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Interspecific competition for phosphorus between the blue-green alga Anabaena flos-aquae and the green alga Staurastrum luetkemuelleri was studied in enclosure experiments. Both algal populations increased in number upon introduction of fish, but addition of herbivorous zooplankton led to an increase solely in the Staurastrum population. This could not be attributed to grazing by zooplankton, chemical or physical conditions (light, pH, inorganic carbon), or the total supply or rate of supply of phosphorus. The observed differences were apparently due to the patchy release of phosphorus by the fish, and the more homogenous release of this nutrient by the zooplankton. This would allow the Anabaena species, which was capable of rapid uptake of temporary pulses of phosphate, to coexist with Staurastrum in the enclosures with fish.

During blooms in Norwegian waters phytoplankton communities are phosphorus limited in fresh and brackish waters, and balanced or even nitrogen limited in high-salinity marine waters. This results from the high N/P ratio for available nutrients before onset of blooms in freshwater relative to seawater (N/P (atoms) >100 and 12–16, respectively). Algal N/P ratios at nutrient saturation vary between 8 and 27 among species, and average 16. A low ratio at nutrient saturation may imply P limitation even in high-salinity waters (e.g. Skeletonema costatum). In general, nutrient deficiency becomes more pronounced as the biomass increases. Addition of sewage shifts natural systems toward N limitation, which may therefore be a secondary effect of accelerated eutrophication. interspecific competition in nutrient-limited communities depends on the nutrient requirement of individual species and the mode of nutrient uptake. In our example the desmid Staurastrum luetkemuellerii outcompetes the cyanobacterium Microcystis aeruginosa in a chemostat when the nutrient supply is continuous; the opposite happens when nutrient supply is pulsed.

The desmid Staurastrum luetkemuellerii Donat et Ruttner and the cyanobacterium Microcystis aeruginosa Kutz. were grown in mixed cultures with various phosphate (Pi) additions. One pulse of Pi each day (semi-continuous cultures) favored M. aeruginosa whereas S. luetkemuellerii was favored when the same quantity of Pi was supplied continuously (chemostats). Both species coexisted under P limitation provided that the nutrient was supplied in an appropriate mode. The ability of each species to compete for P depended on their Pi uptake characteristics and their capability to retain the accumulated Pi. High affinity in uptake at low Pi concentrations contributed considerably to the growth eficiency of S. luetkemuellerii under continuous supply of PiM. aeruginosa was, however, consistently superior to S. luetkemuellerii in accuniulatiug the newly added P, but had a high rate of Pi release. In both -types of cultures, a net high of P went from M. aeruginosa to S. luetkemuellerii. The kinetic characteristics of the two species were used to simulate the outcome of competition experiments. Simulations agreed with the experimental data f both uptake and Pi release were considered in the model. The zlariable P*(the concentration of Pi at which the net uptake is equal to μ·QP is a function of uptake and release of Pi but could not explain the chemostat results. S. luetkemuellerii was the winner in many experiments even if its P*was higher thou that of M. aeruginosa. Thus, in the present case Pc (the concentration at which the net uptake is zero) was a better predictor of the ability to compete for Pi under steady state as well as transient conditions in the Pi supply.

1. 1. A detailed carotenoid analysis of unialgal cultures of Scenedesmus acutus, including mass [1H]NMR, CD and determination of optical purity by the carbamate method, is reported. 2. 2. The detailed carotenoid pattern of Daphnia magna, fed on Scenedesmus acutus as the sole source of nutrients, revealed the metabolic formation of echinenone, canthaxanthin, (2R)-2-hydroxy-echinenone, (2R)-2-hydroxycanthaxanthin and optically pure (3S,3′S)-astaxanthin by D. magna. 3. 3. Feeding of carotenoid depleted D. magna on a yeast diet with canthaxanthin added as the single carotenoid was monitored by HPLC analysis and showed the metabolic conversion to 2-hydroxycanthaxanthin, but no increase in astaxanthin concentration. 4. 4. Feeding of D. magna with yeast supplemented with β,β-carotene, in direct comparison with a carotenoid-free yeast diet, revealed the conversion of β,β-carotene to echinenone, canthaxanthin, 2-hydroxyechinenone and 2-hydroxycanthaxanthin. 5. 5. It is concluded that D. magna in nature derives its ketocarotenoids echinenone, canthaxanthin, (2R)-2-hydroxyechinenone and (2R)-2-hydroxycanthaxanthin from β,β-carotene, selectively resorbed from the algal diet, whereas astaxanthin has a different metabolic origin. 6. 6. D. magna grown on spinach, revealed selective resorption of zeaxanthin. Carotenoid analysis of the spinach is reported. 7. 7. D. magna grown on yeast supplemented with synthetic (3R,3′R)-zeaxanthin converted zeaxanthin to astaxanthin. It is concluded that (3S,3′S)-astaxanthin in D. magna is metabolically derived from (3R,3′R)-zeaxanthin of algal origin.

