THE BELL JAR USE TO ASSESMENT OF THE BENTHIC METABOLISM IN ARMAÇÃO DO ITAPOCOROY: PRELIMINARY RESULTS

Abstract

The purpose of this work was to determine the benthic metabolism and the regeneration of nutrients in the sediments under a mussel culture, using an adaptation of in situ benthic chambers (bell jars). Three experiments were carried out in the the summer. Two bell jars were used in each experiment: one under the mussel culture and another one as reference, outside of the culture area. The benthic domes was shaped on perspex with a volume of 72 l and covering an area of 0.33m2. A submersible pump (Rule 360) was placed on the top of the dome in order to gently stir the incubated water. This pump was fed by three 6V batteries sealed in a plastic tube. On each experiment, water samples were collected from the bell jars every 12 hours, (the samplings were collected always at early morning or at late afternoon), completing 24 hours cycles. On each sample, Total CO2(TCO2), O2, dissolved inorganic nutrients (NH4 +, NO3 -, NO2 - and PO4 3-) and biogenic elements (Particulate Organic Carbon, Total and Dissolved Organic Nitrogen, Total and Dissolved Organic Phosphorus) were analysed. Particulate Organic Nitrogen and Phosphorus were obtained by the difference between the total and dissolved fraction. The fluxes were calculated from the variation of the concentration in the water of these parameters. The fluxes of TCO2 and O2 indicated that the benthic metabolism was in equilibrium (Production~Mineralization) at both sites (mean of - 2,5 mmol TCO2.m-2.d-1 and -0,18 mmol O2.m-2.d-1 under the mussel culture and -1,19 mmol CO2.m- 2.d-1 and -3,2 mmol O2 .m-2.d-1 on the reference site). The averaged values of NH4 +-fluxes were -0,19 mmol N.m-2.d-1 and -1,09 mmol N.m-2.d-1 under the mussel culture and in the reference site respectively. The PO4 3- fluxes were, -0,96 mmol P.m-2.d-1 under the culture and -0,05 mmol P.m-2.d- 1 in the reference site. These fluxes values were small compared to the content of nutrients in the water column, sugesting that the regeneration in the sediment was not very important in this environment in the studied period.