyanobacteria Synechococcus sp. (ca. 2 μm) and Microcystis viridis (A. Brown) Lemmermann (ca. 6 μm) and the green alga Chlorella pyrenoidosa Chick (ca. 13 μm), were investigated in laboratory experiments involving the following treatments: (1) light without added algal prey (autotrophy), (2) light with added algal prey (mixotrophy), and (3) dark with added algal prey (phagotrophy). There were significantly higher cell numbers under mixotrophic and phagotrophic growth than under autotrophic growth. With phytoplankton as food, growth rates under both mixotrophy and phagotrophy were about two or three times higher than those under autotrophy, indicating that the algal diets were readily able to support the population growth of P. malhamensis. There were no significant differences in growth rate between mixotrophic and phagotrophic cultures during exponential growth. The ingestion rate of P. malhamensis with algal prey was also similar under both continuous light and dark. Poterioochromonas malhamensis ingested on average 0.27 M. viridis cells·flagellate^{− 1} ·h^{− 1} and 0.18 C. pyrenoidosa cells·flagellate^{− 1} ·h^{− 1} in continuous light and 0.25 M. viridis cells·flagellate^{− 1} ·h^{− 1} and 0.18 C. pyrenoidosa cells·flagellate^{− 1} ·h^{− 1} in continuous dark during exponential growth. The results showed that light had no effect on the growth and ingestion rates of P. malhamensis for phagotrophy during exponential growth. However, phagotrophic populations of P. malhamensis were incapable of growth in continuous darkness for longer than 5 days. Populations of P. malhamensis showed no increase when prey was added again after 4 days in continuous darkness, indicating that light is necessary for sustained phagotrophic growth of P. malhamensis. The study suggests that P. malhamensis, which has strong tolerance for light, is light dependent for phagotrophy.     

Ultrastructure and Systematics of Two New Species of Dinoflagellate,Paragymnodinium Asymmetricum sp. nov. and Paragymnodinium Inerme sp. nov. (Gymnodiniales,Dinophyceae)1

The genus Paragymnodinium currently includes two species, P. shiwhaense and P. stigmaticum, that are characterized by mixotrophic nutrition and the possession of nematocysts. In this study, two new dinoflagellates belonging to this genus were described based on observations using LM, SEM, and TEM together with a molecular analysis. Cells of P. asymmetricum sp. nov., isolated from Nha Trang Beach, Vietnam, were 7.9–12.6 μm long and 4.7–9.0 μm wide. The species showed no evidence of feeding behavior and was able to sustain itself phototrophically. Paragymnodinium asymmetricum shared many features with P. shiwhaense, including presence of nematocysts, absence of an eyespot, and a planktonic lifestyle, but was clearly distinguished by the asymmetric shape of the hyposome, possession of a single chloroplast, and its nutritional mode. Cells of P. inerme sp. nov., isolated from Jogashima, Kanagawa Pref, Japan, were 15.3–23.7 μm long and 10.9–19.6 μm wide. This species also showed no evidence of feeding behavior. Paragymnodinium inerme was similar to cells of P. shiwhaense in shape and planktonic lifestyle, but its nutritional mode was different. The presence of incomplete nematocysts was also a unique feature. A phylogenetic analysis inferred from concatenated SSU and LSU rDNA sequences recovered the two dinoflagellates in a robust clade with Paragymnodinium spp., within the clade of Gymnodinium sensu stricto. This evidence, together with their morphological similarities, made it reasonable to conclude that these two dinoflagellates are new species of Paragymnodinium.     

Predator–prey interactions between a planktonic ciliate Strombidium sp. (Ciliophora, Oligotrichida) and the dinoflagellate Pfiesteria piscicida (Dinamoebiales, Pyrrophyta)