Natural bacterial communities were grown in chemostats diluted with filtered lake water manipulated to yield phosphorus limitation. The results suggested P limitation of the bacteria when the P : C and the surplus : C ratios were between 17–120 and 2.7–100 µg P(mg C)−1, respectively. These high values indicate that heterotrophic bacteria are net consumers of phosphate and that they sequester a considerably higher fraction of the particulate P in lakes than expected from their biomass in terms of carbon. Estimates of maximal uptake velocity and affinity for phosphate at low concentrations for the mixed bacterial communities were influenced by the degree of P limitation. A review of published data indicates that under P limitation bacteria are superior to phytoplankton in taking up phosphate at low concentrations, whereas their abilities are comparable at saturation.

The flux of phosphorus in a zooplankton-phytoplankton system is described by a simple com- partment model, with differential equations for grazing, growth and reproduction of animals, and release and recycling of P. The mean release rates from zooplankton for a given period are derived for cases restricted to animals feeding on algae at densities abcve the incipient limiting food concentration. The model is used to analyze short term incubat .on experiments with Daphnia pulex feeding on Scenedesmus sp. and Daphnia longispina fcediiig on Rhodomonas lacustris in dim light. When food organisms were saturated with P, total releitse rates were in the range 1. l- 1.5 pg P (mg dry wt-' h-l in accordance with a maximal excretio 1 rate of 0.8 pg P (mg dry wt)-l h-l predicted by the model of Peters. Phosphorus-starved phytoplankton, containing less P, gave much lower release rates, about 0.05 fig P (mg dry wt)-* h-l, only lo'% of the excretion rate predicted - .- - by the model of Peters.

The desmid Staurastrum luetkemuellerii Donat et Ruttner and the cyanobacterium Microcystis aeruginosa Kutz. showed pronounced differences in chemical composition and ability to maintain P fluxes. The cellular P:C ratio (Qp) and the surplus P:C ratio (Qsp) were higher in M. aeruginosa, indicating a lower yield of biomass C per unit of P. The subsistence quota (Qp) was 1.85 μg P·mg C−1in S. luetkemuellerii and 6.09 μg P·mg C−1in M. aeruginosa, whereas the respective Qp of P saturnted organisms (Qs) were 43 and 63 μg P·mg C−1. These stores could support four divisions in S. luetkemuellerii and three divisions in M. aeruginosa, which suggests that the former exhibited highest storage capacity (Qs/Q0). M. aeruginosa showed a tenfold higher activity of alkaline phosphatase than S. luetkemuellerii when P starved. The optimum N:P ratio (by weight) was 5 in S. luetkemuellerii and 7 in M. aeruginosa. The initial uptake of Pi pulses in the organisms was not inhibited by rapid (

The phosphorus status [total phosphorus (P), surplus P, and alkaline phosphatase activity (APA)] of phytoplankton and bacteria were followed in a eutrophic Norwegian lake. On average 36% (range 20-60) of the particulate P and 57% (range 10-l 00) of the surplus P were bound by bacteria. The P : C ratio of phytoplankton and bacteria varied between 2.5 and 55 (median 5.1) and 34 and 360 (median 89) pg P (mg C))‘, indicating that bacteria have a substantially higher P requirement than do phytoplankton. The inverse relation found between surplus P : C and APA : C ratios during a Microcystis aeruginosa bloom and the positive relation between P: C ratio and growth rate of the cryptophytedominated community in summer indicate that the phytoplankton were P limited during most of the ice-free period. This is supported by the fact that the data from the cryptophyte-dominated community could be described by the Droop model for nutrient-limited growth. For the bacteria a positive relation was found between growth rate and cellular P (i.e. the P : C and the surplus P : C ratios), as predicted by the Droop model. Although the bacteria obviously were P subsaturated, it cannot be concluded that they were P limited since C may have been supplied at an even lower rate. Because of their high P requirements the bacteria acted as consumers of inorganic P and their net consumption of P was four times higher than that of the phytoplankton.

A mathematical model of the production process of live feed to fish larvae is presented. The model is used as a design tool to construct a constrol system which ensure stable and controlable conditions for live feed organisms (rotifers). The simulation results demonstrate the effect of possible changes inthe biological parameters and different operational routines.

