Alkalinity, inorganic carbon and CO2 flux variability during extreme rainfall years (2010-2011) in two polluted tropical estuaries NE Brazil

Abstract The susceptibility of coastal environments to shifts in the biogeochemical cycles of carbon and nutrients driven by anthropogenic pressure and climate change is a real challenge for the scientific community. This paper evaluated the effects of an extreme rainfall event over the nutrients and carbonate parameters in two polluted tropical estuaries. Surface water samples were taken seasonally along a salinity gradient in the Capibaribe and Barra de Jangadas estuaries in order to investigate the spatial and seasonal variability of dissolved nutrients, chlorophyll-a, dissolved oxygen, total alkalinity, inorganic carbon, partial pressure of CO2 (pCO2) and CO2 fluxes. The increased riverine influence caused by the fluvial flooding during the extremely rainy season augmented the nitrogen concentrations in the plumes, which also presented reduced salinity, alkalinity and dissolved oxygen values. In the Capibaribe plume it has also shifted the mean CO2 flux value of - 4.01 mmolC m-2 d-1 during the dry season, to a positive mean flux of + 5.7 mmolC m-2 d-1 during the rainy season. Within the estuaries the BOD5,20 and dissolved phosphorus values were higher during the dry season (p<0.0001), they showed positive correlation with the phytoplanktonic blooms that reached a chl-a value of 85 mg m-3 in the Capibaribe. The high alkalinity found in both estuaries, with mean values between dry and wet seasons respectively from 1808 to 1373 µmol kg-1 in the Capibaribe estuary and 1616 to 1058 µmol kg-1 in Barra de Jangadas estuary, may act as a buffer to the process of coastal acidification due to eutrophication. The increased rivers discharge lead to a greater transport of organic matter and nutrients to the coast, decreasing the oxygen availability and shifting the metabolic status of the estuarine plume to heterotrophic, whereas increased the water quality within the estuaries due the flushing promoted by the extreme rainfall event.

摘要：沿海环境在人为压力与气候变化驱动下，其碳、营养盐生物地球化学循环发生转变的敏感性，是科学界亟待应对的现实挑战。本研究针对两处受污染的热带河口，评估了极端降雨事件对营养盐与碳酸盐参数的影响。研究沿卡皮巴里贝（Capibaribe）与巴拉当雅加达（Barra de Jangadas）河口的盐度梯度开展季节性地表水采样，以探究溶解态营养盐、叶绿素a（chlorophyll-a）、溶解氧、总碱度、无机碳、二氧化碳分压（pCO₂）以及碳通量的空间与季节变异特征。极端雨季的河流洪水增强了河流输入影响，使羽流区的氮浓度升高，同时盐度、碱度与溶解氧水平均出现下降。在卡皮巴里贝河口羽流区，其碳通量均值已从旱季的-4.01 mmolC m⁻² d⁻¹转变为雨季的+5.7 mmolC m⁻² d⁻¹的正值通量。河口区域内，五日生化需氧量（BOD₅,₂₀）与溶解态磷浓度在旱季更高（p<0.0001），且与浮游植物水华呈正相关；卡皮巴里贝河口的叶绿素a浓度最高可达85 mg m⁻³。两处河口均呈现较高的总碱度，卡皮巴里贝河口旱季与雨季的均值分别为1808 µmol kg⁻¹与1373 µmol kg⁻¹，巴拉当雅加达河口则分别为1616 µmol kg⁻¹与1058 µmol kg⁻¹；该高碱度可对富营养化引发的海岸酸化过程起到缓冲作用。河流径流量的增加导致更多有机质与营养盐被输送至近岸海域，降低了水体溶解氧可用性，并使河口羽流区的代谢状态转变为异养型；而极端降雨事件带来的冲刷作用则改善了河口内部的水质。