The functional response of a planktonic ciliate, Strombidium sp. feeding on the dinoflagellate Pfiesteria piscicida non-toxic zoospores (NTZ) was experimentally studied with four different prey concentrations (43–3153 cells ml⁻¹). Data from direct observations (NTZ inside individual Strombidium sp.) was used to calculate predator–prey specific ingestion and clearance rates. The ingestion rates varied between 0.68 and 14.26 NTZ ind⁻¹ h⁻¹, and with the predator–prey specific handling time of 2.83 min the U_max was 21.18 NTZ ind⁻¹ h⁻¹. The increase in the prey concentration between approximately 700 and 3000 NTZ ml⁻¹ did not increase the uptake of prey, and at the lowest Pfiesteria NTZ concentrations the feeding efficiency of Strombidium sp. was lowered, possibly indicating a situation of threshold feeding. When data from direct observations of ingested Pfiesteria NTZ were compared with values of total NTZ loss from the experimental water during the experiment, ingestion was found to represent only a fraction of the total NTZ loss in the presence of ciliates. This discrepancy was concluded to be due to other grazer related factors than actual ciliate grazing. The control of the initial growth of Pfiesteria community, in a pre-bloom situation, would require only a small ciliate abundance (less than 5 ml⁻¹), but when the Pfiesteria NTZ are scarce, relatively more ciliates are needed to limit the population growth of the dinoflagellate community because of the apparent feeding threshold. It is concluded that the formation of non-toxic P. piscicida blooms require periods of low grazing pressure or a means to escape grazing.     

Grazer interactions with four species of Lyngbya in southeast Florida

Blooms of the toxic cyanobacteria Lyngbya spp. have been increasing in frequency and severity in southeast Florida in recent years. Lyngbya produces many active secondary metabolites which often act as feeding deterrents to generalist herbivores, possibly increasing the longevity of these nuisance blooms. Whilst diverse arrays of small invertebrate consumers are often found in association with Lyngbya, little is known of their grazing selectivity among species of Lyngbya. This study examines the feeding preference of grazers for four local Lyngbya species (Lyngbya majuscula, Lyngbya confervoides, Lyngbya polychroa and Lyngbya spp.). Stylocheilus striatus and Haminoea antillarum showed no dietary selectivity between L. polychroa, L. majuscula and Lyngbya spp. in multiple choice feeding assays, whereas Bulla striata showed a distinct preference for L. polychroa (P < 0.001). To determine whether preference might be related to species-specific secondary metabolites, L. majuscula, L. confervoides and L. polychroa non-polar and polar extracts were incorporated into artificial diets and offered to a range of mesograzers. No significant difference was noted in feeding stimulation or deterrence amongst extracts and the controls for any of the grazers. When fed a monospecific diet of L. polychroa, S. striatus consumed more (P < 0.001) and attained a higher daily biomass (P = 0.004) than S. striatus fed L. confervoides. As L. polychroa and L. confervoides often co-exist on local coral reefs and yield dense numbers of S. striatus, host switching to a more palatable species of Lyngbya may have important implications regarding top-down control of local blooms leading to proliferation of one species and decimation of another. S. striatus fed a diet of L. polychroa consumed more (P = 0.003), had a greater increase in body mass (P = 0.020) and higher conversion efficiency (P = 0.005) than those fed L. confervoides regardless of host origin. Possible explanations for host switching between species of Lyngbya related to morphology, toxicity and nutrient requirements for growth are discussed.     

INVOLVEMENT OF INDOLE‐3‐ACETIC ACID PRODUCED BY THE GROWTH‐PROMOTING BACTERIUM AZOSPIRILLUM SPP. IN PROMOTING GROWTH OF CHLORELLA VULGARIS1

Involvement of indole‐3‐acetic acid (IAA), produced by the microalgae‐growth‐promoting bacteria Azospirillum brasilens and A. lipoferum, in promoting growth of the microalga Chlorella vulgaris Beij. was studied. Four wildtype strains of Azospirillum and their IAA‐deficient mutants were co‐immobilized with C. vulgaris in alginate beads. Cultures were grown in synthetic growth medium supplemented with tryptophan. Growth promotion of microalgae and production of exogenous IAA by Azospirillum spp. were monitored. All wildtype Azospirillum spp. produced significant but varying amounts of IAA, while their mutant forms produced significantly less. The results demonstrated a significant growth promotion in Chlorella cultures when immobilized with the four wildtype strains of Azospirillum, while very low or no enhanced growth was induced by the four IAA‐deficient mutants, compared to when C. vulgaris is immobilized alone. A complementation experiment, where an IAA‐attenuated mutant (A. brasilense SpM7918) was supplemented with IAA produced by its parental wildtype strain (A. brasilense Sp6), restored growth promotion in the microalgae‐mutant culture.     

