In earlier times many coastal waters were monitored merely for the presence of pollutants and toxic substances. Nowadays, many countries, in addition, monitor their waters for the presence of harmful/toxic microalgae – aquatic micro plants. Eutrophication coupled with favourable climatic condition and certain biotic factors cause some microalgae to proliferate (microalgal bloom), and discolour water. Hence, such connotations as red tides, brown water and green water have been used to describe blooms events. Because some of the proliferating species cause environmental and human health problems, the phenomenon has often been referred-to as harmful algal bloom (HAB). In the course of human history, there are many reports connected with this phenomenon. A good example, and the oldest one, is that reported during the time of the Biblical Moses when red tide organisms infested and impacted the waters of the Nile (Exodus 7: 20-21). HAB incidences and the species that are involved are on the increase, and during the past three decades HAB events have been known to occur in nearly every part of the world. The effects of HABs are many, including anoxia, gill irritation, biota kills, water scums, etc. Some microalgal species produce toxic metabolites (phycotoxins) that can accumulate in grazers and filter feeders of microalgae (fish, shellfish, crustaceans, etc.). Such predators act as vectors of phycotoxins, which may intoxicate human consumers of seafood. When human beings consume seafood that has been tainted by phycotoxins, it may result to human phycotoxin poisonings: ciguatera fish poisoning (CFP), amnesic shellfish poisoning (ASP), diarrhoetic shellfish poisoning (DSP), paralytic shellfish poisoning (PSP), etc. Seafood harvested in phycotoxins-infested areas may cause epidemics in areas very remote from the source of importation. Thus, it is very necessary to monitor coastal waters for the timely detection of HAB species, and to ensure that only safe seafood products reach the market. But awareness about the HAB phenomenon seems to be lacking in the West African region. Within the framework of this thesis, the coastal waters of Nigeria were investigated for the presence of potentially harmful microalgal species (planktonic and benthic species). The level of awareness about the HAB phenomenon and about the human syndromes of phycotoxins poisoning was also evaluated among the coastal dwellers of Nigeria. The findings suggest that potential HAB/toxic species are present in Nigeria’s coastal waters. They also revealed that awareness about the HAB phenomenon and the associated impacts is lacking in the country. This is a clear example of the usefulness of HAB monitoring in coastal waters. DSP is one of the most notorious phycotoxin poisoning syndromes with a wide geographic spread. Laboratory studies that involved in vitro and in vivo experiments were, thus, designed and carried out to ascertain the effects of a DSP-causing dinoflagellate Prorocentrum lima on different aquatic biota, including the microalga Prorocentrum micans, a micro grazer Artemia, and juveniles of the European sea bass Dicentrarchus labrax. The dinoflagellate investigated here, P. lima, is very toxic. It negatively impacted the other aquatic biota: it eliminated P. micans in mixed cultures, as well as caused lethal effects on Artemia nauplii and the juveniles of D. labrax.