**1. Introduction**

In the past few decades there have been massive increase in marine eutrophication globally [1]. The major drivers of marine eutrophication are nitrogen (N) and phosphorus (P) [2]. Eutrophication leads to hypoxia and anoxia, reduced water quality, alteration of food web structure, habitat degradation, loss of biodiversity and noxious and harmful algal blooms [1, 3]. In addition, coastal hypoxia contributes to ocean acidification harming the calcifying organisms for example mollusks and crustaceans [4].

 Nitrogen and P are required to support aquatic plant growth and have been reported as the key limiting nutrients in most aquatic ecosystems. Further, N is needed for protein synthesis while as P is required for DNA, RNA and energy transfers [5]. Marine ecosystem heavily loaded with nutrients can display N limitation, P limitation and co-limitation [6] the limiting nutrient could change both seasonally and spatially [7].

 A number of factors make N more limiting in the marine ecosystem than in fresh water ecosystem with two primary factors being (1) desorption of P bound to clay as salinity increase and (2) reduced/lack of planktonic N fixation as a result of increased salinity, resulting in flux of relatively P rich N poor marine water [4].

 Increased N and P fertilizer and manure application in agricultural production have significantly improved crop yields and food security for the increasing human population, however fertilizer application on farms has led to serious problems with aquatic eutrophication (**Figure 1**) [1, 9]. As a result N and P in fertilizer and manure enter freshwater systems and are transported by streams and rivers to coastal areas resulting in eutrophication of coastal and marine ecosystems globally (**Figure 2**) [10–12]. In addition, atmospheric deposition of N from fossil fuel combustion contributes to the global budget for reactive N and is the largest single source of nitrogen pollution in some regions (**Table 1**) [1]. The chapter addresses the forms of N and P, and their sources. Consequences of eutrophication and mitigation strategies as well as some of the challenges faced during the mitigation process.

**Figure 1.** 

*Period in which the symptoms of eutrophication and hypoxia/anoxia began in developed countries and the more recent evolution of these symptoms in developing countries, modified from Schlesinger [8].* 

#### **Figure 2.**

*Average annual nitrogen export per area of watershed from large regions around the North Atlantic Ocean to the coastal ocean as a function of net anthropogenic nitrogen inputs to the landscape per area. Modified from Howarth [1].* 


*Nitrogen and Phosphorus Eutrophication in Marine Ecosystems DOI: http://dx.doi.org/10.5772/intechopen.81869* 

#### **Table 1.**

*Budgets indicating reactive nitrogen from human sources in United States of America (Tg N per year). Net anthropogenic N inputs indicate use of inorganic fertilizer plus N fixation in agricultural systems plus NOx deposition from fossil fuel combustion minus the net export of Nx in food and feeds. Modified from Howarth et al. [55].* 

#### **2. Forms of nitrogen and phosphorus**

 Most of the N on earth is molecular dinitrogen (N2) and most of it is in the atmosphere, however, a portion of it is dissolved in the ocean [1]. Only ~0.002% of N on earth is present in living tissues and detrital organic matter [8]. Nitrogen is essential for life; however, biologically available forms such as nitrate, nitrite and ammonium are a small proportion of N on earth and as a result, N limits primary productivity in coastal marine ecosystems [13].

Soil P exists in a range of organic and inorganic compounds that differ remarkably in their biological availability in the soil environment [14]. The inorganic P compounds preferentially couple with crystalline and amorphous forms of Al, Fe, and Ca [15] the coupling is highly influenced by soil pH [16]. Organic P in most soils is dominated by a mixture of phosphate diesters (mainly nucleic acids and phospholipids) and phosphate monoesters (example; mononucleotides, inositol phosphates) with smaller amounts of phosphonates (compounds with a direct carbon–phosphorus bond) and organic polyphosphates (for example; adenosine triphosphate) [17]. Plants have the capacity to manipulate their acquisition of P from organic compounds through various mechanisms, some of which allow plants to utilize organic P as efficiently as inorganic phosphate [18]. Alkaline pH can alter the availability of P binding sites on ferric complexes as a result of competition between hydroxyl ions and bound phosphate ions [19]. Anaerobic conditions favor release of P as a result of reduction of ferric to ferrous iron [20]. While, the presence of sulfate could lead to reaction of ferric iron with sulfate and sulfide to form ferrous iron and iron sulfide leading to release of P [21]. Temperature increase can reduce adsorption of P by mineral complexes in the sediment [22]. Other physiochemical processes affecting release of P from the sediment include pH potential, redox, reservoir hydrology and environmental conditions [23]. These physiochemical processes could further be complicated by the influence of biological processes such as mineralization, leading to a complex system governing the release of P across sediment water interface [23].

