Food web manipulation in Lake Zwemlust: Positive and negative effects during the first two years

The hypertrophic Lake Zwemlust, a small water body used as a swimming pool, was characterized by algal blooms in summer, reducing the Secchi disk transparency to less than 0.3 m. Since in The Netherlands a Secchi disk transparency of 1 m is obligatory for swimming waters, corrective measures were called for to improve the light climate of the lake. In March, 1987, as an experiment, the lake was drained by pumping out the water to facilitate fish elimination. Planktivorous and benthivorous fish species, which were predominant, were removed by seine- and electro-fishing. After the lake had refilled by seepage it was restocked by a new simple fish community comprising pike (Esox lucius) and rudd (Scardinius erythrophthalmus) only. Stacks of willow twigs (Salix) and macrophytes (roots ofNuphar lutea and seedlings ofChara globularis) were introduced into the lake as spawning grounds and refuges for the pike against cannibalism and as shelter for the zooplankton. The effects of this food web manipulation on the light climate, phytoplankton, zooplankton, fish, macrophytes, macrofauna and on the nutrient concentrations were monitored during 1987 and 1988. In summer 1987, despite of high nutrient concentrations, the phytoplankton density was low, due to control by zooplankton, causing a Secchi disk transparency of 2.5 m, the maximum depth. Chlorophyll-a concentrations were low (<5 g Chl.l^–1), blooms of cyanobacteria did not occur and a shift from rotifers to cladocerans took place. In 1988, however, also some negative effects were noticed. Macrophytes and filamentous green algae reached a much higher biomass (50–60% cover of the lake bottom) than in 1987; some species, growing through the entire water column, interfered with the lake's recreational use. Associated with the macro-vegetation and possibly with the absence of larger cyprinids, the diet of which also comprises snails, a large scale development of the snail population, among themLymnaea peregra var.ovata took place. This species is known to act as an intermediate host of the bird-parasitizing trematodeTrichobilharzia ocellata, the cercariae of which cause an itching sensation at the spot of penetration of the human skin, accompanied by rash (schistosome dermatitis or swimmers' itch); in July, 1988, about 40% of the bathers complained about this itching. A positive effect of the macrophytes and filamentous green algae was the high uptake of nitrogen, resulting in a low nitrogen concentration in the lake and growth limitation of the phytoplankton population by nitrogen in the summer of 1988. In 1988 the cladocerans were abundant in April only; and unlike in 1987, in the summer of 1988 there was a shift from cladocerans to rotifers. Therefore, only in early spring (April) zooplankton grazing controlled phytoplankton growth and in summer nitrogen limitation was the major controlling factor, keeping chlorophyll-a concentrations low.     

Response of Daphnia to substances released from crowded congeners and conspecifics

The effects of chemicals released from crowded congeners andconspecifics on life history parameters of the freshwater zooplanktersDaphnia cucullata and Daphnia pulex were examined. Length andage at maturity of D. pulex were affected by crowding chemicals.Reproduction was lower in crowded medium, and ephippia wereproduced. Newborn D. pulex in crowded medium were significantlylonger than the controls. The intrinsic rate of population increaseof D. pulex was 14 and 25% lower than the control when exposedto crowded medium from D. cucullata and D. pulex, respectively.Neither urea nor ammonia (at 1 mg l^-1) seemed to be responsiblefor these effects in D. pulex. In D. cucullata, no significanteffect of crowding infochemicals on length and age at maturitywas found. However, crowding chemicals reduced reproduction.No ephippia were produced in crowded medium, but up to 83% non-developingeggs were observed in D. cucullata. Newborns were similarlysized in crowded and standard medium. The intrinsic rate ofpopulation increase of D. cucullata was 44 and 96% lower thanthe control when exposed to crowded medium from D. cucullataand D. pulex, respectively. Clearance rates of D. pulex weresignificantly reduced in crowded media compared with standardmedium, which could partly explain why the animals exposed tocrowding chemicals reacted as if they were food limited.     

