Impacts of Arctic permafrost erosion on nearshore planktonic food webs

Arctic planktonic communities form the foundation of Arctic marine food webs and play a crucial role in the biological carbon pump. Global warming is increasing the thawing and erosion of permafrost coasts in the Arctic. This leads to the discharge of substantial amounts of sediment, carbon, and nutrients into the Arctic Ocean’s nearshore zone, changing the ecosystem conditions. Questions have arisen about how planktonic communities in the nearshore zone are affected by such changes in the environmental conditions. In my thesis, I applied a multiple-study approach to investigate the effects of Arctic coastal erosion and the associated changes in turbidity, carbon, and nutrient levels on planktonic community dynamics, biomass, and interactions within the nearshore zone. I decided on the shallow nearshore due to the fact that these zones represent 20% of the Arctic shelves and 7.5% of the Arctic Ocean, a proportion substantially greater than that of the nearshore zones of other oceans. In Chapter 2, the manuscript, “Future Arctic: How will increasing coastal erosion shape nearshore planktonic food webs?” sets the scene. In this chapter, I assessed how coastal erosion impacts carbon, nutrients, and light regimes in the nearshore zone, and what we can expect for the future. Additionally, I assessed the potential effects on planktonic community structure and food web dynamics. I used published literature and a formal review of our current state of knowledge. The literature data showed that sediment discharge increases turbidity and reduces light penetration into the water column. This darkening is expected to reduce phytoplankton productivity, while additional carbon will support bacterial production and shift the balance between autotrophic and heterotrophic production at the base of the food web. Given the lower energy transfer efficiency in the heterotrophic pathway, its dominance might lower zooplankton biomass with potential negative consequences for higher trophic levels. Drawing some of the testable hypotheses from the in-depth literature synthesis, I investigated the influence of terrigenous input on planktonic community dynamics around Herschel Island-Qikiqtaruk. Located in the Western Canadian Arctic, the permafrost coast around Herschel Island-Qikiqtaruk is one of the highly eroding sites in the Arctic. The results in the manuscript, “Eroding permafrost coasts lead to lower productivity in the Arctic nearshore zone,” in Chapter 3, show that permafrost thaw and erosion impact planktonic biomass. Relative to stable sites, actively eroding sites exhibited higher turbidity, resulting in a 45% reduction in phytoplankton biomass. Moreover, the very nearshore stations zone showed higher heterotrophic dinoflagellates and microzooplankton biomass than the offshore stations, suggesting that the nearshore stations were dominated by heterotrophy, while the offshore stations were dominated by autotrophic energy mobilization. Mesozooplankton abundance decreased by 26% from the nearshore towards offshore stations, suggesting potential utilization of both marine and terrestrial OC sources. In the third manuscript, “Impact of permafrost coastal erosion on Arctic marine food webs”, I investigated the sources, age, and utilization of marine versus terrigenous organic carbon. The results showed that although permafrost erosion discharges a substantial amount of OC into the nearshore zone, only 6% of the old permafrost OC ends up in the planktonic food web. Planktonic consumers are mainly supported by marine production, and the additional terrigenous OC carbon utilized by nearshore consumers largely comes from the active layer, representing modern terrestrial carbon. Overall, this study highlights that Arctic permafrost thaw and erosion influence planktonic community structure by reducing phytoplankton biomass and shifting the balance between autotrophs and heterotrophs in the nearshore zone. These processes might weaken the Arctic Ocean’s capacity as a CO2 sink, and potentially turn it into a net CO2 source.

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