NITROGEN ISOTOPES OF OTOLITH-BOUND ORGANIC MATTER: A NEW TOOL FOR TROPHIC RECONSTRUCTION USING MODERN AND FOSSIL OTOLITHS

Abstract

Global declines in fish biodiversity and abundance, from overfishing, habitat loss, and pollution, are expected to worsen with climate change. Our ability to predict fish response to climate and fishing impacts is hindered by lack of long term data, resulting from a dearth of tools for quantifying past ecological variation. The nitrogen isotopic composition (δ15N) of otolith-bound organic matter (OM) is a potential source of information on the dietary history of bony fishes. In contrast to white muscle tissue δ15N (δ15Nwmt), which is commonly used for ecological studies, the δ15N of otolith-bound OM (δ15Noto) provides a record of whole life history. δ15Noto can be measured in contexts where tissue is not available, for example, in otolith archives and sedimentary deposits. Application of δ15Noto was previously limited by the low N content of otoliths, which precluded routine measurement of individual otoliths. This dissertation reports measurements made by persulfate-oxidation of otolith-bound OM to nitrate followed by bacterial conversion to N2O, which increases sensitivity by 200 fold relative to previous techniques.

Optimization of the new method is described in Chapter two, which also reports the first analysis of δ15Noto in fossil otoliths and shows higher trophic level of Atlantic cod (Gadus morhua) prior to the onset of industrial fishing. The long term analytical precision of 0.3‰ makes it a useful tool for many ecological questions. Chapter three shows that δ15Noto is highly correlated with δ15Nwmt across diverse taxa (25 species investigated). Biomineralization introduces a small but predictable isotope effect in otoliths. Chapter four describes a diet switch experiment, which introduces micro-drilling δ15Noto techniques and validates the ability of δ15Noto to record diet and diet changes. Four appendices are included. The first reports δ15N measurements of nitrate, phytoplankton, zooplankton, and fish otoliths from two oceanographic cruises on the US Northeast Continental Shelf. The second and third report decadal and centennial δ15Noto records of commercially important fish species in the Gulf of Maine, Georges Bank, and Iceland. The fourth reports a global compilation of published metazoan δ15N data, with a first meta-analysis suggesting that anthropogenic impacts can affect the relationship between fish size and δ15Nwmt.