An ocean investigation

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My name’s Phil Hosegood and I’m a physical oceanographer from Plymouth University where I lead the Marine Physics Research Group. This expedition has presented a great opportunity to collaborate with marine scientists with a more biological persuasion and has provided an amazing insight for me personally into how large predators such as sharks are observed throughout this vast ocean. Not that I don’t love looking at the output from my current meters but you can’t beat having a look at the sharks the cameras have filmed at the end of the day! At the opposite end of the spectrum, we’ve been sat opposite Andy Brierley and Roland Proud from the University of St. Andrews in the hot and noisy ‘hangar’ for the whole cruise, and have learnt much about the detection of zooplankton using echo sounders. This is one of the areas in particular that will undoubtedly result in a fruitful collaboration following this expedition as there have been clear stories emerging of how the zooplankton are feeding in discrete layers that are related to the vertical structure of the water column and the physics that result in these layers forming. This is one of the key goals of this multidisciplinary expedition: to bring together marine scientists from a range of different but complimentary disciplines to produce insights into how this amazing ecosystem functions in a way that would not have been able to be achieved by single groups acting independently.

Chagos coral reef

This brings us to what myself and my colleague, Kate Adams, are here to do. We’re here to improve our fundamental understanding of the physics that drive the ocean throughout the archipelago and beyond, in addition to providing the underpinning physical context for the biological observations made by other groups aboard the Pacific Marlin. The key processes we’re particularly interested in are the tides, the impact of the daily warming and cooling of the upper ocean (called the diurnal cycle) during daytime and night-time, respectively, on the upper ocean, and the influence of turbulence on the vertical structure of the water column. So far we’ve seen that despite the tides being weak, they form a very unusual, intense subsurface jet that is likely due to our proximity to the equator. Also, the upper 300 m of the ocean here is separated into discrete layers, each with a thickness of 50 m or less. These layers, which are separated by rapid changes in temperature and salinity with depth, correlate nicely with the distribution of chlorophyll and (here’s the multidisciplinary bit!) the zooplankton.

Life aboard is, as if often the case on scientific cruises, fun but hard work. Kate and I have been working at night a lot as the other camera-based activities require daylight. We want to make sure we use the ship as fully as possible but, by also contributing throughout the day to provide data at the same time as the long-lines and drop-cams go in, does mean that trying to develop any kind of routine has been nearly impossible and fatigue nearly reached a critical point a couple of days before we had a rest day! But since then we ‘ve had a phenomenal couple of days snorkelling and swimming with Manta Rays – I can’t say it was a life changing moment but certainly the best day I’ve ever had at sea and one that instantly made all the hard work that has gone into this trip worthwhile (but the physical oceanography data will of course do that too…). We still have a few more days to go and will finish with the mooring recovery from the seamount. This will be the most nerve-wracking moment as if the acoustic release fails or something got tangled, the whole mooring may be stuck at the bottom with little or no means to recover - Look out for the cruise report to see what happened……

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