Tracing aquatic food webs using fatty acids: From qualitative indicators to quantitative determination

Food web structure, predator-prey dynamics, foraging behavior, and consequences of these factors for individual growth, reproduction and survival are central to our understanding of ecosystem structure and functioning. Moreover, in the current context of understanding (and managing) ecosystems in the face of ongoing environmental change, important questions include: What are the critical prey of key consumers in relation to prey abundance, availability, and nutritional quality? What are the ecosystem processes responsible for food web production? And, how do these processes respond to changes in physical forcing? A fundamental require-ment to understand any of these areas is an accurate assessment of trophic relationships and consumer diets. However, in aquatic, and especially marine ecosystems, such information is generally not easily or reliably obtained. In these systems, the relative inaccessibility of free-ranging organisms and the inability to directly observe species interactions make it difficult to accurately characterize diet. Traditional approaches, such as examining gut contents, have well-recognized biases in addition to representing only snapshots of recent meals and may therefore not be reliable indicators of long-term diet (Iverson et al. 2004). Thus, alternative approaches have been developed, which use various types of trophic markers. One of the most promising of these approaches is the use of lipids and fatty acids (FA) to study food web dynamics.