Nutrient input drive macroalgal blooms and increases in photosynthetic activity in coastal ecosystems. An intense macroalgal photosynthetic activity can increase the surrounding pH and it could prevent the acidification that often follows an eutrophication process. We tested this hypothesis with field sampling and experiments in a macrotidal (up to 9 m in amplitude) coastal system within a semi-desert region with contrasting eutrophic conditions and Ulva lactuca blooms in the northern Argentinean Patagonia (San Antonio Bay). Our results indicate that daily pH variability during low tide could be controlled by the photosynthetic activity of Ulva lactuca under eutrophic conditions. At seasonal scale, the pH variations were related to environmental features, particularly seawater temperature. Both environmental (i.e. high solar radiation, negligible freshwater inputs and large tidal action) and anthropogenic nutrient inputs into the studied area promote the Ulva lactuca blooms, which in turn increases the surrounding pH in well oxygenated seawater through the intense photosynthetic activity. Our study shows that eutrophication instead of being a driver of acidification, could contribute to its prevention in well oxygenated marine coastal systems located within semi-desert regions.

营养输入驱动沿海藻类大量繁殖，并增加沿海生态系统中光合作用的活动。强烈的大型藻类光合作用活性可以提高周围的pH值，并且可以防止经常发生富营养化过程后的酸化。我们通过野外采样和在半沙漠地区的富潮条件下的巨潮（振幅高达9 m）沿海系统进行了试验，并进行了实验，富营养条件相反，阿根廷巴塔哥尼亚北部（圣安东尼奥湾）的Ulva lactuca开花。我们的结果表明，退潮期间日pH值的变化可以通过Ulva lactuca的光合活性来控制。在富营养化的条件下。在季节性尺度上，pH值的变化与环境特征有关，尤其是海水温度。进入研究区域的环境（即高太阳辐射，可忽略的淡水输入和大的潮汐作用）和人为养分输入都促进了Ulva lactuca的开花，从而通过强烈的光合作用提高了含氧量高的海水中的周围pH。我们的研究表明，富营养化而不是酸化的驱动因素，可能有助于其在半干旱地区内含氧量高的海洋沿海系统中的预防。