Applying a lagrangian trajectory model to investigate and describe the distribution of drifting ecklonia maxima within the benguela upwelling system
Coppin, Ross Mark
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Macroalgal morphological variation is determined to a large extent by a combination of environmental factors, with wave exposure and temperature perhaps the main influences, as they are key environmental properties to which a species becomes locally adapted. Macroalgae have shown to exhibit different responses to different magnitudes of exposure to waves, such as reduction in overall size and strength increasing traits. In terms of temperature, warmer environments have been shown to reduce the overall size of resident and transplanted species. However, none of the past studies have identified specific wave and temperature metrics responsible for the morphological adaptation macroalgae exhibit. Past research has often used simple or two-dimensional models of wave exposure, which do not take into account important aspects of the nearshore environment such as wave breaking, refraction and diffraction. Furthermore, past studies have often used satellite-derived datasets as sources for temperature data; however, such data have been shown to have large bias when applied to the nearshore environment. This study used in situ temperature data and wave power metrics calculated from a 3D-numerical model to identify specific temperature and wave metrics responsible for morphological adaptation of the kelp, Ecklonia maxima and Laminaria pallida. Between temperature and wave exposure, the results identify wave exposure as the main influencer of morphological adaptation while identifying specific wave metrics. Furthermore, the results show differences in wave metrics between species, and between deep and shallow populations. The findings from this chapter were used in the next chapter to investigate the role of hydrodynamic and wind drag on floating kelp trajectory. Ocean currents act as an essential dispersal mechanism of natural and anthropogenic material on the ocean surface. Macroalgae are one of the essential natural dispersal vectors of marine organisms and are regarded as the ‘tumble-weed of the ocean.’ Despite many studies on the topic, the relative role of wind and surface currents in influencing the trajectory of macroalgal dispersal is still uncertain. Past studies focused on kelp-rafts, which can vary significantly in size, making it difficult to form a consensus on the relative role of wind versus surface currents. In addition, these studies have not considered surface area in relation to drag characteristics of the macroalgae, both of which have been shown to play a role in the trajectory and accumulation of flotsam. Advances in Lagrangian trajectory modelling have been rapid in recent years and allow the use of numerical experiments to investigate trajectories of flotsam in the ocean. This study aimed to shed light on the relative role of wind versus surface currents and the role of drag in macroalgal trajectory and accumulation. We focused on solitary kelp plants. This was achieved by comparing simulations of virtual kelp ‘particles’ which incorporate drag with that of purely Lagrangian particles (no drag).