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Size matters! Investigating benthic size spectra

Published: 15 October 2015

Bangor University team sort their catchStudying marine ecosystems is a messy job. For instance, if you were to head out on a survey vessel and haul up some of the seabed, you would be presented with a sample of sediment containing thousands of different species of worms, molluscs and other tiny critters. However, if you were to move just a kilometre away and repeat this, you would likely end up with a very different sample, perhaps containing a completely different suite of species, or just as likely, a sample containing hardly any animals at all. This is because the sea is a very dynamic environment.

Samples that strongly differ from each other can result in what we call “noisy data”, which is very difficult to interpret and draw any solid conclusions. Hence, to help simplify these kind of studies we can employ a method known as “size spectra”. In size spectra, we are not interested in how many hundreds of different species there are. Instead, we are just concerned with how many individual animals there are, and how big they are. In using this approach we can effectively simplify an entire ecosystem to just a single graph. From this graph, we can then determine how human impacts, such as fishing pressure, are affecting marine ecosystems and the services they provide.

Different sizes of gurnards caught aboard the Prince MadogUnfortunately, the size spectra of marine ecosystems is still poorly understood. This is because the majority of size spectra studies tend to focus on just one size class of animals. For example, by focusing on just the really small stuff like the plankton, or the really big animals like fish. However, in September 2015, we spent four weeks at sea working aboard the RV Prince Madog, a research vessel owned by Bangor University. Whilst on board, we deployed a series of grabs, dredges and nets which captured the full size spectra of animals ranging in size from the tiniest worm to the largest fish.

As we surveyed sites across a gradient of fishing pressure and primary productivity (i.e. the amount of algae in the water - which are at the base of the marine food chain), we can see just how important the impacts of fishing are, or whether natural processes, such as primary productivity, remain the most important controlling factors. The samples we obtained from this research cruise are currently being analysed at Bangor University. Our results will then be supplied to other MERP scientists who require the data in order to construct their ecosystem models.


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