Twenty-One Years of Phytoplankton Bloom Phenology in the Barents, Norwegian, and North Seas

Abstract

Phytoplankton blooms provide biomass to the marine trophic web, contribute to the carbon removal from the atmosphere and can be deadly when associated with harmful species. This points to the need to understand the phenology of the blooms in the Barents, Norwegian, and North seas. We use satellite chlorophyll-a from 2000 to 2020 to assess robust climatological and the interannual trends of spring and summer blooms onset, peak day, duration and intensity. Further, we also correlate the interannual variability of the blooms with mixed layer depth (MLD), sea surface temperature (SST), wind speed and suspended particulate matter (SPM) retrieved from models and remote sensing. The climatological spring blooms start on March 10th and end on June 19th. The climatological summer blooms begin on July 13th and end on September 17th. In the Barents Sea, years of shallower mixed layer (ML) driven by both calm waters and higher freshwaters input keeps the phytoplankton in the euphotic zone, causing the spring bloom to start earlier and reach higher biomass but end sooner due to the lack of nutrients upwelling from the deep. In the Norwegian Sea, a correlation between SST and the spring blooms is found. Here, warmer waters are correlated to earlier and stronger blooms in most regions but with later and weaker blooms in the eastern Norwegian Sea. In the North Sea, years of shallower ML reduces the phytoplankton sinking below the euphotic zone and limits the SPM increase from the bed shear stress, creating an ideal environment of stratified and clear waters to develop stronger spring blooms. Last, the summer blooms onset, peak day and duration have been rapidly delaying at a rate of 1.25-day year–1, but with inconclusive causes based on the parameters assessed in this study.

Changes in phytoplankton community structure over a century in relation to environmental factors

“…In inner Oslofjorden, the reduced diatom growth in early March may be caused by a combined effect of increased SST during winter and phosphate limited growth of diatoms during early spring, but the driving factor behind this can be another one than those that were analyzed here. Increased SST during spring was correlated to decreased mixed layer depth in the open parts of the North Sea, but not in the coastal areas ( Silva et al. , 2021 ).…”

Threshold indicators of primary production in the north-east Atlantic for assessing environmental disturbances using 21 years of satellite ocean colour

“…Similarly, in the greater North Sea, the spring bloom has been observed to be occurring earlier with a one month difference between 1998 and 2020 and longer duration, which is partially explained by rising seawater temperatures (Alvera-Azcarate et al, 2021). In addition, summer blooms have been observed to start and end later, and to be longer in duration with higher Chl a concentrations (Silva et al, 2021). Using 21 years of merged ocean colour data, we identified that the highest PP in the NE Atlantic and North Sea occurs between May and July (Fig.…”

“…4). By comparison, (Silva et al, 2021) 2020) characterised a double peak in Chl a for the Iceland Basin, with a maximum in April followed by a secondary peak in September. By contrast, Taboada and Anadon ( 2014) observed a single spring-summer peak from merged SeaWiFS-MODIS-Aqua data (with the GSM Chl a algorithm) for the NE Atlantic similar to the climatology that we observe in PP for this region (Fig.…”

“…Distinctive phenological properties derived from remote sensing ocean colour data have been used to assess the impacts of climate change on phytoplankton bloom development in coastal, shelf and oceanic environments (Krug et al, 2018). Recent studies on phenology in the North Sea, based on OC-CCI time series data from 2000 to 2020, have shown that the spring bloom now starts earlier, and the duration is longer (Silva et al, 2021). This has been related to shallower mixed layer depths limiting the resuspension of suspended material to the surface which results in more stratified, clearer waters, which in turn form higher biomass blooms.…”

Trophic niches of macrobenthos: Latitudinal variation indicates climate change impact on ecosystem functioning

“…On the one hand, prolonged stratification is a major driver of the above-described prolonged primary production duration (Silva et al, 2021). On the other hand, stratification can strongly reduce production once nutrients are depleted and the entire production is based on recycled nutrients (Silva et al, 2021).…”