UC Santa Barbara

Human development of watersheds has fundamentally altered nutrient (nitrogen and phosphorus) and sediment regimes in freshwater and marine ecosystems. Watersheds located on steep tropical islands can be particularly susceptible to increases in nutrient and sediment runoff following the replacement of native forest by agriculture, roadways, and buildings. Ultimately, much of this elevated runoff is deposited in the nearshore environment. On tropical islands the nearshore is often characterized by coral reefs. Generally, coral reefs tend to thrive in clear, low-nutrient waters. As such, nutrient enrichment and sediment runoff can have numerous deleterious impacts on the physiology and life cycle of corals and other reef organisms.In Chapter 1, I test the impact of watershed development on nutrient and total suspended solid (TSS) concentrations on the island of Moorea, French Polynesia. To do this, I collected water samples during rainfall events in the rainy and dry seasons in eleven watersheds on the island. Water samples were analyzed for dissolved inorganic nitrogen (DIN = nitrate + nitrite + ammonium) and phosphate concentrations and total suspended solids (TSS). I quantified land use in watersheds by analyzing satellite imagery for the percent of each watershed that is cleared of forest for agriculture or development, and used census data to determine the population of each watershed. PCA and regression analyses of these data indicate strong positive relationships between how much of a watershed is cleared and the nitrogen levels of the rivers, as well as a positive relationship between the amount of sediment in the river and both how much the watershed is cleared and the population of the watershed. Phosphate concentrations were not related directly to watershed development, but positively related to TSS. I then used DIN and TSS con-centration thresholds established in similar tropical island ecosystems to assess watersheds on Moorea for potential concerns to human health and ecosystem function. I found that DIN and TSS concentrations in Moorea’s rivers regularly exceed these thresholds. In Chapter 2, I studied the impacts of land use on nearshore fringing reefs around Moorea. I used a combination of remote sensing, river water sampling, and reef photo transects to quantify land use, river nutrient and sediment concentrations, and reef community structure across a gradient of land use intensity. I found that the percentage of cleared land in a watershed has a negative relationship with coral cover on adjacent fringing reefs. Through ordination analysis I also revealed that land clearing, human population, and river nutrient and sediment concentrations are all correlated with the heterogeneity of community composition on fringing reefs. Reefs situated near more developed watersheds were exposed to higher nutrient and sediment runoff and were more homogenous than those with less anthropogenic stress even hundreds of meters from river outflows. In Chapter 3, I present a new research tool for water chemistry research. Water chemistry conditions in freshwater and marine environments can change rapidly over both space and time. This is especially true in environments that are exposed to anthropogenic impacts such as sedimentation, sewage, runoff and other types of pollution. Through the fieldwork and analyses for Chapters 2 and 3 the challenges associated with accurately collecting water samples across relevant spatial and temporal scales became abundantly clear. In response, I developed an inexpensive, open-source Programmable Autonomous Water Sampler (PAWS) that is compact, robust, highly adaptable and submersible to 40 me-ters. PAWS utilizes a time-integrated sampling approach by collecting a single sample in a syringe slowly over hours to days. Once analyzed, data from the sample collected represents an integrated average of water chemistry conditions over time. Due to its adaptability and low cost, PAWS has the potential to improve the spatial and temporal coverage of many freshwater and marine studies.