DETERMINATION OF THE HISTORICAL TREND OF CARBON AND NITROGEN IN CROSS RIVER ESTUARY

CHAPTER ONE
INTRODUCTION
The chemistry of estuaries is dependent on a combination of tidal pulses, riverine flow, hydrodynamic and autochthonous biological processes (Allanson and Winter, 1999). Coastal ecosystems are places of intense biogeochemical activity where both continental (through freshwater runoff) and oceanic inputs (through upwelling and tidal mixing) contribute to high biological productivity and their important role in the global cycle of carbon, nitrogen and phosphorus (Gattuso et al., 1998). Studying the fate of terrestrial organic matter in these environments is therefore of considerable importance because carbons are buried in coastal sediments, exported to the open ocean or enters coastal food webs. Cole et al., (2007) estimated worldwide that over 0.9 Pg C is annually exported by the rivers to the oceans.
Carbon and nitrogen are the most important elements that sustain life on earth, they are found in the atmosphere, biosphere lithosphere and hydrosphere. Carbon and nitrogen are very essential to all life processes and they play an important part for the flow of energy through aquatic food chain and terrestrial environment (Wetzel 1992) and are the primary component of organic matter in which these organic matter comes majorly from the terrestrial environment, thereby contributing to the abundance of carbon and nitrogen to the aquatic ecosystem. For instance carbon-dioxide from the atmosphere is converted into organic carbon by plant through the process of photosynthesis and it is stored in plants upon the death and decay of these plants and even the animals that feeds on them, organic matter are being released into the terrestrial environment. These organic matters are the major constituent of carbon and nitrogen and some other elements such as phosphorous, hydrogen, silicon etc (Wetzel 1992).
Organic matter in the marine sediment adjacent to coastal plain is mainly dominated by the terrestrial system transported to the estuaries primarily by rivers; the rivers receive, produce, and discharge significant amounts of organic materials. They further integrate into different habitat reaching from alpine regions to coastal and oceanic realms. Soils from different land types are the main source of riverine organic matter, contributing >90% of the buried organic carbon in the marine sediments occurring in “terrigenous-deltaic” regions near river mouth (Berner, 1989; Raymond and Bauer, 2001; Tao et al., 2015).
The organic carbon and nitrogen accumulated in the coastal sediments are heterogeneous and complex mixture of organic materials with different characteristics and from different sources (marine and freshwater phytoplankton, soil, leaf debris, wastewater, kerogen (Cloern et al., 2002; Lamb et al., 2006; Schreiner et al., 2013; Cai et al., 2015). However, depending on riparian vegetation, stream size and discharge of autochthonous materials, microbial production may also contribute significant portion to riverine organic matter. Knowledge of the sources and composition of organic matter in estuarine and coastal sediments and factors controlling its distribution is important for the understanding of global biogeochemical cycles (Wang et al., 2013; Li et al., 2015a, b). The coastal zone and river mouth systems are important interfaces between the continent and ocean, in which change and sequestration of the carbon and nitrogen among land, river and ocean have an impact on the biogeochemistry of these elements at a global scale (Hedges and Keil, 1995; de Haas et al., 2002; Bianchi and Allison, 2009).
Dissolve organic matter is the largest reservoir of organic carbon in the world ocean and the estuary, and life depends on organic carbon. Land use change including reclamation and managed agro-ecosystem development can increase export derived of soil organic carbon stored in aquatic sediments (Bianchi, 2011). The rapid urbanization and economic growth results in the input of agricultural wastes, industrial discharge, sewage and waste water into the ocean thereby contributing to the organic matter in the coastal sediment. Aquaculture and human waste can affect at different levels of the ecosystem reducing the biomass, density and diversity of the benthos, plankton and nekton and modify natural food webs in coastal areas (Gowen et al., 1991).
Due to large quantities of sewages and other untreated and partially-treated pollutants which have been discharged into the estuary, it causes significant toxic impact on the aquatic environment. Urban sewage inputs strongly influence the quantity and quality of incoming materials in the sediments and therefore the nature of organic materials depend on complex physicochemical processes occurring in overlying water layers and in the sediments (Cotano and Villate 2006).
Particulate organic matter and nutrients brought to the coastal ocean by rivers are generally enriched in nitrogen compared to coastal primary producers, due to anthropogenic activities hence allowing efficient characterization of riverine inputs into coastal food webs (McClelland et al., 1997; Riera et al., 2000). Organic matter in sediments has been recognized as the major factor affecting metabolism, distribution and dynamics of benthic organisms (Grant and Hargrave, 1987; Graf 1989; Duineveld et al., 1997) and has been widely employed to evaluate the trophic state of both marine and estuarine ecosystems and portion of sedimentary organic matter which are more readily available to benthic consumers (labile fraction are assessed by estimation of the main biochemical classes of organic compounds (Fichez 1991; Danovaro et al. 1993; Fabiano et al. 1995; Dell’Anno et al. 2002; Cloern 2001; Renjith et al., 2012; Akhil et al. 2013).
The biochemical composition of sedimentary organic matter has been employed to gather information on the nature and parameters controlling the diagenetic fate of organic matter. The concentrations of labile organic biopolymers (proteins, carbohydrates, lipids and chlorophyll pigments) are considered as best tool for the evaluation of nature and quality of organic matter which can provide clear insights into the biogeochemical characterization of sedimentary environment in estuaries (Colombo, 1996; Dell’ Anno et al., 2002; Pusceddu et al., 2009, 2011; Venturni et al., 2012).