We have a plan!
After a morning spent on the Suffolk coast at Cefas, I returned home to reflect over the outcome of the meeting. The purpose of such an assembly was to stimulate the transfer of knowledge between the two differing parties, and to maximise the efficiency of scientific experiments being carried out on a winter cruise set for January 2014.
The cruise is a multi-purpose scientific adventure starting in the North Sea, and then heading down along the South coast of the United Kingdom until reaching our final destination; the Celtic Sea. The plan is to maintain Cefas’s Smart Buoy monitoring stations and combine this trip with a first run of a set of scientific experiments which are to be carried out later in the year as part of the NERC Shelf Sea Biochemistry programme.
January may not seem like a good time to go on a ‘leisurely’ cruise where frequent gales and storms can be encountered; but this is where you’d be wrong! It is during such events that sediment at the bottom of the continental shelf becomes disturbed due to the transfer of energy from the atmosphere to the ocean. Within these sediments exists a large microbial ecosystem which slowly processes all sorts of compounds, and when you mix it up, who knows what oceanic soup you are going to get! This is where I fit in. My experiments will piggy back off in-situ resuspension experiments carried out by other scientists from Cefas, Portsmouth University and Southampton University aimed at simulating such events. One of my tasks will be to analyse the NH4+ (ammonium – remineralised from decaying organic matter) concentration through the course of the experiments.
You may well ask “why is this an important factor?”
Well…… NH4+ is an key component of the nitrogen cycle. Under oxic conditions, it is assimilated (biologically mediated conversion of nutrients to obtain energy) into NO2– (nitrite) by aerobic archea and bacteria. In a separate biochemical pathway under oxic conditions, the nitrite is then assimilated into NO3– (nitrate). The multi-step process is termed nitrification.
Nitrogen forms part of the building block for all organisms; a good example is DNA which is composed of amino acids (C2H4NO2R). Phytoplankton utilise NO3– due to its availability in the ocean; stimulating phytoplankton blooms when other environmental conditions are met. NO3– is considered to be the limiting nutrient for primary production within the European continental shelf. This is down to the decoupling of the nitrogen cycle under anoxic conditions; denitrification. The denitrifying biochemical process occurs predominantly within the sediment (>5mm depth) where the biological oxygen demand would be higher than supply. Any NO3– within the anoxic region would firstly be remineralised to NO2– and then remineralised to di-nitrogen gas (N2). Other anaerobic processes (anammox) that decouple the nitrogen cycle lead to a reduction in bio-available nitrogen.
Now what effect does nature and anthropogenic influences have upon the nitrogen cycle within the continental shelf?
The water column in the North European Shelf becomes stratified during the summer months. As Autumn and Winter months approach, the stratified boundary begins to break down. This is further exacerbated by the increased frequency and intensity of storms due to weather systems developing over the Atlantic. The increased intensity of storms can cause an increase in resuspended sediment; which alters the dynamics of the biochemical processes within the sediment and water column (see sediment resuspension below).
Fishing is a lucrative industry within the UK as targets aim to meet the demand. Trawling is one such method of capturing bottom dwelling marine organisms. During such events, the overlying sediment (up to the top 10cm) is disturbed, and becomes resuspended in the water column. Thus an ‘artificial’ concoction of organic matter, nutrients and organisms is created; changing the biochemical processes and hence the chemistry. Although carried out on a localised scale, the effects posed upon the natural ecosystem and biochemical cycling have been overlooked on the European continental shelf (Pilskaln, Churchill, Mayer, 1998).
Resuspension is a naturally occurring process directly related to the critical threshold value for the sediment. As far as I am aware, there exists limited data on the critical threshold value for (or release of) nitrogen compounds (NO2– NO3– NH4+) which are contained within the sediment. As weather systems pass over, the pressures exerted on the surface of the ocean increases, causing a cascading disturbance down the water column which in turn leads to flushing – the process of water being forced through the sediment. The threshold value where surface influences have an impact of increased flushing through shelf sediments has not been effectively quantified. Tidal cycles would cause a degree of flushing, but it would have a reduced impact when compared with a downward pressure force generated by wave activity (Tolhurst, 2013, unpublished). The increased penetration depth of water would change the chemistry of the sediment: organic matter would increase as longer chain forming planktonic diatoms are forced downwards deeper into the sediment (Ehrenhauss and Huettel, 2004), whilst nutrients could be generated or removed depending on the chemistry of the bottom water, sediment disturbance and organisms present (Karlson, Bonsdorff, Rosenberg, 2007). The oxygen penetration depth would increase as would the BOD.
The planned experiments will drive our understanding of the nitrogen cycle in a continental shelf setting; possibly creating more questions than answers, and that’s what makes it interesting! There is a lot of work to be done between now and the first cruise, so stay tuned for further updates.