Fatty acids distribution in marine, brackish and freshwater plankton during mesocosms experiments

Most studies on the fatty acid composition of phytoplankton are based on laboratory experiments, and numerous studies using fatty acids as food web traces were conducted with monoalgal cultures. However, fatty acid studies with natural phytoplankton communities (food quality) and the passage of these to the next food web levels (biomarkers) are not abundant. It is important to consider the differences between analyses of fatty acids from natural plankton and laboratory studies. Seston does not only contain phytoplankton, but also some portion of bacteria, protozoa and non-living particles (detritus), especially in regions with high nutrient contents. Because it is almost impossible to quantitatively separate algae from the other particles, the fatty acid composition of phytoplankton can be disguised in natural waters. The aim of this studies was to describe and compare the composition of fatty acids in natural phytoplankton and their transfer to the mesozooplankton community under different nutrient availability and grazing pressure. Furthermore the viability of using phytoplankton fatty acid biomarkers found in monoalgal culture studies was tested. The fatty acid composition of planktonic particulate matter (seston and zooplankton) was examined during summer and spring experiments carried out in Lake Schöhsee (Germany), the Hopavågen lagoon (Norway) and the Kiel Fjord (Germany). At all study sites, mesocosm experiments were carried out in polyethylene bags (volume ~1.5 or 3.4 m3) suspended in several floats. Each treatment consisted of a logarithmically scaled gradient of copedod or Daphnia densities. The copepods originated from natural assemblages, the Daphnia were laboratory-reared. In all experiments the nutrient availability influenced the fatty acid contents in seston. These results supported the idea that the ratio between SAFA, MUFA and PUFA change with variable nutrient limitation and may be used as an indicator of the physiological status of the algae. Increasing nutrient limitation lead to reduced food quality of phytoplankton because of the decrease in essential PUFA. Therefore nutrient limitation of phytoplankton can alter trophic interactions. The low food quality can inhibit zooplankton growth, because most zooplankton species are not able to synthesize de novo these fatty acids essential for growth and reproduction. However the results of the Kiel fjord experiment suggested that the lacking quality of seston can partially be compensated by seston quantity

Seasonal variation in carbon sources of mesograzers and small predators in an eelgrass community: stable isotope and fatty acid analyses

We assessed the seasonal change in trophic interactions at the mesograzer and small predator level in a temperate eelgrass system. This was done through stable isotope (δ13C and δ15N) and fatty acid analyses of eelgrass Zostera marina, attached epiphytes, sand microflora, the red alga Delesseria sanguinea, and the 4 most common mesograzer and predator species. Sampling took place monthly in the western part of the Baltic Sea during a whole year (April 2002 to February 2003). The trophic importance of microalgae was corroborated in all studied species. Red algae were consumed to a lesser extent and eelgrass was of minor importance. The degree of dependence on the main carbon sources was species-specific and varied with time. The small gastropod, Rissoa membranacea, mostly grazed epiphytes (73% on average), whereas the fractions of epiphytes and sand microflora showed a more balanced pattern in the other mesograzer species. Stable carbon data and fatty acid composition strongly suggested that epiphytic algae were the primary source of organic matter for mesograzers in late spring and autumn. In summer, sand microflora were of greater importance as carbon sources, except for the amphipod Gammarus oceanicus for which red algae was the most important. Stable nitrogen values indicated that the degree of carnivory was size-dependent in both omnivorous crustacean species studied; larger individuals generally occupied a higher trophic position than did smaller ones. Furthermore, the isopod Idotea baltica was more herbivorous in summer than in the other seasons. Our results confirm the importance of species-specific and temporal variability for the effect of mesograzers in eelgrass systems. The significance of mesograzers as determinants of food-web structure via the reduction of epiphytes is further corroborated by our study, and the observed high plasticity of mesograzers concerning food sources may promote the stability of eelgrass food webs despite strong seasonal variations in the biomass of primary producers

The trophic importance of epiphytic algae in a freshwater macrophyte system (Potamogeton perfoliatus L.): stable isotope and fatty acid analyses

Stable isotope and fatty acid analyses were used to study carbon sources for animals in a submerged plant bed. Epiphytes growing on Potamogeton perfoliatus, sand microflora, and alder leaves were the most important carbon sources. The most abundant macrophyte, P. perfoliatus was unimportant as a food source. Modelling (IsoSource) showed that epiphytes were the most important food source for the most abundant benthic invertebrates, the isopod Asellus aquaticus (annual mean contribution 64%), the amphipod Gammarus pulex (66%), and the gastropod Potamopyrgus antipodarum (83%). The mean annual contributions of sand microflora were, respectively, 21, 19, and 9%; and of alder leaves, 15, 15, and 8% for these three species. The relative importance of carbon sources varied seasonally. The relative contribution of epiphytes was lowest for all three grazer species in July: A. aquaticus 38%, G. pulex 43%, and P. antipodarum 42%. A decline in epiphyte biomass in summer may have caused this switch to less attractive food sources. P. perfoliatus provided habitat and shelter for consumers, but food was mainly supplied indirectly by providing space for attached epiphytes, which are fast-growing and provide a highly nutritious food source

Carbon sources and trophic structure in an eelgrass (Zostera marina L.) bed based on stable isotope and fatty acid analyses

Multiple stable isotope and fatty acid analyses were applied to examine food web dynamics in an eelgrass Zostera marina L. system in the western Baltic Sea. Samples of eelgrass, epiphytic algae, sand microflora, red algae, phytoplankton and main consumer species were collected in June 2002. delta C-13 values of primary producers ranged from -9.6%. for eelgrass to the most depleted value of -34.9%. for the most abundant red alga, Delesseria sanguinea, Epiphyte delta C-13 (-11.3 parts per thousand.) was close to the value for eelgrass, whereas sand microflora and phytoplankton showed intermediate values (-20.0 and -22.6 parts per thousand, respectively). delta C-13 values of consumer species varied from - 12.2 parts per thousand in the gastropod Lacuna vincta to -23.9 parts per thousand in the amphipod Amphitoe rubricata. Epiphytes, sand microflora and phytoplankton had relatively similar fatty acid signatures, indicating a dominance of diatoms. Fatty acid composition of the main consumers included the biomarker fatty acids for diatoms and red algae, whereas those for eelgrass were negligible. The stable isotope data in combination with the results of the fatty acid analysis strongly indicated that the food web in this eelgrass community was based on epiphytes, sand microflora and red algae. Additionally, the continuous distribution of delta N-15 values implied a food web that was characterized by a large proportion of generalist feeders in every group of consumers and by a high degree of omnivory