Unlike N fertilizer, P fertilizer is not volatile, consequently very little P could be distributed from cropland to nearby terrestrial ecosystem [24]. However, excessive

#### **Figure 3.**

*Summer mean benthic chlorophyll concentrations from streams worldwide as a function of summer mean concentrations of total P and total N in the water column. Modified from Dodds and Smith [31].* 

P fertilizer application could result in significant transfer of P to adjacent freshwater bodies, followed by transport to coastal waters [25]. The nitrogen cycle contains diverse gaseous forms, both dissolved and particulate forms of N, while P cycle is dominated by particulate and non-gaseous forms of P [26]. This means that N pool can exchange with and escape to the atmosphere but P is trapped in receiving marine waters. The processes controlling losses of N to the atmosphere include ammonification, denitrification, nitrous and nitric oxide production and products of anaerobic ammonium oxidation (or anammox) reaction, while N fixation represents a gain from the atmosphere [27, 28]. Nitrification and denitrification are regulated by oxygen concentration and potentially can produce nitrous oxide, a climate relevant atmospheric trace gas [29]. However, there are no analogous air-water exchanges that exist in the P cycle. Therefore, while the net effect of the microbially mediated dissolved gaseous fluxes on N is loss of N to the atmosphere, P remains in the system, either as dissolved or as particulate forms [26]. While many efforts have focused on P mitigation, less attention has been given to N mitigation [30]. However, it is clear that N and P together describe eutrophication better than either can alone (**Figure 3**).

#### **3. Sources of nitrogen and phosphorus**

 A century ago, the world reactive N was derived mainly through microorganisms fixation. This is the natural N fixation from the atmosphere. Currently, most of reactive N is derived from anthropogenic activities, mainly synthetic N fertilizers, manure application and fossil fuel combustion [32–36]. In addition, anthropogenic activities have accelerated biological N fixation associated with agriculture [33]. It is estimated that globally deposition of reactive N is ~25–33 Tg N per year from fossil fuel combustion, ~118 Tg N per year from fertilizer, and ~65 Tg N per year from fixation of atmospheric N2 by cultivated leguminous crops and rice. Only ~22% of total human input on N ends up accumulating in soils and biomass, whereas ~35% enter oceans through atmospheric deposition (17%) and leaching through river runoff (18%) [25]. However, the only source of atmospheric P deposition is through mineral *Nitrogen and Phosphorus Eutrophication in Marine Ecosystems DOI: http://dx.doi.org/10.5772/intechopen.81869* 

aerosols and the global flux is estimated at 3–4 Tg P per year [25]. Agricultural and urbanization activities are the major drivers of N pollution in the coastal waters [13]. The green revolution has led to synthetic N fertilizers, creating reactive N at a rate four times greater than fossil fuel combustion [34, 36]. Dinitrogen fixation by planktonic cyanobacteria is less likely in coastal seas compared to lakes, due to high salinity, whereby coastal planktonic N2 fixation has not been observed at salinities higher than 8–10 and normally ocean salinity is ~35 [5].

Phosphorus sources can be natural which includes indigenous soil P, atmospheric deposition and anthropogenic P [37]. Phosphorus sources include both point and non-point sources [38]. Excess phosphorus inputs to lakes/rivers, which are eventually translocated to the marine ecosystem, usually come from industrial discharges, construction sites, urban areas, sewage and runoff from agriculture [39]. Many countries have implemented mechanisms to control point source P, however, controlling non-point P sources especially agricultural sources remains a challenge [38, 40]. The major source of nonpoint P input to water bodies is the excessive application of fertilizer or manure on farms which cause P accumulation in soils [40]. It should be noted that crop and livestock production systems are the major cause of human alteration of the global N and P cycles [41].
