Local-scale feedbacks influencing cold-water coral growth and subsequent reef formation

Abstract

Despite cold-water coral (CWC) reefs being considered biodiversity hotspots, very little is known about the main processes driving their morphological development. Indeed, there is a considerable knowledge gap in quantitative experimental studies that help understand the interaction between reef morphology, near-bed hydrodynamics, coral growth, and (food) particle transport processes. In the present study, we performed a 2-month long flume experiment in which living coral nubbins were placed on a reef patch to determine the effect of a unidirectional flow on the growth and physiological condition of Lophelia pertusa. Measurements revealed how the presence of coral framework increased current speed and turbulence above the frontal part of the reef patch, while conditions immediately behind it were characterised by an almost stagnant flow and reduced turbulence. Owing to the higher current speeds that likely promoted a higher food encounter rate and intake of ions involved in the calcification process, the coral nubbins located on the upstream part of the reef presented a significantly enhanced average growth and a lower expression of stress-related enzymes than the downstream ones. Yet, further experiments would be needed to fully quantify how the variations in water hydrodynamics modify particle encounter and ion intake rates by coral nubbins located in different parts of a reef, and how such discrepancies may ultimately affect coral growth. Nonetheless, the results acquired here denote that a reef influenced by a unidirectional water flow would grow into the current: a pattern of reef development that coincides with that of actual coral reefs located in similar water flow settings. Ultimately, the results of this study suggest that at the local scale coral reef morphology has a direct effect on coral growth thus, indicating that the spatial patterns of living CWC colonies in reef patches are the result of spatial self-organisation.

Tiger reefs: Self‐organized regular patterns in deep‐sea cold‐water coral reefs

“…Aragonite saturation, temperature, and oxygen will, however, not vary regularly on length scales at which regular patterns are found in this study, so these environmental factors are unlikely explanations for the observed patterns. Currents and organic matter availability are important for cold‐water coral occurrence (da Costa Portilho‐Ramos et al, 2022; De Clippele et al, 2021) but are also modified by coral reef presence, as demonstrated by observational, experimental, and modeling studies (e.g., Bartzke et al, 2021; Corbera et al, 2022; Davies et al, 2009; Soetaert et al, 2016; Wagner et al, 2011). We suggest that the regular patterns found in the cold‐water coral reefs at the Logachev cold‐water coral mound province are the result of self‐organization.…”

“…With their dendritic skeletons, corals attenuate and slow down the currents around, within, and behind a coral patch, thus promoting turbulence in their lee (Bartzke et al, 2021; Hennige et al, 2021; Mienis et al, 2019). These flow modifications increase the rate at which coral polyps can filter food particles from the water column (Corbera et al, 2022; Hennige et al, 2021) and increase sediment deposition within the patch, stabilizing the coral framework and stimulating coral reef formation (Bartzke et al, 2021; Douarin et al, 2014; Wang et al, 2021). Downstream of a coral reef, the water is depleted of food particles (Corbera et al, 2022; Wagner et al, 2011), and around a coral reef, erosive scouring can negatively affect reef formation (Huvenne et al, 2009; Lim et al, 2018).…”

“…These ecosystem engineering mechanisms act on different spatial scales. The positive effects on coral growth as described above are effective on a small spatial scale, that is, at or within the coral framework, while negative effects act at a larger spatial scale (Corbera et al, 2022; van der Kaaden et al, 2020). When an organism affects its own growth differently at distinct spatial scales by the so‐called “scale‐dependent feedbacks,” this can give rise to regular spatial patterns in the distribution of the organism, that is, “spatial self‐organization” (Rietkerk & van de Koppel, 2008).…”

Cold-water coral framework architecture is selectively shaped by bottom current flow

“…The local flow regime of a CWC reef habitat is influenced not only by seabed topography, but also by the corals themselves, which are capable of significantly altering the surrounding flow field with their complex structure, deflecting near-bed flow and decreasing turbulence and current speed in their wake (Mienis et al 2019;Bartzke et al 2021;Hennige et al 2021;Corbera et al 2022). This suggests that CWC colonies can mutually influence each other's morphology, by inducing heterogeneous flow patterns that may increase local architectural variability.…”

“…In principle, more compact D. pertusum colonies intercept sinking and transported sediment more effectively and require a lower amount of infill, becoming buried more rapidly and thus enhancing reef and mound growth rates under suitable sediment supply conditions (Wang et al 2021). CWC growth, hydrodynamics and sedimentation are mutually influenced at multiple scales, with development of colonies, reefs and mounds enhancing flow, food supply to living colonies and sediment baffling by dead framework, and thus further growth (Masson et al 2003;Davies et al 2009;Buhl-Mortensen et al 2016;van der Kaaden et al 2021;Corbera et al 2022). The high sensitivity of CWC architecture to water flow may further boost this positive feedback mechanism, with exposure of elevated colonies to stronger currents (and enhanced food supply) resulting in higher compactness, which would in turn facilitate post-mortem sedimentary infilling.…”