Report No FR/D0027

Mar 1997

EXECUTIVE SUMMARY

This report details the results of a shipboard study of the influence of sediment dwelling organisms (macrofauna) on the exchange of nutrients (ammonia, nitrate, nitrite, phosphate and silicate) and trace metals (iron, manganese, zinc, copper, nickel, cadmium and lead) across the sediment-water interface. The work was carried out on six research cruises on RRS Challenger from December 1992 to June 1995, at six stations (two with minimal organisms) in the coastal waters of the Humber, Wash and N. Norfolk region. The effect of the organisms was investigated via measurements on sediment cores of (a) sediment properties (particle size distribution, porosity, redox potential, carbon and nitrogen concentrations), (b) nutrient and trace metal diffusive fluxes, and following sediment remobilisation (in a shipboard microcosm) and (c) pore water concentrations. The populations of macrofauna were determined by sieving sediment cores, identifying and enumerating the species and weighing for biomass. A supplementary mesocosm study was carried out in Solbergstrand, Norway in which the influence of discrete species of bioturbating macrofauna on sediment-water nutrient exchange was determined under controlled conditions.

Macrofaunal species, including known bioturbators, were abundant at four stations and species diversity was high at three of them. No distinct influences of biota on sediment properties, relevant to solute transports, were identified. At three of these stations nutrient fluxes resulted in transport to the water column of ammonia (up to 4721 µM.m^-2.d^-1), nitrate (up to 2057,µM.m^-2.d^-1), nitrite (up to 30µM.m^-2.d^-1), phosphate (up to 392µM.m^-2.d^-1) and silicate (up to 6458µM.m^-2.d^-1), higher in the spring and summer for ammonia and silicate, and in the winter for nitrate, nitrite and phosphate. Attempts to correlate fluxes with macrofaunal numbers or biomass were not successful, probably due to the influence of other variables associated with specific macrofaunal activities. Comparison of measured fluxes with those calculated from pore water concentration gradients indicated bioturbational enhancement of fluxes by 1-3 orders of magnitude. The results of the laboratory mesocosm study demonstrated modifications to nutrient transport in sediment cores with added organisms, compared to unseeded controls. While correlations with biomass were only partially successful, all the effects could be qualitatively related to specific bioturbational activities.

Transport to the water column in UK coastal sediments, was also noted for iron (up to 1450µM.m^-2.d^-1), copper (up to 69µM.m^-2.d^-1), nickel (up to 10µM.m^-2.d^-1) and lead (up to 4µM.m^-2.d^-1) at three of the stations where macrofauna were present; manganese (up to 665µM.m^-2.d^-1) and zinc (up to 197µM.m^-2.d^-1) at two stations and cadmium (up to 0.8µM.m^-2.d^-1) at one station. Outfluxes of zinc, copper, cadmium and lead were also noted at one, low macrofaunal station in the Humber mouth.

Summarising, this multicomponent study has yielded quantitative and qualitative evidence for bioturbationally enhanced transport of nutrient and environmentally important trace metals, across the sediment-water interface in coastal regions. It also provides a seasonal compilation of macrofaunal populations and benthic exchanges in an important UK coastal region. However, it has also demonstrated the difficulties in studying these effects in natural sediments with diverse macrofaunal populations. It is concluded that further understanding of these processes is more likely to result from laboratory mesocosm studies in which animal populations and sediment types can be carefully controlled.

Copies of the report are available from FWR, price £25.00, less 20% to FWR Members.