Abstract
Reef-building species form discrete patches atop soft sediments, and reef restoration often involves depositing solid material as a substrate for larval settlement and growth. There have been few theoretical efforts to optimize the physical characteristics of a restored reef patch to achieve high recruitment rates. The delivery of competent larvae to a reef patch is influenced by larval behavior and by physical habitat characteristics such as substrate roughness, patch length, current speed, and water depth. We used a spatial model, the “hitting-distance” model, to identify habitat characteristics that will jointly maximize both the settlement probability and the density of recruits on an oyster reef (Crassostrea virginica). Modeled larval behaviors were based on laboratory observations and included turbulence-induced diving, turbulence-induced passive sinking, and neutral buoyancy. Profiles of currents and turbulence were based on velocity profiles measured in coastal Virginia over four different substrates: natural oyster reefs, mud, and deposited oyster and whelk shell. Settlement probabilities were higher on larger patches, whereas average settler densities were higher on smaller patches. Larvae settled most successfully and had the smallest optimal patch length when diving over rough substrates in shallow water. Water depth was the greatest source of variability, followed by larval behavior, substrate roughness, and tidal current speed. This result suggests that the best way to maximize settlement on restored reefs is to construct patches of optimal length for the water depth, whereas substrate type is less important than expected. Although physical patch characteristics are easy to measure, uncertainty about larval behavior remains an obstacle for predicting settlement patterns. The mechanistic approach presented here could be combined with a spatially explicit metapopulation model to optimize the arrangement of reef patches in an estuary or region for greater sustainability of restored habitats.
Highlights
Reef-building marine invertebrates, including oysters, mussels, and corals, form discrete clumps or patches that are elevated above the surrounding sediments
Depth-averaged larval settlement probabilities P were consistently highest in shallow water, but for diving larvae Pd had a dome-shaped relationship with current speed Umax (Fig. 2, 3)
The relationship between settlement probability Pp and Umax was sometimes bimodal with local maxima both at intermediate current speeds and at the lowest current speeds
Summary
Reef-building marine invertebrates, including oysters, mussels, and corals, form discrete clumps or patches that are elevated above the surrounding sediments. Some restoration efforts focus on constructing patches of solid substrates to stimulate larval recruitment and growth [1]. Both the size and roughness of constructed substrate patches may influence the ability of artificial reefs to accumulate new settlers. Rougher reef substrates generate more turbulence, which affects the delivery of larvae to the bed by diffusive mixing and can enhance or reduce settlement depending on larval behavior. Patch characteristics influence both behavioral and physical processes delivering larvae to the bed, there have been few attempts to quantify the relationship between substrate patch size or roughness and larval settlement. We develop a quantitative approach for identifying the characteristics of a habitat patch that will maximize settlement both for individual larvae and for the reef patch
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