Bugs, smelly water and marine dead zones-that is no fish zones.

Many, very many, global measurements by oceanographers tell us that the sea almost everywhere from surface to seafloor contains moderate quantities of dissolved oxygen gas. Nonetheless, certain restricted areas either contain far less dissolved oxygen than normal or none at all. Regions with low dissolved oxygen are called hypoxic; those with no measurable dissolved oxygen are anoxic, or anaerobic. Such environments, usually with a sluggish water circulation, are created by an excessive inflow of easily oxidised matter, most usually dead phytoplankton (‘plant’ plankton), which uses up much dissolved oxygen as, sinking, it is decomposed by rapid, efficient bacterial action, in the process recycling the essential major nutrients, carbon, nitrogen and phosphorous.

But why bother about dissolved oxygen at all? Well for a start, all higher living creatures need oxygen to live! Marine inhabitants from the tiniest zooplankton or shellfish to the largest finfish all access dissolved oxygen through their gills. Marine mammals, turtles and so on, of course breathe air from above. However, should hypoxic conditions pertain, those fish that can, will although distressed, swim away; unfortunate sessile (‘fixed’) creatures like limpets or mussels, will suffocate and soon die. Even worse, should anoxia follow hypoxia as often happens, all that survives will be specialised microbes such as anaerobic bacteria and some plants, while the chemistry of the water changes amazingly as dissolved oxygen disappears. These “dead zones” of anoxic waters may be temporary and a good storm with high waves will mix freshly oxygenated sea water down flushing out the anoxic layers, or deeper ocean currents may bring in a new supply of the essential dissolved oxygen. The only large volume of “permanent” anoxic water in the world ocean lies below 100 m or so in the Black Sea, which is cut-off and not really a part of the main ocean system, in fact the lack of incoming oxygenated water is the major reason why it remains anoxic. However, large areas of the near bottom, near coastal seafloor world-wide are becoming more and more anoxic as excess fertilising compounds (mostly nitrate and phosphate) flowing in both from intensive agriculture and from excessive human sewage wastes vastly stimulate natural algal growth, called eutrophication, massively increasing the downward flow of dead matter, putting much extra strain on the deeper dissolved oxygen supply and thus reducing it. Ultimately this creeping anoxia drives away coastal commercial fish stocks and wipes out those marine life forms unable to migrate away from such dead zones. Worse, some anaerobic microbes are pathogenic causing very nasty diseases if ingested. Dead zones are ‘very bad news’, but regrettably are increasing globally both in volume and in area.

Remember: Dead Zone equals No Fish.

But another thought. Geochemists tell us that the ocean has only contained measurable dissolved oxygen for about half its lifetime-before this time it was everywhere anoxic and home to microbes we would definitely not select by ourselves! These specialised microbes used hydrogen gas, then far more abundant, along with much carbon dioxide, and made methane (natural gas) in quantity, some using sunlight as an energy supply; note that unlike today these early photosynthetic organisms did not produce dissolved oxygen. Others used dissolved iron then orders of magnitude more abundant than today as an energy source, still more used elemental sulphur and its compounds like hydrogen sulphide, a “rotten-eggs” smelling gas, extremely toxic to all higher life forms. Then an “environmental crisis” took place. Evolving photosynthetic cyanobacteria near the ocean surface discovered how to create organic matter far more effectively by the photosynthetic process we know today and which process now produced free oxygen gas as a by-product. When such photosynthetically generated dissolved oxygen first arrived in the sea it was like a horrible environmental pollutant since even in minute quantities, it annihilated entire native populations of anaerobic microbes by the trillions. As dissolved oxygen increased in stages, the ocean changed its chemical character to that of today, higher life forms evolved then proliferated. The life giving gas to us-oxygen-thus sounded the death knell for the earliest marine inhabitants which had occupied the young ocean for billions of years. Before long, geologically speaking, oxygen gas seeped into the overlying atmosphere where it oxidised gases like methane and slowly built up, eventually through its making of stratospheric ozone, providing our ultra-violet umbrella shielding us from these harsh rays given out by the sun. This was a truly earth-changing event, permitting life as we know it to flourish-and photosynthetic oxygen was the prime mover.

However, these ancient marine anaerobes have simply gone into hiding and when pollution or natural causes induce anoxic waters, back they flock, the waters soon reek of poisonous hydrogen sulphide gas and even more evil-smelling phosphorous compounds, even traces of the “war gas” phosphine (PH3), are generated; kreef “walk out” of the water and fish move away. Anoxic waters occur naturally in mid-depth layers off our west coast, growing and subsiding at intervals. Near-shore hypoxic waters are increasing globally mainly from man-made effluents, but it is fascinating to recall that such unpleasant smelly “dead” waters are in every sense a “throw-back” to the youthful days of our mighty ocean.

 

Fig: General collection deadzone

 

Fig: Lobster walk-out