Wednesday, 9 July 2008

The plan

I had a request today – to explain what the survey plan was. Hmmm, good question, the plan changes daily, hourly even – all dependent on the weather. However there is a general plan… the survey had been planned so that we’d hit one big spring tide (5th July) and one very low neap tide (13th July). A spring tide occurs when there is a new or full moon (moon lined up with the sun), which makes for bigger tidal currents, and a neap tide occurs half way between the new & full moons with smaller tidal currents. This results in more mixing (& more internal waves) at spring tides than at neaps tides. So the theory is that the spring tides mix the water up and bring nutrients to the surface, but the neap tides let things settle a bit and allow the phytoplankton to stay close to the surface & grow with the sun & make the most of the nutrients. This regular alternating between vigorous mixing and more settled tidal conditions makes our tidal seas as productive as they are… and this is what we’re here to investigate.

So I diverge a little. We cover a big spring tide and a small neap tide, so we should be able to see the contrast between the tides in the physics (we expect stronger & bigger internal waves at springs & lower at neaps), and in the biology (I’d probably expect there to be more surface feeding by seabirds during springs because the internal waves push the food to the surface – but I could be proven wrong). So, we chose a nice productive bank – Jones Bank (see map in previous blog) to look at these internal waves, and the associated physics & biology in three locations: on top of the bank (MS1), on the middle of the slope of the bank (MS2) and on the bottom of the bank (MS3). We also have a control site south of the bank by a few miles in a flat area (MS4) where we don’t expect the abrupt dramatic internal waves we’ve already seen on the bank edge (just some slow gentle internal waves).

So as part of this we do 25 hour stationary monitoring both on the bank and at the flat control site, taking regular measurements of turbulence through the water column (we expect more turbulence in internal waves, at thermocline boundaries etc.), plus a range of other variables through the water column: temperature, salinity, nutrients, dissolved oxygen, bacteria, phytoplankton, zooplankton (see previous blog), and probably other stuff that I haven’t found out about yet! In these sites we also take samples of the sediment, so that we can look at interchange of nutrients between the sediment & the water column & see if the internal waves have any effect on the productivity of the bottom, and therefore of course what critters are living in the sediment (so far it’s been sand or muddy sand at the bottom). We also have a camera that takes photos of the fish at the seabed (more of that later), and we also collect fisheries acoustics data & carry out visual watches for birds & cetaceans. These stations are carried out on the bank at spring & neap tides, and at the control site.

Then we have the ‘boxes’ (described yesterday) – surveying an oval 12 times over a 25 hour period, measuring oceanography (the temperature & salinity through the water column), fish & seabirds/marine mammals (if we ever see any of the latter). We’ll do these boxes on the bank both at spring & neaps, and at the control site.

One final activity is the dye release – but that’s in progress now, and quite cool… so I’ll concentrate on that in tomorrows blog :o)

...since there are no photos for todays blog I thought I'd put in a photo of the sunset I took yesterday...

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