Tropical primary forests have been disappearing quickly to make use of the land for commercial purposes. Land-use change has an impact on downstream aquatic processes, but those impacts have mainly been studied in temperate climate regions. The present article reviews the impacts of various tropical land-use changes caused by human activities on downstream riverine and estuarine water properties and biogeochemical cycles, focusing especially on the behaviors of nitrogen (N) and phosphorus (P). Logging of tropical primary forests, subsequent establishment of pasture lands, and occasional wildfire or intentional burning have decreased terrestrial N fixation and increased the discharge of P combined with soils, which has lowered the N:P ratio of dissolved inorganic nutrients in the adjacent stream waters and downstream rivers. Agricultural fertilizers and aquacultural practices basically cause nutrient enrichment in downstream riverine and estuarine waters, changing the N:P ratio depending on the source. Finally, urbanization causes eutrophication in many tropical estuaries, where a halocline forms easily because of a warm temperature throughout the year and the water at the bottom of the estuary tends to become hypoxic or anoxic. Overall, the impact of land-use change on aquatic processes may be more serious in tropical regions than in temperate or cold climate regions because of (1) a higher biomass and nutrient stock in original tropical forests; (2) higher precipitation, more frequent episodic flooding, and warmer temperatures in tropical regions; and (3) certain practices that are rapidly expanding in tropical regions such as land-based aquaculture. Various land-use changes are causing downstream nutrient enrichment or disturbance of the nutrient balance at tropical land-sea interfaces, and the overall N:P ratios in the aquatic ecosystem seem to be declining. Nonetheless, if proper management is conducted and the discharge of nutrients and soils ceases, tropical aquatic systems may have the potential to recover faster than those in other climate regions because of their abundant precipitation and warm temperature. Long-term monitoring and more attention to elemental stoichiometry are important areas for future research.

中文翻译：

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热带土地利用变化对下游河流和河口水特性和生物地球化学循环的影响：综述

为了将这片土地用于商业目的，热带原始森林正在迅速消失。土地利用变化对下游水生过程有影响，但这些影响主要在温带气候地区进行了研究。本文回顾了人类活动引起的各种热带土地利用变化对下游河流和河口水特性和生物地球化学循环的影响，特别关注氮 (N) 和磷 (P) 的行为。热带原始森林的砍伐、随后的牧场建设以及偶尔的野火或故意燃烧降低了陆地固氮量，增加了与土壤结合的磷的排放，从而降低了邻近河流水中溶解无机养分的氮磷比和下游河流。农业肥料和水产养殖方法基本上会导致下游河流和河口水域的养分富集，根据来源改变氮磷比。最后，城市化导致许多热带河口富营养化，由于全年温度升高，很容易形成盐酸盐，河口底部的水容易缺氧或缺氧。总体而言，热带地区土地利用变化对水生过程的影响可能比温带或寒冷气候地区更严重，因为（1）原始热带森林的生物量和养分存量更高；(2) 热带地区降水量增加、间歇性洪水更频繁、气温升高；(3) 某些在热带地区迅速扩大的做法，例如陆基水产养殖。各种土地利用变化导致下游热带海陆界面养分富集或营养平衡紊乱，水生生态系统中的整体氮磷比似乎正在下降。尽管如此，如果进行适当的管理并停止排放养分和土壤，热带水生系统有可能比其他气候区的水生系统恢复得更快，因为它们降水充沛，气温温暖。长期监测和对元素化学计量的更多关注是未来研究的重要领域。如果进行适当的管理并停止排放养分和土壤，热带水生系统有可能比其他气候区的水生系统恢复得更快，因为它们降水充沛，温度高。长期监测和对元素化学计量的更多关注是未来研究的重要领域。如果进行适当的管理并停止排放养分和土壤，热带水生系统有可能比其他气候区的水生系统恢复得更快，因为它们降水充沛，温度高。长期监测和对元素化学计量的更多关注是未来研究的重要领域。