Phytoplankton along the coastal shelf of an oligotrophic hypersaline environment in a semi-enclosed marginal sea: Qatar (Arabian Gulf)

•Qatari phytoplankton are highly diverse but low in biomass and productivity.•125 Species of diatoms, dinoflagellates and cyanobacteria were identified.•DIN:P and DIN:Si ratios<1.•Nutrient enrichment experiments revealed nitrogen limitation.•Oligotrophic, hypersaline semi-enclosed marginal sea under stress.

Phytoplankton biomass (chlorophyll a concentration), primary production, abundance, species diversity and species groupings were measured in the coastal waters surrounding Qatar (Arabian Gulf) at 13 stations in February 2010, July 2010, February 2011 and May 2011. In addition, a broad suite of physico-chemical characteristics were measured: temperature, salinity, pH, dissolved oxygen, turbidity, and nutrients (dissolved and particulate). Waters surrounding the Qatari peninsula were found to be highly diverse (125 species of diatoms, dinoflagellates and cyanobacteria were identified) but were low in both biomass (0.18–2.19 μg Chl a l−1) and productivity (0.14–0.97 mg C m−2 day−1). Phytoplankton physiology (Fv/Fm, σPSII, τQa, p) revealed acclimation strategies consistent with phytoplankton populations receiving ample light but insufficient nutrients. The finding of low primary production is consistent with water column nutrient ratios (DIN:P and DIN:Si ratios<1) and nutrient enrichment experiments in which the addition of nitrate or the addition of near-bottom waters stimulated biomass production of phytoplankton. This study in an oligotrophic, hypersaline semi-enclosed marginal sea is intended to contribute to the growing body of ecological information on this ecosystem functions.