Phytoplankton and Primary Production in the Red Sea

Abstract

Studies on phytoplankton and primary production in the Red Sea are few and far between, and even in the few that have been conducted, most cover only a limited area. The last review of phytoplankton and primary production by Ismael (2015) reaffirmed the oligotrophic nature of the Red Sea and the north-to-south increasing trend in chlorophyll concentrations and rates of primary production. Also, in the above review the inventory of phytoplankton species was enlarged to 389 from the earlier record of 181 by Halim (1969). Since then, four research cruises undertaken in the Saudi Arabian waters of the Red Sea (2012–2015) have added a considerable amount of data on the patterns of primary production in the Red Sea and this review builds on that of Ismael (2015) by presenting the new findings. The levels of biomass and production in the Red Sea are relatively low, with a discernable north-south gradient. Their distribution is influenced by anticyclonic eddies, which entrain the nutrient-rich Gulf of Aden Intermediate Water across the Red Sea basin. Biomass and production in regions of eddy currents are twice as high as those elsewhere, suggesting that the notion that the Red Sea is oligotrophic needs to be revised. The injection of nutrients into the euphotic zone in the eddy boundary currents favours the proliferation of producers across a range of size classes rather than of a single class. As with any nutrient-poor tropical sea, the primary production in the Red Sea is supported up to 80% by nano- and picoplankton. Though the contributions of microplankton (diatoms and dinoflagellates) appear to be less significant, the phytoplankton diversity is quite high. With additional records of 74 species from the samples in the four cruises, the current inventory of phytoplankton stands at 463 species. The review also provides suggestions on prospective avenues of phytoplankton research in the Red Sea waters. These include extensive spatial and seasonal coverage of primary production, the importance of benthic production, a better evaluation of nitrogen (N) fixation by Trichodesmium spp., the role of allochthonous nutrient sources (such as dust) in increasing the productivity, additional inventories of phytoplankton species, especially those belonging to the nano- and picoplankton size classes, and the assessment of the importance of the heterotrophy and microbial loop in the food chain dynamics. Experimental studies on the physiology of phytoplankton that already live at extreme conditions of temperature and salinity in the Red Sea could also help to understand how phytoplankton in other seas would react to the effects of global warming and climate change.