Maarten Boersma

Predator-Mediated Plasticity in Morphology, Life History, and Behavior of Daphnia: The Uncoupling of Responses

We studied the way 12 traits responded to fish kairomones in a set of 16 Daphnia magna clones derived from four different habitats—two where daphnids co‐occur with fish and two without fish. These clones differed widely in their response to predator kairomones, with none of the clones showing a significant response in all traits and all clones showing a response for at least one trait. Most of the clones showed a significant response in one to four traits, with no evidence for an association between different traits. Clones from fish habitats were slightly more responsive to the presence of fish kairomones than clones from fishless locations. We conclude that most clones show an induced response to the presence of their predators (fish) but that there is a large genetic variability with respect to the traits for which clones show a response. Our results indicate that the major distinction is not between inducible and noninducible genotypes but rather that the genotypes differ in the combination of traits for which they show inducible responses.

TOO MUCH OF A GOOD THING: ON STOICHIOMETRICALLY BALANCED DIETS AND MAXIMAL GROWTH

Nutritional imbalances are of great interest in the ecological stoichiometry literature, in which researchers have focused almost exclusively on cases where nutrients are available in low amounts relative to energy (carbon), and animal growth is impaired due to insufficient nutrient intake. Little attention has been given to situations where food elemental content is higher than the level that satisfies animal requirements. However, most animals are strongly homeostatic with respect to the elemental composition of their body; hence they must excrete the excess of elements that are not in short supply. To date, stoichiometric theory has assumed that excretion of superfluous elements does not come with a cost and, thus, that consumption of food with surplus nutrients does not impair performance. Here we challenge this assumption, based on a compilation of several examples involving food phosphorus content that show that the performance of a wide variety of animals decreases when supplied with food containing high concentrations of (potentially) limiting nutrients. We discuss possible mechanisms for this phenomenon, and suggest that animals most vulnerable to effects of high food nutrient content are those that normally feed on low- quality (low-nutrient: C) food, and have a relatively low body nutrient content themselves, such as herbivores and detritivores.

Extraction of pigments and fatty acids from the green alga Scenedesmus obliquus (Chlorophyceae)

In this paper, the efficiency of pigment and fatty acid extraction from resistant algae using Scenedesmus obliquus as an example was examined. We found that adding quartz sand and solvent to freeze-dried algal material and subsequent extraction in an ultrasound bath for 90min at −4 °C resulted in excellent extraction of these compounds. This extraction method was compared with a method regularly used for extraction of fatty acids and pigments, i.e. addition of solvents to algal material with subsequent incubation. Our extraction using the ultrasound and sand method was about twice as efficient as this method for both pigments and fatty acids. The ultrasound method is simple, extracts over 90% of the different substances in one step and conserves the relationships of pigments and fatty acids. In addition, no alteration- or breakdown products were observed with the new method. Thus, this method allows accurate quantitative extraction of both pigments and fatty acids from Scenedesmus obliquus and other algae. The method was also been found to be as effective for Cryptomonas erosa (Cryptophyceae), Cyclotella meneghiniana (Bacillariophyceae), Microcystis aeruginosa (Cyanophyceae), and Staurastrum paradoxum (Chlorophyceae, Desmidiaceae) and is thus applicable to a wide spectrum of algae.

Life at the edge: Is food quality really of minor importance at low quantities?

There is increasing evidence that the quality of nutrient-limited algae is suboptimal for zooplankton production. These effects of nutrient limitation are supposed to be important mainly at higher concentrations of food because at lower quantities the overall energetic limitation of body growth should hide the effect of mineral limitations. This has been hypothesized in a variety of papers, but experimental evidence is still weak. In the present study we carried out a set of growth experiments investigating the effect of food quality at very low food levels ranging from 30 μg C/L up to 150 μg C/L. In all of the experiments, the growth rates of Daphnia magna neonates were lower when grown on P-limited Scenedesmus obliquus. This effect disappeared when phosphorus was added to the P-limited algae prior to feeding, indicating that mineral limitation can occur even at very low levels of food. Neonates born to mothers raised on either high- or low-P Scenedesmus were analyzed for body mass and lipid content as well as mass-specific phosphorus content to check for possible differential investment into neonates in different environments. Although mass-specific phosphorus content was higher in animals born from mothers grown on high-P algae, when fed low quantities of P-limited algae, growth rates of neonates born under low-P conditions were higher than those of animals born under high-P conditions. This can be explained by an increased body lipid content of low-P neonates, even though there were no differences in neonate body mass between treatments. These results illustrate the importance of the incorporation of low food concentrations in ecological stoichiometry models.

