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Nutrient Dynamics in Sand

Many articles deal with the subject of nutrient dynamics in sediments - decomposition, oxidation, reduction, remineralization, sequestration and many other aspects. I focus here on works that summarize practical understanding of nutrient dynamics in coral reef carbonate sediments in hopes that those who feel sand beds are a sink (or source) for nutrients gain a better understanding of these processes. In particular, it is ironic that many aquarists state that sand beds become “nutrient sinks” that result in a “source” of nutrients in the tank; terms that are technically antithetic to each other. Entsch et al (1983) examined the dynamics of phosphorus and nitrogen in the sediments of coral reefs around Davies Reef, part of the Great Barrier Reef, and Suzumura et al (2002) examined phosphorus cycling in the reef sediments of Ishigaki Island, Japan. These, along with the studies cited therein, can be summarized as below.

Both teams found that surface water typically was extremely devoid of nutrients while the sediments contained an inorganic phosphorus pool (300ppm by weight in Entsch et al 1983). Earlier works had supposed that carbonate substrates strongly bound or absorbed phosphates. Their sediment analyses contained Halimeda, coral, molluscs, coralline algae, forams and other carbonate sediments, similar to those found in many aquariums. They (and others) showed no change in phosphate with water that contacted reef substrates. Entsch et al (1983) examined nitrogen and phosphorous ratios (N:P) of phytoplankton and benthic macroalgae and found the algae to contain higher nitrogen and in some cases, phosphorus, than the phytoplankton whose composition resembled those found in the water column. The majority of phosphorus was found in the inorganic form (>80%), indicating long-term concentration of phosphorus in the sediments representing a nearly uniform pool across depth gradients to 5 meters (also found to be the case at other reefs). The results of Suzumura et al (2002) were similar. The conclusion was that sediments did act as a sink for phosphate.

What about soluble phosphorus and sediments acting as a “source” or, as aquarists often state, sand beds “leaching” phosphates into the water? Including nitrogen and phosphorus, the researchers hypothesized that the large nutrient pool must be large compared to the total biomass, though plants and animals can have 10 and 100 fold higher concentrations. Phosphate solubility is complex in the sediments, and they noted the distinct layering of oxic over anoxic (reducing) sediments 5cm from the sediment/water interface. This distance varies and can be as little as a few millimeters in organically enriched fine sediments. In sediments less than 10cm deep, as would be found in most aquariums, soluble phosphorus was thought to depend mainly on the species of calcium phosphate present and pH, and also to cause nitrogen to occur mainly as ammonia (as was found to be the case). The concentration of phosphate was 30 times higher in the shallow interstitial water of the sediments than the water column and thus available to algae in the sediments. Entsch et al (1983) examined desorption of phosphate from sediments and found that only 1-3% was lost as soluble material, although rapid cycling but containment was found by Suzumura et al (2002). Water moving through the sediments and the subsequent washing away of organisms binding or sequestering phosphorus can increase, even indirectly, phosphorus in the water column. Yet, this increase is not significant in the overlying water in most cases. Inputs to the sediment nutrient pool are likely the accumulation of particulate material (detritus) and its accumulation into reef biota that form carbonate sediments, with biological pathways, probably benthic algae (microphytobenthic communities), retaining the phosphorus absorption onto calcium carbonate with relatively rapid adsorption and desorption resulting in low net loss of phosphate from the sediments as they are taken up during photosynthesis. These benthic algae are then used as food for grazing species. Higher levels of particulate material that could provide input to the sediments are likely minimized by the consumption of those particles by benthic species such as corals and other filter-feeding invertebrates. Furthermore, the loss of nitrogen to the overlying water of sediments is mediated by benthic species’ uptake, and perhaps by never escaping the anoxic areas. Traditional views of the nitrification-denitrification pathway involves the production of both ammonium and nitrate, either of which may escape sediments into the water column. Thamdrup and Dalsgaard (2002), however, showed that with coupled ammonium oxidation-nitrate reduction, ammonium may be largely consumed in the anoxic zone of the sediment and reduce its escape into the overlying water.