Growth responses of the mixotrophic dinoflagellates, Cryptoperidiniopsis sp. and Pfiesteria piscicida, to light under prey-saturated conditions

Recent research emphasis on the ecology of Pfiesteria spp. (Dinophyceae) has led to recognition of several morphologically similar heterotrophic dinoflagellates that often co-occur with Pfiesteria spp. in estuaries along the United States Atlantic coast. These include cryptoperidiniopsoid dinoflagellates, which resemble Pfiesteria spp. in having complex life cycles that include zoospores capable of kleptoplastidy. To examine and compare the role of kleptoplastidy in Cryptoperidiniopsis sp. and Pfiesteria piscicida, we tested the effects of irradiance on growth under prey-saturated (Storeatula major, Cryptophyceae) conditions. Growth of Cryptoperidiniopsis was strongly influenced by light intensity while no major effects were observed in P. piscicida. In Cryptoperidiniopsis, highest cell numbers and specific growth rates, but lowest specific cryptophyte consumption rates, were found at the highest light intensity tested (100 μmol photons m⁻² s⁻¹). A growth model was developed and used to estimate that the average half-life of chloroplasts ingested by Cryptoperidiniopsis decreased 3.4-fold from 12.6 h at high light to 3.7 h in the dark. These results show that light strongly enhances specific growth rate and growth efficiency of Cryptoperidiniopsis feeding on cryptophytes, and suggest that retained kleptochloroplasts may play a quantitatively significant role in carbon and energy metabolism of this organism. Differences in the effects of light between Cryptoperidiniopsis and P. piscicida may reflect different nutritional strategies, and allow these closely related dinoflagellates to occupy different niches and co-exist.     

Responses of four submerged macrophytes to freshwater snail density (Radix swinhoei) under clear‐water conditions: A mesocosm study

Macrophytes play a key role in stabilizing clear‐water conditions in shallow freshwater ecosystems. Their populations are maintained by a balance between plant grazing and plant growth. As a freshwater snail commonly found in shallow lakes, Radix swinhoei can affect the growth of submerged macrophytes by removing epiphyton from the surface of aquatic plants and by grazing directly on macrophyte organs. Thus, we conducted a long‐term (11‐month) experiment to explore the effects of snail density on macrophytes with distinctive structures in an outdoor clear‐water mesocosm system (with relatively low total nitrogen (TN, 0.66 ± 0.27 mg/L) and total phosphorus (TP, 36 ± 20 μg/L) and a phytoplankton chlorophyll a (Chla) range of 14.8 ± 4.9 μg/L) based on two different snail densities (low and high) and four macrophyte species treatments (Myriophyllum spicatum, Potamogeton wrightii, P. crispus, and P. oxyphyllus). In the high‐density treatment, snail biomass and abundance (36.5 ± 16.5 g/m² and 169 ± 92 ind/m², respectively) were approximately twice that observed in the low‐density treatment, resulting in lower total and aboveground biomass and ramet number in the macrophytes. In addition, plant height and plant volume inhabited (PVI) showed species‐specific responses to snail densities, that is, the height of P. oxyphyllus and PVI of M. spicatum were both higher under low‐density treatment. Thus, compared with low‐density treatment, the inhibitory effects of long‐term high snail density on macrophytes by direct feeding may be greater than the positive effects resulting from epiphyton clearance when under clear‐water conditions with low epiphyton biomass. Thus, under clear‐water conditions, the growth and community composition of submerged macrophytes could be potentially modified by the manual addition of invertebrates (i.e., snails) to lakes if the inhibitory effects from predatory fish are minor.     