Stoichiometry: linking elements to biochemicals

Ecological stoichiometry is a useful tool for studying how the elemental composition of organisms and their food affects production, nutrient cycling, and food- web dynamics. Two analyses are presented here that show that the use of simple element ratios in stoichiometric calculations may in certain circumstances prove inadequate because of the influence in animal nutrition of biochemical aspects of diet. In the first, a stoichio- metric analysis of herbivores consuming food with varying carbon to nitrogen (C:N) ratios is undertaken, in which the intake of C is segregated into easily assimilated compounds and fiber. Two herbivore strategies emerge from the analysis, both as a means of minimizing limitation by C, not N: fiber eaters that consume high C:N food and have efficient fiber digestion, and selective feeders that consume low C:N food but that do not possess fiber- digesting enzymes. In the second example, the stoichiometric axiom that a single substrate, the one in least supply relative to demand, limits growth is used to identify potentially limiting essential amino acids in the diets of a range of animals. Large consumer-prey imbalances in amino acids were found in several cases, indicating that, at least in theory, growth should be strongly limited by individual amino acids rather than bulk N. In practice such limitation may be offset in consumers by physiological and other factors such as symbiotic relationships. The two analyses emphasize the simplicity of element stoichi- ometry, highlighting the need to consider biochemical and physiological arguments when undertaking stoichiometric studies of carbon and nutrient transfers in ecosystems.

Genetic markers, genealogies and biogeographic patterns in the cladocera

Cladoceran crustaceans are an important component of zooplankton in a wide range of freshwater habitats. Although the ecological characteristics of several cladoceran species have been well studied, biogeographical studies have been hampered by problematic taxonomic affiliations. However, recently developed molecular techniques, provide a powerful tool to subject aquatic taxa to comparative analyses. Here we highlight recent molecular approaches in aquatic ecology by presenting a simple method of DNA preparation and PCR amplification of the mitochondrial DNA (16S rDNA) in species from nine different families within the cladocera. On a broad taxonomic scale, sequence analysis of this mtDNA fragment has been used to produce the first molecular based phylogeny of the cladocera. This analysis clustered the cladoceran families in a fashion similar to that suggested by previous systematic classifications. In a more detailed analysis of the family Daphniidae, nuclear randomly amplified polymorphic DNA (RAPD), mitochondrial restriction fragment length polymorphism (RFLP) and morphological analyses were combined to identify species and interspecific hybrids within the Daphnia galeata species complex across 50 lakes in 13 European countries and one lake in Africa. The study revealed interspecific hybridization and backcrossing between some taxa (D. cucullata and D. galeata) to be widespread, and species and hybrids to frequently occur in sympatry. Genetic, as well as morphological information, suggests the occurrence of D. hyalina outside the Holarctic.

Differential effects of nutrient-limited primary production on primary, secondary or tertiary consumers

Nutritional imbalances between predator and prey are the rule rather than the exception at the lower end of food webs. We investigated the role of different grazers in the propagation of nutritionally imbalanced primary production by using the same primary producers in a three-trophic-level food chain and a four-trophic-level food chain experimental setup. The three-trophic-level food chain consisted of a classic single-cell primary producer (Rhodomonas salina), a metazoan grazer (the copepod Acartia tonsa) and a top predator (the jellyfish Gonionemus vertens), while we added a protozoan grazer (Oxyrrhis marina) as primary consumer to the food chain to establish the four-trophic-level food chain. This setup allowed us to investigate how nutrient-limitation effects change from one trophic level to another, and to investigate the performance of two components of our experimental food chains in different trophic positions. Stoichiometry and fatty acid profiles of the algae showed significant differences between the nutrient-depleted [no N and no P addition (−P), respectively] and the nutrient-replete (f/2) treatments. The differences in stoichiometry could be traced when O. marina was the first consumer. Copepods feeding on these flagellates were not affected by the nutritional imbalance of their prey in their stoichiometry, their respiration rates nor in their developmental rates. In contrast, when copepods were the primary consumer, those reared on the −P algae showed significantly higher respiration rates along with significantly lower developmental rates. In neither of our two experimental food chains did the signals from the base of the food chains travel up to jelly fish, our top predator.

