If there is global warming, much of the land will be covered in water. If this happens, will the water become warmer? If so, does that mean that there will be fewer zooplankton or whichever plankton creates oxygen?

Ocean temperatures already vary from place to place. If there is significant climate change then the pattern of ocean temperatures is likely to alter. In the event of global warming probably, yes, average ocean temperatures are going to rise, but that fact may be less significant than the altered distribution of ocean temperatures. So plankton which currently flourish in one particular region or latitude may in the future not disappear, but instead flourish in a different region or latitude. I don’t know what effect this might have on oxygen production.

The other factor to bear in mind is that if ocean levels rise then (a) there is more water in the oceans, and (b) the oceans have a greater surface area, and (c) the ratio of deep ocean to shallow ocean changes (in favour of a greater proportion of shallow ocean. All this too could have a greater effect on oxygen-producing plankton than the rise in average water temperatures.

Welcome Purnima

Fresh water fish will be affected by warmth to. Rivers will lose their trout and salmon.

http://oceanlink.island.net/ONews/ONews7/plankton.html

The link above is quite informative if like me you're not an expert on marine life.

I don't know but I believe that if there were remotely a chance that warmer temperatures would threaten the supply of oxygen on earth, we would have been hearing about it from those with a political investment in ACG.

Global warming (if caused by atmospheric carbon) does involve a reduction in oxygen concentration in the atmosphere, not so much as a consequence of reduced plankton activity but for a much more fundamental reason. The greater the concentration of CO2 (and other greenhouse gases) in the atmosphere, the lesser the concentration of nitrogen and oxygen.

But so far as I know none of the models predict a lowering of oxygen concentration to anything like a point which might inconvenience us.

Air is about 21% oxygen. For an adult in normal health, this can fall to below 19% without us suffering any consequences at all. At 17% your judgment is affected. Below 16% breathing and pulse rate increases, and muscular co-ordination is impaired. When it gets down to 12% you suffer abnormal fatigue and emotional upsets, and below 10% nausea, vomiting and some degree of paralysis. By the time it gets down to 6% you’ll have lost consciousness. Much below that, you suffer convulsions, stop breathing and die in a few minutes.

So, a bit like a fun Friday night in King’s Cross, then.

It sounds bad, but even massively increased amounts of fossil fuel consumption are modelled to reduce atmospheric oxygen concentration by only about 0.1% The climate effects of all that atmospheric carbon would finish us long before lowered oxygen concentrations started to bother us.

The initial premise is seriously in error.

1) Global warming / climate change is real. It is however, unlikely to be anthropogenic OR catastrophic.

2) Any realistic rise in the sea level will not noticeably reduce the proportion of the Earth exposed as land.

3) Any realistic increase in average ocean temperatures will not reduce the amount of alga (photosynthesising plant material) but more likely increase it and oxygen production with it. CO2 is plant food, and rising atmospheric CO2 has been demonstrated to increase plant growth quite markedly.