Clearance rates and prey selectivity of the predaceous cladoceran Polyphemus pediculus

Littoral plankton communities have rarely been included in food web dynamics and predator/prey behavior studies of planktonic communities of fresh water ecosystems. Polyphemus pediculus, a typically littoral predaceous cladoceran, is common in lakes and ponds throughout the northern temperate zone.Diel clearance rates of Polyphemus were studied with a differential count method. A multi-chambered in situ feeding trap manipulated Polyphemus densities at three levels above the control ambient densities. Polyphemus generally selected small prey (Vorticella and Keratella) over large prey (nauplii) and more vulnerable prey (Conochilus and Polyarthra) over smaller prey with long protective structures (Kellicottia). Polyphemus clearance rates were inversely correlated with body volume of prey. Polyphemus clearance rates ranged from 4.2 to 4.5 ml h^–1 for Keratella and Vorticella respectively to <1 ml h^–1 for copepod nauplii. Selectivity coefficients were highest for small species, Vorticella and Keratella.     

Food selectivity of the herbivore <Emphasis Type="Italic">Daphnia magna</Emphasis> (Cladocera) and its impact on competition outcome between two freshwater green algae

Freshwater green algae, Chlorella, have heavy cell walls and their size usually exceeds the lower limits of limb size of herbivorous Daphnia (Cladocera). According to the optimal foraging theory, we speculated that Daphnia would graze more exposed and relatively large Clamydomonas rather than Chlorella, and this process would lead to small-sized Chlorella becoming a superior competitor in the presence of Daphnia. We used Daphnia magna, Clamydomonas sajao and Chlorella pyrenoidosa to test this hypothesis. Our grazing experiments showed that Daphnia preferred C. sajao to C. pyrenoidosa, regardless of the concentration and relative abundance of these two algae. The decrease in relative abundance of high-quality Clamydomonas in Clamydomonas–Chlorella assemblages did not diminish the grazing efficiency of Daphnia on this algal species, but increased selectivity of low-quality Chlorella. However, when the concentration of Clamydomonas was extremely high, the grazing of Daphnia on Clamydomonas decreased. In competition experiments, we observed that the presence of Clamydomonas restrained the growth potential of Chlorella; however, the introduction of herbivorous Daphnia into the competing environment weakened this influence and to some extent enhanced the growth ability of Chlorella. Moreover, we also observed that the intensity of herbivory, imposed by different densities of Daphnia, had an obvious influence on the competition outcome between Clamydomonas and Chlorella. At the highest intensity of herbivory (10 Daphnia), C. sajao was eliminated from the culture medium whereas C. pyrenoidosa could persist, but at low cell density.     

Effects of Protoceratium reticulatum yessotoxin on feeding rates of Acartia hudsonica: A bioassay using artificial particles coated with purified toxin

Paralytic shellfish toxins produced by dinoflagellates are known to deter copepod grazing. Dinoflagellate species, including Protoceratium reticulatum, also produce disulfated polyether yessotoxins that were previously referred to as diarrheic shellfish toxins. However, the role of yessotoxins in predator–prey relationships is not yet clear. In the present study, the effects of purified yessotoxin (YTX) on feeding activities of Acartia hudsonica (Copepoda, Calanoida) were experimentally investigated. Polystyrene fluorescent microspheres (10 μm in diameter) colored bright blue or yellow-green were coated with cell extracts of P. reticulatum that do not produce yessotoxins. The bright blue microspheres were further coated with YTX, and the yellow-green microspheres were used as the reference. The microspheres were then given to the copepods separately or in combination to measure clearance rates and feeding selectivity. A. hudsonica was found to feed on the yellow-green microspheres without YTX at twice the rate of the bright blue microspheres with YTX. We also confirmed that microsphere color per se did not affect the feeding rates. The bright blue microspheres adsorbed 1.8–43.3 pg of YTX per microsphere, which is similar to the cell-specific yessotoxin content of toxic P. reticulatum found in natural environments. These results suggest that production of yessotoxin is advantageous for P. reticulatum by deterring predation by copepods.     

