Human activity is increasing salt concentrations in freshwaters worldwide, but effects of freshwater salinity gradients on biogeochemical cycling are less understood than in saline, brackish, or marine environments. Using controlled microcosm experiments, we characterized (1) short-term (one to five days) biogeochemical responses and (2) water column metabolism along a freshwater salinity gradient of multiple salt types. After one day, microcosms were oxic (4.48–7.40 mg O₂ L⁻¹) but became hypoxic (1.20–3.31 mg L⁻¹) by day five. After one day in oxic conditions, microbial respiration in magnesium-, sodium-, and sea salt-based salinity treatments showed a subsidy-stress response, with respiration increasing by over 100% as salinity increased from 30 to 350–800 µS cm⁻¹. Conversely, respiration consistently increased along a calcium-based salinity gradient, peaking at 1500 µS cm⁻¹. By day five, an inverse subsidy-stress response was observed with elevated respiration at upper or lower ends of the gradient except for the magnesium treatment, which had the lowest respiration at the highest salinity. Calcium- and magnesium-based salinity treatments also caused considerable changes in phosphorus concentrations and C:P and N:P. In a separate experiment, microbial respiration and water column primary production also displayed subsidy-stress responses, but imbalances in effect sizes caused consistently declining net community production with increasing salinity. Collectively, our results establish that short-term exposure to different salt ion concentrations can enhance freshwater biogeochemical cycling at relatively low concentrations and alter resource stoichiometry. Furthermore, the nature of effects of freshwater salinization may also change with oxygen availability.

人类活动正在增加全世界淡水中的盐浓度，但与咸水、咸水或海洋环境相比，人们对淡水盐度梯度对生物地球化学循环的影响了解较少。通过受控微观实验，我们表征了（1）短期（一到五天）生物地球化学反应和（2）沿着多种盐类型的淡水盐度梯度的水柱代谢。一天后，微观世界处于含氧状态（4.48–7.40 mg O ₂ L ^-1 ），但到第五天时变得缺氧（1.20–3.31 mg L ^-1 ）。在含氧条件下一天后，基于镁、钠和海盐的盐度处理中的微生物呼吸表现出补贴应激反应，随着盐度从 30 增加到 350–800 µS cm ^-1 ，呼吸增加超过 100% 。相反，呼吸作用沿着基于钙的盐度梯度持续增加，在1500 µS cm ^-1处达到峰值。到第五天，观察到逆补贴应激反应，梯度上端或下端的呼吸增加，但镁处理除外，镁处理在最高盐度下呼吸最低。基于钙和镁的盐度处理也会引起磷浓度以及 C:P 和 N:P 的显着变化。在一项单独的实验中，微生物呼吸和水体初级生产也表现出补贴应激反应，但效应大小的不平衡导致净群落生产随着盐度的增加而持续下降。总的来说，我们的结果表明，短期暴露于不同的盐离子浓度可以在相对较低的浓度下增强淡水生物地球化学循环并改变资源化学计量。此外，淡水盐化影响的性质也可能随着氧气的可用性而改变。