The efficacy of Scenedesmus morphology as a defense mechanism against grazing by selected species of rotifers and cladocerans

Phytoplankton often develop various defense mechanisms in response to zooplankton grazing, such as spines and colonies. While it is now known that increased spine length and cells in a colony of members of the genus Scenedesmus, when zooplankton grazing is intense, helps in reducing zooplankton filtering rates, the effect of these defense mechanisms at the population level has been observed in few studies. Here we present data on the growth rates of four zooplankton species, Brachionus calyciflorus, B. patulus, Ceriodaphnia dubia and Daphnia pulex at two food levels using two species of colony-forming Scenedesmus spp.: S. acutus (cell length = 18.2 ± 0.4 µm; width = 4.2 ± 0.1 µm; average colony length = 90 µm; width: 21 µm) and S. quadricauda (cell length: 21 ± 0.5 width 7.5 ± 0.3 µm; average colony length: 84 µm; width: 30 µm). Whereas S. acutus had no spines, S. quadricauda had spines of 6–10 µm. Population growth experiments of the test rotifers and cladocerans were conducted in 100 ml containers with 50 ml of the medium with test algae. Algae concentrations used were: 13 and 52 mg dw l^–1 of each of the two algal species offered in colonial forms. We used an initial inoculation zooplankter density of 1 ind. ml^–1 for either of the rotifer species and 0.2 ind. ml^–1 for either of the cladoceran species. In all, we had 64 test containers (4 test species of zooplankton × 2 test species of algae × 2 algal densities × 4 replicates). We found a significant effect of algal size on the growth rates of all the four tested species of zooplankton. The population growth rates of zooplankton ranged from –0.58 to 0.66 and were significantly higher on diet of S. acutus than of S. quadricauda. Thus, our study confirms that the larger colony size and the formation of spines in S. quadricauda were effective defenses against grazing by both rotifers and smaller sized cladoceran Ceriodaphnia dubia but that larger-bodied Daphnia pulex could exploit both the algal populations equally.     

Feeding kinetics of Brachionus plicatilis fed Isochrysis galbana

Clearance and ingestion rates of Brachionus plicatilis were measured using ¹⁴C-labeled Isochrysis galbana Tahiti. Experiments were conducted at 20–22 °C, 20 ppt salinity, and algal concentrations ranging from 0.13–64 mg C 1^–1. Clearance rates were constant and maximal at concentrations <2 mg C 1^–1, with maximum rates ranging from 3.4–6.9 µl ind.^–1 hr^–1. The ingestion rate varied with food concentration, and was described by a rectilinear model. The maximum ingestion rate varied considerably, and was dependent on the growth rate of the rotifers. Depending on the pre-conditions, B. plicatilis ingested about 0.5 to 2 times its body carbon per day at saturating food concentrations.     

Effect of Aeolosoma sp. (Aphanoneura: Aeolosomatidae) on the Population Dynamics of Selected Cladoceran Species

Although oligochaete worms naturally coexist with cladocerans in many shallow freshwater ponds and lakes, their influence on the latter is not well established. In this work we studied the effect of Aeolosoma sp. on the population growth of Alona rectangula, Ceriodaphnia dubia, Daphnia pulex, Macrothrix triserialis and Moina macrocopa. Population growth studies were conducted at one algal food density (1 × 10⁶cells ml^–1 of Chlorella vulgaris). The experimental design was similar for all five cladoceran species, where we used 100 ml capacity transparent jars containing 50 ml of EPA medium with the desired algal density and three replicates for each treatment. The test medium was changed daily and fresh algal food was added. The initial density of each of the cladoceran species in the population growth studies was 0.4 ind ml^–1 while that of the worms 1.0 ind ml^–1. Following inoculation, we estimated daily the number of cladocerans and the worms for duration of 21 days. Regardless of the presence of worms, Moina macrocopa and Macrothrix triserialis showed rapid population growth while A. rectangula took more than 2 weeks to reach peak abundances. With the exception of M. triserialis, all the other our cladoceran species declined in the presence of Aeolosoma sp. The lowest peak population density (about 1 ind ml^–1) was observed for M. triserialisin controls. The remaining species had peak densities of about 3–5 ind ml^–1. The rates of population increase per day varied from 0.03 to 0.19 depending on the cladoceran taxa and the treatment. In general we found that pelagic taxa were more adversely affected by the presence of the worms than were the littoral cladocerans.     

