Sverdrup critical depth and the role of water clarity in Norwegian Coastal Water

Abstract

The critical depth concept was first recognized by Gran and Braarud (1935). During summer, in the Bay of Fundy, they observed an unexpected no bloom situation. Their interpretation was that high amounts of detritus of terrestrial origin caused too murky water and insufficient light for the tidally mixed phytoplankton. Almost 20 years later, this was elaborated by Sverdrup (1953) into a hypothesis for the initiation of the spring bloom in the North Atlantic Water (NAW) masses. Since then, variations in mixed layer depth have been a key in phytoplankton modelling. As illustrated by the study of Gran and Braarud, variation in the non-phytoplankton light attenuation coefficient is also a key to understand phytoplankton bloom conditions. Due to lack of accurate parameterizations, however, non-phytoplankton light attenuation is often assumed invariant in phytoplankton modelling. Here, I report spatial variation in a proxy for the pre-bloom light attenuation in Norwegian Coastal Water (NCW). It is shown that this variation can be partially accounted for by variations in salinity and dissolved oxygen. The light attenuation coefficient at 440 nm increased by 0.041 and 0.032 m−1 with drops in salinity and dissolved oxygen of 1PSU and 1 ml O2 l−1, respectively. Consequences for the euphotic depth, Sverdrup critical depth, and the nutricline depth are discussed. I conclude that phytoplankton modelling, particularly across coastal and oceanic waters, such as NCW and NAW, needs to account for variations in the non-phytoplankton light attenuation and that salinity might be a useful proxy for regional parameterizations.