Biogeography of spring phytoplankton communities from the Labrador Sea: drivers, trends, ecological traits and biogeochemical implications

Abstract

The Labrador Sea is a high latitude sea of the sub-Arctic region known to be an important oceanic sink for atmospheric CO2 due to intensive convective mixing during winter and the development of extensive phytoplankton blooms that occur during spring and summer. Therefore, a broad-scale investigation of the response of phytoplankton community composition to environmental forcing is essential for understanding planktonic food-web organization and biogeochemical functioning in the Labrador Sea. The aim of the research included in this thesis is to investigate the biogeographical and biochemical aspects of phytoplankton communities resulting from the contrasting hydrographical zones that divide the Labrador Sea into distinct ecological provinces. In Chapter 2, the phytoplankton community structure from near surface waters during spring and early summer (2011 to 2014) was investigated in detail, including species composition and environmental controls. This initial results demonstrated that the Labrador Sea spring and early summer blooms were composed of contrasting phytoplankton communities, for which taxonomic segregation appeared to be controlled by the physical and chemical characteristics of the dominant water masses. In Chapter 3, further work included an investigation of spring phytoplankton communities from surface waters of the Labrador Sea using pigment-based data and CHEMTAX analysis over ten-years (2005-2014). The photophysiological parameters (derived from photosynthesis-irradiance curves) and biochemical (particulate organic carbon to nitrogen ratio (POC:PON)) values differed among distinct phytoplankton communities. These results have provided a baseline of current distributions and an evaluation of the biogeochemical role of spring phytoplankton communities in the Labrador Sea, which will improve our understanding of potential long-term responses of phytoplankton communities in high-latitude oceans to a changing climate. In Chapter 4, potential indicator phytoplankton species of Atlantic and Arctic waters were investigated during spring in the Labrador Sea to identify possible functional traits driving biogeography. Future implications in trait biogeography and species distributions under a global warming scenario are discussed. In Chapter 5, a synthesis of the main findings of each result Chapter is included, in addition to schematic representation of the environmental controls on phytoplankton communities and species/classes succession from May to June in the Labrador Sea. Work that would increment our knowledge of phytoplankton from the Labrador Sea is suggested together with a final conclusion