Phytoplankton primary production in estuarine ecosystems is often concentrated in distinct zones where high nutrient loads coincide with optimal conditions for phytoplankton growth. These zones are called chlorophyll maxima and produce a disproportionate amount of carbon for estuarine food webs. These areas are zones of high ecological importance because they can be used as indicators of estuarine condition and function. The focus of our study, the Neuse River Estuary (NRE), has a well defined chlorophyll maxima during the spring-fall. The NRE is highly affected by human activities, in particular excess nutrient input from agricultural and urban areas. The results of this research will provide baseline information about how these areas behave under future nutrient management, hydrologic (e.g., droughts, storms) and altered food web scenarios.
Estuaries are among the most productive, yet highly impacted ecosystems on Earth. The chlorophyll maxima is an important location in many temperate estuaries worldwide, thus our result will be applicable beyond our study system, the Neuse River estuary. Further knowledge of the behavior of this important feature will enhance our ability to better manage highly impacted estuarine ecosystems. This project will provide diverse educational opportunities for graduate students who will develop independent research projects. The NSF-supported Summer Pre-graduate Research Experience Program (SPGRE) at UNC-CH and regional universities educating under-represented groups will provide students for summer internships. Finally, high school students and teachers will be employed as interns during the summer field season, giving them a range of newfound experiences and knowledge to bring back to the classroom.
Phytoplankton are important indicators of ecological change in the coastal zone, such as the degree of nutrient (nitrogen and phosphorus) loading experienced by a system. Many estuaries worldwide are experiencing harmful symptoms of eutrophication, or nutrient overloading, which may include excessive phytoplankton growth and low oxygen conditions among others. This process of nutrient overloading, eutrophication, is typically synonymous with nitrogen (N) loading because estuarine phytoplankton are generally N-limited. In addition to N availability, loss of important fish or shellfish or species invasions and the subsequent changes in zooplankton grazing pressure can also affect phytoplankton growth by possibly creating "windows of opportunity" for certain phytoplankton species to bloom. However the singular or interactive effects of nutrient loading and changes in food web dynamics on estuarine phytoplankton growth and community structure are not well understood. Likewise, the fate of the phytoplankton carbon is also poorly understood. In this study, we used field sampling and experimental approaches to determine the relative influence of bottom-up (N availability and form) and top-down (grazing) controls on phytoplankton growth and taxonomic composition at different times during the growing season (March, June, August) in North Carolina’s Neuse River Estuary. Results from this study show an overarching influence of N availability on phytoplankton growth in the Neuse River Estuary, at least during warm summer months. Only minor differences were observed in the estuarine phytoplankton response to various N forms, including nitrate and urea. Although the importance of N availability has been previously shown in this and many other temperate estuaries, what is surprising is the relative lack of grazing influence compared to N availability in the system. For example, experimental results firmly demonstrate that larger zooplankton (>153 µm) have a minimal effect on phytoplankton growth in the Neuse River Estuary. Furthermore, peak levels of phytoplankton biomass and mesozooplankton abundance were consistently separated from one another. This finding has important implications for not only phytoplankton bloom dynamics, but also for the fate of phytoplankton production, which apparently either passes through the microzooplankton or sinks to bottom waters where it may fuel low oxygen conditions. Two M.S. students were trained as part of this study. Three peer-reviewed manuscripts have been published or are in preparation, and project results have been presented at three national/international conferences.
