Remote sensing the phytoplankton seasonal succession of the Red Sea

Abstract

The Red Sea holds one of the most diverse marine ecosystems, primarily due to coral reefs. However, knowledge on large-scale phytoplankton dynamics is limited. Analysis of a 10-year high resolution Chlorophyll-a (Chl-a) dataset, along with remotely-sensed sea surface temperature and wind, provided a detailed description of the spatiotemporal seasonal succession of phytoplankton biomass in the Red Sea. Based on MODIS (Moderate-resolution Imaging Spectroradiometer) data, four distinct Red Sea provinces and seasons are suggested, covering the major patterns of surface phytoplankton production. The Red Sea Chl-a depicts a distinct seasonality with maximum concentrations seen during the winter time (attributed to vertical mixing in the north and wind-induced horizontal intrusion of nutrient-rich water in the south), and minimum concentrations during the summer (associated with strong seasonal stratification). The initiation of the seasonal succession occurs in autumn and lasts until early spring. However, weekly Chl-a seasonal succession data revealed that during the month of June, consistent anti-cyclonic eddies transfer nutrients and/or Chl-a to the open waters of the central Red Sea. This phenomenon occurs during the stratified nutrient depleted season, and thus could provide an important source of nutrients to the open waters. Remotely-sensed synoptic observations highlight that Chl-a does not increase regularly from north to south as previously thought. The Northern part of the Central Red Sea province appears to be the most oligotrophic area (opposed to southern and northern domains). This is likely due to the absence of strong mixing, which is apparent at the northern end of the Red Sea, and low nutrient intrusion in comparison with the southern end. Although the Red Sea is considered an oligotrophic sea, sporadic blooms occur that reach mesotrophic levels. The water temperature and the prevailing winds control the nutrient concentrations within the euphotic zone and enable the horizontal transportation of nutrients.

Figure 1. Bathymetry and general circulation of the Red Sea.

a) Bathymetry of the Red Sea. At the southern part the Red Sea connects with the Gulf of Aden via the straits of Bab-el-Mandeb. b) Schematic representation of the general circulation of the Red Sea (figure copied and updated from Johns et al. [34]). The circular features represent the main cyclonic and anti-cyclonic eddies at the Red Sea.

Figure 2. Phytoplankton biomass (Chlorophyll-a) in four provinces of the Red Sea.

a) MODIS Chl-a (mg/m³) annual composite of the during (2003-present), at the Red Sea Chl-a. The four provinces consecutively starting from the North to South are the NRS, NCRS, SCRS, SRS. b) Weekly climatology of MODIS-Aqua Chl-a in the four provinces.

Figure 3. Red Sea seasonal climatologies of MODIS Chl-a (mg/m³) and SST (°C).

The Local Area Coverage (LAC) dataset are presented for the period 2003–2011.

Figure 4. Red Sea seasonal climatologies of QuikSCAT wind speed (m/s), for the period 2000–2009.

Winter climatology is based on averages from October to April and summer climatology from May to September). The black arrows highlight the different wind direction between the two seasonal climatologies, whereas the red arrow indicates the intrusion of water masses through the facilitation of intensive northwards winds.

Figure 5. Vertical distribution of Chl-a profiles during October 2008 at the SRCS domain of the Red Sea.

The average of 35 stations/profiles is depicted for each depth, along with the minimum and maximum profile range. The horizontal dashed-lines represent the 1^st optical depth, the mixed layer depth and the euphotic depth.