Water toxicity and fertility of a drain wastewater discharging wastewater directly to the
Damietta branch of the River Nile were seasonally investigated from winter to autumn,
2016. Nutrient enrichment bioassays to define the algal growth limiting nutrients, their
bioavailability and heavy metal toxicity of the sampled water, were carried out with the ISO
standard test alga Pseudokirchneriella subcapitata. The algal growth potential (AGP) test
exhibited a wide range of growth potential ranging from 0.4 mg.l-1 to 19.42 mg.l-1 algal dry
weight. At drain, mixing and recovery stations the algal growth was mainly limited by
heavy metals toxicity with inhibition percent at day 14 of growth (%I14) ranged between
27.9 and 58.6 while at the upstream reference station, %I14 ranged between 20 and 37.The
algal growth was also limited by N and P depending on site and season. In all seasons, but
only at drain station, the expected Chl. a was significantly higher than the observed Chl. a,
however, the reverse was held true for other sampling stations during this study. The
relationships between the chemically analyzed nutrients (P and N) and their bioavailable
concentrations were greatly affected by heavy metal toxicity. The results indicated that the
observed chlorophyll a showed significant correlations with TDP(r=0.82), while the
expected chlorophyll a correlated significantly with NH4-N(r=0.73). The algal growth
potential test proved a sensitive and reliable mean to assess water fertility and toxicity of
polluting drains and the receiving waters.