Publication Abstract

Modelling potential impacts of bottom traw fisheries on soft sediment biogeochemistry in the North Sea

D.E.Duplisea, S. Jennings, S.J. Malcolm , R. Parker and D.B. Sivyer

Physics, chemistry and biology are intertwined in the benthic boundary layer such that activities in one are closely coupled to the state of another. This close coupling in time and space causes self-reinforcing or positive feedback processes in sediments. Processes such as bioturbation by metazoans oxygenates sediments and oxidises toxic sulphides. As bioturbation is positively related to metazoan biomass, metazoans create a more favourable environment for themselves. Contrary positive feedbacks also exist, however, such as production of sulphide by bacteria which use sulphate to oxidise organic carbon. The production of sulphides which are toxic to many other organisms, reduces competition for their carbon source. There are limits to these processes, mostly driven by exogenous factors such as carbon input and temperature; however, direct human physical impacts are also likely to be significant. For example, the Southern North Sea is estimated to be dragged over by trawling gear about 5 times per year on average. The implications of this for mixing sediment layers and increasing faunal mortality is likely to be important to the state of the benthic boundary layer.

Model simulations show that high levels of benthic trawling are likely to create instability in the benthic community. Physical mixing caused by benthic trawling is likely to differ from that caused by macrobenthos in that anaerobic carbon mineralisation pathways can be relatively enhanced as the trawling induced mortality of macrofauna means that macrofauna are unimportant carbon mineralisers themselves. Furthermore, the shift to microbial mineralisation pathways means that turnover times are fast and responses of the sediments to changes in the carbon pool are large and rapid. These explicable model results still need verification from field measurements.

Reference:

D.E.Duplisea, S. Jennings, S.J. Malcolm , R. Parker and D.B. Sivyer, 2001. Modelling potential impacts of bottom traw fisheries on soft sediment biogeochemistry in the North Sea. Geochemical Transactions, 2(14): 1-6.