1. The biogeochemical cycles can be divided in two basic types: 1.Cycles of gaseous type 2.Cycles of sedimentary types In the gaseous cycles, the main storage is the atmosphere and there is little (or no) loss of the nutrient during the relatively fast process of re-circulation or recycling. The nutrient is distributed widely in the atmosphere and relatively accessible to the organisms. Typical examples are the oxygen, carbon and the nitrogen cycles.

2. In the sedimentary cycles, the nutrient is stored in the sedimentary rocks or earth’s crust. These cycles are slow and have a limiting influence upon living organisms. In order that nutrients reach for example a plant, the rocks need weathering, the nutrient is slowly wash-out, transported to the surface soil and by means of run-off (for example a stream, river) the nutrient reach the ocean and is deposited finally at the bottom of the ocean (or lake) as sediment. The trapped element is not accessible to the organisms, until a disturbance (i.e. water turbulences, volcanic eruptions) occurs and thus expose the nutrients back to the atmosphere or dissolve them into the water column. This implies that these nutrients can constitute a limiting factor. Examples are the phosphorus and sulphur cycles.

3. Phosphorus Cycle P cycle is a typical sedimentary cycle, the major reservoir of P is stored in rocks and can come back to the sea only through weathering and run off of water

4. The second largest reservoir of P is land and then the deep ocean from where P can settled at the bottom and be buried with a global rate of 1.9 Tg per year. Note that turnover time of surface ocean P is less than 3 yrs compared to 1,500 yrs of the deep ocean.

6. By far marine sediments contain the largest pool of P, but uplifting of the ocean sediments is not a substantial process to give back P to the water column, with the exception of very shallow environments. Instead rivers, which receive excess of fertilizers, and sewage effluents have a significant role in increasing both inorganic and organic P, particularly in the coastal shallow areas.

7. The other animals that play a unique role in the phosphorous cycle are marine birds. These birds take phosphorous containing fish out of the ocean and return to land, where they defecate. Their guano contains high levels of phosphorous and in this way marine birds return phosphorous from the ocean to the land. The guano is often mined and may form the basis of the economy in some areas (Chile, Perù).

8. Focusing at the sea: inorganic P (DIP) is present almost exclusively as orto-phosphate (PO 4 -2 ). Organic phase is divided in particulate (POP) and dissolved (DOP) compounds. Algae and bacteria use DIP to build up their biomasses which is eaten by consumers. These excrete P as DOP and POP. DOP is transformed very quickly in DIP by bacteria (hrs). Uptake of P by producers is also very fast so that DIP disappears in few min. P is found principally incorporated in living particles. Considering the equilibrium among DIP,DOP and POP, the rate of transformation are such that the concentrations of POP exceeds those of DIP and DOP.

9. Sea water P concentrations are usually very low, and beside uptaken by producers, there are two aspects of PO 4 -2 chemistry very important: the facility of adsorption onto amorphous oxyhydroxides, calcium carbonate and clay particles, and the propensity to form insoluble compounds with certain metals (Al, Fe). Particles settle at the bottom and can be buried and become permanent depositions so that the geological cycle closes. In anaerobic environments due to the presence of bacteria and H 2 S, ferric iron is reduced to ferrous iron, which is less effective in adsorbing PO 4 -2 thus resulting in greater availability of DIP in anaerobic environments, which in turn can diffuse to the water column.

10. Climatology of ortophosphates in the Gulf of Trieste (1998-2005). Note that only occasionally PO 4 -2 exceed 0.2 M. There are two peaks of maximum in March and in October, at both the surface and the bottom (15m), concomitant with maximum river discharge. Values are slightly higher at the bottom, particularly in December when uptake by bottom algae becomes negligible because of photolimitation

11. Nitrogen Cycle This is a typical gaseous cycle: in the atmosphere there are 3.87 x 10 21 g of N, which corresponds to the 78% of the total volume. Most of the atmospheric N is present as N 2, but NO x are a significant part. N 2 can enter the sea and be fixed by cyanobacteria, but there is still not conclusive data on the fixation rates, which are definitely less intense than in fresh waters.

