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.