Restoration by biomanipulation in a small hypertrophic lake: first-year results

Biomanipulation was carried out in order to improve the water quality of the small hypertrophic Lake Zwemlust (1.5 ha; mean depth 1.5 m). In March 1987 the lake was drained to facilitate the elimination of fish. Fish populations were dominated by planktivorous and benthivorous species (total stock c. 1500 kg) and were collected by seine- and electro-fishing. The lake was subsequently re-stocked with 1500 northern pike fingerlings (Esox lucius L.) and a low density of adult rudd (Scardinius erythrophthalmus). The offspring of the rudd served as food for the predator pike. Stacks of Salix twigs, roots of Nuphar lutea and plantlets of Chara globularis were brought in as refuge and spawning grounds for the pike, as well as shelter for the zooplankton.The impact of this biomanipulation on the light penetration, phytoplankton density, macrophytes, zooplankton and fish communities and on nutrient concentrations was monitored from March 1987 onwards. This paper presents the results in the first year after biomanipulation.The abundance of phytoplankton in the first summer (1987) after this biomanipulation was very low, and consequently accompanied by increase of Secchi-disc transparency and drastic decline of chlorophyll a concentration.The submerged vegetation remained scarce, with only 5 % of the bottom covered by macrophytes at the end of the season.Zooplankters became more abundant and there was a shift from rotifers to cladocerans, comprised mainly of Daphnia and Bosmina species, the former including at least 3 species.The offspring of the stocked rudd was present in the lake from the end of August 1987. Only 19% of the stocked pike survived the first year.Bioassays and experiments with zooplankton community grazing showed that the grazing pressure imposed by the zooplankton community was able to keep chlorophyll a concentrations and algal abundance to low levels, even in the presence of very high concentrations of inorganic N and P. The total nutrient level increased after biomanipulation, probably due to increased release from the sediment by bioturbation, the biomass of chironomids being high.At the end of 1987 Lake Zwemlust was still in an unstable stage. A new fish population dominated by piscivores, intended to control the planktivorous and benthivorous fish, and the submerged macrophytes did not yet stabilize.     

Detection of hydrogen peroxide by lactoperoxidase-mediated dityrosine formation

The aim of this work was to study the dityrosine-forming activity of lactoperoxidase (LPO) and its potential application for measuring hydrogen peroxide (H₂O₂). It was observed that LPO was able to form dityrosine at low H₂O₂ concentrations. Since dityrosine concentration could be measured in a simple fluorimetric reaction, this activity of the enzyme was utilized for the measurement of H₂O₂ production in different systems. These experiments successfully measured the activity of NADPH oxidase 4 (Nox4) by this method. It was concluded that LPO-mediated dityrosine formation offers a simple way for H₂O₂ measurement.     

Strong interactions between stoichiometric constraints and algal defenses: evidence from population dynamics of Daphnia and algae in phosphorus-limited microcosms

The dynamic interactions among nutrients, algae and grazers were tested in a 2 × 3 factorial microcosm experiment that manipulated grazers (Daphnia present or absent) and algal composition (single species cultures and mixtures of an undefended and a digestion-resistant green alga). The experiment was run for 25 days in 10-L carboys under mesotrophic conditions that quickly led to strong phosphorus limitation of algal growth (TP ? 0.5 μM, N:P 40:1). Four-day Daphnia juvenile growth assays tested for Daphnia P-limitation and nutrient-dependent or grazer-induced algal defenses. The maximal algal growth rate of undefended Ankistrodesmus (mean ± SE for three replicate microcosms; 0.92 ± 0.02 day^?1) was higher than for defended Oocystis (0.62 ± 0.03 day^?1), but by day 6, algal growth was strongly P-limited in all six treatments (molar C:P ratio >900). The P-deficient algae were poor quality resources in all three algal treatments. However, Daphnia population growth, reproduction, and survival were much lower in the digestion-resistant treatment even though growth assays provided evidence for Daphnia P-limitation in only the undefended and mixed treatments. Growth assays provided little or no support for simple threshold element ratio (TER) models that fail to consider algae defenses that result in viable gut passage. Our results show that strong P-limitation of algal growth enhances the defenses of a digestion-resistant alga, favoring high abundance of well-defended algae and energy limitation of zooplankton growth.     

Unusual transformations in the biosynthesis of the antibiotic phosphinothricin tripeptide

Phosphinothricin tripeptide (PTT, phosphinothricylalanylalanine) is a natural-product antibiotic and potent herbicide that is produced by Streptomyces hygroscopicus ATCC 21705 (ref. 1) and Streptomyces viridochromogenes DSM 40736 (ref. 2). PTT has attracted widespread interest because of its commercial applications and unique phosphinic acid functional group. Despite intensive study since its discovery in 1972 (see ref. 3 for a comprehensive review), a number of steps early in the PTT biosynthetic pathway remain uncharacterized. Here we report a series of interdisciplinary experiments involving the construction of defined S. viridochromogenes mutants, chemical characterization of accumulated intermediates, and in vitro assay of selected enzymes to examine these critical steps in PTT biosynthesis. Our results indicate that early PTT biosynthesis involves a series of catalytic steps that to our knowledge has not been described so far, including a highly unusual reaction for carbon bond cleavage. In sum, we define a pathway for early PTT biosynthesis that is more complex than previously appreciated.