Changing Occurrences of Fall Blooms Associated With Variations in Phytoplankton Size Structure in the Pacific Arctic

Hisatomo Waga,Toru Hirawake

doi:10.3389/fmars.2020.00209

Abstract

Phytoplankton blooms in the Pacific Arctic have been characterized as a huge single bloom in spring. However, several studies have reported recent increases in the occurrence of fall blooms during the period after dissipation of the spring bloom. Here, we explored spatiotemporal variations in the occurrence of fall blooms in the region, using satellite remote-sensing data for 2003–2017. Seasonal time-series variation in satellite-derived chlorophyll-a concentrations (chla) was modeled using a Gaussian function to distinguish whether a peak in chla was evident in fall; an occurrence of a fall bloom was recognized if the model detected the presence of a chla peak in fall. In addition, phytoplankton size structure was estimated from the satellite data to examine the influence of fall blooms on seasonal variations in the size structure. The results indicate that fall blooms occurred in a wide area of the Pacific Arctic, and larger phytoplankton were predominant during fall blooms relative to the phytoplankton present before/after the bloom or in the absence of a fall bloom. Examining interannual variation in occurrences of fall blooms revealed clear increasing and decreasing trends in the southern Chukchi Sea and the St. Lawrence Island polynya region, respectively, possibly associated with temporal variations in atmospheric forcing as well as in the water-column structure. Because the cell sizes of phytoplankton largely determine their sinking rate, temporal trends in the occurrence of fall blooms modulating seasonal variations in phytoplankton size structure could significantly influence marine ecosystems. These results suggest that spatiotemporal monitoring of phytoplankton communities not only in spring but also after the period or their dissipation would improve our understanding about processes causing variations within marine ecosystems, as might occur in the Pacific Arctic.

Highlights

The marine environment in the Arctic is sensitive to climate forcing
Our results depict the presence of fall blooms in a wide area of the Pacific Arctic (Figure 6c)
It is noteworthy that relatively scarce occurrences of fall blooms were found in the regions with higher climatological chla values, such as the Bering Strait, Norton Sound, and Kotzebue Sound (Figure 4a)

Summary

Introduction

The marine environment in the Arctic is sensitive to climate forcing. the Arctic is experiencing significant ocean warming of approximately three times the global average (Steele et al, 2008). Reductions in sea-ice cover are being further amplified by multiple factors: increased volume fluxes of warm Pacific water into the Arctic through the Bering Strait (Woodgate, 2018); warming of North Atlantic water that is transported to the Arctic through the Fram Strait (Comiso, 2012); and ice-albedo feedbacks heating the open ocean and melt ponds (Kashiwase et al, 2017) Because these key patterns cause reductions in the extent of sea ice, and early sea-ice retreat in spring and late sea-ice growth in fall, marine organisms in the Arctic are experiencing drastic changes in their surrounding environment owing to the recent sea-ice dynamics (Grebmeier, 2012). Several studies have reported recent occurrences of an evident fall bloom, when light availability is still relatively high and storm-driven mixing replenishes nutrients in the upper well-lit layer, which is accompanied by an increased proportion of larger phytoplankton such as diatoms (Nishino et al, 2015; Fujiwara et al, 2018)

Objectives

Methods

Results

Conclusion