Dörthe C. Müller-Navarra

Fatty acid trophic markers in the pelagic marine environment

Fatty acids have been used as qualitative markers to trace or confirm predator-prey relationships in the marine environment for more than thirty years. More recently, they have also been used to identify key processes impacting the dynamics of some of the worlds major ecosystems. The fatty acid trophic marker (FATM) concept is based on the observation that marine primary producers lay down certain fatty acid patterns that may be transferred conservatively to, and hence can be recognized in, primary consumers. To identify these fatty acid patterns the literature was surveyed and a partial least squares (PLS) regression analysis of the data was performed, validating the specificity of particular microalgal FATM. Microalgal group specific FATM have been traced in various primary consumers, particularly in herbivorous calanoid copepods, which accumulate large lipid reserves, and which dominate the zooplankton biomass in high latitude ecosystems. At higher trophic levels these markers of herbivory are obscured as the degree of carnivory increases, and as the fatty acids originate from a variety of dietary sources. Such differences are highlighted in a PLS regression analysis of fatty acid and fatty alcohol compositional data (the components of wax esters accumulated by many marine organisms) of key Arctic and Antarctic herbivorous, omnivorous and carnivorous copepod species. The analysis emphasizes how calanoid copepods separate from other copepods not only by their content of microalgal group specific FATM, but also by their large content of long-chain monounsaturated fatty acids and alcohols. These monounsaturates have been used to trace and resolve food web relationships in, for example, hyperiid amphipods, euphausiids and fish, which may consume large numbers of calanoid copepods. Results like these are extremely valuable for enabling the discrimination of specific prey species utilized by higher trophic level omnivores and carnivores without the employment of invasive techniques, and thereby for identifying the sources of energetic reserves. A conceptual model of the spatial and temporal dominance of group-specific primary producers, and hence the basic fatty acid patterns available to higher trophic levels is presented. The model is based on stratification, which acts on phytoplankton group dominance through the availability of light and nutrients. It predicts the seasonal and ecosystem specific contribution of diatom and flagellate/microbial loop FATM to food webs as a function of water column stability. Future prospects for the application of FATM in resolving dynamic ecosystem processes are assessed.

A highly unsaturated fatty acid predicts carbon transfer between primary producers and consumers.

The factors that regulate energy transfer between primary producers and consumers in aquatic ecosystems have been investigated for more than 50 years (refs 1,2,3). Among all levels of the food web (plants, herbivores, carnivores), the plant–animal interface is the most variable and least predictable link. In hypereutrophic lakes, for example, biomass and energy transfer is often inhibited at the phytoplankton–zooplankton link, resulting in an accumulation of phytoplankton biomass instead of sustaining production at higher trophic levels, such as fish. Accumulation of phytoplankton (especially cyanobacteria) results in severe deterioration of water quality, with detrimental effects on the health of humans and domestic animals, and diminished recreational value of water bodies. We show here that low transfer efficiencies between primary producers and consumers during cyanobacteria bloom conditions are related to low relative eicosapentaenoic acid (20:5ω3) content of the primary producer community. Zooplankton growth and egg production were strongly related to the primary producer 20:5ω3 to carbon ratio. This indicates that limitation of zooplankton production by this essential fatty acid is of central importance at the pelagic producer–consumer interface.

Unsaturated fatty acid content in seston and tropho-dynamic coupling in lakes

Determining the factors that control food web interactions is a key issue in ecology. The empirical relationship between nutrient loading (total phosphorus) and phytoplankton standing stock (chlorophyll a) in lakes was described about 30 years ago and is central for managing surface water quality. The efficiency with which biomass and energy are transferred through the food web and sustain the production of higher trophic levels (such as fish) declines with nutrient loading and system productivity, but the underlying mechanisms are poorly understood. Here we show that in seston (fine particles in water) during summer, specific ω3-polyunsaturated fatty acids (ω3-PUFAs), which are important for zooplankton, are significantly correlated to the trophic status of the lake. The ω3-PUFAs octadecatetraenoic acid, eicosapentaenoic acid (EPA) and docosahexaenoic acid, but not α-linolenic acid, decrease on a double-logarithmic scale with increasing total phosphorus. By combining the empirical relationship between EPA-to-carbon content and total phosphorus with functional models relating EPA-to-carbon content to the growth and egg production of daphnids, we predict secondary production for this key consumer. Thus, the decreasing efficiency in energy transfer with increasing lake productivity can be explained by differences in ω3-PUFA-associated food quality at the plant–animal interface.

Perspectives on marine zooplankton lipids

We developed new perspectives to identify important questions and to propose approaches for future research on marine food web lipids. They were related to (i) structure and function of lipids, (ii) lipid changes during critical life phases, (iii) trophic marker lipids, and (iv) potential impact of climate change. The first addresses the role of lipids in membranes, storage lipids, and buoyancy with the following key question: How are the properties of membranes and deposits affected by the various types of lipids? The second deals with the importance of various types of lipids during reproduction, development, and resting phases and addresses the role of the different storage lipids during growth and dormancy. The third relates to trophic marker lipids, which are an important tool to follow lipid and energy transfer through the food web. The central question is how can fatty acids be used to identify and quantify food web relationships? With the fourth, hypotheses are presented on effects of global warmi...

A global database of lake surface temperatures collected by in situ and satellite methods from 1985–2009

Global environmental change has influenced lake surface temperatures, a key driver of ecosystem structure and function. Recent studies have suggested significant warming of water temperatures in individual lakes across many different regions around the world. However, the spatial and temporal coherence associated with the magnitude of these trends remains unclear. Thus, a global data set of water temperature is required to understand and synthesize global, long-term trends in surface water temperatures of inland bodies of water. We assembled a database of summer lake surface temperatures for 291 lakes collected in situ and/or by satellites for the period 1985–2009. In addition, corresponding climatic drivers (air temperatures, solar radiation, and cloud cover) and geomorphometric characteristics (latitude, longitude, elevation, lake surface area, maximum depth, mean depth, and volume) that influence lake surface temperatures were compiled for each lake. This unique dataset offers an invaluable baseline perspective on global-scale lake thermal conditions as environmental change continues.

Seston essential fatty acids and carbon to phosphorus ratios as predictors for Daphnia pulex dynamics in a large reservoir, Lake Berryessa

We examined whether seston food quality was a significant factor to predict zooplankton, specifically Daphnia pulex, biomass dynamics in Lake Berryessa, California, U.S.A. The seston composition changed dramatically during the study period. Total macrozooplankton to seston biomass ratios were high during the springtime, as was D. pulex biomass. In monthly growth experiments using natural seston, eicosapentaenoic acid (EPA) concentration was best in explaining D. pulex growth, although α-linolenic acid (ALA) also showed considerable relationship with D. pulex growth. We built D. pulex growth models using combinations of EPA concentration, C:P ratio, and water temperature. The model using EPA concentration, C:P ratio and water temperature was best in explaining D. pulex biomass in springtime in Lake Berryessa. However, this model did not work for other seasons, suggesting that resource control via seston food quality was prevalent in springtime, but other factors may also become important in other seasons in this reservoir.