The Faroe shelf spring bloom onset explained by a ‘Critical Volume Hypothesis’

Abstract

Abstract The variable annual primary production on the Faroe Shelf drives changes in higher trophic levels. However, the dynamics triggering the onset of the on-shelf bloom have previously not been well understood. Here we investigate how solar radiation and stratification on the outer periphery of the shelf affect the primary production on the central part of the shelf in spring, and we introduce a Critical Volume Hypothesis (CVH) to explain the initial onset of the bloom. The water masses on the entire shelf are vertically well mixed in winter but horizontally, the Central Shelf is separated from the deeper Outer Shelf by a tidal front. While the Central Shelf is always vertically mixed, the Outer Shelf becomes stratified during the spring which causes the front to change character and find a new position nearer shore. Simultaneously, the increasing radiation induces net phytoplankton accumulation in increasingly deeper water extending out from the shelf – i.e. an increased critical volume. In the CVH the onset of the spring bloom occurs when the net primary production within the Central Shelf becomes positive, which in an idealized circular symmetric version of the CVH translates to an average distance out to the front (Central Shelf width) becoming narrower than the critical width – analogous to a mixed layer becoming shallower than the critical depth in Sverdrup's vertical critical depth hypothesis. Utilizing comprehensive observations and simulations from 1997 to present, we show that more often than not, the CVH helps explain the onset time of the Faroe shelf spring bloom.