Lipid and fatty acid composition of diatoms revisited: Rapid wound-activated change of food quality parameters influences herbivorous copepod reproductive success

Lipid and fatty acid composition are considered to be key parameters that determine the nutritive quality of phytoplankton diets for zooplanktonic herbivores. The fitness, reproduction and physiology of the grazers are influenced by these factors. The trophic transfer of lipids and fatty acids from algal cells has been typically studied by using simple extraction and quantification approaches, which, as we argue here, do not reflect the actual situation in the plankton. We show that cell disruption, as it occurs during a predators grazing on diatoms can drastically change the lipid and fatty acid content of the food. In some algae, a rapid depletion of polyunsaturated fatty acids (PUFAs) is observed within the first minutes after cell disruption. This fatty acid depletion is directly linked to the production of PUFA‐derived polyunsaturated aldehydes (PUA); these are molecules that are thought to be involved in the chemical defence of the algae. PUA‐releasing diatoms are even capable of transforming lipids from other sources if these are available in the vicinity of the wounded cells. Fluorescent staining reveals that the enzymes involved in lipid transformation are active in the foregut of copepods, and therefore link the depletion processes directly to food uptake. Incubation experiments with the calanoid copepod Temora longicornis showed that PUFA depletion in PUA‐producing diatoms is correlated to reduced hatching success, and can be compensated for by externally added single fatty acids.

Seasonal patterns in the mortality of Daphnia species in a shallow lake

To assess the impact of predation by young-of-the-year (0+) fish on the population dynamics of Daphniaspecies, we made independent estimations of the mortality of Daphniaspecies during the year, and of the predation pressure exerted by the juvenile fish. Mortality of daphnids was computed using a model that allowed us to differentiate between different size-classes, while total fish consumption was estimated from the temperature-dependent daily weight increase and the population development of the 0+ fish. The predation pressure on the different size-classes of Daphniaspecies was estimated by combining the total fish consumption with estimates of the selective feeding behaviour of the fish. To make the estimates of fish consumption independent of our current (1989–1991) zooplankton data set, we estimated fish species-specific and fish length-dependent selectivity indices on zooplankton using a different data set (1976–1977).Daphniapopulation densities usually increased in spring and decreased rapidly in early summer. Predation by 0+ fish was not severe enough to explain the large mortality that caused the summer decline; later in the year most of the mortality in the larger size-classes of the daphnids (>1.0 mm) could be explained by fish predation. Resumé: Pour mesurer l’impact de la prédation par les jeunes poissons de l’année (0+) sur la dynamique des populations des espèces de Daphnia, nous avons fait des estimations indépendantes de la mortalité des espèces de Daphniapendant l’année, et de la pression de prédation exercée par les jeunes poissons. Nous avons calculé la mortalité des daphnies à l’aide d’un modèle qui nous permettait de différencier les classes de taille, tandis que la consommation totale des poissons était estimée à partir de l’augmentation de poids quotidienne dépendant de la température et du développement de la population de poissons 0+. Nous avons estimé la pression de prédation sur les différentes classes de taille de Daphniaen combinant la consommation totale des poissons à des estimations du comportement d’alimentation sélective des poissons. Pour rendre les estimations de la consommation des poissons indépendantes de notre série présente (1989–1991) de données sur le zooplancton, nous avons calculé les indices de sélectivité à l’égard du zooplancton qui sont propres à l’espèce de poisson et dépendants de la longueur des poissons, en nous servant d’une série différente de données (1976–1977). Les densités de la population de Daphnia augmentaient généralement au printemps et baissaient rapidement au début de l’été. La prédation par les poissons 0+ n’était pas assez forte pour expliquer l’importante mortalité qui causait le déclin estival; pendant le reste de l’année, la majeure partie de la mortalité chez les plus grandes classes de taille des daphnies (>1,0 mm) pouvait s’expliquer par la prédation des poissons. [Traduit par la Rédaction]

The first occurrence of the ctenophore Mnemiopsis leidyi in the North Sea

After the discovery of large densities of Mnemiopsis leidyi in the Baltic Sea near Kiel by Javidpour et al. (First record of Mnemiopsis leidyi A. Agassiz 1865 in the Baltic Sea, 2006) in October 2006, we investigated the gelatinous zooplankton in the North Sea near Helgoland and recorded Mnemiopsis leidyi for the first time in the North Sea, albeit in much lower densities than those recorded in the Baltic Sea.