Nutrient remineralization

The inefficiency of microbial heterotrophy does have a side benefit as it enhances nutrient remineralization rates. This serves to increase the availability of inorganic nitrogen and phosphorus for the photoautotrophs. The multiple roles of bacteria in the marine food web were shown in Figure 23.2, with the component of the food web controlled by the algal herbivores depicted on the left side and the microbial loop on the right. The viral shunt acts on both pathways. 

Because of its high organic content, the marine snow acts as a microhabitat that supports enhanced rates of heterotrophic microbial activity. The associated nutrient remineralization causes the seawater within and aroimd the marine snow to be characterized by elevated nitrogen and phosphorus concentrations and low levels of O2. The importance of these suboxic and anoxic microzones to the marine cycling of the biolimiting elements is unknown but potentially significant. 

Nowicki, B.L., and Nixon, S.W. (1985) Benthic nutrient remineralization in a coastal lagoon ecosystem. Estuaries 8 182-190. 

Roques, P.F. (1985) Rates and stoichiometry of nutrient remineralization in an anoxic estuary, the Pettaquamscutt River. Ph.D. dissertation, University of Rhode Island, Narragansett, RI. 

Oceanic surface waters are efficiently stripped of nutrients by phytoplankton. If phytoplankton biomass was not reconverted into simple dissolved nutrients, the entire marine water column would be depleted in nutrients and growth would stop. But as we saw from the carbon balance presented earlier, more than 90% of the primary productivity is released back to the water column as a reverse RKR equation. This reverse reaction is called remineralization and is due to respiration. An important point is that while production via photosynthesis can only occur in surface waters, the remineralization by heterotrophic organisms can occur over the entire water column and in the underlying sediments. 

It follows that deep seawater contains nutrients from two sources. First, it may contain nutrients that were present with the water when it sank from the surface. These are called preformed nutrients." Second, it may contain nutrients derived by the in situ remineralization of organic particles. These are called oxidative nutrients. 

Anderson, L. A. and Sarmiento, J. L. (1994). Redfield ratios of remineralization by nutrient data analysis. Glob. Biogeochem. Cycles 8,65-80. 

This equation describes the ratios with which inorganic nutrients dissolved in seawater are converted by photosynthesis into the biomass of "average marine plankton" and oxygen gas 02. The opposite of this reaction is respiration, or the remineralization process by which organic matter is enzymatically oxidized back to inorganic nutrients and water. The atomic ratios (stoichiometry) of this reaction were established by... 

Plankton produce biogenic particles in the surfece waters of all the ocean basins. Most of these particles sink into the deep sea and are then remineralized. The rain of biogenic particles causes the nutrient concentration of the deep-water masses to increase as they move through the ocean basins for two reasons. First, the further a deep-water mass has traveled from its site of formation, the greater the amount of particles it will... 

Nitrate versus phosphate concentrations at 2500 m in the (a) Atlantic, (b) Indian, and (c) Pacific Oceans. Dissolved oxygen versus phosphate concentrations at 2500 m in the (d) Atlantic, (e) Indian, and (f) Pacific Oceans. The slopes of these lines represent the proportions by which these constituent concentrations are altered by the remineralization of POM in the deep sea. These data are replotted from Figure 10.1. Source From Conkright, M. E., et al. (2002). World Ocean Atlas 2001, Volume 4 Nutrients, NOAA Atlas NESDIS 52,... 

The problem with this fixed nitrogen is that its use as a fertilizer requires land application. While some of the nitrogen is retained by the plants, much is carried off the land as stormwater runoff This increases the concentration of DIN in groundwater and river-water. Drainage of these waters into the coastal ocean supplies nutrients that stimulate plankton growth. Remineralization of the plankton biomass can lead to development of hypoxic and anoxic conditions in coastal waters. 

The impact of anthropogenic nutrient emissions in the coastal zone is heightened by its chemical speciation. Pollutant nitrogen and phosphorus are delivered to the coastal waters primarily in inorganic form, whereas most of the natural riverine dissolved nitrogen and phosphorus are components of organic compounds, i.e., DON and DOE Thus, the pollutant nutrients are delivered to the coastal waters in a chemical form that can be directly assimilated by coastal plankton, whereas the organically bound (natural) forms must first be remineralized. 

Remineralization The dissolution of hard parts or the degradation of POM that leads to solubilization of nutrients and micronutrients. 

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