Tomahawk Lagoon Research

By Josie Crawshaw   |   April 2015

It has been a craaazy busy 2 weeks. I was back home week before last for cousins wedding, which was absolutely beautiful! I made the most of being home and went to the beach with my dog Tip, and enjoyed the warm sunshine after the recent snowfall in Dunedin!


2 days after getting back it was into my Autumn sampling of Tomahawk Lagoon. I was so lucky to get a weather window, and it was sunny and calm the whole 3 days. 


Tomahawk Lagoon is made up of 2 small, shallow (~ 70 cm deep) predominantly fresh lagoons located by Tomahawk beach in Dunedin. We are interested in learning how the system can remove nitrate from the water column, and how efficient the sediment microbial denitrifiers are at turning the nitrate into nitrogen gas (thereby removing it from the system). The core structures I use isolate a section of the water column for us to be able to then extrapolate out to the rest of the lagoon.


At the end of the experiment we can pull up the cores, and check out what invertebrates were in the sediment, and look at potential correlations between their abundances and the nitrogen transformation rates. When sieving the cores, I saw some massive colour changes around the chironomid larvae burrows, showing that the sediment surrounding their burrows was highly oxygenated, compared to the anoxic sediment around the burrows. This creates a larger surface area for denitrification to occur, and the water transport into the burrows also brings nitrate to deeper sites where denitrification could occur. It will be interesting to see whether these guys make a difference in my research!


I also collected some cores to measure sediment oxygen microprofiles, looking at oxygen changes in the sediment on the micrometer scale. This lets us calculate oxygen diffusion rates into the sediment, which may then tell us something about how fast the nitrate could enter the sediment. 


Porosity is another factor which may affect this, which is a measure of the empty space between the sediment particles, which I measure back in the lab - almost looks like I am baking cupcakes…

Hopefully going to get out diving this week, so watch this space for some underwater photos soon!

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