Ecological problems of Lake Ladoga: causes and solutions

We studied the outcome of competition between a large (Brachionus calyciflorus) and a small (Anuraeopsis fissa) rotifer species at five algal (Scenedesmus acutus) concentrations (0.5 × 10⁶ to 40.5 × 10⁶ cells ml^–1) and with varying initial densities in mixed populations (100 to 0% of B. calcyciflorus or A. fissa), the combined initial biomass being 0.2 µg ml^–1 in all test jars. Experiments were conducted at 28 ± 1 °C.Regardless of food concentration, B. calcyciflorus showed a greater increase in biomass than A. fissa, peak densities (mean ± standard error) at the lowest food concentration in the controls being 1.34 ± 0.31 µg dry weight ml^–1 and 0.82 ± 0.08 dry weight ml^–1, respectively. At the lower food concentrations, A. fissa displaced B. calyciflorus and vice versa at the higher food concentrations. At the intermediate food concentrations of 4.5 × 10⁶ cells ml^–1, B. calyciflorus outcompeted A. fissa only if its initial population density was three times higher. The rates of population growth in controls varied from 0.792 ± 0.06 d^–1 to 1.492 ± 0.13 d^–1 for B. calyciflorus and 0.445 ± 0.04 to 0.885 ± 0.01 for A. fissa depending on food level. When both species were introduced together, low food levels favoured higher abundance of A. fissa than B. calyciflorus, suggesting, in nature, it is likely that small Anuraeopsis colonize oligotrophic water bodies more successfully than larger Brachionus. The results also suggest that the outcome of competition depends not only on the size of the competing species and food availability but also on their colonizing density.     

Population growth of some genera of cladocerans (Cladocera) in relation to algal food (Chlorella vulgaris) levels

We studied the patterns of population growth of 7 cladoceran species (Alona rectangula, Ceriodaphnia dubia, Daphnia laevis, Diaphanosoma brachyurum, Moina macrocopa, Scapholeberis kingi and Simocephalus vetulus) using 6 algal densities, viz. 0.05×10⁶, 0.1×10⁶, 0.2×10⁶, 0.4×10⁶, 0.8×10⁶ and 1.6×10⁶ cells ml^–1, of Chlorella vulgaris for 18 – 30 days. In terms of carbon content these algal concentrations corresponded to 0.29, 0.58, 1.16, 2.33, 4.65 and 9.31 g ml^–1, respectively. Cladocerans in the tested range of algal levels responded similarly, in that increasing the food concentrations resulted in higher numerical abundance and population growth rates (r). The peak population densities were (mean±standard error) 71±5; 17.1±0.4, 3.6±0.3, 12.7±1.1, 18.2±2.7, 15.8±1.0 and 10.9±0.02 ind. ml^–1, respectively for A. rectangula, C. dubia, D. laevis, D. brachyurum, M. macrocopa, S. kingi and S. vetulus. In general, the lowest r values were obtained for D. laevis (0.01±0.001) at 0.05×10⁶ cells ml^–1 food level while the highest was 0.283±0.004 for A. rectangula at 1.6×10⁶ cells ml^–1 of Chlorella. When the data of peak population density for each cladoceran species were plotted against the body length, we found an inverse relation, broadly curvilinear in shape. From regression equations between the food level and rate of population increase, we calculated the theoretical food quantity (the threshold level) required to maintain a zero population growth (r = 0) for each cladoceran species, which varied from 0.107 to 0.289 g ml^–1 d^–1 depending on the body size. When we plotted the cladoceran body size against the corresponding threshold food levels, we obtained a normal distribution curve. From this it became evident that for up to 1300 m body size, the threshold food level increased with increasing body size; however, beyond this, the threshold level decreased supporting earlier observations on rotifers and large cladocerans.     

