A comparative study of the pelagic food webs in two Newfoundland fjords using stable carbon and nitrogen isotope tracers

Dickson, Mary-Lynn (1986) A comparative study of the pelagic food webs in two Newfoundland fjords using stable carbon and nitrogen isotope tracers. Masters thesis, Memorial University of Newfoundland.

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Abstract

A comparative study was initiated to investigate the role of environmental variability on the trophic organization and structure of pelagic food chains in two fjords, Bay d'Espoir and Fortune Bay, along the south coast of Newfoundland, Canada. Although in close proximity, these fjords are biologically and physically distinct. On the basis of physical oceanographic studies, Bay d'Espoir was considered to be a relatively constant environment, while Fortune Bay had a dynamic water column, undergoing bi-annual deep-water renewal. Fauna, particulate organic matter (POM) and sediment were collected during summer and winter and analyzed for their stable carbon and nitrogen isotopic compositions. -- This study entailed the elucidation of the trophic organization of the most complex ecosystems yet analyzed using dual stable isotope tracers. Stable carbon and nitrogen isotopes used in combination provided better resolution than either could have given singly. However, they only allowed the assignment of species to a trophic level and did not in themselves provide information on the linkages between and within levels. -- The structure of the pelagic food webs and their principal food chains were determined for winter and summer. The trophic organization of the food webs was similar in both fjords within and between seasons; however, differences were observed in the food chains. With the exception of Bay d'Espoir in August, the top predators in both fjords occupied the fourth trophic level. The isotope data indicated which species were at intermediate trophic levels and the trophic position of the microzooplankton. -- Three pelagic food chains were present in Bay d'Espoir during both seasons, while Fortune Bay had three in the winter and two in summer. On the basis of the carbon and nitrogen isotopic composition of the fauna and POM it is suggested these pelagic food chains are affiliated with specific water types. This study is the first to draw attention to a relationship between the isotopic composition of the fauna and POM deeper than the subsurface layer. -- The trophic composition of the fauna differed between the two fjords; carnivores were dominant in Bay d'Espoir, while omnivores prevailed in Fortune Bay. A decrease in the proportion of omnivores in Fortune Bay was found between seasons, due to increased water column stability in the summer. -- A progressive enrichment of ¹³C and ¹⁵N in the fauna was found with increasing trophic level. Trophic level enrichments calculated for each food web varied slightly between fjords and seasons. These values corresponded closely to those reported in the literature and suggest similar mechanisms are involved in the fractionation of stable isotopes in food chains, independent of geographic location. Although minor differences were noted in the food chains, the overall trophic organization of the two fjords was similar to the Bering Sea (McConnaughey 1978) and the Scotian Shelf (Mills and Fournier 1979, Mills et al. 1984) ecosystems. This confirms the claims of Dickie (1972) and Mills (1975) that coastal ecosystems in general are similarly structured.

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