Fatores que influenciam na produtividade dos oceanos: a importÃncia do fluxo de difusÃo dos nutrientes para a biomassa do fitoplÃncton na regiÃo oceÃnica do nordeste brasileiro

AUTOR(ES)
DATA DE PUBLICAÇÃO

2003

RESUMO

In the tropical oceanic region the phytoplankton primary biomass is controlled by light availability and inorganic dissolved nutrients, besides other physical factors as turbulence and sedimentation. The water properties in the photic zone are subjected by a number of different forcing intensities opposite to the deeper waters which varies regularly. In Northeastern Brazil the presence of a permanent and deep thermocline limits the nutrients flux forming a concentration gradient opposite to the light. The aim of the present work was to characterize the oceanic region of the Economic Exclusive Zone of the Northeastern Brazil, between the north littoral of Rio Grande do Norte State (5ÂS) and Salvador-BA (14ÂS), from 12 to 200 nautic miles. It was intended to assess the nutrients concentration dynamics and their temporal and spatial variations; their effect in the primary productors distribution in terms of chlorophyll a and the trophic level of the region; and the vertical diffusion of the nutrients through the nutricline and their influence in the primary biomass. A Principal Component Analysis was performed and explained 60% of the total variance being utilized all sampling levels (100%, 50%, 1% of light penetration; thermocline beginning, mid and end, and 500 m), showing an inverse correlation between temperature, salinity, pH, dissolved oxygen and phosphate- P, nitrate-N, silicate-Si. This analysis indicated the increase in nutrients mineralization in relation to the depth until the nutricline stabilization and the great relationship between chlorophyll a, and nitrate-N and phosphate-P. The median concentration at the photic and aphotic layers were respectively to ammonia-N ≤0,03 μmol.L-1, to nitrate ≤0,6 and ≤12,56 μmol.L-1, to phosphate-P ≤0,14 and ≤1,09 μmol.L-1 and to silicate-Si <12,0 and ≤20,5 μmol.L-1. The chlorophyll a presented median values <1,0 μg.L-1 at surface and ≤1,45 μg.L-1 at 1% light penetration depth. It was defined that the liquid regeneration rate of the C component (nitrate-N or phosphate-P) is a result of the balance among the input of the organic material degradation, the diffusive flux and the removal by phytoplankton absorption, as the sedimentation of the total particulate material (nutrients adsorption), being these the main mechanism in the nutrient vertical balance. This suggests that the ascendant diffuse flux is counterbalanced by sedimentation and phytoplankton consumption. To calculate the turbulent diffusion (νt) it was used the semi-empirical formulation of Kitaigorodskii (1960), which better represented the diffusive profiles obtained in the field with a SCAMP sound (Self- Contained Autonomous MicroProfiler). The vertical distribution of nitrate-N and phosphate-P concentration was normalized in relation to the concentration and depth verified at the end of the thermocline (ZFT), which resulted in a non dimensional representation. This methodology allows an analytic representation of the curves from the logistic model of Verhulst-Pearl (JÃrgensen, 1986). The maximum diffusive depth was at 0.82 ZFT (214 m), decreasing to 1/3 at the photic layer base. The nitrate-N maximum flux was nearly 1.5 more intense than the phosphate-P one. This explain the oligothrophic condition of the area. The increase in chlorophyll a during the autumn in some sampling points resulted from the increase in the photic layer with a diffusive increase in this depth bringing additional flux of available nutrients, showing the importance of the nitrate physical transport as the main source of nitrate-N to the new phytoplankton production at the oceanic superficial layer

ASSUNTO(S)

fluxo de difusÃo biomassa fitoplanctÃnica nutriclina revizee oceanografia nutrientes

ACESSO AO ARTIGO

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