The distribution and biochemical composition of biogenic particles across the subtropical Front in June 1993 (Azores-Madeira region, Northeast Atlantic)

Abstract

Water samples were collected in the north-east Atlantic Ocean between the Azores and Madeira (33°N-36°N and 24°W-26°W) during the Oceanographic Cruise SEMAPHORE in June 1993. Temperature, salinity, nutrients (nitrate and phosphate) and particulate organic matter (organic carbon, organic nitrogen, carbohydrates, proteins and phytopigment) were investigated in the water column to a depth of 2000 m. The presence of the subtropical front (STF) separating warmer more saline Western Atlantic Water (WAW) from colder and fresher Eastern Atlantic Water (EAW) in the upper 100 m, and a tongue of salt water arising from the influence of Mediterranean Water (MW) at a depth of 1000 m, were well identified by the physical and chemical parameters. POC and PON concentrations, in the surface layer (0-100 m), ranged between 23.3-64.5 and 2.9-9.1 µg l^-1 respectively, while concentration between 12.4-30.5 and 1.1-4.0 µg l^-1 prevailed below the thermocline (100-2000 m). The very low POC and PON concentrations together with the low nutrient and chlorophyll-a concentrations confirmed the oligotrophic nature of the Azores-Madeira region. Statistical analysis was carried out to investigate the difference in the quantity and quality of POM between water masses. Regression analysis showed a high correlation between POC and PON but the slopes and intercepts of the regression lines did not differ significantly between WAW and EAW. In contrast, an examination of vertical profiles as well as mean integrated values of biochemical variables in the upper 100 m suggest a difference in the quantity and quality of biogenic particles between the water masses. WAW showed the lowest integrated concentrations of particulate organic carbon, particulate organic nitrogen, particulate protein and particulate carbohydrate. In contrast, frontal stations showed the highest values while EAW stations showed intermediate values. All these results, coupled with the occurrence of the highest POC:Chl-a ratio in EAW, suggest a temporal offset in the production cycle between the two water masses. Alternatively, the WAW is a more stratified and oligotrophic body of water than is EAW, and it always has lower biomass and nutrients than the more productive EAW. High chlorophyll-a concentrations were linked to the STF; moreover the average depth of the subsurface chlorophyll maximum (DCM) appeared to shift to shallower values at the frontal stations, suggesting a close association between the STF-Azores Current (AC) physical structure and the distribution of chlorophyll-a in this area.