**4. Consequences of nitrogen and phosphorus eutrophication**

Eutrophication leads to excessive plant production, blooms of harmful algae, increased frequency of anoxic events, and death of fish. These conditions lead to health implications and economic losses, including losses of fish and wildlife production and losses of recreational amenities [38, 42].

#### **4.1 Ocean acidification**

Coastal hypoxia contributes to ocean acidification harming the calcifying organisms such as mollusks and crustaceans [4]. Anoxic and hypoxic water are associated with elevated carbon dioxide which causes acidification accelerating perturbation of ocean chemistry and influencing carbon dioxide emission into the atmosphere [4].

#### **4.2 Dead zones**

Hypoxia and anoxia lead to dead zones whereby fauna is eliminated or diversity and abundance is reduced. Dead zones in the coastal area have spread significantly since 1960s (**Figure 4**) and the increases are triggered by increases in primary production as a result of increased marine eutrophication fueled by riverine runoff of fertilizers and burning of fossil fuels [44]. The increased primary production lead to the accumulation of particulate organic matter which, accelerate microbial activity and consumption of dissolved oxygen in bottom waters resulting in death of fish and other marine fauna.

#### **4.3 Human/animal health**

Ranging from United States to Japan, the Black Sea and Chinese coastal waters increased nutrient loading in marine waters has been attributed to development of biomass blooms, which lead to toxic or harmful impacts on ecosystems, human health and/or recreation [45]. Approximately 60–80 species of about 400 known phytoplankton are toxin producing and capable of producing harmful algal blooms [46]. Toxin producing algae could cause mortalities of fish, birds, and marine

**Figure 4.** 

*Period of the explosive increase in coastal eutrophication in relation to global additions of anthropogenically fixed N from Boesch [43].* 

mammals as well as human illness through consumption of fisheries [47]. In humans, toxins arising from harmful algal blooms have mainly been reported from shellfish consumption [47] since bivalve shellfish (*Mollusca*) graze on algae and concentrate toxins effectively. In May and June 1998 the mortality of over 200 California sea lions (*Zalophus californianus*) and signs of neurological dysfunction in surviving sea lions along the central California coast was attributed to a harmful algal blooms [48].

#### **4.4 Tourism**

Coastal areas are hotspots for tourism, and are an important economic source for tourism [49]. The algal bloom resulting from N and P eutrophication have degraded the investment environment and damaged the tourism and hospitality industry (**Figure 3**) [50].
