Monday, August 2, 2010: 4:00 PM
320, David L Lawrence Convention Center
Background/Question/Methods The application of stable nitrogen isotopes (δ15N) has increased understanding of the mechanisms that govern freshwater food chain length (FCL). However, less is known about what factors influence marine FCL. For example, northern marine ecosystems that are seasonally ice-covered experience fluctuating levels of primary productivity, temperature, and light annually, but it is unclear how this variability influences food web structure. There is a need to identify the important factors that govern northern marine food webs, especially in light of decreasing ice cover and duration. The present study compared FCL between open-water and ice-cover in a northern marine ecosystem, and tested the hypothesis that FCL would be longer in open-water (i.e. during times of high productivity). If FCL is longer during times of open-water, then inputs of primary productivity could be an important mechanism governing the length of northern marine food chains. The trophic position (TP) (based on δ15N) of Greenland sharks (Somniosus microcephalus), which consume fish and marine mammals, was used as an indicator of maximum FCL. Ringed seal (Pusa hispida) TP was also calculated to investigate potential changes in food web structure with season that may have been obscured by the omnivorous feeding behavior of Greenland sharks.
Results/Conclusions Food chain length based on Greenland shark muscle was 3.79 ± 0.32 (mean ± 1S.E.) in open-water and 4.00 ± 0.03 in ice-cover. Food chain length based on Greenland shark blood plasma, which should reflect more recent feeding, was also similar between seasons (open-water = 4.11 ± 0.07, ice-cover = 3.96 ± 0.07). Thus, maximum FCL did not vary with season. However, ringed seal TP based on both muscle and liver δ15N was lower in open-water (muscle = 3.05 ± 0.06, liver = 3.44 ± 0.06) than ice-cover (muscle = 3.52 ± 0.10, liver = 3.89 ± 0.09). Lower TP of ringed seals in open-water could have resulted from their exploitation of herbivorous zooplankton that increase in numbers during phytoplankton blooms. Seals appeared to switch to a higher TP during ice-cover, although this behavior was not reflected in the maximum FCL calculated from Greenland shark δ15N. Overall, results from this study provided no evidence to support the hypothesis that higher inputs of primary productivity in open-water yield longer food chains in a northern marine ecosystem.
Results/Conclusions Food chain length based on Greenland shark muscle was 3.79 ± 0.32 (mean ± 1S.E.) in open-water and 4.00 ± 0.03 in ice-cover. Food chain length based on Greenland shark blood plasma, which should reflect more recent feeding, was also similar between seasons (open-water = 4.11 ± 0.07, ice-cover = 3.96 ± 0.07). Thus, maximum FCL did not vary with season. However, ringed seal TP based on both muscle and liver δ15N was lower in open-water (muscle = 3.05 ± 0.06, liver = 3.44 ± 0.06) than ice-cover (muscle = 3.52 ± 0.10, liver = 3.89 ± 0.09). Lower TP of ringed seals in open-water could have resulted from their exploitation of herbivorous zooplankton that increase in numbers during phytoplankton blooms. Seals appeared to switch to a higher TP during ice-cover, although this behavior was not reflected in the maximum FCL calculated from Greenland shark δ15N. Overall, results from this study provided no evidence to support the hypothesis that higher inputs of primary productivity in open-water yield longer food chains in a northern marine ecosystem.