Effects of some environmental factors on the population density and species diversity of phytoplankton in Bitter Lakes, Egypt

Abstract

The assessment of the phytoplankton community structure in the Bitter Lakes during 2012–2013 indicated about 150 species belonging to 73 genera. These are classified as 89 species of diatoms, 27 dinoflagellates, 19 chlorophytes, 13 cyanophytes, and one of euglenophytes as well as one rare species of silicoflagellates. Diatoms were the leading and most dominant group forming about 81.67 % of the total number of individuals due to the high flourishing of Rhizosolenia alata variety gracillima, Thalassionema nitzschioides, Skeletonema costatum, Thalassiothrix frauenfeldii, and Chaetoceros lorenzianus. Abu-Rommanh was the most productive area in the phytoplankton counts, whereas the area of Shandoura was the lowest productive one, coincided principally with the types and amounts of pollutants as well as the highest human and fishing activities at this area of the Bitter Lakes. Seasonally, summer 2012 was the most productive season with an average of 8464 unit/L followed by autumn 2012 with an average of 6199 unit/L. Whereas, the winter and spring of 2013 were observed with approximately similar averages of 5527 and 5194 unit/L, respectively. Analysis of diversity indicated that the Bitter Lakes sustained good variations of phytoplankton species. It fluctuated between the averages of 2.4 in the spring of 2013 and 3.21 during the autumn of 2012, which may reflect the healthy state and ecosystem stability of Bitter Lakes during autumn. The results revealed that the total density of phytoplankton slightly increased from the average counts of 6286 unit/L during 2002–2003 to 6346 unit/L in the present study, while the number of species significantly increased from 108 species in 2002–2003 to 150 species in the present study. This may be due to the relative increase in the eutrophication state of the Bitter Lakes during the last 10 years. Generally, the seasonal fluctuations of phytoplankton positively correlated with the water temperature (22–24.2 °C), dissolved oxygen (4.85–11.11 mg/L), and reactive nitrate (0.04–1.00 mg/L), but it inversely correlated with the ammonium (0.05–0.2 mg/L) and reactive silicate (0.10–1.44 mg/L). The stepwise multiple regressions indicated that the reactive silicate, nitrate, and ammonium were the most effective factors that controlled the seasonal fluctuations and species diversity of phytoplankton in the Bitter Lakes during 2012–2013.