Examples

Baltic Sea - several anoxic basins, e.g Gotland Deep. At irregular intervals of several years ousaline water is pucshed through the belts and fills the deep basins with oxygenated water.

Black Sea at >100-240 m depth, caused by saline Mediterranean water pssing through the bosphorous and sinking to the botto causing vertical stratification. The flux of organic matter to the bottom maintains the saline water anoxic.

Cariaco Trench - off Venuzula at >380 m. The trench is 200 x 50 km and 1400 m deep. A cill prevents horizontal transport of water in the lower layers.

Caspian Sea at > 300 m depth

Mediterranean - deep water basins, e.g. Bannock Basin.

Fjords with cills, e.g Mariager Fjord, Denmark.

Ecosystems
The ecosystems over anoxic basins vary from oxic ones in the surface layers to hypoxic in the mixing or micro-aerobic zone and to anoxic ones below. In shallow water basins the zooplankton in the oxic zone has a different species dstribution from neighbouring areas since species with sinking eggs, or a benthopelagic phase, will not develop.

In the zone where oxygen and sulphide co-occur chemosythetic production may give rise to a higher biomass than immediately above or below.

Black Sea
Sediment dating suggests that the onset of water-column anoxia at 7.5 ka BP was virtually synchronous across the basin over a depth range of to 200 - 2250 m. The trigger for this anoxia was the spillover of saline waters through the Bosporus.
Particulate organic carbon, nitrogen, and sulfur are produced in surface waters from photosynthesis and recycled via oxic respiration. When this organic matter reaches anoxic marine environments, regeneration is largely coupled with microbial sulfate reduction, producing hydrogen sulphide.
The upper boundary of the sulphide-containing water may oscillate up and down over a range of 30-50 m during the year. Correlation between
the depth of the sulphide onset and the density structure of the layer of the main pycnocline is significant for the period 1910-1995. Oscillations in the average depth of the sulphide onset range over 24 m and indicate that a steady-state trend does not occur. The period of these oscillations may occur over a century timescale.
The carbon-14 age of the total inorganic dissolved carbonate in the deep water is 2000 years in the Black Sea.
Biomass and activities of planktonic microorganisms (bacteria, nanoplankton and microplankton) were measured in the Black Sea during summer 1995. Fluorescently labeled prey were chosen to determine the ingestion rate of bacteria and nanoplankton by phagotrophic microorganisms. This method revealed the presence of mixotrophic organisms such as 'plastid-retaining ciliates' in the whole coastal area. Mixotrophic ciliates were dominated by micro-sized forms and maximum biomasses were recorded in the water masses characterised by low nutrient concentrations but high food particle concentrations. Mixotrophic nanoflagellates were absent and mixotrophic dinoflagellates were observed at one station only. Mixotrophic ciliates were shown to ingest preferably bacteria while mixotrophic dinoflagellates were grazing almost exclusively on nanoflagellates. Although the biomass of mixotrophic organisms were significantly lower than those of aplastidic protozoa, their feeding activity contributed to 14 and 24% of the ingestion of bacteria and nanoplankton, respectively.
Female Calanus euxinus begin migration to the upper phytoplankton-rich layer approximately 3 or 4 hours before sunset. They grazed for between 7.5 h and 10.5 in the euphotic zone removing 9.5 to 14.5% of the primary production.
An analysis of the main constituents of the pelagic ecosystem of the Black Sea changed following the outbreak of the ctenophore, Mnemiopsis leidyi, when Aurelia aurita declined. The total gelatinous biomass increased from 250 g m(-2) in 1980 to 2500 g m(-2) in 1995. M. leidyi condumed 7% of the available biomass per day at its peak and 50% of the daily production of zooplankton. At this time there was a reduction in numbers of planktivorous fishes (Sprattus sprattus, Engraulis encrasicholus and Trachurus mediterraneus), the main competitors of M. leidyi. The reduction in these fish could be a reason for the upsurge in abundance of M. leidyi. Following the increase of M. leidyi, there was a decline in the abundance and species diversity of ichthyoplankton and mesozooplankton. An assessment of data collected during the period 1992-1997 showed that the number of fish eggs and larvae and of zooplankton was negatively related to M. leidyi abundance. After the decrease of M. leidyi in the period 1995-1997, there was an increase in abundance and diversity of fish eggs, fish larvae, and zooplankton, which together with an increased catch of planktivorous fish indicates that there has been a recovery of the ecosystem.

Chemocline
Five strains of the phototrophic sulphur bacterium Chlorobium phaeobacteroides occurs at the chemocline, all contain bacteriochlorophyll e as the main photosynthetic pigment. These strains are adapted to low light and can grow at <4mE m-2 s-1 due to an increase in chlorophyll by 175-fold compared with the type strain. At the Black Sea chemocline yhere is a light transmission of 0.0005% of the surface irradiance. Anoxygenic photosynthesis is believed to acount for between 4 and 13% of total sulphide oxidation.

Fjords
Sulphide removal rates were as high as and 240 /l/day were measured at the oxic/anoxic interface in Framvaren Fjord, Norway, some of the highest in the world (compare with hydrothermal vents at the Juan de Fuca Ridge - 164 µmol mol/l/day)

Mariager fjord

Zooplankton
44 spp. of heterotrophic flagellates are found, 9 from the anoxic zone, 6 restricted to the microaerobic and 29 exclusively or predominantly in the oxic zone.
37 spp. of ciliates were identified, only 6 were confined to the anaerobic zone.
Ciliates dominated the top of the oxic zone with flagellates below.