The filamentous blue-green alga Nostoc calcicola Geitler, strain 79WA01, showed a peak production of ∼70% of its biomass as a mixture of exocellular proteoglycan complexes, containing 3–30% of a polypeptide moiety and >70% of a complex glycuronoglycan. The former contained high proportions of Asp, Glu, Arg, and amido-NH 3 , in addition to ∼35% of “hydrophobic” amino-acids. The latter varied in composition in different fractions: GalA (2.5–10.3%), GlcA (4.7–11.5%), Glc (11.7–39.0%), Xyl (5.7–17.9%), Man (2.7–9.5%), Gal (5.7–9.5%), Fuc (1.5–11.1%), Ara (1.9–4.3%), and Rha (1.4–4.4%). None of the fractions showed a stoichiometric ratio of sugar residues. Palmelloid cells of three unicellular green soil-algae of the genus Chlamydomonas yielded ∼70% of their dry weight as capsular mucilage. About 50% of the sodium salt of this material was soluble in water, and contained 3–12% of polypeptide and 88–97% of glycuronoglycan (GlcA:Glc:Xyl = 1:1:3 for C. humicola Lucksch, and GlcA:Gal = 1:2 for C. peterfii Gerloff and C. sajao Lewin). These categorical differences in sugar composition, together with narrow composition distributions, suggested regular structures for the glycuronoglycans. The remainder of the mucilages contained essentially the same glycuronoglycan chains, but a higher proportion of polypeptide. These materials did not form true solutions in water, but dispersed as microgel particles.

In two enclosure experiments, Daphnia pulex ingested cryptophytes, bacteria, and probably detritus particles. The specific clearance rate of the zooplankton increased when the concentration of food decreased. The P : C ratio of the food also increased. More than 92% of the particulate phosphorus was located in the living cells (algae and bacteria); the detritus was practically phosphorus-free. The specific release rate of phosphorus estimated for the daphnids by use of the recycling model increased as the P : C ratio of the food increased and became zero at a critical low P : C ratio, Q. of 6-8 pg P mg-’ C. At this concentration, all the ingested phosphorus is needed for growth and reproduction, and no release of the element can be expected. This indicates that Daphnia may experience P limitation in nature, since the P : C ratio of P-starved algae and detritus may be considerably ~6-8 pg P mg-l C. The use of mass balance also appears to be suitable for modeling phosphorus and carbon fluxes through zooplankton in pelagic ecosystems.

The efficiency of carbon assimilation in Daphnia magna was estimated as a function of the 14C-labeling time of the food alga Scenedesmus acutus. The assimilation efficiency of 14C was up to 24% higher at low labeling times (≤ 1.4 generations) (P < 0.05) than at high labeling times (≥ 4.2 generations). In uniformly labeled cells the assimilation efficiency in D. magna was 71 ± 2% SE. Variation in the assimilation efficiency with labeling time is associated with change in the relative distribution of 14C among the different cell compartments in the food alga. Uniform labeling of algae is achieved effectively via exponential growth over several generations in batch cultures from sparse inocula.

Moderately starved rotifers exhibited a two-phased increase in n-3 fatty acids when they were fed a diet rich in these fatty acids. The first 20–30 min of enrichment, the increase in n-3 fatty acids was primarily due to increased gut content. The subsequent slow increase was due to an incorporation of n-3 fatty acids into rotifers tissues. Saturation was achieved before 24 h of exposure and the saturation level was independent of the initial content of n-3 fatty acids in the rotifers.

A detailed study of the life history of the rotifer Brachionus plicatilis was done at 20 °C, 20 ppt salinity, and 90 mg C 1−1 food concentration. Rotifers were grown individually in culture plate wells (150 µl culture volume) and fed Isochrysis galbana Tahiti, Tetraselmis sp., Nannochloris atomus, or a l : 1 mixture (weight) of two of the algae. Observations were made every 2–8 hr and rotifers were sized and transferred to new food daily. A total of 19 different parameters were compared. Rotifers fed Isochrysis averaged 21 offspring per female, a 6.7 day reproductive period, a lifespan of 10.5 days and a mean length of 234 µm. After Isochrysis, the foods giving the highest growth, survival, and reproduction in decreasing order were Isochrysis + Nannochloris, Nannochloris, Isochrysis + Tetraselmis, Tetraselmis + Nannochloris, and Tetraselmis. Although the small volume culture system used in this study seems appropriate for studying life history of B. plicatilis, the results cannot always be directly applied to larger cultures.