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The Eastern Harbour of Alexandria, on the Egyptian Mediterranean coast, is characterized by environmental complications due to different types of anthropogenic stressors associated with water dynamics inside the harbor as well as the rapid water exchange with the open sea. These conditions caused chronic eutrophication conditions, with variable levels in the long term. The present study followed daily some physicochemical parameters, nutrients, and phytoplankton biomass, for a complete year. The results indicate coincidence on the short-time scale between the nutrients, phytoplankton biomass, pH, and dissolved oxygen. Spearman's correlation illustrated strong positive correlations between algal blooms and both pH and dissolved oxygen. The present study recorded twelve separate algal blooms, with an average of chlorophyll-a > 16.7 μg/L, confirming the continuity of high eutrophication in the Eastern Harbour. The seasonal Mann-Kendall tests showed that summer attained significant increasing trends for chlorophyll-a, silicate, nitrite, and nitrate, while winter has a significant decreasing trend for chlorophyll-a and pH.

The hydrography, nutrient levels and plankton abundance were investigated monthly for a complete annual cycle in the southwestern part of Abu Qir Bay, the most polluted and biologically productive area on the Egyptian Mediterranean coast. Intense temporal and spatial variability was observed in all measured parameters characteristic of the effects of several effluents discharged into the bay. Based on the present investigation, the southwestern Bay can be divided ecologically and biologically into two parts: one including the near shore strip, which is directly affected by the waste waters, and a second comprising the southwestern part of the coastal strip and the offshore stations, both of which are relatively far away from the land-based effluents.The Bay water was characterized by low transparency (monthly average: 64-280 cm), dissolved oxygen (monthly average 2.0-6.8 mg/l) and surface salinity (monthly average: 24.8-37.9 ppt), the highest limits usually being in the offshore section. Water fertility and plankton production were high in the Bay indicating an occasionally acute degree of eutrophication, particularly nearshore. Great variations occurred in the concentrations of nutrients throughout the year, with monthly averages of 0.8-50.88 mM for ammonia, 0.42-3.28 mM for nitrite, 1.29-17.36 mM for nitrate, 0.32-3.61 mM for reactive phosphate and 1.09-33.34 mM for reactive silicate. Similarly, the abundance of both phytoplankton and zooplankton showed pronounced temporal and spatial variability, whereas the monthly average chlorophyll-a fluctuated between 2.06 and 52.64 mg/l and zooplankton between 31x103 and 248.6x103 ind./m3. However, the absolute values of all parameters indicated remarkably wider ranges of variations.Significant correlation was found between chlorophyll-a and some ecological parameters like temperature, salinity, transparency, dissolved oxygen, nitrite and between zooplankton and temperature, while there was a significant correlation between nutrient concentrations and salinity changes.

Eutrophication as one of the importunate environmental hazards in the aquatic ecosystems causes pronounced deterioration of the water quality and represents serious threat to the biotic components of this ecosystem. The main environmental effects of eutrophication are increase of suspended particles owing to extensive macroalgal blooms, decrease of water clarity, and increase in the rate of precipitation that led to the destruction of benthic habitat by shading of submerged vegetation. In addition, other important effects are also known such as the bottom-water hypoxia, production of CO2 associating the decomposition of intensive produced organic matter which enhances water acidification, and altering biogeochemical processes, including sediment anoxia, accumulation of deleterious hydrogen sulfide, and nutrient cycling. Shift in the phytoplankton community was frequently reported in numerous eutrophic coastal waters owing to the variable nutrient requirement of different phytoplankton groups and the ratios between the different nutrients in these waters. Eutrophication is often accompanied by algal blooms which are frequently harmful and cause various injuries to the aquatic animals, such as clogging of fish gills, poisoning by toxins secretion, and localized anoxia, which consequently lead to detrimental effect on the fishing resources and the national economy through mass mortality of variable aquatic animals. The hypoxia conditions in bottom waters cause escape of sensitive demersal and other benthic fishes, mortality of bivalves, echinoderms and crustaceans, and extreme loss of benthic diversity, which led to changes in the diet of bottom-feeding fishes as well as shift in dominance among demersal fish species. Increase of algal growth/organic production rates led to changes in the benthic community structure, such as replacement of hermatypic corals with coralline algae, filamentous algae, macroalgae, and/or a variety of filter feeders and increase of bioerosion in some forms. Trophic linkages between pelagic and benthic communities are affected by eutrophication in the coastal waters, where the feeding habit of higher consumers such as benthic fish changes to derive high percentage of their energy from pelagic primary production sources. Shellfishes as an intermediate link between the water column and demersal fish could also be affected by eutrophication and will impact (as prey) on the demersal fish production. Meanwhile, increasing turbidity with eutrophication led to a shift in fish species owing to change of feeding on zooplankton to benthic organisms. Severe shading and light attenuation caused by blooms of both macroalgae and phytoplankton in eutrophic conditions hinder the photosynthetic processes in benthic plants and has led to the decline of seagrass habitats. High nutrient levels may lead to disturbance in nitrogen and phosphorus metabolism in seagrass and consequently cause a change in plant communities. Coral reefs are affected by eutrophication in different aspects. The organic compounds released from algal blooms promote microbial activity on coral surfaces and cause coral mortality, while synergistic effect of both the dissolved organic matter and rates of bioerosion has a pronounced role in reef degradation. Harmful algal blooms caused a complete loss of the branching corals, and substantial reductions in the abundance, richness, and trophic diversity of the associated coral reef fish communities. Eutrophication and siltation have severely stressed many fringing and offshore reefs that prefer to grow in nutrient-poor waters, and cause physiological changes in growth and skeletal strength, decrease of reproductive effort, and a reduced ability to withstand disease. In many marine eutrophic habitats, zooplankton community experienced a decline in species richness and abundance, change in structure, size, reproduction rate, and feeding habits. Size change in zooplankton occurs owing to the replacement of small species by another relatively large species of the same group, while the structure may change because of the trophic relationship of zooplankton with their prey (primary producers) and predators (fishes). Although benthic foraminiferans have been widely used as indicators of eutrophication in coastal marine ecosystems, low species diversity and high population densities of several benthic foraminiferans were reported in eutrophic area. On the other hand, smaller opportunistic benthic foraminiferal species dominate in the coral reef ecosystems and lead to a decline of larger endosymbiont-bearing taxa, while the hypoxia-tolerant foraminiferan species increased in abundance against the disappearance of the more sensitive species.