Phenology and Environmental Control of Phytoplankton Blooms in the Kong Håkon VII Hav in the Southern Ocean

Abstract

Knowing the magnitude and timing of pelagic primary production is important for ecosystem and carbon sequestration studies, in addition to providing basic understanding of phytoplankton functioning. In this study we use data from an ecosystem cruise to Kong Håkon VII Hav, in the Atlantic sector of the Southern Ocean, in March 2019 and more than two decades of satellite-derived ocean color to study phytoplankton bloom phenology. During the cruise we observed phytoplankton blooms in different bloom phases. By correlating bloom phenology indices (i.e., bloom initiation and end) based on satellite remote sensing to the timing of changes in environmental conditions (i.e., sea ice, light, and mixed layer depth) we studied the environmental factors that seemingly drive phytoplankton blooms in the area. Our results show that blooms mainly take place in January and February, consistent with previous studies that include the area. Sea ice retreat controls the bloom initiation in particular along the coast and the western part of the study area, whereas bloom end is not primarily connected to sea ice advance. Light availability in general is not appearing to control the bloom termination, neither is nutrient availability based on the autumn cruise where we observed non-depleted macronutrient reservoirs in the surface. Instead, we surmise that zooplankton grazing plays a potentially large role to end the bloom, and thus controls its duration. The spatial correlation of the highest bloom magnitude with marked topographic features indicate that the interaction of ocean currents with sea floor topography enhances primary productivity in this area, probably by natural fertilization. Based on the bloom timing and magnitude patterns, we identified five different bloom regimes in the area. A more detailed understanding of the region will help to highlight areas with the highest relevance for the carbon cycle, the marine ecosystem and spatial management. With this gained understanding of bloom phenology, it will also be possible to study potential shifts in bloom timing and associated trophic mismatch caused by environmental changes.

First phytoplankton community assessment of the Kong Håkon VII Hav, Southern Ocean during austral autumn

“…2a), and Chl a data showing the highest biomass in the area (Fig. 3; Kauko et al, 2021). Most of the sampling areas were dominated by diatoms in terms of average abundances, most notably for the area represented by station 53 and Maud Rise (74 and 89 %, respectively), whereas at station 54 or Astrid Ridge the dominance was less pronounced (62 and 56 %), and the area along the 6° E transect was slightly dominated by flagellates (45 % flagellates compared to 36 % diatoms).…”

“…1). In addition, two stations were sampled in between the areas: station 53 at 68.1° S, 6.0° E and station 54 at 68.5° S, 8.3° E. Station 53, though geographically close to the 6° E transect, showed much higher biomass and a distinct bloom event (Kauko et al, 2021;Moreau et al, in prep. ) and was therefore considered separately.…”

“…Trials indicated that dividing the Maud Rise samples into depth clusters may bring further improvements but as the number of samples was relatively small (in total 12 CTD samples from Maud Rise) they were kept as one cluster. Astrid Ridge had a larger number of samples (55 in total) and was divided into two clusters (above and below including 40 m; average mixed layer depth (MLD) was 34 m, Kauko et al, 2021) and separated from the rest, which reduced the error. For the 6° E transect, separating the surface samples did not reduce the error.…”

“…Some of the data types and analyses indicated that the phytoplankton communities differed along the depth gradient, in addition to the spatial variability discussed in the next sections. Besides differences in biomass or that the phytoplankton communities above and below the MLD (the average for all the stations was 36 ±13 m, Kauko et al, 2021) differed to some degree. As species richness correlated positively with biomass (Fig.…”

“…Irigoien et al, 2005;Löder et al, 2011;Smetacek et al, 2004). The cruise observations and satellite chlorophyll a (Chl a) data have shown distinct phytoplankton phenologies in the region, such as between Astrid Ridge and Maud Rise (Kauko et al, 2021). Knowledge on the community composition complements our understanding of this regional variability.…”