What Bhagavata Purana text mentions apropos this question? Is Bhagavan Krishna the source of all incarnations or Bhagavan Vishnu as traditionally believed?
एतौ भगवतः साक्षात् हरेर्नारायणस्य हि । अवतीर्णाविहांशेन वसुदेवस्य वेश्मनि ॥ Bhagavatam 10.43.23 ॥
Meaning: These two brothers are Avataras of rays of the almighty Hari whose Abode is cosmic waters, descended on the earth in the home of Vasudeva.
अप्यद्य विष्णोर्मनुजत्वमीयुषो भारावताराय भुवो निजेच्छया ।Bhagavatam 10.38.10
Meaning: ..Lord Viṣṇu, the Abode of beauty and excellence, who by His own will has incarnated as a human being (Krishna) for lightening the burden of the earth.
प्रधानपुरुषावाद्यौ जगद्धेतू जगत्पती । अवतीर्णौ जगत्यर्थे स्वांशेन बलकेशवौ ॥ ३२ ॥
Meaning: They (Purusha and Prakriti) had descended on the earth in their Amshas as Balarama and Krishna for the good of the world.