Lifetable demography of four cladoceran species in relation to algal food (Chlorella vulgaris) density

Algal food density is known to influence life history variables of cladoceran species. It is not, however, well established whether both littoral and planktonic cladocerans show similar trends when exposed to increasing food concentrations. In the present work, we studied the life table demography of four cladoceran species (Ceriodaphnia cornuta, Moina macrocopa, Pleuroxus aduncus and Simocephalus vetulus) in relation to three algal food concentrations (low: 0.5 × 10⁶, medium: 1.5 × 10⁶ and high: 4.5 × 10⁶ cells ml^–1 of Chlorella vulgaris) (in terms of carbon content, these were equivalent to 0.15, 0.45 and 1.35 g ml^–1, respectively) at 25 °C. In general, for all the tested cladoceran species, values of average lifespan, gross reproductive rate, net reproductive rate, generation time and the rate of population growth were higher at lower food concentrations. Furthermore, high food concentration resulted in a negative population growth rate (mean ± standard error: –0.091 ± 0.026) for P. aduncus. The highest population growth rate (0.602 ± 0.014) was recorded for M. macrocopa at low food density. S. vetulus had the longest average lifespan (40 ± 1 d) while M. macrocopa had the lowest (5 ± 1 d). C. cornuta showed better performance at medium food concentration. We conclude that among the algal concentrations used here, 0.5 × 10⁶ – 1.5 × 10⁶ was beneficial not only to the planktonic species but also to the littoral P. aduncus and S. vetulus while 4.5 × 10⁶ cells ml^–1 was unsuitable for all the cladocerans tested.     

Induction of diapausing amictic eggs in Synchaeta pectinata

Amictic females of a clone of S. pectinata from Star Lake (Norwich, Vermont) may produce diapausing as well as non-diapausing (subitaneous) eggs. The proportion of diapausing eggs produced in cultures was unaffected by temperature (12 vs 19 °C) or rotifer population density (minima of 0.33 vs 3 ind. ml^–1) at 19 °C. However, at 19 °C this proportion was higher in cultures maintained at a low food level suppressing reproduction (5 × 10³ cells ml^–1 Cryptomonas erosa) than in those maintained at a high food level (2 × 10⁴ cells ml^–1); the treatment effect was marginally significant (p=0.067). Consistent with the effect of low food availability, a period of starvation was very effective in inducing the development of diapausing eggs. None of 19 females cultured individually from hatching at 19 °C on C. erosa (2 × 10⁴ cells ml^–1) in 1-ml volumes produced any diapausing eggs in 4 days (0 out of 349 eggs), while 13 out of 16 females subjected to a 15-hour starvation period 6 hours after birth produced one or more diapausing eggs during that time (34% of the 158 eggs produced by the 16 females were diapausing). Diapausing eggs produced and left at 19 °C hatched after 4 to 13 days. Those produced in cultures with a low food level took significantly longer to hatch (9.7 days) than those produced in cultures with a high food level (8.1 days) (p=0.022). In natural communities, S. pectinata should be able to respond directly and rapidly to poor food conditions by producing eggs that undergo an obligatory dormant period before resuming development.     

A laboratory study of feeding and assimilation in Euchlanis dilatata lucksiana

Feeding of Euchlanis dilatata lucksiana, a brachionid rotifer isolated from the Loosdrecht Lakes (The Netherlands), was examined in the laboratory using ¹⁴C-labelled food. The gut-filling time at a food concentration of 9.6 µg C ml^–1 was about 15 minutes. Animals which were fed on 3 size fractions of the lake seston (< 7 µm, 7–15 µm, and 15–33 µm) showed a clear preference (Jacobs' selectivity index) for the largest size fraction. This fraction was composed predominantly of filamentous cyanobacteria. For animals weighing 0.37–0.49 µm C ind.^–1 the daily ration (daily food consumption per unit body weight × 100) ranged from 50 to 100% at food levels of 2 mg C l^–1 and below, but increased to 150–250% at food concentrations of 5 mg C l^–1 and above. The assimilation efficiency was 100% up to 5 mg C l^–1 of food, but decreased to about 80% at higher food levels.     

