Material Residence Time in Marine Canopies Under Wave-Driven Flows

Maryam Abdolahpour,Kathryn Mcmahon,Paul Lavery,Marco Ghisalberti

doi:10.3389/fmars.2020.00574

Maryam Abdolahpour, Kathryn Mcmahon + Show 2 more

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https://doi.org/10.3389/fmars.2020.00574

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Abstract

Coastal canopies (e.g. seagrasses, coral reefs, and kelp forests) are vitally important ecosystems that provide a range of ecological services (e.g. oxygen production, sediment stabilization and trapping and recycling of nutrients). The long-term health, productivity and survival of these canopies rely heavily on the residence time of ecologically-significant materials in these environments. Recent studies have shown that the presence of submerged canopies induces strong mean current over the canopy top, even in purely wave-dominated environments. Thus, in addition to vertical mixing, the horizontal flushing of materials (resulting from these canopy-induced currents) will dictate rates of water renewal and, therefore, residence time in wave-dominated flows over submerged canopies. Building on this recently-improved understanding, this paper provides (for the first time) a framework for estimation of material residence time (Tres) and its variation with core system parameters, including both canopy and wave characteristics. This is done through consideration of a P'eclet number (Pe) which is the ratio of mixing to advective time scales. Prediction of residence time for a wide and realistic range of marine canopies (i.e. mixing dominated with Pe>1, and transitional (Pe ~ O (1)) reveals that while Tres decreases with wave height and increases with water depth, it has a complex relationship with canopy density and height. Importantly, residence time can vary from orders of seconds to hours, depending on wave and canopy properties. This has considerable ecological implications for marine canopies through the direct impact on a range of chemical and biogeochemical processes within the canopy. The framework presented here provides a critical step forward in being able to predict residence time in coastal canopies and test the interacting set of factors that influence the residence time in real, complex systems.

Highlights

Coastal canopies, such as those formed by seagrass meadows, coral communities, and kelp forests (Duarte, 2002; Rosman et al, 2007), provide food (Connolly et al, 2005), nursery, habitat (Fonseca et al, 1992), and shelter (Scott et al, 2000) for a wide range of marine organisms (Gambi et al., 1990; Koch et al, 2006)
While the results presented here nicely demonstrate how residence time varies in a typical coastal canopy, the method described in section 2 must be followed for residence time estimation for specific wave and canopy conditions
The predicted values are in excellent agreement with those observed in the real coastal canopies

Summary

Introduction

Coastal canopies, such as those formed by seagrass meadows, coral communities, and kelp forests (Duarte, 2002; Rosman et al, 2007), provide food (Connolly et al, 2005), nursery, habitat (Fonseca et al, 1992), and shelter (Scott et al, 2000) for a wide range of marine organisms (Gambi et al., 1990; Koch et al, 2006) These canopies influence water quality through the direct uptake of nutrients (Moore, 2004; Larkum et al, 2006), particulate materials (Gacia et al, 2002), as well. In addition to the implications for macrophytes that take up nutrients from the water column, residence time can play an important role for biogeochemical cycling within the hyporheic zone, one of the most significant regions for material storage (Harvey et al, 2003; Zarnetske et al, 2011)

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