Chinese 节气 are certainly not defined or tracked by natural phenomena such as fish becoming visible under the ice or plants blooming, though. As Wikipedia notes, they are defined astronomically and are used to calculate when an extra month needs to be inserted into the calendar.
(Each 节气 is assigned a polarity, yin or yang. A month is supposed to include one yin point and one yang point. Since lunar months are too short, eventually there will be a month with only one point instead of the correct two; that month is immediately repeated. I think this system is very cool because just a little thought will show that it is guaranteed to stay within one month of the "true" position of the earth wrt the sun, using just the dead simple method of "Oh, were we too slow this month? We'll put in some overtime.")