Temporal variability of a benthic food web: patterns and processes in a low-diversity system

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

“…Nevertheless, there are significant changes in carbon sources detectable at the predator level in the course of the year. This is supported by Nordström et al (2009), who found that δ 15 N values and thus trophic position changed significantly from month to month.…”

“…Earlier studies with lower temporal resolution than ours indicated that these processes are relevant in macrophyte systems in which epiphytes and/or annual macroalgae are important food sources (Orav-Kotta & Kotta 2003, Vizzini & Mazzola 2003, Connolly et al 2005, Nordström et al 2009). Trophic interactions in benthic food webs that depend more on macrophyte detritus seem to be characterised by relatively stable dynamics (Akin & Winemiller 2006, Douglass 2008.…”

“…Gut content analyses showed that amphipods and isopods are important parts of the nutrition of these predators (Bobsien 2006. Nordström et al (2009) found that seasonal fluctuations of carbon contribution at the herbivore level were strongly diminished at the predator level, but they sampled only from June to September, a period of time when our data suggested similarly stable carbon sources. As the importance of seasonal variation in basic δ 13 C values depends on the growth rates of the species, the observed low variability in carbon sources of small predators is in accordance with their lower turnover rate compared with the smaller fast-growing mesograzers.…”

Incorporating temporally dynamic baselines in isotopic mixing models

“…In aquatic ecosystems, the d 13 C (ratio of heavy to light carbon isotope expressed as %) value of phytoplankton and periphyton is influenced by the composition and source of assimilated dissolved inorganic carbon (e.g., CO 2 vs. HCO 3 À , atmospheric vs. respired CO 2 ), nutrient availability (e.g., flow regime, boundary layer dynamics), algal metabolism and biomass, and fractionation kinetics (Fry and Sherr 1984, MacLeod and Barton 1998, Trudeau and Rasmussen 2003, Finlay 2004, Hill and Middleton 2006, Rasmussen and Trudeau 2007. Many of these influences are seasonally variable, resulting in dynamical d 13 C patterns in aquatic primary producers (Rolff 2000, Gu et al 2006) that can propagate to the next trophic level (i.e., primary consumers; Kline 1999, Grey et al 2001, Nordstro¨m et al 2009). Trophic studies in which samples are collected over limited time intervals may fail to identify such systematic seasonal shifts in the isotope value of baseline organisms (Nordstro¨m et al 2009).…”

“…A common application of these endmembers is the delineation of trophic baselines Rasmussen 1999, Post 2002), benchmark values used to identify ultimate nutrient sources or standardize data to common scales of reference (e.g., Anderson andCabana 2007, Takimoto et al 2008). Most isotope mixing models require that baselines be static over timescales relevant to the equilibration rate of the modeled consumer, an important assumption that is rarely tested rigorously (Syvara¨nta et al 2006, Nordstro¨m et al 2009). To satisfy mixing models that rely on the assumption of temporal equilibrium, primary consumers are typically selected as baseline organisms due to their known trophic position and capacity to dampen the temporally variable isotopic values of autotrophic resources Rasmussen 1996, Post 2002).…”