Feeding in Euchlanis dilatata lucksiana Hauer on filamentous cyanobacteria and a prochlorophyte

Ingestion and assimilation rates of Euchlanis dilatata lucksiana Hauer, isolated from Lake Loosdrecht (The Netherlands) and cultured on lake water (seston < 33 µm), were measured in the laboratory using the ¹⁴C-tracer technique. The five taxa used as tracer foods, together with 6–7 mg C l^–1 of lake seston in each case, included four species of filamentous cyanobacteria (Oscillatoria redekei, O. limnetica, Aphanizomenon flos-aquae, Anabaena PCC 7120) and a prochlorophyte (Prochlorothrix hollandica). Except Anabaena, they are all commonly encountered in eutrophic Loosdrecht lakes, including Lake Loosdrecht, and their dimensions ranged between 150 and 250 µm in length and 2 and 3.5 µm in width. The small and large Euchlanis used as experimental animals had mean lengths of 217–223 µm and 276–305 µm, respectively. Euchlanis fed on all the taxa offered as food. Clearance rates ranged from 51 to 99 µl ind^–1 d^–1 for the large animals and from 22 to 41 µl ind^–1 d^–1 for the small animals. The highest ingestion rate observed, 1.7 µg ind^–1 d^–1, was for the large animals feeding on Aphanizomenon. The daily ration for both size classes far exceeded 100% of body weight, reaching up to 690% for the small animals feeding on Aphanizomenon. The small animals generally appeared to assimilate the ingested food more efficiently (assimilation efficiencies: 37–100%) than the large animals (34–77%). Compared with an earlier study in which only lake seston (<33 µm) was used as food, the specific clearance rates of Euchlanis on the cyanobacteria and Prochlorothrix were generally somewhat lower.     

Extracellular DNA in aquatic ecosystems may in part be due to phycovirus activity

In a eutrophic lake, a crash of the algal population was followed by a significant increase in the number of virus-like particles (from ca. 1 10⁶ ml^–1 to ca. 26 10⁶ ml^–1), and soon thereafter by an increase of the amount of extracellular DNA (from ca. 20 µg l^–1 to ca. 40 µg l^–1). The same pattern of correlation between decrease of algae and increase of viruses and extracellular DNA could be demonstrated by an in vitro experiment with a Chlorella-virus-system. Lysis of algae by viruses increased both the number of viruses and the amount of DNA in the culture medium. Extracellular DNA mainly consisted of material with a molecular weight below 500 bp.The Chlorella-virus-system is discussed. It could be used as a model-system for studying the dynamics of interaction of viruses and algae in aquatic ecosystems.     

Feeding the fairy shrimp Streptocephalus (Anostraca - Crustacea) with the rotifer Anuraeopsis

Laboratory-cultured Streptocephalus torvicornis were offered 8 concentrations (from 6 to 800 ind. ml^–1) of Anuraeopsis fissa for periods of 2 h 30 min. Two size classes, small (male: 14.7 mm± 1.6, female: 15.4 mm± 1.3) and large (male: 20.0 mm±2.0, female: 23.1 mm± 1.5), of S. torvicornis were used. Functional response for large S. torvicornis (both sexes) plateaued at 400 rotifers ml^–1, while in small specimens it did so at 200 prey ml^–1. Females consumed significantly more (30%) prey than males. Large males consumed maximum 4730 rotifers h^–1, females 6560 h^–1.     

Utilization of cyanobacteria by Brachionus calyciflorus: Anabaena flos-aquae (NRC-44-1) as a sole or complementary food source

The rotifer Brachionus calyciflorus can utilize the cyanobacterium Anabaena flos-aquae as either a sole or supplementary food source in laboratory culture. Positive population growth rates accompany food densities of 10 or 100 µg dry weight ml^–1, but slightly negative rates are found at a lower density (1.0 µg ml^–1). These results are consistent for rotifers feeding on two strains of A. flos-aquae, UTEX-1444 and NRC-44-1, with slightly enhanced survivorship and reproduction with the latter food. A 1:1 mixture (by dry weight) of Euglena gracilis and A. flos-aquae (NRC-44-1) produces survivorship comparable to that of control rotifer cohorts fed E. gracilis alone, but elicits significantly greater fecundity and population growth rates than found with the control food suspension at the same biomass density.     

