Bill Wells, of Dunedin, asks :-
I noticed several floating mats of white, plastic-like material scattered along the shore where the wind had pushed them close to the bank. From the bobbing in the waves, one could see that they were coherent and flexible, up to 2 metres in diameter, and they had bits of weed and other flotsam trapped in the apparently amorphous matrix. The material was bright white and floated high. Could it have been from a whale?
Chris Hepburn, a marine botanist at the University of Otago, responded.
I study seaweed and from the description it sounds like what is described is more mundane than whale related flotsam. In Otago Harbour and many others the green seaweed Ulva (sea lettuce) blooms due to high nutrient levels. Ulva grows on soft sediment environments in sheltered areas and when it is windy beds of Ulva are washed away and eventually gather together to form rafts of weed. These can be extensive and you can even walk on them when they are floating over deep water (I've tried but fell through, I smelt like rotting seaweed for a few days after that).
As the Ulva degrades it is bleached and sometimes the green pigment that gives Ulva it's colour is completely stripped away leaving a transparent plastic appearance that you describe.
The rafts of Ulva and other seaweeds are actually quite important in feeding grazers like paua and providing food for invertebrates that support fisheries and birdlife. Paua pin down floating pieces of seaweed with their foot and eat it, you can often see paua clamped on to all sorts of seaweed as they eat it. We are currently studying how this food is used by grazing communities. If nutrient levels become too high and blooms more frequent, Ulva can cause major problems to the environment.