When microbial action during decomposition in aquatic ecosystems removes too much oxygen, the body of water experiences

N and P limitation is strongest in aquatic ecosystems (because light and water isn't) governed by same principles as terrestrial ecosystems radically different environmental controls streams have been treated as "sewers of terrestrial ecosystems" yet stream ecosystems have their own characteristics generated by the interaction of biota with the physical and chemical properties of the system unlike terrestrial ecosystems, nutrients "spiral" rather than cycle. The more limiting the nutrient, the tighter the spiral. Detritus/heterotrophs <---> autotrophs. 

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

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i'd say that the moon rock is about 2.2 billion years old.