Lagrangian_simulations_code&settings

Compressed archive (.zip) containing the source code and settings used for Lagrangian simulations

Understanding large-scale, long-term larval connectivity patterns: The case of the Northern Line Islands in the Central Pacific Ocean

<div><p>Protecting key hotspots of marine biodiversity is essential to maintain ecosystem services at large spatial scales. Protected areas serve not only as sources of propagules colonizing other habitats, but also as receptors, thus acting as protected nurseries. To quantify the geographical extent and the temporal persistence of ecological benefits resulting from protection, we investigate larval connectivity within a remote archipelago, characterized by a strong spatial gradient of human impact from pristine to heavily exploited: the Northern Line Islands (NLIs), including part of the Pacific Remote Islands Marine National Monument (PRI-MNM). Larvae are described as passive Lagrangian particles transported by oceanic currents obtained from a oceanographic reanalysis. We compare different simulation schemes and compute connectivity measures (larval exchange probabilities and minimum/average larval dispersal distances from target islands). To explore the role of PRI-MNM in protecting marine organisms with pelagic larval stages, we drive millions of individual-based simulations for various Pelagic Larval Durations (PLDs), in all release seasons, and over a two-decades time horizon (1991–2010). We find that connectivity in the NLIs is spatially asymmetric and displays significant intra- and inter-annual variations. The islands belonging to PRI-MNM act more as sinks than sources of larvae, and connectivity is higher during the winter-spring period. In multi-annual analyses, yearly averaged southward connectivity significantly and negatively correlates with climatological anomalies (El Niño). This points out a possible system fragility and susceptibility to global warming. Quantitative assessments of large-scale, long-term marine connectivity patterns help understand region-specific, ecologically relevant interactions between islands. This is fundamental for devising scientifically-based protection strategies, which must be space- and time-varying to cope with the challenges posed by the concurrent pressures of human exploitation and global climate change.</p></div

Study area.

<p>Filled dots indicate positions and areal extents of the five considered islands of the Northern Line Islands archipelago, from where larvae are released for Lagrangian simulations, while dashed circles enclose the area used as retention zone of each island (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0182681#sec002" target="_blank">Methods</a>). Colors represent the magnitude of the surface velocity field (global average 1991–2010), while arrows indicate average flow direction. The map is highlighted as a white square in the Central Pacific region (inset).</p

Relationship between ENSO SST anomaly and connectivity.

<p>ENSO SST anomaly (top-left panel) and connectivity patterns for the period 1991–2010. Black solid lines represent monthly connectivity scores (obtained with <i>β</i> = 10 km) averaged over the set of PLDs considered in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0182681#pone.0182681.g006" target="_blank">Fig 6</a>, while the gray shaded areas report the min-max range of the connectivity scores. Insets are reported only for statistically significant correlations (<i>p</i> < 0.05) between yearly averages of the SST anomaly (〈<i>ΔSST</i>〉) and yearly averages of connectivity scores (〈<i>P</i>_<i>ij</i>〉, dots; year 2000 is highlighted in black). Regression lines obtained after the removal of outliers (white filled dots), identified by computing confidence intervals for each observation based on the associated (externally) studentized residuals (see e.g. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0182681#pone.0182681.ref045" target="_blank">45</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0182681#pone.0182681.ref047" target="_blank">47</a>]), are shown as solid lines within insets.</p

Connectivities for Palmyra Atoll.

<p>Self- (<i>P</i>₂₂), out- (<i>P</i>_2<i>j</i>) and in-bound (<i>P</i>_<i>i</i>2) connectivity for Palmyra Atoll as a function of spawning season and PLD. Results refer to year 2000 and are obtained for <i>β</i> = 10 km. Connectivity patterns to Washington, Fanning and Christmas Islands are not shown because connectivity scores are less than 1% for every combination of PLD and release season.</p

Evaluation of different interpolation algorithms for sub-grid reconstruction of the oceanic velocity fields.

<p>(a–b) Median absolute reconstruction errors for the longitudinal (a) and the latitudinal (b) components of the velocity fields. (c–d) As in a–b, for the relative reconstruction errors. See Methods for details on the cross-validation procedure used to evaluate interpolation errors and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0182681#pone.0182681.s001" target="_blank">S1 Fig</a> for a schematic representation of the different interpolation schemes.</p

Simulation of particle dispersal with a PLD of 2 months and release on March 1st, 2000.

<p>(a) Sample trajectories of Lagrangian particles released from each of the NLIs; filled circles indicate positions and areal extents of the islands, while dashed circles mark their retention zones. (b) Analysis of sample trajectories for Lagrangian particles released from Palmyra Atoll (2, sky blue) and Christmas Island (5, burgundy); first, the minimum distance from each of the NLIs is computed for all trajectories, then ensemble statistics are evaluated. (c) Frequency distribution (50 km bins) of the minimum distances to Palmyra Atoll of the trajectories starting at Christmas Island (, burgundy), and viceversa (, sky blue; inset shows the latter distribution at a finer spatial grain); solid, dashed, dotted and dash-dotted lines mark respectively the positions of the average, median, 10th and 25th percentiles of the frequency distributions (for : 157, 110, 13, 43 km; for : 618, 619, 613, 615 km). (d) Histogram visualization of the between-island connectivity matrix; bars represent the fraction of particles released at source island <i>i</i> (horizontal axis) that cross the retention zone of island <i>j</i> (<i>β</i> = 10 kilometers, island numbers and colors as in panel a).</p

Minimum distances to target islands of particles starting from (or ending to) Palmyra Atoll.

<p>Cumulative distributions of the minimum distances to/from the other NLIs (numbered as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0182681#pone.0182681.g001" target="_blank">Fig 1</a>) for trajectories having Palmyra Atoll as a source/sink (top/bottom panels). Results are shown for a PLD of 60 days and different release months (see legend), and refer to year 2000.</p

The effect of settlement buffer on connectivity.

<p>Connectivity scores of Palmyra Atoll for different sizes of the settlement buffer (<i>β</i>) that defines the retention region of each island. Connectivity patterns are evaluated from Lagrangian simulations run with Palmyra Atoll as a source (panels a–b) or a sink (panels c–d), using the interpolation scheme S. Results refer to year 2000, and have been averaged over different PLDs and release seasons.</p

Circulation fields and ENSO anomalies.

<p>Daily- and yearly-averaged (colored and white, respectively) directions and intensities of the local circulation field for a representative location in the study area (160°W, 5°N, close to Washington Island), for two different years (1997, red, and 1998, blue). Year 1997 was characterized by a strong El Niño anomaly, while 1998 marked the beginning of a protracted La Niña event, with remarkable reverberations on superficial ocean currents.</p