Analysis of the Factors Affecting Red Sea Phytoplankton Phenology

By Laura ThomsonDec 17 2021

The Red Sea basin is a biodiversity hotspot and small variations in climate change make a huge impact. The phytoplanktons of the area play a major role in the marine ecosystem and hence the phytoplankton phenology needs to be sustained. Kheireddine, M. et al. recently published a study detailing the use of a clustering technique to better comprehend the factors regulating the phytoplankton phenology.

Image Credit: Boris Stroujko/Shutterstock.com

The Red Sea, a marine biodiversity hotspot is enclosed by mangroves and coral reefs favoring the formation of specific ecological niches. The phytoplanktons present in this basin play a major role in the pelagic marine ecosystem's functioning. The phytoplankton cells also impact the production and vertical export of organic carbon—a major component of the carbon cycle influencing the atmospheric concentration of CO₂.

Recent research works have revealed that the Red Sea is a quickly warming region of the global ocean, which likely affects the dynamic of the phytoplankton, particularly its phenology. This would in turn impact marine food webs and the efficacy of the biological carbon pump. The changes in phytoplankton phenology might induce a mismatch between primary producers, grazers, and apex predators which affects the export of organic carbon aggregates.

As the Red Sea is a small, semi-enclosed basin and these alterations affect the whole ecosystem, there is an enormous interest in understanding phytoplankton dynamics, and the Red Sea is considered to be the natural laboratory as this oceanic basin has conditions occurring in various tropical areas of the global ocean.

Some studies attempted to examine the phytoplankton phenology in the Red Sea and one study pinpointed the use of ocean color remote sensing as a robust tool to analyze the water column dynamics and the phytoplankton phenology. The study utilized ocean color remote sensing findings of chlorophyll-a concentration (CHL) to analyze Red Sea phytoplankton phenology.

The current research reinvestigates the ocean color remote sensing dataset of CHL to offer information on phytoplankton phenology in various areas of the basin using a prior method which was proved efficient. The study pinpoints bio-regions depending on observed annual cycles of phytoplankton biomass and puts forth mechanisms enhancing the phytoplankton dynamic and its inter-annual variability.

Methodology

Ocean color measurements of CHL were obtained from the Ocean Colour Climate Change Initiative (OC-CCI). Satellite images from 1998–2018 were collected. The researchers started with the regionalization of the Red Sea basin utilizing eight-day climatological satellite composite images of CHL. Pixels with the same CHL annual cycle were grouped into clusters that are defined as distinct bio-regions.

The temporal gaps were filled by a linear interpolation method. A cluster center was outlined as the mean of all CHL annual cycles and some phytoplankton phenology metrics were obtained from the four cluster centers—bloom initiation, termination and duration, and its maximum intensity utilizing the threshold criterion.

Later, annual datasets of normalized CHL cycles were clusterized and each yearly annual cycle was normalized by its maximum value. The yearly clustering of the Red Sea was carried out only in the north of the Red Sea. The R scripts utilized to carry out these clustering analyses are made publicly available on GitHub.

The Mixed Layer Depth (MLD) information from the Mercator model and the monthly values of the multivariable El Niño Southern Oscillation (ENSO) Index (MEI) were downloaded.

Results and Discussion

The climatological clustering method pinpointed four diverse bio-regions in the Red Sea (see Figure and Table 1). The presence of these bio-regions indicates high variability in the annual cycle of phytoplankton biomass. The spatial distribution also indicates that the dynamics might be due to the basin circulation dynamic and/or the accessibility of nutrients.

Figure 1. Regionalization of the phytoplankton phenology in the Red Sea. (a) Spatial distribution of the four bio-regions obtained with a fuzzy c-means clustering method applied to a dataset of climatological annual cycles of normalized CHL. (b) Annual cycle of normalized CHL of each bio-region. Thick lines depict averages and thin lines indicate the 1σ intervals. Image Credit: Kheireddine, et al., 2021

Table 1. Main characteristics of each Bio-region presented in Figure 1. Source: Kheireddine, et al., 2021

Figure 2 illustrates the phytoplankton phenology between these bio-regions.

Figure 2. The chlorophyll-a seasonality (i.e., amplitude) (mg m⁻³) associated to a zoom on Bio-regions 3 and 4 (a), the annual mean chlorophyll-a concentration (mg m⁻³), the timing of the bloom maximum where the black dots correspond to the most frequent month where we observed the highest bloom intensity (month; y-axis) (c) and annual mean MLD (d). Image Credit: Kheireddine, et al., 2021

It was observed that Bio-regions 3 and 4 were predominant in the northern half of the Red Sea basin. The annual cycle associated with Bio-region 3 and 4 varied owing to the differences in air-sea heat fluxes and the dynamics of the basin. The phytoplankton seasonal cycle seen in Bio-region 3 was related to vertical mixing taking place in winter (see Figures 2d and 3).

