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

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 tidallymixedphytoplankton. Almost 20 years later, thiswas elaboratedby Sverdrup (1953) into ahypothesis for the initiationof the spring bloom in the North AtlanticWater (NAW)masses. Since then, variations inmixed layer depth have been a key in phytoplanktonmodelling. 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 lackof 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 canbepartially accounted for by variations in salinity anddissolvedoxygen. The light attenuation coefficient at 440 nm increased by 0.041 and 0.032 m with drops in salinity and dissolved oxygen of 1PSU and 1 ml O2 l , respectively. Consequences for the euphotic depth, Sverdrup critical depth, and thenutricline depth are discussed. I conclude that phytoplanktonmodelling, particularly across coastal and oceanicwaters, such asNCWandNAW, needs to account for variations in the non-phytoplankton light attenuation and that salinitymight be a useful proxy for regional parameterizations.