12. Nevertheless the biological fixation, N in the atmosphere acts as a conservative element, its turnover time being in the order of 10 7 yrs. Exchanges are mostly controlled by reciprocal partial pressure. Marine biomass is 2 orders of magnitude less then the terrestrial one, but it has a shorter turn over time.

13. A simplified N cycle: atmospheric N 2 is fixed by N fixing bacteria and enters in the plant tissues, which are eaten by consumers; the both can be decomposed at their death by decomposers that produce ammonium (NH 3 ). By a two steps nitrification processes bacteria transform NH 3 in nitrate (NO 3 - ), which can be taken up by plants again. Via denitrifying bacteria the 3 forms of N can be transform in gaseous N 2 and returns to the atmosphere.

14. The most important source of N for the marine realm is the run off, which transports from land N of different origin. Dissolved inorganic N (DIN) is taken up by algae, which preferentially take up NH 4 + over NO 3 -, and take up NO 3 - when NH 4 + is depleted, because it must be reduced within the cell by assimilation processes involving several enzymes, and thus requiring energy. Algae are eaten by consumers that excrete N as dissolved organic N (DON) or as particulate (PON). PON can settles at the bottom and be buried, more frequently it is modified by bacterial action in DON, which again is transformed by bacterial processes in NH 4 +.

15. In aerobic compartment nitrification occurs, which transforms NH 4 + in a two steps process eventually in NO 3 - or alternatively in NO 2. In anaerobic environments, beside bacterial degradation of organic matter, which produces NH 4 +, denitrification can occur, which transforms again NO 3 - in N 2 or alternatively in NO 2. Both N forms are given back to the atmosphere in the gaseous phase.

16. N 2 is fixed by primary producers to build up their proteins, which are decomposed by heterotrophic bacteria, producing NH 4 +, which in aerobic environments via nitrification is firstly transformed in NO 2 - and then in NO 3 -. Via assimilative reduction it is converted again in NH 4 +, or directly in N 2 gas back to the atmosphere. In anaerobic environments dissimilative ammonia production makes the same conversion with the difference that N is not use to produce proteins.

17. More specifically in the diagram are reported the specific species of bacteria involved in fixation, nitrification and denitrification. Note that N assimilation is a process requiring energy, while nitrification produce energy.

18. In the aquatic environments N can be found in 7 different states of oxidation, which are biologically mediated.The most oxidized step is NO 3 - and when there is not uptake by producers is this form to prevail. In cold temperate environments this is the most abundant form at the end of winter when, because of photo-limitation, PP halts.

21. Because NO 3 - availability is typical of the beginning of a phytoplankton bloom after the winter pause of uptake by producers or after an upwelling event we call this “ new ” production compared to the “ regenerated ” one, which takes place later in the season and is mostly based on regeneration processes made by all living organisms. The ratio between “ new ” and “ regenerated ” production is called f ratio and can give an indication about N exported to the bottom.

22. Because of denitrification and dissimilative production all inorganic forms of N can be released by anoxic sediments (or water), as well as gaseous N 2.

23. Climatology of NH 4 + and NO 3 - in the Gulf of Trieste (1998-2005). The bulk of total N is mostly made by nitrates that show at the surface a clear annual pattern with maxima corresponding to river discharges. Ammonium is more abundant at the bottom, particularly at the end of summer due to regenerative processes.

24. Humans often mine rock rich in P. The phosphate is then used as fertilizer. This mining of phosphate and use of the phosphate as fertilizer greatly accelerates the P cycle and may cause local overabundance of P, particularly in coastal regions, at the mouths of rivers, and any place where there is a lot of sewage released into the water. Local abundance of phosphate can cause overgrowth of algae in the water; the algae can use up all the oxygen in the water and kill other aquatic life. This is called eutrophication. From Paers (2003).