Susceptibilities of different-sized ciliates to direct suppression by small and large cladocerans

• 1 Laboratory experiments compared the susceptibilities of six ciliates and the rotifer Keratella cochlearis to predation and interference from Daphnia pulex and Bosmina longirostris.
• 2 Susceptibilities of the ciliates to D. pulex were similar to or less than that of the rotifer, and decreased with increasing ciliate size. Most ciliates were just as susceptible to B. longirostris as to the much larger D. pulex. The jumping response of the oligotrich Strobilidium gyrans appeared to be an effective defence against B. longirostris.
• 3 Clearance rates of B. longirostris and D. pulex on different ciliate species at a density of 1,3 ciliates ml^?1 ranged from 1–30 to 5–24ml ind.^?1 day^?1, respectively. In natural plankton communities, cladocerans could impose high mortality rates on ciliates and shift the size structure of ciliate assemblages towards larger and less susceptible species.

     

The phytotoxicity of selected mycotoxins on mature,germinatingZea mays embryos

Mature maize (Zea mays) embryos were exposed to 5, 10 and 25 µg ml^–1 of deoxynivalenol (DON), zearalenone (ZEA), ochratoxin A (OA) and a mixture of zearalenone and deoxynivalenol (ZEA/DON) for 9 days. DON and the ZEA/DON combination were consistently more inhibitory of the measured parameters than either ZEA or OA. Based on the predicted additive values, it would appear that, in combination, ZEA and DON act synergistically to inhibit root and shoot growth. For ZEA alone, a concentration of 5 µg ml^–1 ZEA was generally inhibitory of root and shoot elongation and fresh mass accumulation, while at 10 and 25 µg ml^–1, this toxin had a stimulatory effect on these parameters. For OA, the measured effects on root and shoot growth at 5 and 25 µg ml^–1 were stimulatory, while at 10 µg ml^–1 OA, an inhibitory effect was observed. For all toxins, inhibitory/stimulatory effects were generally more marked for root parameters than for shoot elongation or mass.Abbreviations ADON acetyldeoxynivalenol - AFB₁ aflatoxin B₁ - DAS diacetyoxyscirpenol - DON deoxynivalenol - FB₁ fumonisin B₁ - FHB Fusaium head blight - MON moniliformin - NIV nivalenol - OA ochratoxin A - ZEA zearalenone     

Enhanced nisin and pediocin production on whey supplemented with different nitrogen sources

Production of nisin and pediocin were followed, respectively, in Lactococcus lactis subsp. lactis CECT 539 and Pediococcus acidilactici NRRL B-5627 grown with lactose and four different nitrogen sources. Neither NH₄Cl nor glycine improved production of the bacteriocins. Both yeast extract and Casitone increased pediocin production from 55 BU ml^–1 to 195 BU ml^–1 and 185 BU ml^–1, respectively. Nisin increased from 21 BU ml^–1 to 74 BU ml^–1 and 59 BU ml^–1 with these nitrogen sources.     

Clearance rates of sessile rotifers: in vitro determinations

We measured laboratory clearance rates of 10 rotifer and one unidentified bryozoan species from 3 different lakes using ³²P labeled algae (Chlamydomonas) or yeast (Rhodotorula). Clearance rates for all rotifers fed yeast ranged from < 2.0 to > 260 µl · animal^–1 · h^–1 depending on species. The in vitro clearance rates of two sessile rotifers (Ptygura crystallina and P. pilula) were not significantly different from previously measured in situ rates (Wallace and Starkweather 1983). Clearance rates for 5 rotifers fed algae ranged from < 5.0 to > 90.0 µl · animal^–1 · h^–1. Ptygura beauchampi, P. crystallina, P. pilula, Floscularia conifera, and F. melicerta ingested both cell types but their clearance rates varied substantially among species and between cell types. There was a substantial time-dependent loss of 32P from formalin-fixed animals (Sinantherina socialis) awaiting processing. This loss stabilized at approximately 20 hours and was estimated to be about 40% of the initial ingested label. Clearance rates for the bryozoan fed yeast or algae were highly variable, ranging from < 1.0 to > 3 000 µl · animal^–1 · h^–1.     

