Conservation Planning for Coral Reefs Accounting for Climate Warming Disturbances

<div><p>Incorporating warming disturbances into the design of marine protected areas (MPAs) is fundamental to developing appropriate conservation actions that confer coral reef resilience. We propose an MPA design approach that includes spatially- and temporally-varying sea-surface temperature (SST) data, integrating both observed (1985–2009) and projected (2010–2099) time-series. We derived indices of acute (time under reduced ecosystem function following short-term events) and chronic thermal stress (rate of warming) and combined them to delineate thermal-stress regimes. Coral reefs located on the Brazilian coast were used as a case study because they are considered a conservation priority in the southwestern Atlantic Ocean. We show that all coral reef areas in Brazil have experienced and are projected to continue to experience chronic warming, while acute events are expected to increase in frequency and intensity. We formulated quantitative conservation objectives for regimes of thermal stress. Based on these objectives, we then evaluated if/how they are achieved in existing Brazilian MPAs and identified priority areas where additional protection would reinforce resilience. Our results show that, although the current system of MPAs incorporates locations within some of our thermal-stress regimes, historical and future thermal refugia along the central coast are completely unprotected. Our approach is applicable to other marine ecosystems and adds to previous marine planning for climate change in two ways: (i) by demonstrating how to spatially configure MPAs that meet conservation objectives for warming disturbance using spatially- and temporally-explicit data; and (ii) by strategically allocating different forms of spatial management (MPA types) intended to mitigate warming impacts and also enhance future resistance to climate warming.</p></div

Methods of this paper divided into three major phases.

<p>(A) Data collation involved acquisition of habitat data (green box), boundaries of marine protected areas (MPAs) (blue box), and observed and projected data (red and orange boxes, respectively) on sea-surface temperature (SST). In the selection and calculation of metrics of thermal stress (B), we derived metrics of chronic and acute stress from observed and projected datasets and combined them to define thermal-stress regimes. Regimes were delineated based on upper and lower terciles labelled as “high” (highest 33% of values, dark red or orange) and “low” (lowest 33% of values, light red or orange), respectively. The incorporation of warming disturbances into conservation planning (C) consisted of setting conservation objectives for each thermal-stress regime, evaluating their achievement in existing MPAs, and identifying priority areas that would achieve unmet objectives. Arrows in gray indicate the flow of information and lighter boxes linked by dashed lines depict types of data or analyses involved in each step.</p

Annual maximum SST and Degree Heating Weeks (DHWs) for five reef cells within different thermal-stress regimes.

<p>Approximate locations of the five cells are shown as a-e in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0140828#pone.0140828.g002" target="_blank">Fig 2A</a>. Observed data (satellite NOAA) are shown by black solid lines (SST values) and filled bars (DHWs) while projections (GCM PCM1 output) are shown by gray solid lines (SST values) and filled bars (DHWs). Warming trends (in °C per decade) are shown for observed (ɷ^O) and projected (ɷ^P) time series. The thermal-stress regime allocated to each reef cell is indicated in the top right of each graph, and defined in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0140828#pone.0140828.g003" target="_blank">Fig 3</a>.</p

Conceptual illustration of the effects of acute thermal stress on coral-reef ecosystem state described by a logistic function.

<p>Empty circles indicate values used to fit the model. The form of this function assumes that the time that coral reefs spend with reduced ecosystem function (capacity to grow, repair, and reproduce), <i>t</i>_<i>c</i>, is short at low DHW values because we expect that corals would recover quickly (within one year). This is followed by a steeper increase in <i>t</i>_<i>c</i>; when widespread mortality begins (DHW reaches 8°C-weeks), the time that corals would spend recovering increases rapidly as bleaching-level events intensify above this level. When almost the entire community is extirpated over large spatial scales (above about 16°C-weeks), we expect to have small increments of <i>t</i>_<i>c</i> with increasing DHW for the ecosystem as a whole because only stress-tolerant species that can withstand greater acute disturbance are present. The function then reaches an upper bound (i.e., in the formulae—asymptotic value—equal to 20 years) which is the maximum time required to regenerate a fully functional ecosystem after bleaching causes massive mortality and extirpates all organisms.</p

The acute thermal stress metric.

<p>Accumulated time for which the ecosystem is under reduced ecosystem function from acute stress events for all reef cells according to observed (A) and projected (B) time series. Times are derived from the logistic function used to relate intensity of acute stress events to recovery time (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0140828#pone.0140828.g003" target="_blank">Fig 3</a>) and summed through each time-series and presented as years per decade. Panels for reefs in the northern, central, and southern sectors of the study area correspond to insets in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0140828#pone.0140828.g002" target="_blank">Fig 2A</a>.</p

Thermal-stress regimes.

<p>The nine thermal-stress regimes defined within our study area (A), defined by combinations of high and low values for observed (Ob) and projected (Pr) chronic (Ch) and acute (Ac) stress. The rationale for management of each regime is summarised in (B). (C) Conservation objectives (dark green vertical lines) for each thermal-stress regime and their coverage by MPAs (green bars). Objectives prescribe the percentage of the total extent of reef cells in the regime (100%, 50%, or 30%) requiring management and the type of management required (no-take—solid green lines; multiple-use areas—dashed green lines). Horizontal bars indicate the percentage of each thermal-stress regime covered by the two types of MPAs: no-take MPAs are indicated by green bars; multiple-use MPAs are indicated by light green bars. The green checkmark symbol indicates that the conservation objective has been fully achieved in both extent and management type; the red “x” indicates that the conservation objective has not been attained. Objectives were formulated for explictiness in the design of MPAs to account for resilience to warming impacts, considering supporting evidence in the literature (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0140828#pone.0140828.s002" target="_blank">S2 Fig</a> for further details about rationales to protect all regimes).</p

Summary of observational bleaching data.

<p>Summary of observational bleaching data.</p

The study area and the chronic stress metric.

<p>(A) Sectors (northern, central, southern), reef cells (n = 428), and the existing MPA boundaries along the Brazilian coast. MPAs are classified according to their main management categories: no-take areas and multiple-use areas. Letters a-e with stars denote approximate locations of reef cells selected to depict temperature variability (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0140828#pone.0140828.g006" target="_blank">Fig 6</a>). (B) Decadal SST trends describe observed chronic stress for each reef cell from NOAA satellite data. (C) Decadal SST trends describe projected chronic stress for each reef cell, downscaled from PCM1 general circulation model output.</p

Examples of bleaching observations and probabilities for the Caribbean in 2005 (a,c,e) and central-west equatorial Pacific in 2004 (b,d,f).

<p>Top panels (a,b) show the raw observations of moderate and severe bleaching (level 2 and 3) from the database; middle panels (c,d) show the interpolated bleaching probabilities; and bottom panels (e,f) show the semi-variograms for the region and year.</p

Annual number of bleaching observations for 1985–2010.

<p>Blue bars represent original ReefBase reports; orange bars represent new reports. Shading reflects bleaching level, from mild (1–10%) or unknown extent of bleaching (levels 1 and -1 in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0175490#pone.0175490.t002" target="_blank">Table 2</a>, lightest colour) to severe (>50%) bleaching (level 3, darkest colour).</p