False Bay Creek watershed water quality and Ulva abundance

The False Bay Creek watershed is the largest watershed on San Juan Island which drains into False Bay on the southwestern coast of the island. Much of the lower watershed is comprised of agriculture and cattle ranching some of which is managed by the San Juan County Land Bank. False Bay is a large open sand flat home to many spcies including infuanal invertebrates, fishes, crabs, and birds. A conspicuous feature of False Bay is the accumulation of Ulva sp. (sea lettus) on the upper shore during spring and summer. Over time these mats grow to considerable size, often over 20cm deep. Over the summer, these mats decompose producing a strong oder that is detecable from kilometers away. Some False Bay and Island residents claim that summer time Ulva abundance has increased over time and suggest that this is due to the upland agriculture. However, other observations from locals indicate little change in Ulva aubndnace and resulting odor. This monitoring program was devised to address these opposing views. Specifically, we wanted to see if nutrients from the creek were correlated with Ulva abundance. We identified 9 sampling location (later expanded to 11) through out the watershed at key locations. At each sampling location we collected water temperature, salinity, dissolved oxygen, and conductivity data from a handheld meter and water samples approximaly monthly from April 2021 to November 2022. All water samples were processed for total nitirgen and phospohorus (TNP) and a subset were also processed for nitrate, nitrite, ammonia, ortho-phosphate, and silica. From ~March to ~September the biomass and cover of Ulva sp. was also measured in a fixed area near the head of the bay. These were paired with orthomosaics captured by a unmanned aerial system ('drone'), to estimate Ulva sp. cover and biomass in the whole bay. A subset of water and Ulva sp. samples were processed for nitrogen and oxygen stable isotopes to examine the ultimate source of nitrogen in Ulva tissues.

假湾溪流域（False Bay Creek watershed）是圣胡安岛（San Juan Island）面积最大的流域，其最终汇入该岛西南海岸的假湾（False Bay）。流域下游大片区域为农业与肉牛牧场用地，其中部分地块由圣胡安县土地银行（San Juan County Land Bank）管理。假湾为一处大型开放式沙质滩涂，栖息着诸多物种，涵盖底栖无脊椎动物（infaunal invertebrates）、鱼类、蟹类与鸟类。假湾的典型特征之一是，春夏季期间，石莼属（Ulva sp.，俗称海生菜sea lettuce）会在潮间带上岸区域大量聚集。随着时间推移，这些藻垫可生长至可观规模，厚度常超过20厘米。夏季时段，这些藻垫会发生分解，散发出可在数公里外被感知的浓烈异味。部分假湾及圣胡安岛居民表示，夏季石莼属藻类的丰度逐年攀升，并推测这一现象与上游陆地农业活动有关。但当地居民的其他观测结果则显示，石莼属藻类的丰度及由此产生的异味并无显著变化。本监测项目正是为厘清这两种对立的观点而设立，核心目标为探究溪流携带的营养盐是否与石莼属藻类的丰度存在相关性。我们在流域内的关键点位共划定9个采样点，后扩增至11个。2021年4月至2022年11月期间，我们每月约采集一次水样，并使用手持仪表在每个采样点同步收集水温、盐度、溶解氧及电导率数据。所有水样均用于检测总氮与总磷（TNP），其中部分子样本还额外开展了硝酸盐、亚硝酸盐、氨氮、正磷酸盐及二氧化硅的检测。在每年约3月至9月期间，我们还在湾顶附近的固定区域内，测定了石莼属藻类的生物量与覆盖度。我们将该实测数据与无人机系统（unmanned aerial system，简称无人机drone）采集的正射镶嵌图（orthomosaics）相结合，以此估算整个海湾内石莼属藻类的覆盖度与生物量。此外，我们还选取了部分水样及石莼属藻类样本，开展氮与氧的稳定同位素（stable isotopes）检测，以分析石莼组织中氮元素的最终来源。