Figure 3. For each bio-region, average ± standard deviation of the annual cycles of the mixed layer depth (MLD). The dashed dark gray line represents the yearly average value of the MLD. Image Credit: Kheireddine, et al., 2021

The Bio-regions 3 and 4 also showed a recurrent summer peak in CHL in June, which might be due to the presence of anti-cyclonic mesoscale eddies. The spatial distribution of the bio-regions was greatly erratic with three bio-regions (Bio-regions 1, 2, and 3) in the southern half of the Red Sea.

Even though Bio-region 3 was found in the southern part of the basin, the core was seen in the northern part (see Figure 4) and its presence in the southern areas may be due to mesoscale features.

Figure 4. Probability of bio-region membership. (a) Probabilities of membership of each pixel to its assigned bio-region. Trend lines on the right depict (b) the frequency of occurrence of each bio-region by latitude, and (c) the average probability of membership by latitude for each bio-region. Latitudinal bands of 2 were used. Image Credit: Kheireddine, et al., 2021

Bio-region 1 had a summer phytoplankton bloom from the end of May and peaked in July, collapsing in August. The bio-region 2 was characterized by a peak in CHL in summer (from the end of May and ceased in September) and in fall-winter (October–February), which might be the consequences of the summer southwest monsoon and the winter northeast monsoon, respectively. The shallow waters and the onshore coastal habitats of the southern Red Sea had higher CHL values compared to the northern bio-regions (see Figures 2a, 2b, and 5), suggesting that the northern half is more oligotrophic.

Figure 5. Climatological annual cycles vs. yearly annual cycles. For each bio-region, the climatological annual cycles chlorophyll-a concentration (thick lines) is compared with all 21 annual cycles available (thin lines, from 1998 to 2018). The two groups of years mentioned in “Inter-annual variability in the annual cycle of CHL” within the manuscript and Table 1 are highlighted, Group 1: 1998 to 2001 and 2012 to 2018; Group 2: 2002 to 2011. Image Credit: Kheireddine, et al., 2021

Analysis of the yearly annual cycles revealed two diverse groups of years (see Figure 5 and Table 2) that had distinct CHL annual cycles in Bio-regions 1 and 2 and only small variations in Bio-regions 3 and 4. These differences might be due to the lesser availability of the yearly CHL annual cycle in the Red Sea’s southern part.

Table 2. Years and MEI phase associated to Group 1 and Group 2 highlighted in Figure 5. Source: Kheireddine, et al., 2021

Earlier research has indicated that ENSO could influence the global climate and impact the phytoplankton phenology. Positive ENSO phases showed a deeper nutricline, an increase in stratification, and thus a decreased CHL concentration in the ocean.

The research focused on the spatiotemporal and inter-annual variability of the phytoplankton phenology in the basin’s northern part. For this, an annual clustering technique was employed (see Figure 6).

Figure 6. Annual clustering results from the northern part of the Red Sea. (left) Annual spatial distribution of bio-regions #3 and #4 from 1998 to 2018, and (right) the average map. Image Credit: Kheireddine, et al., 2021

Figure 7 illustrates the annual clustering results of Bioregions 3 and 4.

Figure 7. Annual clustering results for the northern half of the Red Sea (above 20°N). The average annual cycles of normalized chlorophyll-a concentration (thick lines) with all annual cycles (thin lines) for bio-regions #3 and #4. Image Credit: Kheireddine, et al., 2021

The annual cycles of CHL from 1998–2018 demonstrated that the Bio-region 3 showed greater variability in the phytoplankton phenology which might be due to the differences in surface circulation, sea surface temperature, and/or mesoscale activity.

The phytoplankton bloom seen majorly in the Bioregion 3 takes place when the mixing is deep enough to provide surface layers with nutrients from deep waters. Figure 8 illustrates the MLD of the northwestern Red Sea from 1998 to 2018.

Figure 8. Average ± standard deviation of the winter Mixed Layer Depth within the northwestern Red Sea from 1998 to 2018. The dashed dark gray line represents the average value of the MLD. Image Credit: Kheireddine, et al., 2021

The winter bloom observed in the central part of the Red Sea might be due to the convergence zone developed by opposite surface winds and also from the vertical mixing that distributed phytoplankton from the deep CHL maximum.

Moreover, earlier works revealed that physical-like mesoscale features also supply nutrients to the region which induces phytoplankton growth.

Conclusion

The research revealed the potential of employing ocean color data to analyze phytoplankton phenology. The observations offer a deeper understanding of phytoplankton phenology in the Red Sea. However, in situ measurements are also required for the complete comprehension of the mechanisms that bestow to phytoplankton seasonal cycle in the basin.

Journal Reference:

Kheireddine, M., Mayot, N., Ouhssain, M., Jones, B. H. (2021) Regionalization of the Red Sea Based on Phytoplankton Phenology: A Satellite Analysis. Journal of Geophysical Research: Oceans, 126(10), p. e2021JC017486. Available online: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021JC017486.

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