Nutrients / Chemistry
Water chemistry and nutrients are another important component of our project. Zabe Premo (PhD student, USM) explain some of the sampling we do aboard the R/V Point Sur as part of the Sargassum project.
As part of the Sargassum study, water samples are collected at surface (within the Sargassum feature itself), at 1.0-meter depth, and at chlorophyll maximum depth. Surface samples are obtained using a beta-sampler, while the two other depths are collected using niskins affixed to the CTD unit deployed from the vessel. Water is filtered in the field shortly after collection using a glass syringe and filter to remove large particles and detritus in the water sample. Filtered water is then immediately frozen onboard to ensure nutrients dissolved within the water do no volatilize or decompose before they may be analyzed. Upon return to the laboratory, the samples are analyzed for nitrate, nitrite, ammonia, phosphate, and dissolved carbon. As this study aims to evaluate the foodweb structure associated with Sargassum features, it is essential to understand primary production and nutrient availability within the system. In open water locations of the Gulf of Mexico, large portions of the water column are often nutrient-limited. This gives the water the
distinctive cobalt blue hue as there is minimal primary production (phytoplankton) in many areas. Macronutrients such as nitrogen, phosphorous, and carbon in the open water are derived from biogenic pathways where chemical species are utilized and become bioavailable to sustain discrete ecosystems such as those found surrounding Sargassum features. By sampling the water for these compounds, we seek to better understand nutrient cycling within the Sargassum features and to compare these processes with those found in open water without Sargassum present and at the chlorophyll maximum layer. The chlorophyll maximum is a subsurface layer of high primary production usually associated with the thermocline of the water column and the depth at which sufficient sunlight penetrates to support photosynthetic processes.