Factors influencing depth distribution of soft bottom inhabiting Laminaria saccharina (L.) Lamour. in Kiel Bay,Western Baltic

The kelp Laminaria saccharina dominates soft bottoms in 4–10 m depth in Kiel Bay. Experimental sporophytes transplanted to 2 and 5 m depth showed the typical annual growth pattern of Laminaria species. Surprisingly, 2 m plants died after the first resting phase, whereas 5 m plants survived and showed outgrowth of a new blade generation. Thalli at both depths were infected with the brown algal endophyte Streblonema aecidioides, with host deformations being significantly stronger in 2 m plants. Growth rates of infected sporophytes were reduced. Exclusion of UV light in 2 m depth resulted in less infected thalli. Discs excised from L. saccharina and cultivated in different photon fluence rates from 10–600 µmol m^–2 s^–1 did not differ in growth rate, photosynthesis or dark respiration. Hence, an exclusion of L. saccharina from shallow depths caused by high light cannot be concluded. We suggest the biological interaction with the endophyte S. aecidioides, amplified by UV light, to be most important for the exclusion of L. saccharina from shallow depths in the western Baltic.     

Filtering impacts of larval and sessile zebra mussels (Dreissena polymorpha) in western Lake Erie

Summary We assessed the feeding biology of veliger larvae of the introduced zebra mussel (Dreissena polymorpha Pallas) in laboratory experiments using inert microspheres as food analogues. Mean clearance rate on 2.87-m beads ranged between 247 and 420 L veliger^–1 day^–1. Clearance rate was unrelated to bead concentration up to 100 beads L^–1, but was positively correlated with veliger shell length. Clearance rates of Dreissena veligers are within the range of those reported for marine bivalve veligers of similar size and for herbivorous Great Lakes microzooplankton, but are orders of magnitude lower than those of settled, conspecific adults. The impact of settled zebra mussel grazing activities on phytoplankton stocks may be up to 1162 times greater than that exerted by veliger populations in western Lake Erie. Based on 1990 size-frequency distributions and associated literature-derived clearance rates, reef-associated Dreissena populations in western Lake Erie (mean depth 7 m) possess a tremendous potential to filter the water column (up to 132 m³ m^–2 day^–1) and redirect energy from pelagic to benthic foodwebs. Preliminary analyses indicate that chlorophyll a concentration is strongly depleted (<1 g L^–1) above Dreissena beds in western Lake Erie.     

Continuous production of lignin-degrading enzymes by Bjerkandera adusta immobilized on polyurethane foam

Continuous production of lignin-degrading enzymes by Bjerkandera adusta immobilized on polyurethane foam gave maximum activities of 220 U lignin peroxidase ml^–1, 150 U manganese peroxidase ml^–1, 50 U laccase ml^–1 and 6.2 U protease ml^–1 at the retention time of 24 h for 60 days. Protease secretion destabilized the produced lignin peroxidase, manganese peroxidase and laccase.     

Predicting Population Recovery Rates for Endangered Western Atlantic Sawfishes Using Demographic Analysis

Sawfish are a group of endangered elasmobranchs that were common in tropical inshore, estuarine and freshwaters. The demography of two species of sawfish that occur in the western Atlantic – Pristis pectinata and P. perotteti – was investigated using age-structured life tables. Life history parameters for use in the life tables were obtained from published data. Five scenarios were tested for P. pectinata to incorporate uncertainties about life history data. Values of intrinsic rate of increase ranged from 0.08 to 0.13yr^–1, and population doubling times from 5.4 to 8.5 yrs. Eight scenarios were tested for P. perotteti. The most likely range for the intrinsic rate of increase was 0.05–0.07yr^–1, with population doubling times of 10.3–13.5yrs. Four scenarios investigating the sensitivity to methods of estimating natural mortality produced similar results. The demographic results were sensitive to changes in reproductive periodicity and natural mortality. The results indicate that if effective conservation plans can be implemented for sawfish and sawfish habitats, recovery to levels where there is little risk of extinction will take at least several decades.