Prey selection by larvae of Prochilodus lineatus (Pisces: Curimatidae): indigenous zooplankton versus veligers of the introduced bivalve Limnoperna fortunei (Bivalvia: Mitilidae)

We studied experimentally the feeding selectivity of larvae of Prochilodus lineatus (Pisces), with particular emphasis on the role of veligers of the exotic bivalve Limnoperna fortunei. Three concentrations of veligers were offered to three developmental stages of P. lineatus. Veliger concentrations were: (1) higher than in the field (“enriched”, 0.09 ind. ml⁻¹), (2) unmodified from field conditions (“normal”, 0.06 ind. ml⁻¹), and (3) lower than in the field (“low”, 0.02 ind. ml⁻¹). Fish developmental stages were protolarvae (approx. 10 days old), mesolarvae (17 days), and metalarvae (25 days). Proportions (in terms of numbers and biomass) and selectivity values were calculated for each prey item evaluated: veligers, small cladocerans + nauplii, medium-sized cladocerans, copepodits, and large cladocerans + copepods. Protolarvae and mesolarvae consumed veligers almost exclusively (88–90%, both in numbers and in biomass) when offered prey enriched in veligers, whereas for metalarvae veligers represented only 16.0% of the food consumed. At lower veliger concentrations, only protolarvae preferred Limnoperna veligers, whereas older fishes switched gradually to crustacean plankton. We conclude that veligers are preferred by the early fish developmental stages, and we speculate that this may be because their slower swimming makes them easier to capture than planktonic crustaceans. However, as fish larvae grow larger, veligers become too small a prey for their energetic needs, and they switch to larger items like cladocerans and copepods. We anticipate that this new and abundant food resource has an important impact on the survival and growth of P. lineatus.     

Bioenergetics and growth in the ctenophore <Emphasis Type="Italic">Pleurobrachia pileus</Emphasis>

Knowledge of how energetic parameters relate to fluctuating factors in the natural habitat is necessary when evaluating the role of gelatinous zooplankton in the carbon flow of coastal waters. In laboratory experiments, we assessed feeding, respiration and growth of the ctenophore, Pleurobrachia pileus, and constructed carbon budgets. Clearance rates (F, l d⁻¹) of laboratory-reared Acartia tonsa as prey increased as a function of ctenophore polar length (L, mm) as F = 0.17L ^1.9. For ctenophores larger than about 11 mm, clearance rate was depressed in containers of 30–50 l volume. Clearance rates on field-collected prey were highest on the copepod, Centropages typicus, intermediate on the cladoceran, Evadne nordmanni and low on the copepods, Acartia clausi and Temora longicornis. Specific growth rates of 8–10 mm P. pileus increased with increasing prey concentrations to a maximum of 0.09 d⁻¹ attained at prey carbon densities of 40 and 100 μg C l⁻¹ of Artemia salina and A. tonsa, respectively. Weight-specific respiration rates increased hyperbolically with prey concentration. From experiments in which growth, ingestion and respiration were measured simultaneously, a carbon budget was constructed for individuals growing at maximum rates; from the measured parameters, the assimilation efficiency and net growth efficiency were estimated to be 22 and 37%, respectively. We conclude that the predation rates of P. pileus depend on ctenophore size, prey species, prey density and experimental container volume. Because the specific growth rates, respiration, assimilation and net growth efficiencies all were affected by food availability, knowledge of the ambient prey field is critical when evaluating the role of P. pileus in the carbon flow in coastal waters.     

The Newly Described Heterotrophic Dinoflagellate Gyrodinium moestrupii,an Effective Protistan Grazer of Toxic Dinoflagellates

Few protistan grazers feed on toxic dinoflagellates, and low grazing pressure on toxic dinoflagellates allows these dinoflagellates to form red‐tide patches. We explored the feeding ecology of the newly described heterotrophic dinoflagellate Gyrodinium moestrupii when it fed on toxic strains of Alexandrium minutum, Alexandrium tamarense, and Karenia brevis and on nontoxic strains of A. tamarense, Prorocentrum minimum, and Scrippsiella trochoidea. Specific growth rates of G. moestrupii feeding on each of these dinoflagellates either increased continuously or became saturated with increasing mean prey concentration. The maximum specific growth rate of G. moestrupii feeding on toxic A. minutum (1.60/d) was higher than that when feeding on nontoxic S. trochoidea (1.50/d) or P. minimum (1.07/d). In addition, the maximum growth rate of G. moestrupii feeding on the toxic strain of A. tamarense (0.68/d) was similar to that when feeding on the nontoxic strain of A. tamarense (0.71/d). Furthermore, the maximum ingestion rate of G. moestrupii on A. minutum (2.6 ng C/grazer/d) was comparable to that of S. trochoidea (3.0 ng C/grazer/d). Additionally, the maximum ingestion rate of G. moestrupii on the toxic strain of A. tamarense (2.1 ng C/grazer/d) was higher than that when feeding on the nontoxic strain of A. tamarense (1.3 ng C/grazer/d). Thus, feeding by G. moestrupii is not suppressed by toxic dinoflagellate prey, suggesting that it is an effective protistan grazer of toxic dinoflagellates.     

