Summary
- Zebra mussels (Dreissena polymorpha) derive their energy from the pelagic energy pathway by filtering plankton. Because zebra mussels occur in high densities in littoral habitats, they potentially constitute an important trophic link between littoral consumers and pelagic energy sources. Northern map turtles (Graptemys geographica) are widespread in North America and consume zebra mussels.
- We used stable isotopes analyses to quantify the flow of energy from the pelagic pathway to northern map turtles and to infer the contribution of zebra mussels to map turtle biomass. We then built a bioenergetic model to estimate the annual intake of zebra mussels by northern map turtles in Lake Opinicon, Ontario, Canada.
- Stable isotopes analyses indicated that zebra mussels constitute between 0% and 14% of the diet of males and between 4% and 36% of the diet of females. Assuming that zebra mussels account for all of the pelagic contribution, we estimated that map turtles consume 3200 kg of zebra mussels annually. Because female map turtles are much larger than males and consume more zebra mussels, they are responsible for 95% of the zebra mussel biomass ingested annually.
- The pelagic pathway supports an important part of the standing crop biomass of map turtles in Lake Opinicon. We highlight the importance of freshwater turtles in lake ecosystems. Unravelling the trophic interactions mediated by freshwater turtles will lead to a more integrated picture of lake ecosystems.
Methodology
Study site and turtle biomass
We conducted this study in Lake Opinicon at the Queen's University Biological Station, 100 km south of Ottawa, Ontario, Canada (Fig. 1). Lake Opinicon is a small (788 ha) and shallow (mean depth 4.9 m) mesotrophic lake. The littoral zone of lake Opinicon constitutes 69% of the surface of the lake and 80% of the bottom is covered by macrophytes (Karst & Smol, 2000). This lake has been in a clear‐water state since its development (>11 000 years BP) indicating relatively low pelagic primary productivity (Karst & Smol, 2000). Zebra mussels became noticeable in lake Opinicon during the mid‐1990s (G. Blouin‐Demers, pers. obs.) and have now reached a mean density of 2962 individuals m−2 (range: 16–6912, n = 9 sites) in the littoral zone (G. Bulté, unpubl. data).
We sampled map turtles in lake Opinicon between 2003 and 2006 with basking traps and by snorkelling. Every captured individual was measured, weighed and given a unique mark by drilling small holes in the marginal scutes. We used the software capture (Rexstad & Burham, 1991) to estimate population size. We used a sampling interval of 1 year and counted one capture per year for individuals that were captured multiple times in the same year. We estimated standing crop biomass with the following equation:
∑(Ni × Wi)
where Ni is the number of individuals in mass class i and Wi is the midpoint of the mass class i .
Stable isotopes analyses
In lakes, pelagic primary producers (phytoplankton) are depleted in 13C (more negative δ 13C) compared to littoral primary producers (periphyton). The boundary layer present around the periphyton impedes the diffusion of dissolved inorganic carbon, resulting in a smaller isotopic fractionation by benthic primary producers compared to pelagic primary producers (Hecky & Hesslein, 1995). Those isotopic differences at the base of the food web are maintained across trophic levels due to limited trophic fractionation of carbon isotopes (France, 1996) and the differences can thus be used to track the proportion of each energy source contributing to the biomass of a predator (Post, 2002; Vander Zanden & Vadeboncoeur, 2002). Zebra mussels consume phytoplankton and thus integrate a more negative δ 13C ratio than benthic grazers such as snails (Post, 2002). Dreissenid mussels are the only pelagic consumer reported to be frequently consumed by northern map turtles (Lindeman, 2006a) and faeces analyses in our population support this observation (G. Bulté, unpubl. data). Other prey items complementing the diet of map turtles in our population are putative benthic consumers: caddisfly larvae and viviparid snails (G. Bulté, unpubl. data).
From May to August 2005, we collected blood (0.05 mL) from the caudal vein of males (n = 20) and females (n = 39) for stable isotopes analyses. Sampled individuals were chosen to represent the size distribution of our study population. We also sampled at three sites specimens of the three prey (trichoptera, zebra mussels and trap‐door snails) most commonly consumed by map turtles in our study population (G. Bulté, unpubl. data). For each prey type, we measured the isotopic ratio on composite samples composed of at least 10 individuals from each site.