Nutrient Chemistry in the Tropical North Pacific DOMES Sites A, B, and C
The distributions of nutrient chemistry in the DOMES area are controlled by a balance between advective and diffusive mixing and biological reactions. The intensity of mixing changes with depth, defining 6 unique layers in terms of a T-S diagram, These include: the mixed layer, thermocline, oxygen minimum layer (OML), upper deep water (UDW), North Pacific deep water (NPDW), and Pacific bottom water (PBW).
Within each layer chemical profiles deviate slightly from linearity and at the layer interfaces the profiles have distinct changes in slope, Horizontal gradients are weak, except for oxygen in the OML where a tongue-like distribution of low oxygen extends westward along 15°N, No clear temporal patterns are evident when concentrations are referenced to density surfaces.
The interface concentrations of oxygen in the upper layers are principally controlled by the surface saturation value, thermocline diffusion and respiration in the OML. The interface concentrations of nitrate and phosphate in the upper layers are principally controlled by uptake in the thermocline and mixed layer and horizontal flux near the OML-UDW interface.
Mixed layer productivity can be depressed by a reduced nitrate flux into the layer caused by productivity associated nitrate uptake in the thermocline. With productivity occurring within the thermocline, the thermocline acts as a nutrient sink and with little or no productivity in the thermocline it acts as a nutrient filter.
The oxygen minimum tongue in the OML can be established by horizontal diffusion and respiration with the boundaries set by subsurface zonal flows north and south of the tongue.
The phosphate and nitrate maximums at the OML-UDW interface, between 800 and 1000 m, are produced by a depth decreasing oxidative nutrient profile and a depth increasing preformed nutrient profile. Horizontal advection and diffusion along density surfaces are important in maintaining the vertical nutrient maximum.
A silicate maximum in the middle of the NPDW (3000 m) is produced by a combination of deep-water circulation and a high silica dissolution rate throughout the water column. A high silicate content is acquired as the PBW flows north into the high latitudes of the North Pacific and upwells into the level of the NPDW. The NPDW flows south out of the North Pacific creating a silicate maximum at 3000 m in the DOMES area.
A high oxygen and lower nutrient content in the bottom layer are maintained by the northward flow of PBW.
Ammonia near the top of the thermocline is produced by excretion from zooplankton and is consumed by photosynthesis or by nitrifying bacteria.
The nitrite peak in the thermocline is probably produced by leakage of nitrite from phytoplankton, and consumed by phytoplankton. Ammonia and nitrite peaks are not normally observed in the mixed layer, probably because of larger vertical mixing in the layer.
In extremely low oxygen waters, nitrite is produced as an intermediate in the reduction of nitrate to molecular nitrogen in denitrification. In the process the nitrogen compounds are used in place of oxygen as terminal electron acceptors in bacterial respiration.
KeywordsMixed Layer Particulate Organic Carbon Particulate Nitrogen Oxygen Profile Nutrient Profile
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