Marine and not terrestrial resources support nearshore food webs across a gradient of glacial watersheds in the northern Gulf of Alaska
AuthorSchloemer, James W.
Gulf of Alaska
Stable isotopes in ecological research
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AbstractEstuaries are among the most productive ecosystems on Earth, yet are especially at risk in high-latitude regions due to climate-driven effects on the connected terrestrial and marine realms. Warming in these regions exceeds the global average and is a major cause of the rapid melting of glaciers. As a result, the timing and magnitude of freshwater discharge into estuaries are subject to increase during the peak in glacial meltwater, ultimately affecting the riverine flux of nutrients and organic matter (OM) from the land to coastal environments. Intertidal communities near the outflow of rivers often rely on supplementing local (marine) food sources with allochthonous (terrestrial) subsidies, despite the fact that terrestrial OM can be problematic for marine consumers to assimilate. We investigated if terrestrial matter subsidizes nearshore food webs in northern Gulf of Alaska watersheds, and if the relative proportion of terrestrial versus marine OM supporting these food webs differed with watershed glaciation characteristics and with seasonal glacial discharge regimes. We employed a Bayesian stable isotope mixing model to determine the contribution of marine (phytoplankton, particulate OM, macroalgae) and terrestrial (vascular plant) sources to the diets of grazing/detritivore and filter/suspension-feeding coastal invertebrates at the outflows of watersheds of varying glacial influence and across various discharge periods. Additionally, we conducted a distance-based redundancy analysis to investigate the effects of watershed-characteristic sourcing and transport of terrestrial OM on nearshore consumer diets. The diets of both feeding groups were predominantly marine (>90%) and varied little among sites or glacial discharge periods. However, consumers were depleted in ¹³C isotopes with increasing glaciation; the significant watershed descriptors suggest that this change was more associated with discharge effects on marine primary production rather than consumption of terrestrial OM by the invertebrates. These results suggest that, while watershed exports may influence the stable isotope composition of OM sources, the diets of these feeding groups are mostly decoupled from terrestrial influence during the time of sampling. It is possible that marine OM availability in the study system is not limiting, and terrestrial OM subsidies in such productive systems are not needed to support nearshore food webs.
DescriptionThesis (M.S.) University of Alaska Fairbanks, 2022
Table of Contents1. Introduction -- 2. Materials and methods -- 2.1 Study sites and design -- 2.2 Primary consumer and diet-source collection -- 2.3 Stable isotope measurements -- 2.4 Stable isotope mixing models -- 2.5 Watershed habitat characteristics -- 2.6 Habitat drivers of stable isotope composition and trophic niched metrics -- 3. Results -- 3.1 Source and consumer stable isotope values -- 3.2 Estimated source contributions to POMestu and invertebrate consumers -- 3.3 Trophic niche metrics -- 3.4 Habitat drivers of stable isotope composition and trophic niche metrics -- 4. Discussion -- .1 Stable isotope composition of consumers -- 4.2 Estimated source contributions to invertebrate consumers and POMmix -- 4.3 Watershed drivers of consumer stable isotope composition -- 5. Conclusion -- References -- Appendix.
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