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Day 10 Bringing some of the details together: Christian Halverson

Posted by: she395

October 4, 2017

Tuna
Photo: Eric Woehler

The study I have been most involved with should be a little obvious to those who know me—spatial and temporal variability in the distribution and abundance of seabirds with Eric Woehler, which is quite a mouthful. I might have called it “why are seabirds found in particular areas?,” but then that is why I am not a scientist.

Today was a serious Attenborough moment for me. On Monkey Island, or the Monkey Bridge, where Investigator’s resident bird expert and assistants hang out, we saw at least three tuna-bird feeding assemblages. These are those scenes you see in the documentaries—60 plus predatory fish leaping out of the ocean, chasing the small bait fish, and another 40 plus birds diving into the ocean amongst it all. The life of a small fish must not be fun at times. Why is this exciting? It is spectacular for a start, when you realise the size of a fish is a metre or so, and they are launching themselves out of the water, it adds a real element of wonder. They must be the messiest of feeders though, shattering the little fish and leaving chunks for terns and boobies to devour.

The Arafura Sea, where we are, and the Gulf of Carpenteria, which we have just sailed through, are known areas for tuna and prawn industries. The tuna makes sense, and maybe the bait fish are feeding on the prawns and other smaller zoo and phytoplankton. Prawns feed on detritus, the dead materials sinking to the bottom of the ocean. There must be quite a bit of photosynthesis occurring to support the top predators like the terns and the tuna.

Water test

Cassie in hydrochemistry has set up an ingenious water sampler that takes the seawater out from the underway seawater collector. She then runs the water through her analyser and processes the data. It is interesting to see the preliminary results—very low levels of nitrates, nitrites and phosphorus, which is typical of this area. The one that did seem a bit different was a drop in silica. Maybe this is the limiting factor, not enough silica to build the bodies of huge numbers of diatoms. Maybe there are different plankton that act as producers at the bottom of the food web.

We also sailed through a green-brown nebulous smear in the water, and watching the dissolved oxygen meter we noticed a little jump in oxygen. This should happen if the smear was algae or phytoplankton. Even the flurometer jumped too, again an indicator of chlorophyll. We lifted the continuous plankton recorder and carefully poured the formalin over the silk to preserve what was collected, I would have loved to have grabbed a microscope and popped some on a slide, but it would have ruined the collection process.

So it’s not quite the desert I thought it was up here, more like a rainforest. Lots of matter is produced and cycled around, but not much is left, sinking to the bottom or brought in from elsewhere. So many questions to ponder, so much more science to do, so many more measurements to take, and still a few more projects to discover as we head ever closer to Broome and the end of my journey on Investigator.