Ovipositional preferences and larval survival of annual bluegrass weevil,Listronotus maculicollis,on Poa annua and selected bentgrasses (Agrostis spp.)

The annual bluegrass weevil (ABW), Listronotus maculicollis Kirby (Coleoptera: Curculionidae), is a serious and expanding pest of short‐cut turfgrass on golf courses in eastern North America. Increasing problems with the development of insecticide resistance in this pest highlights the need for more sustainable management approaches. Plant resistance is one of the most promising alternative strategies. Bentgrasses are the dominant grass species on golf course fairways, tees, and putting greens in the areas affected by ABW. But Poa annua L. (Poaceae), a highly invasive weed, often constitutes a large percentage of turf stands in short‐mown golf courses and is thought to be particularly susceptible to ABW. We studied resistance to ABW in four cultivars of creeping bentgrass, Agrostis stolonifera L., and two cultivars each of colonial bentgrass, Agrostis capillaris L., and velvet bentgrass, Agrostis canina L. (Poaceae), in comparison with P. annua by addressing the three major components of resistance: antixenosis (adult ovipositional and feeding preferences), antibiosis (larval survival and growth), and grass tolerance (grass damage). Our findings suggest that antixenosis/non‐preference is at least partially involved in bentgrass resistance to ABW. Even though oviposition was observed in all tested grasses, females laid significantly fewer eggs in Agrostis spp. than in P. annua. Compared to P. annua, Agrostis spp. were also less suitable for larval development with lower numbers of ABW immatures recovered and larvae weighing less and being less advanced in development. Resistance levels to ABW larvae varied significantly among Agrostis spp. and cultivars. Agrostis canina was least preferred by females for oviposition and A. stolonifera was the least suitable for larval survival and development. Agrostis spp., especially A. stolonifera, were more tolerant to ABW feeding than P. annua. Our findings suggest that reduction in P. annua and replacement with Agrostis spp., especially A. stolonifera, wherever feasible should be integral to more sustainable approaches to ABW management.     

Feeding by the Newly Described Mixotrophic Dinoflagellate Paragymnodinium shiwhaense: Feeding Mechanism,Prey Species,and Effect of Prey Concentration

ABSTRACT. To investigate the feeding by the newly described mixotrophic dinoflagellate Paragymnodinium shiwhaense (GenBank accession number=AM408889), we explored the feeding process and the kinds of prey species that P. shiwhaense is able to feed on using several different types of microscopes, including a transmission electron microscope and high‐resolution video‐microscopy. In addition, we measured the growth and ingestion rates of P. shiwhaense on its optimal algal prey Amphidinium carterae as a function of prey concentration. We also measured these parameters for edible prey at a single concentration at which the growth and ingestion rates of P. shiwhaense on A. carterae were saturated. Paragymnodinium shiwhaense feed on algal prey using a peduncle after anchoring the prey by a tow filament. Among the algal prey offered, P. shiwhaense ingested small algal species that had equivalent spherical diameters (ESDs) ≤11 μm (e.g. the prymnesiophyte Isochrysis galbana, the cryptophytes Teleaulax sp. and Rhodomonas salina, the raphidophyte Heterosigma akashiwo, and the dinoflagellates Heterocapsa rotundata and A. carterae). However, it did not feed on larger algal species that had ESDs ≥12 μm (e.g. the dinoflagellates Prorocentrum minimum, Heterocapsa triquetra, Scrippsiella trochoidea, Alexandrium tamarense, Prorocentrum micans, Gymnodinium catenatum, Akashiwo sanguinea, and Lingulodinium polyedrum) or the small diatom Skeletonema costatum. The specific growth rates for P. shiwhaense feeding upon A. carterae increased rapidly with increasing mean prey concentration before saturating at concentrations of ca. 350 ng C/ml (5,000 cells/ml). The maximum specific growth rate (i.e. mixotrophic growth) of P. shiwhaense on A. carterae was 1.097/d at 20 °C under a 14:10 h light–dark cycle of 20 μE/m²/s, while its growth rate (i.e. phototrophic growth) under the same light conditions without added prey was ?0.224/d. The maximum ingestion and clearance rates of P. shiwhaense on A. carterae were 0.38 ng C/grazer/d (5.4 cells/grazer/d) and 0.7 μl/grazer/h, respectively. The calculated grazing coefficients for P. shiwhaense on co‐occurring Amphidinium spp. was up to 0.07/h (i.e. 6.7% of the population of Amphidinium spp. was removed by P. shiwhaense populations in 1 h). The results of the present study suggest that P. shiwhaense can have a considerable grazing impact on algal populations.     

