Winter Algal Blooms in Abu Dhabi Waters: Definition, Mechanisms and Link to Eutrophication

The frequency, magnitude and distribution of harmful algal blooms (HAB) have increased in Arabian Gulf countries over recent decades. Coastal environments are being degraded by rapid economic growth, resulting in eutrophication and subsequent HABs. The density of harmful algal species tends to increase in winter (December – February) producing significant blooms during the past several years in Abu Dhabi waters. Winter blooms were mostly dominated by dinoflagellates particularly the Prorocentrum species, except in 2015. Prorocentrum triestinum was first seen in the Abu Dhabi waters in 2011 when it dominated with concentrations between 8 x10⁵ cells/L and 2.2 x10⁷ cells/L. Prorocentrum minimum was found in concentrations from 2.5 x10⁴cells/L to 2.63 x10⁷cells/L. and dominated during 2012 and 2014. The 2013 bloom was a mixture of Oscillatoria sp and P.minimum, whereas the 2015 bloom was dominated by Heterocapsa triquetra, with cell concentrations between 4.5 x10⁵ cells/L and 1 x10⁷ cells/L. In Abu Dhabi waters the frequency and intensity of HABs has increased annually since 2002, corresponding to increases in nutrient concentrations. Bloom frequency appears to be linked to nitrate and phosphate concentrations, whereas bloom intensity appears to depend upon other site-specific factors. Bloom intensity in the Mussafah channels was lower in early 2016. Variations in nutrient concentrations and the relationship between the nitrate-phosphate ratio and HAB intensity were studied in detail.