Stable isotopes show food web changes after invasion by the predatory cladoceran Cercopagis pengoi in a Baltic Sea bay

Abstract

Cercopagis pengoi, a recent invader to the Baltic Sea and the Laurentian Great Lakes, is a potential competitor with fish for zooplankton prey. We used stable C and N isotope ratios to elucidate trophic relationships between C. pengoi, zooplankton (microzooplankton, 90-200 microm, mostly copepod nauplii and rotifers; mesozooplankton, >200 microm, mostly copepods), and zooplanktivorous fish (herring, size range 5-15 cm and sprat, 9-11 cm) in a coastal area of the northern Baltic Sea. The isotope ratios in C. pengoi and fish were much higher than those of zooplankton, showing general trends of enrichment with trophic level. Young-of-the-year (YOY) herring had a significantly higher (15)N/(14)N ratio than C. pengoi, suggesting of a trophic linkage between the two species. To evaluate the possible relative importance of different food sources for C. pengoi and YOY herring, two-source isotope-mixing models for N were used, with micro- and mesozooplankton as prey for C. pengoi and mesozooplankton and C. pengoi as prey for YOY herring. These models indicate that mesozooplankton was the major food source of both species. However, microzooplankton may be important prey for young stages of C. pengoi. Comparative analyses of the herring trophic position before and after the invasion by C. pengoi showed a trophic level shift from 2.6 to 3.4, indicating substantial alterations in the food web structure. Our findings contribute to a growing body of evidence, showing that C. pengoi can modify food webs and trophic interactions in invaded ecosystems.

Predation of the introduced cladoceran Cercopagis pengoi on the native copepod Eurytemora affinis in the northern Baltic Sea

“…To quantify this competition, feeding preferences and consumption rates of C. pengoi must be evaluated. However, apart from a few published experimental studies (Laxson et al 2003, Lehtiniemi & Lindén 2006, C. pengoi food preferences have been inferred only indirectly from alterations in demographic structure of zooplankton communities (Benoît et al 2002, Ojaveer et al 2004, Litvinchuk & Telesh 2006 or using stable isotope analysis (Gorokhova et al 2005). These studies suggest that C. pengoi feeds on small zooplankton (Laxson et al 2003, Lehtiniemi & Lindén 2006 and may therefore have negative effects on these populations by direct predation (Benoît et al 2002, Ojaveer et al 2004, Witt et al 2005.…”

“…In the Gulf of Finland, especially in the eastern parts, a collapse of E. affinis, a key species in the pelagic system, would cause major changes in virtually all trophic levels, because the food web structure is simple with only a few species of zooplankton, planktivores, and piscivores. 362: 193-200, 2008 alterations in demographic structure of zooplankton communities (Benoît et al 2002, Ojaveer et al 2004, Litvinchuk & Telesh 2006 or using stable isotope analysis (Gorokhova et al 2005). These studies suggest that C. pengoi feeds on small zooplankton (Laxson et al 2003, Lehtiniemi & Lindén 2006 and may therefore have negative effects on these populations by direct predation (Benoît et al 2002, Ojaveer et al 2004, Witt et al 2005.…”

Is diatom size selection by harpacticoid copepods related to grazer body size?

“…Copepods are either directly consumed by top predators (e.g. Gee, 1989;De Troch et al, 1998;Turner, 2004;Gorokhova et al, 2005) or linked to them through the food-web (e.g. HaroGaray, 2003).…”

The effect of ethanol fixation on stable isotope signatures in benthic organisms

“…However, if the effect of fixation is known, fixed organisms can also become useful samples for isotope analyses (e.g. Vanderzanden et al 2003, Gorokhova 2005.…”

“…Such a retrospective approach has been adopted for estimating ecosystem changes using archived samples fixed by other preservatives (e.g. Vanderzanden et al 2003, Gorokhova 2005. Ethanol-fixed samples should be useful for the same purpose.…”