When is flow re-entrainment important for the flushing time in coastal reef systems?

Abstract

The rates of water exchange between coastal reef systems and the surrounding ocean are key physical drivers of water quality and reef ecosystems. It is generally assumed that water exiting a reef system through reef channels is predominantly replaced by 'new' water from offshore. However, exiting water may also recirculate back into the reef system reducing the rate of exchange between the reef and the ocean, which has implications for reef water temperatures, nutrient fluxes and population connectivity. To quantify flow re-entrainment at a rocky reef site in southwestern Australia, flow patterns were measured with GPS-tracked drifters during a two-week field experiment. The field observations were extended via a set of idealized numerical experiments to determine the effect of variable oceanic forcing and reef geometry on flow re-entrainment. The observations demonstrate that re-entrainment can vary significantly and the numerical results support the hypothesis that re-entrainment increases with increasing offshore wave height, increasing alongshore currents outside of the reef, and decreasing reef channel spacing but is largely not impacted by reef roughness. Reentrainment was correlated with a predictor variable R, which is a measure of wave forcing versus the total offshore flow cross-section, and alongshore currents outside the reef. For large values of R and strong alongshore currents, flow re-entrainment increases the effective flushing time by a factor of three or more. The results suggest that flow re-entrainment may be particularly important in small-scale reef systems or reefs exposed to an energetic wave climate and/or strong alongshore currents.

Hydrodynamic drivers of fine‐scale connectivity within a coral reef atoll

“…Reef‐scale hydrodynamic processes and morphology affect export and retention mechanisms (Leichter et al 2013; Storlazzi et al 2018) and play a fundamental role in the dispersal of coral reef larvae. The oscillatory movement of the tide, which vary over ebb and flood phases, tended to reduce export from the reef and generate recirculation cells that can recirculate existing water back onto the reef (Winter et al 2020). Mean wave‐driven flows, however, tended to increase export from the reef, due to the persistent forces created by waves breaking on the western reef flat.…”

Loss of reef roughness increases residence time on an idealized coral reef

“…The fringing reef type studied here is representative of reefs across the globe and has been studied extensively previously (e.g. 2 , 7 , 12 , 20 , 21 ). For these reefs, this study identifies a potential positive feedback mechanism where wide-spread bleaching events and coral death decreases reef roughness by lowering the geometric complexity of the sea bed, which in turn can transition a reef system from efficient flushing and short residence times to inefficient flushing and long residence times.…”

“…The fringing reef type studied here is representative of reefs across the globe and has been studied extensively previously (e.g. 2,7,12,20,21 ). For these reefs, this study identifies a potential positive feedback mechanism where wide-spread bleaching events and coral death decreases reef roughness by lowering the geometric complexity…”

“… 3 , 19 , as well as the numerical models by Refs. 20 , 21 . The model spans approximately 0.85 km by 2.4 km, with a horizontal grid size of 8 m, and has 10 equally spaced, vertical -layers (each occupying of the water column).…”

The Effects of Light Intensity and Flow Speed on Biogeochemical Variability Within a Fringing Coral Reef in Onna‐Son, Okinawa, Japan

“…Additionally, the range of variability in reef seawater biogeochemistry appears to partly scale with depth (Cyronak et al., 2020; Page et al., 2019), but with strong influence from the local hydrodynamics (Falter et al., 2013; Kekuewa et al., 2021; Mongin & Baird, 2014; Zhang et al., 2011, 2012). The magnitude of biogeochemical variability is also affected by seawater flow rate and trajectory (i.e., the path the water takes across the reef), which determine the habitats seawater intersects, the time the seawater is in contact with the benthos, and the amount of mixing and dilution that occurs between open ocean and other water masses (Falter et al., 2013; Gruber et al., 2017; Lowe & Falter, 2015; Winter et al., 2020). Seawater flow also affects the rates of metabolic processes by alleviating mass‐transfer limitation (Dennison & Barnes, 1988; Falter et al., 2004), which may influence the response of corals to ocean warming (Edmunds & Burgess, 2017; Rogers et al., 2016) and acidification (Comeau et al., 2014).…”