Cold polar waters may not becomestratified at all and may continue to pro-duce at very high levels throughout theshort summer when the sun shines mostof the day (Fig. 15.28
c
). Primary produc-tion falls as the days shorten and isgreatly reduced during the long wintermonths. Polar waters, which are not nutrient-limited during the summer, canbe extremely productive, as is true around Antarctica (see Table 10.1, p.228). The Arctic Ocean, on the other hand, lies atsuch a high latitude and is so extensively covered by ice that it is generally light-limited and unproductive in spite of over-turn and mixing