Breaching the Barrier: Transition Pathways of Coral Larval Connectivity Across the Eastern Pacific

Abstract

Genetic analyses indicate minimal gene flow across the so-called Eastern Pacific Barrier (EPB) in larvae of the reef-building coral \emph{Porites lobata}. Notably, Clipperton Atoll, situated on the eastern side of the EPB, is the only site that exhibits detectable genetic connectivity with the Line Islands, which lie to the west of the EPB. To elucidate the relationship between this genetic signal and large-scale Pacific Ocean circulation, we analyze historical trajectories of surface-drifting buoys from the Global Drifter Program (GDP). We first discretize the GDP drifter trajectories into a Markov chain representation and subsequently apply transition path theory (TPT) in combination with Bayesian inference. The TPT analysis identifies reactive trajectories -- pathways that connect the Line Islands to Clipperton Atoll with minimal detours -- whose travel times do not exceed 5 months, which is taken as an upper bound for the larval survival time of \emph{P. lobata}. Consistently, the posterior distribution of transport from Pacific islands west of the EPB to Clipperton Atoll attains a local maximum in the Line Islands at a travel time of approximately 2.5 months. Our probabilistic characterization of the Lagrangian dynamics therefore supports a scenario of weak, but non-negligible, permeability of the EPB, in agreement with the genetic evidence, and it motivates a refined dynamical definition of the EPB based on the remaining duration of reactive trajectories. Furthermore, our results indicate that the connectivity between the Line Islands and Clipperton Atoll is governed primarily by the seasonal modulation of the North Equatorial Countercurrent, rather than by the phase of the El Niño--Southern Oscillation (ENSO). Finally, Clipperton Atoll's role as a terminal sink for trajectories is relevant to the planned mining operations.

Figures (8)

• Figure 1: Partition of the Pacific Ocean domain of interest into Voronoi cells resulting from $k$-means clustering of trajectories of satellite-tracked, drogued surface drifting buoys from the NOAA Global Drifter Program (GDP). Indicated are the cells that form the source $A$ and target $B$ for the transition path theory (TPT) calculation. See text for acronyms and details.
• Figure 2: (left panel) Stationary distribution of the Markov chain on cells, constructed via Ulam's discretization method and applied to GDP trajectories assumed to be produced by a stationary stochastic process, showing where the trajectories accumulate in the long run. (right panel) Density of reactive trajectories or normalized joint probability that the chain is in state (cell) $i$ while transitioning from $A$ to $B$, indicating where trajectories spend most of their time.
• Figure 3: Reactive currents representing the net average flux of trajectories passing through states $i$ and $j$ at two consecutive times while on a direct route from $A$ to $B$.
• Figure 4: Arrival (left) or departure (right) rate, or probability per time step, of a reactive trajectory reaching each state $i$ that constitutes $A$ or leaving each state $i$ that constitutes $b$, computed by summing the reactive flux entering that particular state $i$ or exiting that particular state $i$.
• Figure 5: Remaining duration of reactive trajectories, or the expected first entrance time to $B$ from any state $i$ after leaving $A$ on a direct route to $B$, with the 5-month level set highlighted.