Simulating the transport of broadcast coral larvae among the Southern Islands of Singapore

Abstract

Singapore's coral reefs have experienced significant anthropogenic impacts for at least 4 decades. Ongoing reef restoration efforts, however, may not be sustainable if there is no natural coral recruitment. Knowledge of coral reef connectivity, which can be identified using hydrodynamic-advection and individual-based models, can help inform reef management decisions. Here, a 2-dimensional, hydrodynamic, flexible mesh model (MIKE 21 FM) coupled with a Lagrangian particle-tracking module was used to simulate larval transport among Singapore's Southern Islands. In each simulation, neutrally buoyant, passive particles representing coral planulae were released into the hydrodynamic conditions present during the coral multi-species synchronous spawning event of April 2007. When the number of larvae released was proportional to live coral cover (between 2400 and 46 200 particles), 3 islands: Pulau Sudong, Pulau Pawai and Pulau Senang, which all lie within a military Live Firing Area, were identified as the most robust sources of larvae seeding the rest of the Southern Islands. However, when equal numbers of larvae (18 000 larvae per site) were released from all sites in an effort to identify nursery areas with the greatest potential to seed other reefs, 2 different and upstream islands, Sisters' Islands and Kusu Island, were found to be better sources of larvae. We suggest all 5 of these sites should be identified for conservation. Additional effort to enhance coral cover, and hence larval export, at Sisters' Islands and Kusu Island may help increase recruitment on downstream reefs.

Modeling Fine-Scale Coral Larval Dispersal and Interisland Connectivity to Help Designate Mutually-Supporting Coral Reef Marine Protected Areas: Insights from Maui Nui, Hawaii

“…If this is the case, the short time scales of dispersal shown here are important because both larval density ( Figure 6) and recruitment success decrease quickly with time and distance (Sammarco and Andrews, 1988) due to mixing and horizontal dispersion (Wolanski et al, 1989;Oliver et al, 1992). It has been demonstrated (Andutta et al, 2012;Tay et al, 2012) that reefs and islands in close proximity often result in decreased flushing and thus more exposure time in areas where settlement can occur successfully; these features are generally not captured with large-scale models (Vaz et al, 2013;Wood et al, 2014). In this case, the flushing between islands appears to be somewhat offset by areas in the lee of the high islands that result in increased retention that was not captured in previous modeling studies (Andutta et al, 2012;Tay et al, 2012) where topographic steering of winds was not a factor.…”

“…Insolation-driven heating and nocturnal cooling of the islands cause the general Trade wind speeds in the vicinity of the islands to vary from almost negligible at night to more than 15 m s −1 in the afternoon, compared to about 5 m s −1 in the open ocean. These relatively rapid diurnal variations in wind speed result in a very thin (<1 m) winddriven surface layer (Storlazzi et al, 2006b) that constrains positively-buoyant particles, such as coral larvae during their planktonic dispersal stage (Storlazzi et al, 2006a), varies greatly over small spatial and temporal scales, and is not captured in larger-scale oceanographic models (Tay et al, 2012;Vaz et al, 2013;Wood et al, 2014). The wave climate in Maui Nui during the period of coral spawning in the boreal summer is dominated by two wave regimes: northeast Trade wind waves and Southern Ocean swell (Moberly and Chamberlain, 1964).…”

“…A number of previous studies have investigated local or regional-scale coral spawning and larval dispersal using tools such as oceanographic instruments, (Sammarco and Andrews, 1988;Wolanski et al, 1989;Oliver et al, 1992;Tay et al, 2012;Thomas et al, 2014), and/or satellite-or radio-tracked drifters (Lugo-Fernandez et al, 2001;Nadaoka et al, 2002;Storlazzi et al, 2006a). Most of the previous experiments, however, have either been long-term simulations with no field validation or relatively short-term, generally an individual spawning event or two in the same year, thus making it unclear if the short-term model runs and/or drifter trajectories are representative of dominant patterns of dispersal that yield connectivity (Largier, 2003;Oliver et al, 2015) over timescales relevant to management.…”

Rehabilitation of Coral Reefs Exposed to Urbanization and Climate Change Impacts

“…Despite exposure to continued anthropogenic pressure, the remaining reefs remain vigorous in terms of growth [4] , reproduction [5] and recruitment. There is no indication of scleractinian coral species decline [6] and modelling studies showed that the reefs are self-seeding [7] with adequate larval supply. New challenges are posed by climate change impacts.…”

Effectiveness of Reef Restoration in Singapore’s Rapidly Urbanizing Coastal Environment

“…They are considered viable despite the continued anthropogenic impacts to the marine environment, as annual coral mass spawning events which occur with a high degree of predictability contribute significantly to the self-seeding of Singapore's reefs [13]- [15]. The overall approach to reef restoration since the 1990s is aimed at circumventing the problems of high sedimentation and destabilized substrate, and comprises physical (e.g.…”