Mohamed Moussa Dorgham

Effects of Eutrophication

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.

The phytoplankton communities in two eutrophic areas on the Alexandria coast, Egypt

The phytoplankton community was studied in historically and ecologically important area on the southeastern Mediterranean coast at Alexandria, Egypt. Monthly...

Diversity and ecology of crustaceans from shallow rocky habitats along the Mediterranean coast of Egypt

Spatio-temporal patterns of the distribution of crustaceans from shallow hard bottoms along the Alexandria coast (Egypt, Mediterranean Sea) were studied during a complete year cycle and also in relation to potential drivers of change (both biotic and abiotic), including variation in habitat-forming species. Overall, the crustacean assemblages appeared poor, including only 14 species belonging to Amphipoda (five species), Isopoda (five species), Tanaidacea (two species), Cirripedia and Decapoda (one species each). The distribution patterns of crustacean assemblages appeared significantly variable both in the spatial and in the temporal dimension on a rather unpredictable basis, albeit variation was related to changes in dominant algal and invertebrate habitat formers. High variability and low species richness observed suggest that the analyzed assemblages are selected by local unfavorable environmental conditions. In fact, the crustacean hard bottom fauna is composed by a bulk of tolerant forms, including the dominant Tanais dulongi, Apohyale perieri, Dynamene bidentata, Sphaeroma serratum, Elasmopus pectenicrus, and Jassa marmorata. Their spatio-temporal dynamics, as well as those of the remaining species, and correlations with the variation of habitat formers and environmental variables are reported. This is a baseline assessment of the crustacean diversity along the Mediterranean coast of Egypt, thus having paramount importance for understanding the predicted future changes of biodiversity for the area.