The Pacific diatom Neodenticula seminae reappeared in the North Atlantic in May 1999, carried by a pulse of Pacific-origin water through the Arctic, and has since been recorded across the subpolar North Atlantic from the Labrador Sea to the Norwegian Sea. The source study (Reid et al. 2007) reported this as the first North Atlantic record of the species in about 800 000 years and framed it as evidence that a circulation threshold between the North Pacific and North Atlantic had been passed, predicting that trans-Arctic species migrations would increasingly affect Atlantic biodiversity and the biological pump, with feedbacks to the carbon cycle. Later literature documents the spread but leaves the consequences unmeasured. The open problem asks for: whether N. seminae now persists in the subpolar North Atlantic through self-sustaining reproduction, through continuous advection, or both; the ecological consequences of the establishment (competitive interactions with endemic phytoplankton, effects on zooplankton and higher trophic levels, net outcome for biodiversity); its quantitative biogeochemical impact (biogenic silica export and oceanic CO2_2 uptake); the frequency and magnitude of future Pacific-water pulses capable of delivering further organisms; and the environmental thresholds controlling past introductions and extinctions, including the regional extinction near 0.84 million years ago and the absence of re-establishment in the North Atlantic during later interglacials.

Background

Diatoms are single-celled algae with cell walls of biogenic silica (opal); they dominate spring blooms in subpolar seas and, because their dense silica cell walls (frustules) ballast organic carbon, are a principal agent of the biological pump, the export of photosynthetically fixed carbon to the deep ocean. Neodenticula seminae is a large, heavily silicified planktonic diatom endemic to the subarctic Pacific Ocean. Its fossil record shows that it also lived in the North Atlantic during the Early Pleistocene, from roughly 1.2 to 0.84 million years ago, and then disappeared from the Atlantic while persisting in the Pacific; this regional extinction is broadly associated with Pleistocene cooling and sea-level change, which repeatedly isolated the Pacific from the Arctic Ocean through the Bering Strait, a shallow (about 50 m sill depth) gateway that falls dry during glacial sea-level lowstands.

The Continuous Plankton Recorder (CPR) survey, a ship-towed plankton sampling programme that has monitored the North Atlantic since 1958 (monthly in the Northwest Atlantic since 1962), found large numbers of N. seminae in Labrador Sea samples in May 1999 — which Reid et al. (2007) reported as the first North Atlantic record in about 800 000 years. Reid et al. (2007) linked the arrival to the exceptional ice-free conditions north of Canada in 1998 and to increased flows of Pacific water into the Northwest Atlantic via the Canadian Arctic Archipelago (the island passages of the Northwest Passage) and the Fram Strait, the deep gateway between Greenland and Svalbard through which Arctic water exits into the Nordic Seas. They concluded that a threshold in Pacific-Atlantic exchange had been crossed and that trans-Arctic migrations of Pacific species into the Atlantic would likely intensify as Arctic sea ice continues to melt, affecting Atlantic biodiversity and the biological pump, with consequent feedbacks to the carbon cycle. In the source's discussion this projection raises several unmeasured quantities: the ecological fate of the returning diatom, its effect on silica export and CO2_2 uptake, the recurrence of Pacific-water pulses, and the environmental thresholds that governed the species' Pleistocene extinction and the failure of earlier interglacials to reintroduce it into the North Atlantic.

Later work shows that the 1999 arrival was not an isolated event. N. seminae has since been recorded in the Gulf of St. Lawrence, where morphological and molecular comparisons indicate a recent migration from the Pacific rather than a long-separated relict population; in the northern Nordic Seas; and in Labrador Sea spring-bloom communities sampled in 2013-2014. Compilations of surface-sediment data show it is now a regular element of the modern diatom flora of the Labrador, Irminger and Nordic Seas, and it ranks among the predominant invasive diatoms in sinking particles collected by sediment traps (moored instruments that sample the downward particle flux) in the Lofoten Basin of the Norwegian Sea. The same compilations also record earlier, intermittent occurrences of N. seminae in Arctic and Nordic seas sediments — from the Chukchi and East Siberian shelves to the Fram Strait and the Vøring Plateau — during latest-Pleistocene and Holocene warm intervals, most of which lack direct radiocarbon age control. Ocean-model studies using Lagrangian particle tracking, in which virtual particles are followed as they are carried by the simulated currents, map the trans-Arctic pathways, their multiannual transport timescales, and a shift of export towards the Fram Strait, while reviews of non-indigenous species record a sharp rise of introductions into the Arctic and adjacent seas from the late 1990s. Bering Strait inflow, the ultimate source of the Pacific signal, increased from about 0.8 to up to 1.2 sverdrups (1 Sv = 106 m3/s10^{6}\ \mathrm{m^{3}/s}) between the late 1990s and 2015. What this observational record still lacks is any quantification of what the source paper identified as the stakes: the competitive and trophic interactions of the returning diatom within endemic North Atlantic plankton communities, the net outcome for regional biodiversity, and the measurable effect on silica export and oceanic CO2_2 uptake.

Problem Statement

Determine the establishment, ecological and biogeochemical consequences, recurrence, and environmental thresholds of the renewed trans-Arctic exchange of plankton between the Pacific and Atlantic Oceans that began in the late 1990s, taking the established return of the Pacific diatom Neodenticula seminae as the documented case. The open problem comprises the following parts:

  1. Establishment mechanism: whether N. seminae now persists in the subpolar North Atlantic through self-sustaining reproduction, through continuous advection (passive transport by ocean currents) in Pacific-origin water, or both, and in which regions and seasons it maintains viable populations.

  2. Ecological consequences: quantitative estimates of its competitive interactions with endemic North Atlantic phytoplankton (including consequences for bloom composition and for endemic species it may displace), its utilization by zooplankton grazers and any effects propagating to higher trophic levels, and the net outcome for regional plankton biodiversity.

  3. Biogeochemical consequences: the measurable contribution of N. seminae and other Pacific-origin siliceous producers to biogenic silica export and to net oceanic CO2_2 uptake in the North Atlantic, with attribution to the species and to the Pacific-water inflow that carries it.

  4. Recurrence: the expected frequency and magnitude of Pacific-water pulses capable of delivering additional Pacific organisms to the North Atlantic.

  5. Thresholds: the environmental thresholds that controlled past introductions and extinctions of the species — in particular why it went regionally extinct in the North Atlantic near 0.84 million years ago and why subsequent interglacial floods of the Bering Strait did not reintroduce it into the North Atlantic.

The detection question — whether the 1999 arrival was an isolated pulse — is settled and is not part of the open problem.

References

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