You'd figure that in a given human body only has so much capacity to convert stored potential energy into kinetic energy....the rest turns to heat. Now the question is whether cycling offers a better mechanical advantage for the conversion than rowing. Arguably rowing uses more muscle mass, but the question is how well does one sport convert energy to mechanical work using the various muscle groups vs the others. Given that rowing uses more muscles, you'd think rowing would more easily offer a high cardio load.

This one is an interesting discussion for me, cause I am intrigued by what goes on in XC skiing vs cycling. XC skiing involves much larger body mass. I feel I can load my cardio much better on a consistent basis on XC skis than biking cause I "run out of legs" more often riding and my legs just need more recovery time between hard sessions. Between hard XC ski sessions, where I am always cardio limited, I can always bounce back the next day and go hard again.

The only other sport where I can go "that hard that often" is swimming, but even then I can never do a 15 hour week of swimming only (because I am technique limited)....on XC skis I can....and I'd imagine in rowing you can too.