Biogeochemical Cycles Sub-Project

Objectives

To determine the spatial and temporal distribution of nutrients and primary production along the continental shelf.

To quantify the fluxes of carbon and related elements (N, P, Si and trace elements) between dissolved and particulate phases in living and non-living reservoirs.

To develop biogeochemical mass balance models describing long term flows of carbon, nutrients, trace metals and their exchange fluxes between the open ocean and the shelf seas to predict man's impact on land-ocean exchange processes.

Phytoplankton production and fertility on the shelf is critically controlled by the influx of nutrients from rivers and from the open ocean. On an annual basis, the on-shelf flux of nutrients including nitrate, phosphate and silicate are of major significance. Nutrient fingerprints and their seasonal flows were measured during OMEX-I cruises for the different water masses that contribute to exchange at the shelf edge.

Productivity

Phytoplankton photosynthesis rates along shelf edge sections were measured in terms of size-fractionated components, and in terms of nitrate fuelled 'New Production' and ammonium fuelled Regenerated Production. New production can support the fallout or export of biogenic carbon to deep waters. Bacterial degradation and microzooplankton grazing on the phytoplankton carbon recycles organic carbon back to CO₂ for exchange with the atmosphere. These competing processes were investigated simultaneously to allow the seasonal mapping of productivity to be completed and to relate these to pigment distributions determined by shipboard HPLC and remotely sensed by colour sensing satellites. The sources and speciation of carbon and nitrogen were tracked using natural abundance of ¹³C and ¹⁵N in particulate matter. Bio-assimilation of carbon, nitrogen and phosphorus were determined by tracer additions of the isotopes ¹⁴C, ¹⁵N, ³²P and ³³P.

Carbon Cycle

The inventories and flows of dissolved and particulate organic carbon were tracked using novel ultra-clean shipboard analysers for dissolved organic carbon and deep in situ pumping systems for particulate organic carbon. Studying the ¹³C isotope ratios made it possible to distinguish organic matter from terrestrial and marine sources. Molecular biomarkers, including pigments and biopolymers, were measured to distinguish phytogenic from trophically reprocessed forms of organic matter. Sediment traps collected the fallout of shelf and marine organic material and allowed measurements of the carbon export rate from the slopes to the deep sea to be made. During upwelling events at the shelf edge, supersaturated CO₂ may be ventilated to the atmosphere, and so depending on the origins and hydrography, shelf edge systems can switch from being net sinks to being a net source of CO₂ to the atmosphere. During OMEX I, measurements were made of the carbonate system to investigate this.

Bioreactive Elements

The inventories and fate of bioreactive elements (Ba, Ca, Al, Zn, Pb, Cd, Cu, Fe, Mn, Ni, Co) and their vertical and frontal distributions at the shelf edge were mapped to allow residence times, reactivities and shelf budgets for these geochemically and toxicologically important elements to be estimated.