Factors influencing algal consumption and feeding rate in Heterotrissocladius changi Saether and Polypedilum nebeculosum (Meigen) (Chironomidae: Diptera)

Summary Factors influencing the consumption of algae and the rate of feeding by the larvae of Heterotrissocladius changi Saether and Polypedilum nebeculosum (Meigen) (Chironomidae: Diptera) were determined from collections made between July 1975 and May 1977 in Yellowknife Bay, situated in the Canadian subarctic. Although both species fed heavily on algae during the summer, few cells were ingested in the winter, coincident with an overall reduction in feeding intensity. H. changi fed primarily on benthic species (Achnanthes minutissima, Achnanthes pinnata, Fragilaria pinnata, Fragilaria vaucheriae, Navicula spp., Scenedesmus spp.) Whereas P. nebeculosum was restricted to planktonic forms (Dinobryon spp., Scenedesmus spp.). The consumption of algae by H. changi depended largely on the density of the microflora in the environment. This factor had little influence on P. nebeculosum and was replaced by size selection, which prevented the ingestion of many planktonic algal species. The importance of algae to both chironomids also depended on the susceptability of certain algal forms to digestion. Temperature generally regulated the intensity of feeding of both H. changi and P. nebeculosum. However, changing day-length initiated heavy feeding during April and May, despite low water temperatures. The depth of water and the organic content of the substrate had no detectable effect on the amount of material in the guts.     

Feeding strategies and elemental composition in Ponto‐Caspian peracaridans from contrasting environments: can stoichiometric plasticity promote invasion success?

1. Ponto‐Caspian peracaridans, and mysids and amphipods in particular, are among the most successful aquatic invaders. However, species differ in the trophic‐status range of ecosystems they can invade while establishment rates and impacts can vary substantially between habitats. There is limited knowledge of the environmental factors and species characteristics that drive such variation in invasion success. 2. Here we test how trophic level and body stoichiometry vary among peracaridan species and in relation to body size. The amphipod Pontogammarus robustoides and the mysids Limnomysis benedeni and Paramysis lacustris were investigated in ecosystems differing considerably in productivity and nutrient supply, namely an N‐limited eutrophic lagoon and P‐limited mesotrophic lakes. 3. As revealed by stable isotope (¹⁵N/¹⁴N) analysis, herbivory was inferred to be the main feeding mode of L. benedeni. In contrast, the mysid P. lacustris and the amphipod P. robustoides displayed a higher propensity for predatory feeding at larger body sizes, a pattern that was more pronounced in the eutrophic lagoon than in the mesotrophic lakes. 4. Their mean stoichiometric composition (P. robustoides C:N:P 108:20:1, L. benedeni 92:21:1 and P. lacustris 93:22:1) demonstrates that these peracaridans are rich in nutrients, especially nitrogen. They all exhibited the same ontogenetic pattern of reduced stoichiometric regulation during juvenile stages and stricter homoeostasis at older stages. 5. The higher P content in juveniles of all peracaridan species from the lagoon indicates higher potential somatic and population growth rates than those in the mesotrophic lakes. Such a difference may explain the substantially faster rates of invader establishment observed in the lagoon in comparison with lakes of low trophy. 6. Due to differences in ontogenetic and habitat‐induced variation, the study species differed significantly in stoichiometric variability, which was lowest in L. benedeni and highest in P. robustoides. The ranges of species‐specific variation in stoichiometric ratios corresponded to the trophic (by chlorophyll a) and nutrient stoichiometry (N:P) ranges of lentic waters successfully invaded by these species in Lithuania. 7. Stoichiometric plasticity, which should be associated with flexibility of feeding strategy, may enhance the potential of peracaridan species to successfully invade habitats with differing trophy and nutrient supply. The optimal feeding strategy should be omnivory with a propensity for predatory feeding, which can be adjusted with respect to ontogenetic nutrient demands and resource availability. Invading species may have a stronger effect on the local biota in ecosystems with high P levels, which promote growth, and N limitation that should favour predation.     

Bridge under troubled water: Turbulence and niche partitioning in fish foraging

The coexistence of competing species relies on niche partitioning. Competitive exclusion is likely inevitable at high niche overlap, but such divide between competitors may be bridged if environmental circumstances displace competitor niches to enhance partitioning. Foraging‐niche dimension can be influenced by environmental characteristics, and if competitors react differently to such conditions, coexistence can be facilitated. We here experimentally approach the partitioning effects of environmental conditions by evaluating the influence of water turbulence on foraging‐niche responses in two competing fish species, Eurasian perch Perca fluviatilis and roach Rutilus rutilus, selecting from planktonic and benthic prey. In the absence of turbulence, both fish species showed high selectivity for benthic chironomid larvae. R. rutilus fed almost exclusively on zoobenthos, whereas P. fluviatilis complemented the benthic diet with zooplankton (mainly copepods). In turbulent water, on the other hand, the foraging‐niche widths of both R. rutilus and P. fluviatilis increased, while their diet overlap simultaneously decreased, caused by 20% of the R. rutilus individuals turning to planktonic (mainly bosminids) prey, and by P. fluviatilis increasing foraging on littoral/benthic food sources. We show that moderate physical disturbance of environments, such as turbulence, can enhance niche partitioning and thereby coexistence of competing foragers. Turbulence affects prey but not fish swimming capacities, with consequences for prey‐specific distributions and encounter rates with fish of different foraging strategies (pause‐travel P. fluviatilis and cruise R. rutilus). Water turbulence and prey community structure should hereby affect competitive interaction strengths among fish species, with consequences for coexistence probability as well as community and system compositions.     

Copepod feeding response to varying Alexandrium spp. cellular toxicity and cell concentration among natural plankton samples

We challenged four species of copepod grazers (Acartia hudsonica, Centropages hamatus, Eurytemora herdmani, Calanus finmarchicus) with natural water samples containing non-toxic algae mixed with one of three clones of Alexandrium spp.—A. tamarense GTCN16 (non-toxic), A. fundyense GTCA28 (moderate toxicity), and A. fundyense BC1 (higher toxicity), each at relatively high (10⁵ cells L⁻¹) and low (10⁴ cells L⁻¹) concentrations. Within any one copepod species, significant differences existed in copepod clearance rates and total food ingested between high and low Alexandrium cell concentrations, and between levels of toxicity, but feeding response did not follow a predictable relationship proportional to toxin levels—rather, the presence or absence of toxin was more important than the level of toxicity. C. finmarchicus behaved differently from the smaller coastal copepods, showing less selectivity and greater concentration dependence. In low Alexandrium concentration treatments, copepod clearance rates on Alexandrium were usually higher, and electivity indices for Alexandrium less negative (indicating less avoidance), compared to high concentration treatments. In high toxicity (BC1) treatments of all copepod species (except C. finmarchicus), total food consumption was consistently less at high Alexandrium concentrations compared to low concentrations, suggesting that high toxicity and concentration suppress overall feeding, while in non-toxic (GTCN 16) treatments total consumption was always higher at high Alexandrium concentrations. Copepod grazers do not follow predictable feeding rules throughout a continuum of conditions, but become more predictable at extremes of concentration and toxicity of prey, consistent with the conclusion that both factors are important. Results support the hypothesis that grazer deterrence imparted by toxicity is only effective at high cell concentrations, but even then will not protect against all grazers.